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Composition of Certain Financial Statement Captions - Other Current Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Other Current Assets [Abstract]
Prepaid services		$ 644	$ 2,999
Prepaid insurance		636	698
Total other current assets	$ 989	$ 1,280	$ 3,697

Income Taxes - Unrecognized Tax Benefits Activity (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Schedule of reconciliation of unrecognized tax benefits activity [Roll Forward]
Unrecognized Tax Benefits – Beginning	$ 209	$ 81
Gross decreases – tax positions in prior period	(16)	(1)
Gross increase – current-period tax positions	215	129
Unrecognized Tax Benefits – Ending	$ 408	$ 209

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2023

Income Tax Disclosure [Abstract]

Reconciliation of Total Income Tax Provision

A reconciliation of the total income tax provision tax rate to the statutory federal income tax rates of 21% for the years ended December 31, 2023 and 2022, respectively, is as follows:


	2023			2022
Income tax expense (benefit) at federal statutory rate		(21.0	)%		(21.0	)%
Change in valuation allowance		25.5	%		22.5	%
Income tax expense (benefit) at state statutory rate		(0.2	)%		(0.2	)%
Share based compensation		1.0	%		0.9	%
NOLs expiring and adjustments to NOL		(0.1	)%		0.5	%
Research credit		(5.1	)%		(2.5	)%
Return to provision		(0.1	)%		(0.1	)%
Change in state rate		—			(0.1	)%

		0.0	%		0.0	%

Deferred Tax Assets and Liabilities

The tax effects of temporary differences that give rise to significant portions of our deferred tax assets and deferred tax liabilities as of December 31, 2023 and 2022 are as follows (in thousands):


	2023			2022
Deferred tax assets:
Accrued expenses	$	269		$	262
Share based compensation		99			107
Net operating loss carryforwards		13,397			12,605
Income tax credit carryforwards		1,630			956
Property and equipment, principally due to differences in depreciation		154			89
Intangible assets		3,527			2,073
Other, net		453			53

		19,529			16,145
Valuation allowance		(19,486	)		(16,092	)

Total deferred tax assets, net of allowance		43			53


	2023			2022
Deferred tax liabilities:
Other		(43	)		(53	)

Total deferred tax liability		(43	)		(53	)

Net deferred tax assets (liability)	$	—		$	—

Unrecognized Tax Benefits Activity

Following is a tabular reconciliation of the unrecognized tax benefits activity during the years ended December 31, 2023 and 2022 (in thousands):


	2023			2022
Unrecognized Tax Benefits – Beginning	$	209		$	81
Gross decreases – tax positions in prior period		(16	)		(1	)
Gross increase – current-period tax positions		215			129

Unrecognized Tax Benefits – Ending	$	408		$	209

Income Taxes - Reconciliation of Total Income Tax Provision (Details)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Reconciliation of the total income tax provision tax rate [Abstract]
Income tax expense (benefit) at federal statutory rate	(21.00%)	(21.00%)
Change in valuation allowance	25.50%	22.50%
Income tax expense (benefit) at state statutory rate	(0.20%)	(0.20%)
Share based compensation	1.00%	0.90%
NOLs expiring and adjustments to NOL	(0.10%)	0.50%
Research credit	(5.10%)	(2.50%)
Return to provision	(0.10%)	(0.10%)
Change in state rate	0.00%	(0.10%)
Total income tax provision	0.00%	0.00%

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2023

Commitments And Contingencies Disclosure [Abstract]

Summary of Lease Costs

The table below summarizes the Company’s operating lease costs from its statements of operations, and cash payments from its statements of cash flows.


	Year Ended December 31,
	2023		2022
Lease expense:
Operating lease expense	$	141	$	159

Total lease expense	$	141	$	159

Cash payment information:
Operating cash used for operating leases	$	141	$	159

Total cash paid for amounts included in the measurement of lease liabilities	$	141	$	159

Summary of Future Minimum Annual Lease Payments under Operating Leases

The Company’s future minimum annual lease payments under operating leases at December 31, 2023 are as follows (in thousands):


	Operating Leases
2024		146
2025		60
2026		11

Total minimum lease payments	$	217

Less: amount representing interest		(12	)

Present value of obligations under leases		205
Less: current portion		(120	)

Noncurrent lease obligations	$	85

Organization and Operations	12 Months Ended
Organization and Operations	Dec. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Organization and Operations	1. Organization and Operations The Company Plus Therapeutics, Inc. is a clinical-stage pharmaceutical company focused on the development, manufacture and commercialization of complex and innovative treatments for patients battling cancer and other life-threatening diseases. Certain Risks and Uncertainties The Company’s prospects are subject to the risks and uncertainties frequently encountered by companies in the early stages of development and commercialization, especially those companies in rapidly evolving and technologically advanced industries such as the biotech/medical device field. The Company’s future viability largely depends on its ability to complete development of new products and receive regulatory approvals for those products. No assurance can be given that the Company’s new products will be successfully developed, regulatory approvals will be granted, or acceptance of these products will be achieved. Going Concern The Company incurred net losses of $13.3 million for the year ended December 31, 2023, and as of December 31, 2023, the Company had an accumulated deficit of $480.5 million and cash and cash equivalents of $8.6 million. Additionally, the Company used net cash of $12.9 million to fund its operating activities for the year ended December 31, 2023. The Company’s term loan has an outstanding principal of $0.8 million principal and a $3.2 million final payment fee due June 1, 2024 (Note 8). The Company expects that its research and development expenditures will increase in absolute dollars in 2024 and beyond. These factors raise substantial doubt about the Company’s ability to continue as a going concern. As disclosed in more detail in Note 12, the Company has entered into various financing agreements and raised capital by issuing its common stock. Based on the Company’s stockholders’ deficit of $1.3 million as of December 31, 2023, the Company does not meet the minimum stockholders’ equity requirement for continued listing on the Nasdaq Capital Market under Nasdaq Listing Rule 5550(b)(1). The Company expects to receive written notice from Nasdaq staff to that effect following the filing of the Annual Report on Form 10-K in which these financial statements are included. The Company continues to seek additional capital through strategic transactions and from other financing alternatives. Without additional capital, current working capital and cash generated from sales will not provide adequate funding to make debt repayments, for research, sales and marketing efforts and product development activities at their current levels. If sufficient capital is not raised, the Company will at a minimum need to significantly reduce or curtail its research and development and other operations, and this would negatively affect its ability to achieve corporate growth goals. Should the Company fail to raise additional cash from outside sources, this would have a material adverse impact on its operations. The accompanying financial statements have been prepared assuming the Company will continue to operate as a going concern, which contemplates the realization of assets and settlement of liabilities in the normal course of business, and do not include any adjustments to reflect the possible future effects on the recoverability and classification of assets or the amounts and classifications of liabilities that may result from uncertainty related to its ability to continue as a going concern. Amendments to Certificate of Incorporation and Reverse Stock Split At the Annual Meeting of Stockholders of the Company held on April 20, 2023 (the “Annual Meeting”), the stockholders of the Company approved an amendment to the Company’s Amended and Restated Certificate of Incorporation to implement a reverse stock split of the Company’s common stock, par value $0.001 per share, with the ratio to be determined by the Board of Directors (the “Board”) of the Company, within a range of not less than 1-for-3 and not greater than 1-for-15 . Subsequently, on April 21, 2023, the Board determined to fix the ratio for the reverse stock split at 1-for-15 , without any change to its par value (the “Reverse Stock Split”). On April 27, 2023, following stockholder and Board approval, the Company filed a Certificate of Amendment to its Charter (the “Amendment”), with the Secretary of State of the State of Delaware to effectuate the Reverse Stock Split. The Amendment became effective on May 1, 2023. Upon effectiveness of the Reverse Stock Split, the number of shares of the Company’s common stock (x) issued and outstanding decreased from approximately 37.4 million shares to approximately 2.5 million shares; (y) reserved for issuance upon exercise of outstanding warrants and options decreased from approximately 2.0 million shares to approximately 0.1 million shares, and (z) reserved but unallocated under the Company’s current equity incentive plans decreased from approximately 3.0 million common shares to approximately 0.2 million common shares. The Company’s common stock began trading on the NASDAQ Capital Market on a post-split basis on May 1, 2023. The Company’s 5,000,000 shares of authorized Preferred Stock were not affected by the Reverse Stock Split. No fractional shares were issued in connection with the Reverse Stock Split, and accordingly, the outstanding number of shares post Reverse Stock Split was adjusted down by approximately 1,310 (post-effect of Reverse Stock Split) shares. Proportional adjustments for the reverse stock split were made to the Company’s outstanding stock options, warrants and equity incentive plans for all periods presented in the financial statements in this Form 10-K. The Company’s financial statements, and all references thereto have been retroactively adjusted to reflect the reverse split unless specifically stated otherwise.

Composition of Certain Financial Statement Captions - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Composition Of Certain Financial Statement Captions [Abstract]
Depreciation expense	$ 0.6	$ 0.5
Amortization	$ 0.1	$ 0.1

Use of Estimates	3 Months Ended
Use of Estimates	Mar. 31, 2024
Use Of Estimates [Abstract]
Use of Estimates	2. Use of Estimates The preparation of financial statements in conformity with U.S. generally accepted accounting principles requires management to make estimates and assumptions affecting the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts of r evenue and ex penses during the reporting period. The Company’s most significant estimates and critical accounting policies involve reviewing a ssets for impairment and determining the assumptions used in measuring stock-based compensation expense.

Subsequent Events - Additional Information (Details) - USD ($)	May 31, 2024	May 05, 2024	Apr. 26, 2024	May 09, 2024
Private Placement [Member]
Subsequent Event [Line Items]
Warrant liability				$ 7,250,000
Subsequent Event [Member] \| Biocept Asset Purchase [Member]
Subsequent Event [Line Items]
Payments to acquire assets			$ 400,000
Subsequent Event [Member] \| May 2024 Private Placement [Member]
Subsequent Event [Line Items]
Issue and sale of share		3,591,532
Warrant exercise price (in dollars per share)		$ 0.001
Class of warrant exercise price initial subscription		1.772
class of warrant or right, exercise price additional subscription		1.908
Shares Issued, Price Per Share		$ 1.79
Warrant liability				$ 7,250,000
Subsequent Event [Member] \| May 2024 Private Placement [Member] \| Series A Warrant [Member]
Subsequent Event [Line Items]
Issue and sale of share		3,591,532
Subsequent Event [Member] \| May 2024 Private Placement [Member] \| Series B Warrant [Member]
Subsequent Event [Line Items]
Issue and sale of share		3,591,532
Subsequent Event [Member] \| Private Placement [Member]
Subsequent Event [Line Items]
Class of warrant or right purchase price		$ 2.022
Right offering price		2.04
Subsequent Event [Member] \| Pre-funded Warrant [Member]
Subsequent Event [Line Items]
Class of warrant or right purchase price		$ 2.158
Subsequent Event [Member] \| Pershing Credit Facility [Member]
Subsequent Event [Line Items]
Line of credit facility	$ 3,300,000
Line of credit facility, periodic payment	$ 3,300,000
Interest rate	5.50%
Basis variable rate	1.75%
Effective interest rate	0.50%
Debt instrument, interest rate	3.00%
Line of credit facility, interest rate	91.50%

Fair Value Measurements (Tables)

3 Months Ended

12 Months Ended

Mar. 31, 2024

Dec. 31, 2023

Fair Value Disclosures [Abstract]

Summary of Financial Assets Measured at Fair Value on Recurring Basis

The following table summarizes the Company’s fair value hierarchy for its financial assets measured at fair value on a recurring basis as of March 31, 2024 and December 31, 2023, respectively (in thousands).


	Fair Value Measurements Using
March 31, 2024	Fair Value		Level 1		Level 2		Level 3
Money market	$	176	$	176	$	—	$	—
Treasury bills and government agency bonds		323		323		—		—

	$	499	$	499	$	—	$	—


	Fair Value Measurements Using
December 31, 2023	Fair Value		Level 1		Level 2		Level 3
Money market	$	5,449	$	5,449	$	—	$	—

The following table summarizes the Company’s fair value hierarchy for its financial assets measured at fair value on a recurring basis as of December 31, 2023 and December 31, 2022, respectively.


	Fair Value Measurements Using
As of December 31, 2023	Fair Value		Level 1		Level 2		Level 3
Money market	$	5,448,990	$	5,448,990	$	—	$	—

	Fair Value Measurements Using
As of December 31, 2022	Fair Value		Level 1		Level 2		Level 3
Money market	$	17,573,584	$	17,573,584	$	—	$	—

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Income Taxes [Line Items]
Provision or benefit for income taxes	$ 0	$ 0
Income Tax Contingency [Abstract]
Valuation allowance	19,500,000
Increase (decrease) in valuation allowance	$ 3,400,000
Cumulative change in ownership	50.00%
Period of net operating loss and R&D tax credit carry forwards to offset future taxable income	3 years
Liability for uncertain tax positions	$ 0	$ 0
Period for material changes of liability for uncertain tax benefits	12 months
Interest related to unrecognized tax benefits in interest expense and penalties	$ 0
Federal [Member]
Income Tax Contingency [Abstract]
Tax loss carryforwards	$ 63,200,000
Net operating loss carryforwards, Expire date	begin to expire in 2037
Federal tax carry forward	$ 1,900,000
Tax credit carryforward, expiration date	begin to expire in 2039
Net operating losses	$ 59,800,000
Federal [Member] \| Maximum [Member]
Income Tax Contingency [Abstract]
Percentage of federal net operating losses used to offset of future taxable income	80.00%
State [Member]
Income Tax Contingency [Abstract]
Tax loss carryforwards	$ 2,600,000
Net operating loss carryforwards, Expire date	2038
State tax credit carryforwards	$ 200,000
Tax credit carryforward, expiration date	begin to expire in 2043

Grant Revenue - Additional Information (Details) - USD ($)		3 Months Ended			12 Months Ended
	Sep. 19, 2022	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2022	Dec. 31, 2023	Dec. 31, 2022
Grant [Member]
Disaggregation of Revenue [Line Items]
Revenue recognized		$ 1,677,000	$ 506,000		$ 4,913,000	$ 224,000
CPRIT Contract [Member]
Disaggregation of Revenue [Line Items]
Maximum grant amount to be received as per grant contract	$ 17,600,000
Grant funding term	3 years
CPRIT grant funding term, description		The CPRIT Grant is subject to customary CPRIT funding conditions, including, but not limited to, a matching fund requirement (one dollar for every two dollars awarded by CPRIT), revenue sharing obligations upon commercialization of rhenium (186Re) obisbemeda based on specific dollar thresholds and tiered low single digit royalty rates until CPRIT receives the aggregate amount of 400% of the proceeds awarded under the CPRIT Grant, and certain reporting requirements.			The CPRIT Grant is subject to customary CPRIT funding conditions, including, but not limited to, a matching fund requirement (one dollar for every two dollars awarded by CPRIT), revenue sharing obligations upon commercialization of rhenium (186Re) obisbemeda based on specific dollar thresholds and tiered low single digit royalty rates until CPRIT receives the aggregate amount of 400% of the proceeds awarded under the CPRIT Grant, and certain reporting requirements.
Contract effective date	Aug. 31, 2022
Contract termination date	Aug. 30, 2025
Percentage of aggregate amount of proceeds awarded under CPRIT grant	400.00%				400.00%
Percentage of grant proceeds with interest not exceed annually	5.00%
Identified eligible reimbursable costs			168,000	$ 637,000
Increase in grant revenue					$ 637,000
CPRIT Contract [Member] \| Grant [Member]
Disaggregation of Revenue [Line Items]
Revenue recognized		$ 1,700,000	$ 500,000		$ 4,900,000	$ 200,000

Loss per Share

3 Months Ended

12 Months Ended

Mar. 31, 2024

Dec. 31, 2023

Earnings Per Share [Abstract]

Loss per Share

Basic per share data is computed by dividing net income or loss applicable to common stockholders by the weighted average number of common shares outstanding during the period. Diluted per share data is computed by dividing net income or loss applicable to common stockholders by the weighted average number of common shares outstanding during the period increased to include, if dilutive, the number of additional common shares

that would have been outstanding as calculated using the treasury stock method. Potential common shares were related to outstanding but unexercised options, multiple series of convertible preferred stock, and warrants for all periods presented.

The following were excluded from the diluted loss per share calculation for the periods presented because their effect would be anti-dilutive:


	As of March 31,
	2024		2023
Outstanding stock options		303,133		134,283
Preferred stock		28,190		28,190
Outstanding warrants		142,733		142,733

Total		474,056		305,206

4. Loss per Share

The following were excluded from the diluted loss per share calculation for the periods presented because their effect would be anti-dilutive:


	For the Year Ended December 31,
	2023		2022
Outstanding stock options		140,109		78,334
Preferred stock		28,190		28,190
Outstanding warrants		142,733		142,758

Total		311,032		249,282

Commitments and Contingencies - Additional Information (Details) - USD ($)			3 Months Ended	12 Months Ended
	Mar. 31, 2022	Jan. 08, 2021	Mar. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Jul. 01, 2023	Mar. 31, 2023
Recorded Unconditional Purchase Obligation [Line Items]
Operating leases expiry year			2025	2025
Lease liability				$ 205,000
Operating lease right-use-of assets			$ 171,000	$ 202,000	$ 248,000
Operating lease, weighted average discount rate				10.67%
Operating lease, weighted average remaining term				1 year 7 months 6 days
Rent expense				$ 200,000	200,000
Lorem Vascular Pte Ltd [Member]
Recorded Unconditional Purchase Obligation [Line Items]
Litigation settlement, expense					$ 1,400,000
Pre-clinical Research Study Obligations [Member]
Recorded Unconditional Purchase Obligation [Line Items]
Contractual obligation			$ 0	$ 0
Charlottesville [Member]
Recorded Unconditional Purchase Obligation [Line Items]
Lessee, operating lease, term of contract			12 months	12 months
Lessee, operating lease, existence of option to extend [true false]			true	true
Lessee, operating lease, option to extend			The Charlottesville Lease has a term of 12 months and the Company has the ability to renew for three additional one-year periods.	The Charlottesville Lease has a term of 12 months and the Company has the ability to renew for three additional one-year periods.
Lessee, operating lease, renewal term			1 year	1 year
Lease liability							$ 80,000
Operating lease discount rate						13.47%	12.76%
Operating lease right-use-of assets						$ 19,000
Operating leases, expiration date				Mar. 31, 2024
Medidata Solutions, Inc. [Member] \| Services Agreement [Member]
Recorded Unconditional Purchase Obligation [Line Items]
Sales order term	6 months
Piramal Pharma Solutions, Inc. [Member]
Recorded Unconditional Purchase Obligation [Line Items]
Master service agreement date		Jan. 08, 2021
Master services agreement initial term		5 years
Master services agreement terms, Description			The MSA has a term of five years and will automatically renew for successive one-year terms unless either party notifies the other no later than six months prior to the original term or any additional terms of its intention to not renew the MSA. The Company has the right to terminate the MSA for convenience upon thirty days’ prior written notice.	The MSA has a term of five years and will automatically renew for successive one-year terms unless either party notifies the other no later than six months prior to the original term or any additional terms of its intention to not renew the MSA. The Company has the right to terminate the MSA for convenience upon thirty days’ prior written notice.

Stockholders' Equity - Common stock (Details) - USD ($)						2 Months Ended	3 Months Ended		4 Months Ended			7 Months Ended	9 Months Ended	12 Months Ended		16 Months Ended	19 Months Ended
Stockholders' Equity - Common stock (Details) - USD ($)	Mar. 08, 2024	Sep. 09, 2022	Aug. 02, 2022	May 16, 2022	Jan. 14, 2022	Feb. 26, 2024	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2022	Mar. 31, 2024	Sep. 30, 2020	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2023	Mar. 31, 2024	Oct. 31, 2023	Aug. 18, 2023	Aug. 17, 2022	Jul. 14, 2022
Class of Stock [Line Items]
Sale of common stock, net	$ 2,500,000							$ 895,000						$ 5,005,000	$ 15,278,000
Common stock, shares authorized (in shares)							100,000,000		100,000,000	100,000,000	100,000,000	100,000,000		100,000,000	100,000,000	100,000,000	100,000,000
Proceeds from sale of common stock, net								$ 895,000						$ 5,527,000	$ 15,832,000
Remained authorized amount for repurchase							$ 0					$ 0					$ 0
Common Stock [Member]
Class of Stock [Line Items]
Issue and sale of share								168,164						2,230,493	1,206,090
Sale of common stock, net														$ 3,000	$ 1,000
Number of stock repurchased							179,866							78,559
Treasury stock							$ 400,000		$ 126,000			$ 400,000		$ 126,000		$ 126,000	$ 400,000
Common Stock [Member] \| Subsequent Event [Member]
Class of Stock [Line Items]
Number of stock repurchased						168,015
Treasury stock						$ 340,000
Common Stock [Member] \| Maximum [Member]
Class of Stock [Line Items]
Authorized amount of repurchase of stock																		$ 500,000
2022 Purchase Agreement [Member] \| Lincoln Park Capital Fund, LLC [Member] \| Maximum [Member]
Class of Stock [Line Items]
Common stock, shares authorized (in shares)				57,500,000
2022 Purchase Agreement [Member] \| Lincoln Park Capital Fund, LLC [Member] \| Common Stock [Member]
Class of Stock [Line Items]
Maximum value of shares to be issued under agreement			$ 50,000,000
Issue and sale of share							0		150,000		266,666	150,000		410,500
Period exercisable from the date of issuance							36 months							36 months
Number of maximum common shares can be resale																				633,333
Initial commitment shares			32,846																		32,846
Initial commitment fee paid in cash			$ 125,000
Aggregate gross proceeds from sales of common stock			$ 25,000,000
Proceeds from sale of common stock, net											$ 3,200,000			$ 1,000,000
2022 Purchase Agreement [Member] \| Lincoln Park Capital Fund, LLC [Member] \| Common Stock [Member] \| Minimum [Member]
Class of Stock [Line Items]
Percentage issuance of common stock				19.99%
2022 Purchase Agreement [Member] \| Lincoln Park Capital Fund, LLC [Member] \| Common Stock [Member] \| Maximum [Member]
Class of Stock [Line Items]
Number of maximum common shares can be resale																			1,500,000
Number of shares reserved for issuance and sale																				600,486
2020 Purchase Agreement [Member] \| Lincoln Park Capital Fund, LLC [Member] \| Common Stock [Member]
Class of Stock [Line Items]
Maximum value of shares to be issued under agreement													$ 25,000,000
Issue and sale of share																527,166	527,166
Sale of common stock, net															377,666
Period exercisable from the date of issuance													36 months
Proceeds from sale of common stock, net															$ 7,000,000
Remaining common stock issued and sold (in shares)										0	0				0
September 2022 Distribution Agreement [Member] \| Canaccord Genuity LLC [Member] \| At The Market Offering Program [Member]
Class of Stock [Line Items]
Reimburse of fee and disbursements expenses		$ 50,000
September 2022 Distribution Agreement [Member] \| Canaccord Genuity LLC [Member] \| Common Stock [Member] \| At The Market Offering Program [Member]
Class of Stock [Line Items]
Issue and sale of share										68,758				1,819,993
Proceeds from sale of common stock, net										$ 600,000				$ 4,300,000
September 2022 Distribution Agreement [Member] \| Canaccord Genuity LLC [Member] \| Common Stock [Member] \| Maximum [Member] \| At The Market Offering Program [Member]
Class of Stock [Line Items]
Proceeds from sale of common stock, net		$ 5,000,000
Percentage of commission on gross proceeds from sale of common stock		3.00%
January 2022 Distribution Agreement [Member] \| Canaccord Genuity LLC [Member] \| Common Stock [Member]
Class of Stock [Line Items]
Issue and sale of share														460,151	460,151
Proceeds from sale of common stock, net														$ 4,800,000	$ 4,800,000
January 2022 Distribution Agreement [Member] \| Canaccord Genuity LLC [Member] \| Common Stock [Member] \| At The Market Offering Program [Member]
Class of Stock [Line Items]
Remaining availability under financing facility					$ 0
January 2022 Distribution Agreement [Member] \| Canaccord Genuity LLC [Member] \| Common Stock [Member] \| Maximum [Member] \| At The Market Offering Program [Member]
Class of Stock [Line Items]
Proceeds from sale of common stock, net					$ 5,000,000

Stock-based Compensation - Weighted-average Assumptions (Details) - $ / shares	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Black-Scholes-Merton option valuation model based on weighted-average assumptions [Abstract]
Expected term	6 years	6 years
Risk-free interest rate	4.06%	2.83%
Expected volatility	127.00%	123.40%
Dividends	0.00%	0.00%
Resulting fair value	$ 4.47	$ 7.05

Stock-based Compensation - Stock Option Activity (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Options [Roll Forward]
Balance as of Options (in shares)	140,109	78,334
Granted (in shares)	163,025	68,422
Cancelled/forfeited (in shares)	(1)	(6,647)
Balance as of Options (in shares)	303,133	140,109	78,334
Vested and expected to vest (in shares)	278,083	133,810
Exercisable (in shares)	84,908	72,843
Weighted Average Exercise Price [Roll Forward]
Balance as of Weighted Average Exercise Price (in dollars per share)	$ 37.48	$ 68.16
Granted (in dollars per share)	2.03	5
Cancelled/forfeited (in dollars per share)	289,125	283.76
Balance as of Weighted Average Exercise Price (in dollars per share)	17.46	37.48	$ 68.16
Vested and expected to vest (in dollars per share)	18.64	38.62
Exercisable	$ 50.13	$ 60.17
Weighted Average Remaining Contractual Life (years) [Roll Forward]
Balance as of Weighted Average Remaining Contractual Life (years)	8 years 11 months 8 days	8 years 25 days	8 years
Vested and expected to vest	8 years 10 months 13 days	8 years 7 days
Exercisable	7 years 4 months 13 days	7 years 4 months 28 days
Aggregated Intrinsic Value [Roll Forward]
Balance as of Aggregated Intrinsic Value	$ 7,600	$ 7,000
Vested and expected to vest	6,800
Exercisable	$ 500	$ 6,000

Commitments and Contingencies - Summary of Future Minimum Annual Lease Payments under Operating Leases (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Commitments And Contingencies Disclosure [Abstract]
2024		$ 146
2025		60
2026		11
Total minimum lease payments		217
Less: amount representing interest		(12)
Present value of obligations under leases		205
Less: current portion	$ (115)	(120)	$ (110)
Noncurrent operating lease liability	$ 59	$ 85	$ 141

Stock-based Compensation (Tables)

3 Months Ended

12 Months Ended

Mar. 31, 2024

Dec. 31, 2023

Share-Based Payment Arrangement [Abstract]

Stock Option Activity

A summary of activity for the three months ended March 31, 2024 is as follows:


	Options			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (years)		Aggregate Intrinsic Value
Balance as of December 31, 2023		140,109		$	37.48		8.07
Granted		163,025		$	2.03
Cancelled/forfeited		(1	)	$	289,125

Balance as of March 31, 2024		303,133		$	17.46		8.94	$	7,600

Vested and expected to vest at March 31, 2024		278,083		$	18.64		8.87	$	6,800

Exercisable at March 31, 2024		84,908		$	50.13		7.37	$	500

A summary of activity for the year ended December 31, 2023 is as follows:


	Options			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (years)		Aggregate Intrinsic Value
Balance as of December 31, 2022		78,334		$	68.16		8.00
Granted		68,422			5.00
Cancelled/forfeited		(6,647	)		283.76

Balance as of December 31, 2023		140,109		$	37.48		8.07	$	7,000

Vested and expected to vest at December 31, 2023		133,810		$	38.62		8.02	$	—

Exercisable at December 31, 2023		72,843		$	60.17		7.41	$	6,000

Weighted-average Assumptions

The fair value of each option awarded during the years ended December 31, 2023 and 2022 was estimated on the date of grant using the Black-Scholes-Merton option valuation model based on the following weighted-average assumptions:


	December 31, 2023			December 31, 2022
Expected term		6.0 years			6.0 years
Risk-free interest rate		4.06	%		2.83	%
Expected volatility		127.0	%		123.4	%
Dividends		0	%		0	%
Resulting fair value	$	4.47		$	7.05

Summary of Share-Based Compensation Recognized

The following table summarizes share-based compensation recognized during the years ended December 31, 2023 and 2022 in the statement of operations:


	2023		2022
Research and development	$	66	$	87
General and administrative		503		519

Total share-based compensation	$	569	$	606

Composition of Certain Financial Statement Captions

12 Months Ended

Dec. 31, 2023

Composition Of Certain Financial Statement Captions [Abstract]

Composition of Certain Financial Statement Captions

5. Composition of Certain Financial Statement Captions

Other Current Assets

As of December 31, 2023 and 2022, other current assets were comprised of the following (in thousands):


	December 31,
	2023		2022
Prepaid services	$	644	$	2,999
Prepaid insurance		636		698

	$	1,280	$	3,697

Property and Equipment, net

As of December 31, 2023 and 2022, property and equipment, net, were comprised of the following (in thousands):


	December 31,
	2023			2022
Office and computer equipment	$	1,632		$	1,474
Leasehold improvements		1,810			1,810

		3,442			3,284
Less accumulated depreciation		(2,536	)		(1,960	)

	$	906		$	1,324

Depreciation expense totaled $0.6 million and $0.5 million for the year ended December 31, 2023 and 2022, respectively.

Intangible Assets, net

As of December 31, 2023, intangible assets included the net book value of costs incurred for software upgrades. Amortization expenses totaled $0.1 million for each of the years ended December 31, 2023 and 2022.

Accounts Payable and Accrued Expenses

As of December 31, 2023 and 2022, accounts payable and accrued expenses were comprised of the following (in thousands):


	December 31,
	2023		2022
Accounts payable	$	4,758	$	8,364
Accrued payroll and bonus		987		989
Accrued professional fees		128		147
Accrued vacation and compensation		370		325
Accrued R&D studies		388		309

	$	6,631	$	10,134

CONDENSED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022
Cash flows used in operating activities:
Net loss	$ (3,261)	$ (4,805)	$ (13,316)	$ (20,275)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	155	158	628	619
Amortization of deferred financing costs and debt discount	16	66	190	389
Common stock issued for research and development			75	0
Change in fair value of liability instruments			0	(1)
Share-based compensation expense	146	140	569	606
Accretion of discount on short-term investments	1
Reduction in the carrying amount of operating lease right-of-use assets	31	29	117	93
Loss on disposal of property and equipment		2	2	0
Increases (decreases) in cash caused by changes in operating assets and liabilities:
Other current assets	150	2,791
Other assets			2,397	(2,369)
Accounts payable and accrued expenses	(43)	(3,639)	(3,677)	6,452
Change in operating lease liabilities	(31)	(29)	(117)	(129)
Deferred grant liability	(1,677)	(506)	281	1,643
Net cash used in operating activities	(4,513)	(5,793)	(12,851)	(12,972)
Cash flows used in investing activities:
Purchases of property and equipment and intangible assets			(160)	(509)
In process research and development acquired			0	(250)
Purchases of property and equipment	(40)	(97)
Purchase of short-term investments	(324)
Net cash used in investing activities	(364)	(97)	(160)	(759)
Cash flows used in/provided by financing activities:
Principal payments of term loan obligation	(402)	(402)	(1,608)	(1,608)
Purchase of treasury stock	(374)		(126)	0
Proceeds from sale of common stock, net		895	5,527	15,832
Payment of offering costs related to sale of common stock			(348)	(773)
Net cash (used in) provided by financing activities	(776)	493	3,445	13,451
Net decrease in cash and cash equivalents	(5,653)	(5,397)	(9,566)	(280)
Cash and cash equivalents at beginning of period	8,554	18,120	18,120	18,400
Cash and cash equivalents at end of period	2,901	12,723	8,554	18,120
Cash paid during period for:
Interest	23	73	222	327
Supplemental schedule of non-cash investing and financing activities:
Unpaid offering cost	$ 141	25	174	0
Common stock issued in payment for in process research and development			75	0
Right-of-use assets acquired by assuming operating lease liabilities			$ 71	$ 0
Right-of-use assets obtained in exchange for operating lease liability		$ 51

Stock-based Compensation - Additional Information (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock shares to be issued (in shares)	163,025	68,422
Stock option contractual term	10 years	10 years
Stock option vesting percentage	25.00%	25.00%
Stock options exercised		$ 0	$ 0
Dividend yield assumed		0
Total unamortized compensation cost related to outstanding unvested stock options and restricted stock awards	$ 800,000	$ 700,000
Weighted average period for recognition of cost	2 years 3 months 18 days	1 year 9 months 18 days
Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option vesting period	2 years	2 years
Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option vesting period	4 years	4 years
2015 Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Securities remaining and available for future issuances (in shares)	6,024	6,024
2020 Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Securities remaining and available for future issuances (in shares)	17,582	180,607
Common stock shares to be issued (in shares)	236,667	3,550,000

Commitments and Contingencies

3 Months Ended

12 Months Ended

Mar. 31, 2024

Dec. 31, 2023

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

8. Commitments and Contingencies

Leases

The Company leases laboratory, office and storage facilities in San Antonio, Texas, under operating lease agreements that expire in 2025. The Company also leases certain office space in Austin, Texas under a

month-to-month

operating lease agreement and certain office space in Charlottesville, Virginia (the “Charlottesville Lease”). The Charlottesville Lease has a term of 12 months and the Company has the ability to renew for three additional

one-year

periods. On March 31, 2023, Company believed that it was reasonably

certain that the Charlottesville Lease will be renewed through March 31, 2026, and as a result, it remeasured the related lease liability as of March 31, 2023 to be $80,000 using the

then-in-effect

discount rate of 12.76%. Effective July 1, 2023, the Company added additional office lease premises in Charlottesville, which was accounted for as a separate operating lease contract with a lease liability and corresponding

right-of-use

asset of $19,000, as a discount rate of 13.47%.

Piramal Master Services Agreement

On January 8, 2021, the Company entered into a Master Services Agreement (the “MSA”) with Piramal Pharma Solutions, Inc. (“Piramal”), for Piramal to perform certain services related to the development, manufacture, and supply of the Company’s rhenium (

186

Re) obisbemeda intermediate drug product. The MSA includes the transfer of analytical methods, development of microbiological methods, process transfer and optimization, intermediate drug product manufacturing, and stability studies for the Company, which has been initiated at Piramal’s facility located in Lexington, Kentucky.

The MSA has a term of five years and will automatically renew for successive

one-year

terms unless either party notifies the other no later than six months prior to the original term or any additional terms of its intention to not renew the MSA. The Company has the right to terminate the MSA for convenience upon thirty days’ prior written notice. Either party may terminate the MSA upon an uncured material breach by the other party or upon the bankruptcy or insolvency of the other party.

Other commitments and contingencies

The Company has entered into agreements with various research organizations for

pre-clinical

and clinical development studies, which have provisions for cancellation. Under the terms of these agreements, the vendors provide a variety of services including conducting research, recruiting and enrolling patients, monitoring studies and data analysis. Payments under these agreements typically include fees for services and reimbursement of expenses. The timing of payments due under these agreements is estimated based on current study progress. As of March 31, 2024, the Company did not have any clinical research study obligations.

Legal proceedings

From time to time, the Company is subject to legal proceedings and claims, whether asserted or unasserted, that arise in the ordinary course of business. Due to their nature, such legal proceedings involve inherent uncertainties including, but not limited to, court rulings, negotiations between affected parties and governmental actions. Management assesses the probability of loss for such contingencies and accrues a liability and/or discloses the relevant circumstances, as appropriate.

6. Commitments and Contingencies

Leases

At the inception of a contractual arrangement, the Company determines whether the contract contains a lease by assessing whether there is an identified asset and whether the contract conveys the right to control the use of the

identified asset in exchange for consideration over a period of time. If both criteria are met, the Company calculates the associated lease liability and corresponding

right-of-use

asset upon lease commencement using a discount rate based on the rate implicit in the lease or an incremental borrowing rate commensurate with the term of the lease. Lease renewable options are included in the estimation of lease term when it is reasonably certain that the Company will exercise such options.

The Company records lease liabilities within current liabilities or long-term liabilities based upon the length of time associated with the lease payments. The Company records its operating lease

right-of-use

assets as long-term assets. Leases with an initial term of 12 months or less are not recorded on the balance sheets. Instead, the Company recognizes lease expense for these leases on a straight-line basis over the lease term in the statements of operations.

month-to-month

operating lease agreement and certain office space in Charlottesville, Virginia (the “Charlottesville Lease”). The Company’s existing operating lease agreements generally provide for periodic rent increases, and renewal and termination options. The Company’s lease agreements do not contain any material variable lease payments, residual value guarantees or material restrictive covenants.

The Charlottesville Lease has a term of 12 months and the Company has the ability to renew for three additional

one-year

periods. The Charlottesville Lease is currently set to expire on March 31, 2024, and is renewable twice for twelve months each time. On March 31, 2023, Company believed that it was reasonably certain that the Charlottesville Lease will be renewed through March 31, 2026, and as a result, it remeasured the related lease liability as of March 31, 2023 to be $80,000 using the

then-in-effect

right-of-use

asset of $19,000, at a discount rate of 13.47%.

Certain leases require the Company to pay taxes, insurance, and maintenance. Payments for the transfer of goods or services such as common area maintenance and utilities represent

non-lease

components. The Company elected the package of practical expedients and therefore does not separate

non-lease

components from lease components.

The Company’s operating lease liabilities and corresponding

right-of-use

assets are included in the balance sheets. As of December 31, 2023, the weighted average discount rate used to measure operating lease liabilities and the operating leases remaining term were 10.67% and 1.6 years, respectively.

The table below summarizes the Company’s operating lease costs from its statements of operations, and cash payments from its statements of cash flows.


	Year Ended December 31,
	2023		2022
Lease expense:
Operating lease expense	$	141	$	159

Total lease expense	$	141	$	159

Cash payment information:
Operating cash used for operating leases	$	141	$	159

Total cash paid for amounts included in the measurement of lease liabilities	$	141	$	159

Total rent expenses for each of the years ended December 31, 2023 and 2022 was $0.2 million, which includes leases in the table above,

month-to-month

operating leases, and common area maintenance charges.

The Company’s future minimum annual lease payments under operating leases at December 31, 2023 are as follows (in thousands):


	Operating Leases
2024		146
2025		60
2026		11

Total minimum lease payments	$	217

Less: amount representing interest		(12	)

Present value of obligations under leases		205
Less: current portion		(120	)

Noncurrent lease obligations	$	85

Services Agreement and Sales Order with Medidata

On March 31, 2022, the Company and Medidata Solutions, Inc. (“Medidata”) entered into a Sales Order (the “Sales Order”), pursuant to which Medidata will build a Synthetic Control Arm

(“SCA”) platform that facilitates the use of historical clinical data to incorporate into the Company’s Phase 2 clinical trial of rhenium (

186

Re) obisbemeda in recurrent glioblastoma (“GBM”). The Sales Order is governed under the terms of a services agreement, dated November 5, 2021. The Sales Order had a term of six months, and work under the Sales Order has been completed.

Piramal Master Services Agreement

186

Re) obisbemeda-Liposome Intermediate Drug Product. The MSA includes the transfer of analytical methods, development of microbiological methods, process transfer and optimization, intermediate drug product manufacturing, and stability studies for the Company, which has been initiated at Piramal’s facility located in Lexington, Kentucky.

The MSA has a term of five years and will automatically renew for successive

one-year

Other Commitments and Contingencies

The Company has entered into agreements with various research organizations for

pre-clinical

and clinical development studies, which have provisions for cancellation. Under the terms of these agreements, the vendors provide a variety of services including conducting research, recruiting and enrolling patients, monitoring studies and data analysis. Payments under these agreements typically include fees for services and reimbursement of expenses. The timing of payments due under these agreements is estimated based on current study progress. As of December 31, 2023, the Company did not have any clinical research study obligations.

Legal proceedings

On December 9, 2022, the Company entered into a settlement agreement (the “Settlement Agreement”) with Lorem Vascular, Pte. Ltd. (“Lorem”) to settle a prior litigation matter. Under the terms of the Settlement

Agreement, the Company made a payment to Lorem, and Lorem moved to dismiss the lawsuit with prejudice. The Settlement Agreement released the Company from all claims made by Lorem. The parties to the Settlement Agreement recognized that it did not constitute an admission of liability, wrongdoing, or any matter of fact or law. The Settlement was conditioned on the customary terms contained in the Settlement Agreement and was approved by the Court and the case was dismissed on January 17, 2023. As of December 31, 2022, the Company accrued the settlement amount, as well as the accounts that the Company has confirmed to be recoverable under its insurance claims on the matter. The net amount of $1.4 million that was not recoverable under the Company’s insurance has been reflected as an expense in the statement of operations for the year ended December 31, 2022. The full settlement amount was paid in January 2023. All legal costs related to the Lorem Claim were expensed as incurred.

The Company is subject to various claims and contingencies related to legal proceedings. Due to their nature, such legal proceedings involve inherent uncertainties including, but not limited to, court rulings, negotiations between affected parties and governmental actions. Management assesses the probability of loss for such contingencies and accrues a liability and/or discloses the relevant circumstances, as appropriate.

Summary of Significant Accounting Policies - Additional Information (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2023 USD ($)	Dec. 31, 2023 USD ($) ReportableUnit Segment	Dec. 31, 2022 USD ($) Segment	Mar. 31, 2024 USD ($)	Dec. 31, 2021 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Impairment loss		$ 0	$ 0
Stockholders' deficit	$ 2,676,000	(1,348,000)	6,445,000	$ (4,837,000)	$ 10,836,000
Impairment charges		$ 0
Number of reporting units \| ReportableUnit		1
Interest or penalties related to income tax matters	$ 0		$ 0
Number of operating segment \| Segment		1	1
ASU 2016-13
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted		true
Change in accounting principle, accounting standards update, adoption date		Jan. 01, 2023
Change in accounting principle, accounting standards update, immaterial effect		true
In Process Research and Development [Member]
Summary Of Significant Accounting Policies [Line Items]
Impairment charges		$ 0	$ 0
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Useful lives of the assets		3 years
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Useful lives of the assets		5 years

Summary of Significant Accounting Policies	12 Months Ended
Summary of Significant Accounting Policies	Dec. 31, 2023
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Use of Estimates The preparation of financial statements in conformity with U.S. generally accepted accounting principles requires management to make estimates and assumptions affecting the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts of revenue and expenses during the reporting period. The most significant estimates and critical accounting policies involve reviewing assets for impairment, and determining the assumptions used in measuring share-based compensation expense. Actual results could differ from these estimates. Management’s estimates and assumptions are reviewed regularly, and the effects of revisions are reflected in the financial statements in the periods they are determined to be necessary. Cash and cash equivalents The Company considers all highly liquid investments with maturities of three months or less at the time of purchase to be cash equivalents. Cash and cash equivalents include cash in readily available checking, savings accounts and money market accounts. The Company maintains deposits in federally insured financial institutions in excess of federally insured limits. The Company has not experienced any losses in such accounts and believes it is not exposed to significant risk on its cash balances due to the financial position of the depository institution in which those deposits are held. Financial Instruments Financial instruments include cash equivalents, other current assets, accounts payable, accrued expenses, other liabilities and long-term debt. The carrying values of cash equivalents, other current assets, accounts payable, accrued expenses and other liabilities generally approximate fair value due to the short-term nature of these instruments. Based on level 3 inputs and the borrowing rates currently available for loans with similar terms, the Company believes the fair value of the long-term debt is materially consistent with its carrying value. Property and Equipment Property and equipment are stated at cost, net of accumulated depreciation. Depreciation expense, which includes the amortization of capitalized leasehold improvements, is provided for on a straight-line basis over the estimated useful lives of the assets, or the life of the lease, whichever is shorter, and range from three to five years. When assets are sold or otherwise disposed of, the cost and related accumulated depreciation are removed from the accounts and the resulting gain or loss, if any, is included in operations. Maintenance and repairs are charged to operations as incurred. Impairment The Company assesses its property and equipment for potential impairment when there is a change in circumstances that indicates carrying values of assets may not be recoverable. Such long-lived assets are deemed to be impaired when the undiscounted cash flows expected to be generated by the asset (or asset group) are less than the asset’s carrying amount. Any required impairment loss would be measured as the amount by which the asset’s carrying value exceeds its fair value and would be recorded as a reduction in the carrying value of the related asset and a charge to operating expense. The Company recognized no impairment losses during any of the periods presented in these financial statements. Goodwill The Company’s goodwill represents the excess of the cost over the fair value of net assets acquired from its business combinations. The determination of the value of goodwill arising from business combinations requires extensive use of accounting estimates and judgments to allocate the purchase price to the fair value of the net tangible and intangible assets acquired. Goodwill is not amortized; however, it is assessed for impairment using fair value measurement techniques on an annual basis or more frequently if facts and circumstance warrant such a review. Goodwill is considered to be impaired if the Company determines that the carrying value of the reporting unit exceeds its fair value. The Company performs its impairment test annually during the fourth quarter by comparing the Company’s estimated fair value, calculated from the Company’s market capitalization, to its carrying amount. The Company’s annual evaluation for impairment of goodwill consists of one reporting unit. The Company completed its most recent annual evaluation for impairment as of December 31, 2023, when the Company had stockholders’ deficit within its sole reporting unit of approximately $1.3 million, and concluded that no impairment existed. Grant Revenue Recognition In applying the provisions of Accounting Standards Codification (“ASC”) Topic 606, Revenue from Contracts with Customers (“ASC 606”), the Company has determined that government grants are out of the scope of ASC 606 because the funding entities do not meet the definition of a “customer,” as defined by ASC 606, as there is not considered to be a transfer of control of goods or services. With respect to the grant, the Company determines if it is a collaboration arrangement in accordance with ASC Topic 808, Collaborative Arrangements (“ASC 808”). For grants outside the scope of ASC 808, the Company applies International Accounting Standards No. 20, Accounting for Government Grants and Disclosure of Government Assistance, by analogy, and revenue is recognized when the Company incurs expenses related to the grant for the amount the Company is entitled to under the provisions of the contract. The Company also considers the guidance in ASC Topic 730, Research and Development (“ASC 730”), which requires an assessment, at the inception of the grant, of whether the agreement is a liability. If the Company is obligated to repay funds received regardless of the outcome of the related research and development activities, then the Company is required to estimate and recognize that liability. Alternatively, if the Company is not required to repay the funds, then payments received are recorded as revenue or contra-expense as the expenses are incurred. Deferred grant liability represents grant funds received or receivable for which the allowable expenses have not yet been incurred as of the balance sheet date. Research and Development Research and development expenditures, which are charged to operations in the period incurred, include costs associated with the design, development, testing and enhancement of the Company’s products, regulatory fees, the purchase of laboratory supplies, and pre-clinical and clinical studies as well as salaries and benefits for the Company’s research and development employees. Acquired In-Process Research and Development (IPR&D) Acquired IPR&D represents the value assigned to research and development assets that have not reached technological feasibility. Upon the acquisition of IPR&D, the Company completes an assessment of whether the acquisition constitutes the purchase of a single asset or group of assets. The Company considers multiple factors in this assessment, including the nature of the technology acquired, the presence or absence of separate cash flows, the development process and stage of completion, quantitative significance, and the Company’s rationale for entering into the transaction. The Company tests IPR&D assets for impairment as of December 31 of each year or more frequently if indicators of impairment are present. The authoritative accounting guidance provides an optional qualitative assessment for any indicators that indefinite-lived intangible assets are impaired. If it is determined that it is more likely than not that the indefinite-lived intangible assets, including IPR&D, are impaired, the fair value of the indefinite-lived intangible assets is compared with the carrying amount and impairment is recorded for any excess of the carrying amount over the fair value of the indefinite-lived intangible assets. There was no impairment of the Company’s IPR&D assets during 2023 or 2022. Deferred Financing Costs and Other Debt-Related Costs Deferred financing costs are capitalized, recorded as an offset to debt balances and amortized to interest expense over the term of the associated debt instrument using the effective interest method. If the maturity of the debt is accelerated because of default or early debt repayment, then the amortization would be accelerated. Income Taxes Income taxes are accounted for under the asset and liability method. Deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases and operating loss and tax credit carry forwards. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income (loss) in the years in which those temporary differences are expected to be recovered or settled. Due to our history of losses, a full valuation allowance has been recognized against our deferred tax assets. The Company’s policy is to recognize interest and penalties related to income tax matters in income tax expense. For the years ended December 31, 2023 and 2022, the Company has not recorded any interest or penalties related to income tax matters. The Company does not foresee any material changes to unrecognized tax benefits within the next twelve months. Share-Based Compensation The Company recognizes the fair value of all share-based payment awards in our statements of operations over the requisite vesting period of each award, which approximates the period during which the employee and non-employee director is required to provide service in exchange for the award. The Company estimates the fair value of these options using the Black-Scholes option pricing model using assumptions for expected volatility, expected term, and risk-free interest rate. Expected volatility is based primarily on historical volatility and is computed using daily pricing observations for recent periods that correspond to the expected term of the options. The expected term represents the period of time that options are expected to be outstanding. Because the Company does not have historical exercise behavior, it determines the expected life assumption using the simplified method which is an average of the contractual term of the option and its vesting period. The risk-free interest rate is the interest rate for treasury instruments with maturities that approximate the expected term. Segment Information For the years ended December 31, 2023 and 2022, the Company is managed as a single operating segment, and therefore reports its results in one operating segment. Loss Per Share Basic per share data is computed by dividing net income or loss applicable to common stockholders by the weighted average number of common shares outstanding during the period. Diluted per share data is computed by dividing net income or loss applicable to common stockholders by the weighted average number of common shares outstanding during the period increased to include, if dilutive, the number of additional common shares that would have been outstanding as calculated using the treasury stock method. Potential common shares were related entirely to outstanding but unexercised options, warrants and convertible preferred stocks for all periods presented. The Company excluded all potentially dilutive securities from the calculation of diluted loss per share attributable to common stockholders for the years ended December 31, 2023 and 2022, as their inclusion would be antidilutive. Concentration Risk Although the Company’s contracts with its vendors are not exclusive, the Company currently uses sole source providers for core materials used in its clinical trials. Recently Issued Accounting Pronouncements In June 2016, the FASB issued ASU 2016-13, Financial Instruments—Credit Losses (Topic 326), Measurement of Credit Losses on Financial Instruments. The standard amends the impairment model by requiring entities to use a forward-looking approach based on expected losses to estimate credit losses for most financial assets and certain other instruments that are not measured at fair value through net income. For available-for-sale debt securities, entities will be required to recognize an allowance for credit losses rather than a reduction in carrying value of the asset. Entities will no longer be permitted to consider the length of time that fair value has been less than amortized cost when evaluating when credit losses should be recognized. This new guidance is effective in the first quarter of 2023 for calendar-year SEC filers that are smaller reporting companies as of the one-time determination date. Early adoption was permitted beginning in 2019. The Company adopted the new guidance as of January 1, 2023, which did not have a material impact on its financial statements and related disclosures.

Composition of Certain Financial Statement Captions - Property and Equipment, Net (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Property and Equipment, net [Abstract]
Office and computer equipment		$ 1,632	$ 1,474
Leasehold improvements		1,810	1,810
Property and equipment, gross		3,442	3,284
Less accumulated depreciation		(2,536)	(1,960)
Property and equipment, net	$ 800	$ 906	$ 1,324

License Agreements - Additional Information (Details) - USD ($)			1 Months Ended	3 Months Ended			12 Months Ended
	Sep. 07, 2023	Mar. 29, 2020	Jan. 31, 2022	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022
Research and development expense				$ 2,763,000		$ 2,983,000	$ 9,690,000	$ 9,698,000
Voting stock, value				5,000	$ 5,000		5,000	$ 2,000
Biocept Agreement [Member]
Shares issued	53,381
Research and development expense				75,000	75,000
Payment for tech transfer and validation fee	$ 300,000
Option for exclusive worldwide license	$ 1,000,000
UTHSA License Agreement [Member]
License fee payment in cash			$ 300,000
Nano Tx Licenses Agreement [Member]
License fee payment in cash		$ 400,000
Voting stock, value		$ 300,000
Percentage of non-dilutive monetary awards required to pay		15.00%
Accrued payments due to NanoTx				$ 500,000	$ 500,000		$ 500,000

Summary of Significant Accounting Policies (Policies)	3 Months Ended	12 Months Ended
Summary of Significant Accounting Policies (Policies)	Mar. 31, 2024	Dec. 31, 2023
Accounting Policies [Abstract]
Use of Estimates		Use of Estimates The preparation of financial statements in conformity with U.S. generally accepted accounting principles requires management to make estimates and assumptions affecting the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts of revenue and expenses during the reporting period. The most significant estimates and critical accounting policies involve reviewing assets for impairment, and determining the assumptions used in measuring share-based compensation expense. Actual results could differ from these estimates. Management’s estimates and assumptions are reviewed regularly, and the effects of revisions are reflected in the financial statements in the periods they are determined to be necessary.
Cash and cash equivalents		Cash and cash equivalents The Company considers all highly liquid investments with maturities of three months or less at the time of purchase to be cash equivalents. Cash and cash equivalents include cash in readily available checking, savings accounts and money market accounts. The Company maintains deposits in federally insured financial institutions in excess of federally insured limits. The Company has not experienced any losses in such accounts and believes it is not exposed to significant risk on its cash balances due to the financial position of the depository institution in which those deposits are held.
Financial Instruments		Financial Instruments Financial instruments include cash equivalents, other current assets, accounts payable, accrued expenses, other liabilities and long-term debt. The carrying values of cash equivalents, other current assets, accounts payable, accrued expenses and other liabilities generally approximate fair value due to the short-term nature of these instruments. Based on level 3 inputs and the borrowing rates currently available for loans with similar terms, the Company believes the fair value of the long-term debt is materially consistent with its carrying value.
Property and Equipment		Property and Equipment Property and equipment are stated at cost, net of accumulated depreciation. Depreciation expense, which includes the amortization of capitalized leasehold improvements, is provided for on a straight-line basis over the estimated useful lives of the assets, or the life of the lease, whichever is shorter, and range from three to five years. When assets are sold or otherwise disposed of, the cost and related accumulated depreciation are removed from the accounts and the resulting gain or loss, if any, is included in operations. Maintenance and repairs are charged to operations as incurred.
Impairment		Impairment The Company assesses its property and equipment for potential impairment when there is a change in circumstances that indicates carrying values of assets may not be recoverable. Such long-lived assets are deemed to be impaired when the undiscounted cash flows expected to be generated by the asset (or asset group) are less than the asset’s carrying amount. Any required impairment loss would be measured as the amount by which the asset’s carrying value exceeds its fair value and would be recorded as a reduction in the carrying value of the related asset and a charge to operating expense. The Company recognized no impairment losses during any of the periods presented in these financial statements.
Goodwill		Goodwill The Company’s goodwill represents the excess of the cost over the fair value of net assets acquired from its business combinations. The determination of the value of goodwill arising from business combinations requires extensive use of accounting estimates and judgments to allocate the purchase price to the fair value of the net tangible and intangible assets acquired. Goodwill is not amortized; however, it is assessed for impairment using fair value measurement techniques on an annual basis or more frequently if facts and circumstance warrant such a review. Goodwill is considered to be impaired if the Company determines that the carrying value of the reporting unit exceeds its fair value. The Company performs its impairment test annually during the fourth quarter by comparing the Company’s estimated fair value, calculated from the Company’s market capitalization, to its carrying amount. The Company’s annual evaluation for impairment of goodwill consists of one reporting unit. The Company completed its most recent annual evaluation for impairment as of December 31, 2023, when the Company had stockholders’ deficit within its sole reporting unit of approximately $1.3 million, and concluded that no impairment existed.
Grant Revenue Recognition	Grant Revenue Recognition In applying the provisions of Accounting Standards Codification (“ASC”) Topic 606, Revenue from Contracts with Customers (“ASC 606”), the Company has determined that government grants are out of the scope of ASC 606 because the funding entities do not meet the definition of a “customer”, as defined by ASC 606, as the Company does not consider there to be a transfer of control of goods or services. With respect to each grant, the Company determines if it has a collaboration in accordance with ASC Topic 808, Collaborative Arrangements (“ASC 808”). For grants outside the scope of ASC 808, the Company applies International Accounting Standards No. 20 (“IAS 20”), Accounting for Government Grants and Disclosure of Government Assistance, by analogy, and revenue is recognized when the Company incurs expenses related to the grant for the amount the Company is entitled to under the provisions of the contract. The Company also considers the guidance in ASC Topic 730, Research and Development, which requires an assessment, at the inception of each grant, of whether each grant agreement is a liability. If the Company is obligated to repay funds received regardless of the outcome of the related research and development activities, then the Company is required to estimate and recognize that liability. Alternatively, if the Company is not required to repay the funds, then payments received are recorded as revenue or contra-expense as the expenses are incurred. Deferred grant liability represents grant funds received or receivable for which the allowable expenses have not yet been incurred as of the balance sheet date.	Grant Revenue Recognition In applying the provisions of Accounting Standards Codification (“ASC”) Topic 606, Revenue from Contracts with Customers (“ASC 606”), the Company has determined that government grants are out of the scope of ASC 606 because the funding entities do not meet the definition of a “customer,” as defined by ASC 606, as there is not considered to be a transfer of control of goods or services. With respect to the grant, the Company determines if it is a collaboration arrangement in accordance with ASC Topic 808, Collaborative Arrangements (“ASC 808”). For grants outside the scope of ASC 808, the Company applies International Accounting Standards No. 20, Accounting for Government Grants and Disclosure of Government Assistance, by analogy, and revenue is recognized when the Company incurs expenses related to the grant for the amount the Company is entitled to under the provisions of the contract. The Company also considers the guidance in ASC Topic 730, Research and Development (“ASC 730”), which requires an assessment, at the inception of the grant, of whether the agreement is a liability. If the Company is obligated to repay funds received regardless of the outcome of the related research and development activities, then the Company is required to estimate and recognize that liability. Alternatively, if the Company is not required to repay the funds, then payments received are recorded as revenue or contra-expense as the expenses are incurred. Deferred grant liability represents grant funds received or receivable for which the allowable expenses have not yet been incurred as of the balance sheet date.
Research and Development		Research and Development Research and development expenditures, which are charged to operations in the period incurred, include costs associated with the design, development, testing and enhancement of the Company’s products, regulatory fees, the purchase of laboratory supplies, and pre-clinical and clinical studies as well as salaries and benefits for the Company’s research and development employees.
Acquired In-Process Research and Development (IPR&D)		Acquired In-Process Research and Development (IPR&D) Acquired IPR&D represents the value assigned to research and development assets that have not reached technological feasibility. Upon the acquisition of IPR&D, the Company completes an assessment of whether the acquisition constitutes the purchase of a single asset or group of assets. The Company considers multiple factors in this assessment, including the nature of the technology acquired, the presence or absence of separate cash flows, the development process and stage of completion, quantitative significance, and the Company’s rationale for entering into the transaction. The Company tests IPR&D assets for impairment as of December 31 of each year or more frequently if indicators of impairment are present. The authoritative accounting guidance provides an optional qualitative assessment for any indicators that indefinite-lived intangible assets are impaired. If it is determined that it is more likely than not that the indefinite-lived intangible assets, including IPR&D, are impaired, the fair value of the indefinite-lived intangible assets is compared with the carrying amount and impairment is recorded for any excess of the carrying amount over the fair value of the indefinite-lived intangible assets. There was no impairment of the Company’s IPR&D assets during 2023 or 2022.
Deferred Financing Costs and Other Debt-Related Costs		Deferred Financing Costs and Other Debt-Related Costs Deferred financing costs are capitalized, recorded as an offset to debt balances and amortized to interest expense over the term of the associated debt instrument using the effective interest method. If the maturity of the debt is accelerated because of default or early debt repayment, then the amortization would be accelerated.
Income Taxes		Income Taxes Income taxes are accounted for under the asset and liability method. Deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases and operating loss and tax credit carry forwards. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income (loss) in the years in which those temporary differences are expected to be recovered or settled. Due to our history of losses, a full valuation allowance has been recognized against our deferred tax assets. The Company’s policy is to recognize interest and penalties related to income tax matters in income tax expense. For the years ended December 31, 2023 and 2022, the Company has not recorded any interest or penalties related to income tax matters. The Company does not foresee any material changes to unrecognized tax benefits within the next twelve months.
Share-Based Compensation		Share-Based Compensation The Company recognizes the fair value of all share-based payment awards in our statements of operations over the requisite vesting period of each award, which approximates the period during which the employee and non-employee director is required to provide service in exchange for the award. The Company estimates the fair value of these options using the Black-Scholes option pricing model using assumptions for expected volatility, expected term, and risk-free interest rate. Expected volatility is based primarily on historical volatility and is computed using daily pricing observations for recent periods that correspond to the expected term of the options. The expected term represents the period of time that options are expected to be outstanding. Because the Company does not have historical exercise behavior, it determines the expected life assumption using the simplified method which is an average of the contractual term of the option and its vesting period. The risk-free interest rate is the interest rate for treasury instruments with maturities that approximate the expected term.
Segment Information		Segment Information For the years ended December 31, 2023 and 2022, the Company is managed as a single operating segment, and therefore reports its results in one operating segment.
Loss Per Share		Loss Per Share Basic per share data is computed by dividing net income or loss applicable to common stockholders by the weighted average number of common shares outstanding during the period. Diluted per share data is computed by dividing net income or loss applicable to common stockholders by the weighted average number of common shares outstanding during the period increased to include, if dilutive, the number of additional common shares that would have been outstanding as calculated using the treasury stock method. Potential common shares were related entirely to outstanding but unexercised options, warrants and convertible preferred stocks for all periods presented. The Company excluded all potentially dilutive securities from the calculation of diluted loss per share attributable to common stockholders for the years ended December 31, 2023 and 2022, as their inclusion would be antidilutive.
Concentration Risk		Concentration Risk Although the Company’s contracts with its vendors are not exclusive, the Company currently uses sole source providers for core materials used in its clinical trials.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In December 2023, the Financial Accounting Standards Board (the “FASB”) issued Accounting Standards Update (ASU) No. 2023-09 Income Taxes (Topic 740): Improvements to Income Tax Disclosure. This ASU includes amendments that further enhance income tax disclosures, primarily through standardization and disaggregation of rate reconciliation categories and income taxes paid by jurisdiction. The ASU is effective for years beginning after December 15, 2024, but early adoption is permitted. This ASU should be applied on a prospective basis, although retrospective application is permitted. Management is currently evaluating the impact of the changes required by the new standard on the Company’s financial statements and related disclosures. In November 2023, the FASB issued Accounting Standard Update (ASU) No. 2023-07, Segment Reporting (Topic 280): Improvements to Reportable Segment Disclosures. The new standard is intended to improve annual and interim reportable segment disclosure requirements regardless of number of reporting units, primarily through enhanced disclosures of significant expenses. The amendment requires public entities to disclose significant segment expenses that are regularly provided to the C ODM and included within each reported measure of segment profit and loss. This update is effective for fiscal years beginning after December 15, 2023, and interim periods within those fiscal years starting after December 15, 2024. This ASU must be applied retrospectively to all prior periods presented. Management is currently evaluating the impact of the changes required by the new standard on the Company’s financial statements and related disclosures .	Recently Issued Accounting Pronouncements In June 2016, the FASB issued ASU 2016-13, Financial Instruments—Credit Losses (Topic 326), Measurement of Credit Losses on Financial Instruments. The standard amends the impairment model by requiring entities to use a forward-looking approach based on expected losses to estimate credit losses for most financial assets and certain other instruments that are not measured at fair value through net income. For available-for-sale debt securities, entities will be required to recognize an allowance for credit losses rather than a reduction in carrying value of the asset. Entities will no longer be permitted to consider the length of time that fair value has been less than amortized cost when evaluating when credit losses should be recognized. This new guidance is effective in the first quarter of 2023 for calendar-year SEC filers that are smaller reporting companies as of the one-time determination date. Early adoption was permitted beginning in 2019. The Company adopted the new guidance as of January 1, 2023, which did not have a material impact on its financial statements and related disclosures.

Subsequent Events	3 Months Ended
Subsequent Events	Mar. 31, 2024
Subsequent Events [Abstract]
Subsequent Events	12. Subsequent Events Repayment of Term Loan and Use of Pershing Credit Facility On May 31, 2024, the Company drew $3.3 million on the Pershing Credit Facility, to repay the Oxford Term Loan in full, a payment amount that totaled approximately $3.3 million, which included both the balance of outstanding principal and interest and the final payment fee due. The repayment in full of the Oxford Term Loan terminated Oxford’s security interest in the Company’s existing and after-acquired assets, as well as all other certain restrictions and covenants under the Oxford Term Loan. Borrowings under the Pershing Credit Facility bear interest at the target interest rate set by the Federal Open Market Committee, subject to a floor of 5.5%, plus a spread of 1.75% and applicable fees of 0.5%, subject to a maximum interest rate of the then applicable Prime Rate as published in The Wall Street Journal plus 3.0%. Interest payments thereunder are calculated on a monthly basis and, unless paid, are added to the outstanding balance under the Pershing Credit Facility. The proceeds under the Pershing Credit Facility are available for working capital needs and other general corporate purposes. The Pershing Credit Facility includes certain covenants and restrictions, which may require the Company, on an on-going basis, to deposit additional funds or marketable securities in order to maintain the line of credit extended by Pershing. The available credit line limit under the Pershing Credit Facility fluctuates based on the Company’s request for extensions of credit from time to time, subject to the value of the collateralized marketable securities that the Company holds with Pershing, provided that the amount available to draw under the Pershing Credit Facility cannot exceed 91.5% of the value of the Company’s collateralized marketable securities deposited with Pershing (the “Collateral”). Depending on the value of the Collateral that the Company holds with Pershing, Pershing may require the Company, on an on-going basis, to deposit additional funds or marketable securities in order to restore the level of the Collateral to an acceptable level. The amounts borrowed under the Pershing Credit Facility are due on demand. The Pershing Credit Facility is secured by a general lien and security interest in the Company’s Collateral held in its account with Pershing. Upon certain customary events of default, Pershing has the right, in its discretion, to liquidate, withdraw or sell all or any part of the Collateral and apply the proceeds to the amounts outstanding under the Pershing Credit Facility. The Pershing Credit Facility is also subject to certain customary terms and conditions. May 2024 Private Placement On May 5, 2024, the Company entered into a securities purchase agreement (the “Securities Purchase Agreement”) with certain investors, including certain of the Company’s directors and executive officers (“Company Insiders”) (collectively, the “Purchasers”), for the sale and issuance by the Company of its securities (the “Initial Subscription”). On May 8, 2024, the Company entered into a first amendment to the Securities Purchase Agreement (the “Amendment”) for the sale and issuance by the Company of additional securities to two of the Purchasers (the “Additional Subscription”, and together with the Initial Subscription, the “May 2024 Private Placement”). The Securities Purchase Agreement, as amended, provides for the sale and issuance by the Company of an aggregate of 3,591,532 shares (the “Private Placement Shares”) of the Company’s common stock or, at the election of each Purchaser, pre-funded warrants (the “Pre-Funded Warrants”), exercisable immediately at an exercise price of $0.001 per share (the “Pre-Funded Warrant Shares”), with each Private Placement Share or Pre-Funded Warrant accompanied by (i) a Series A common warrant (“Series A Warrants”) to purchase one share of common stock (the “Series A Warrant Shares”), for an aggregate of 3,591,532 Series A Warrants, and (ii) one Series B common warrant (“Series B Warrants”) to purchase one share of common stock (the “Series B Warrant Shares,” and together with the Series A Warrant Shares, the “Common Warrant Shares”), for an aggregate of 3,591,532 Series B Warrants. The combined purchase price for each Private Placement Share and Pre-Funded Warrant from the Initial Subscription was $2.022, and $2.158 from the Additional Subscription, in each case together with one accompanying Series A Warrant and one accompanying Series B Warrant, provided, that the Company Insiders participated in the Initial Subscription at an offering price of $2.04 per Private Placement Share and accompanying Series A Warrant and Series B Warrant. The exercise price of each Series A Warrant and Series B Warrant from the Initial Subscription is $1.772 per share and $1.908 per share in the Additional Subscription, provided that the exercise price for the Series A Warrants and Series B Warrants issued to the Company Insiders is $1.79 per share. Subject to certain ownership limitations, the Series A Warrants will be exercisable until the five-year anniversary of issuance. Subject to certain ownership limitations, the Series B Warrants will be exercisable until the one-year anniversary of the declaration of effectiveness of a registration statement to be filed with the Securities and Exchange Commission covering the resale of the Common Warrant Shares. The Pre-Funded Warrant will not expire until exercised in full. The May 2024 Private Offering closed on May 9, 2024 (the “May 2024 Private Placement Closing”). The aggregate gross proceeds at the May 2024 Private Placement Closing were approximately $7.25 million, before deducting certain expenses payable by the Company, and excluding the proceeds, if any, from the exercise of the Series A Warrant, the Series B Warrant, and Pre-Funded Warrant. The Company is evaluating the accounting treatment of the Series A Warrants, Series B Warrants and the Pre-Funded Warrants under the authoritative accounting guidance. Biocept Asset Purchase On April 26, 2024, the Company acquired from Biocept, for a total cash payment of $400,000, substantially all of the right, title and interest in CNside, including (i) intellectual property, (ii) inventory and raw materials, and (iii) data, information, results and reports pertaining to the completed and on-going clinical studies involving the use of the CNside test (including, but not limited to, the FORESEE clinical study), related to the development, making, selling, and exporting or importing of CNside, after the Company’s bid was approved by the United States Bankruptcy Court for the District of Delaware.

Organization and Operations - Additional Information (Details)						3 Months Ended		12 Months Ended
	Apr. 21, 2023	Apr. 20, 2023 $ / shares	Mar. 29, 2020	Mar. 28, 2020	May 29, 2015 USD ($)	Mar. 31, 2024 USD ($) $ / shares shares	Mar. 31, 2023 USD ($)	Dec. 31, 2023 USD ($) $ / shares shares	Dec. 31, 2022 USD ($) $ / shares shares	May 01, 2023 shares	Apr. 30, 2023 shares	Apr. 04, 2023 $ / shares	Dec. 31, 2021 USD ($)
Organization and Operations [Line Items]
Net loss						$ (3,261,000)	$ (4,805,000)	$ (13,316,000)	$ (20,275,000)
Accumulated deficit						(483,762,000)		(480,501,000)	(467,185,000)
Cash and cash equivalents						2,901,000		8,554,000	18,120,000
Net cash used in operating activities						(4,513,000)	(5,793,000)	(12,851,000)	(12,972,000)
Stockholders' deficit						$ (4,837,000)	$ 2,676,000	$ (1,348,000)	$ 6,445,000				$ 10,836,000
Common stock, par value (in dollars per share) \| $ / shares		$ 0.001				$ 0.001		$ 0.001	$ 0.001			$ 0.001
Stock split conversion ratio	0.06
Common stock, shares issued (in shares) \| shares						4,522,656		4,522,656	2,240,092	2,500,000	37,400,000
Preferred stock, shares authorized (in shares) \| shares						5,000,000		5,000,000	5,000,000
Common stock, reverse stock split, fractional shares issued \| shares						0		0
Stock issued during period reverse stock splits, shares \| shares						1,310		1,310
Equity Incentive Plans [Member]
Organization and Operations [Line Items]
Common Stock, Capital Shares Reserved for Future Issuance \| shares										200,000	3,000,000
Outstanding Warrants and Options [Member]
Organization and Operations [Line Items]
Common Stock, Capital Shares Reserved for Future Issuance \| shares										100,000	2,000,000
Minimum [Member]
Organization and Operations [Line Items]
Stock split conversion ratio		0.33
Maximum [Member]
Organization and Operations [Line Items]
Stock split conversion ratio		0.06
Term Loan [Member]
Organization and Operations [Line Items]
Long term debt outstanding threshold amount								$ 800,000
Fees amount associated with loan					$ 3,200,000	$ 3,200,000		$ 3,200,000
Maturity date			Jun. 01, 2024	Sep. 01, 2021	May 01, 2021			Jun. 01, 2024
Term Loan [Member] \| Maximum [Member]
Organization and Operations [Line Items]
Long term debt outstanding threshold amount						$ 400,000		$ 800,000

Fair Value Measurements

3 Months Ended

12 Months Ended

Mar. 31, 2024

Dec. 31, 2023

Fair Value Disclosures [Abstract]

Fair Value Measurements

4. Fair Value Measurements

Fair value measurements are market-based measurements, not entity-specific measurements. Therefore, fair value measurements are determined based on the assumptions that market participants would use in pricing the asset or liability. The Company follows a three-level hierarchy to prioritize the inputs used in the valuation techniques to derive fair values. The basis for fair value measurements for each level within the hierarchy is described below:

•

Level 1: Quoted prices in active markets for identical assets or liabilities.

•

Level 2: Quoted prices for similar assets or liabilities in active markets; quoted prices for identical or similar instruments in markets that are not active; and model-derived valuations in which all significant inputs are observable in active markets.

•

Level 3: Valuations derived from valuation techniques in which one or more significant inputs are unobservable in active markets.

The Company has investments in money market accounts, which are included in cash and cash equivalents on the balance sheets. Fair value inputs for these investments are considered Level 1 measurements within the fair value hierarchy since money market account fair values are known and observable through daily published floating net asset values.


	Fair Value Measurements Using
March 31, 2024	Fair Value		Level 1		Level 2		Level 3
Money market	$	176	$	176	$	—	$	—
Treasury bills and government agency bonds		323		323		—		—

	$	499	$	499	$	—	$	—


	Fair Value Measurements Using
December 31, 2023	Fair Value		Level 1		Level 2		Level 3
Money market	$	5,449	$	5,449	$	—	$	—

3. Fair Value Measurements

asset or liability. The Company follows a three-level hierarchy to prioritize the inputs used in the valuation techniques to derive fair values. The basis for fair value measurements for each level within the hierarchy is described below:

•

Level 1: Quoted prices in active markets for identical assets or liabilities.

•

Level 3: Valuations derived from valuation techniques in which one or more significant inputs are unobservable in active markets.

The following table summarizes the Company’s fair value hierarchy for its financial assets measured at fair value on a recurring basis as of December 31, 2023 and December 31, 2022, respectively.


	Fair Value Measurements Using
As of December 31, 2023	Fair Value		Level 1		Level 2		Level 3
Money market	$	5,448,990	$	5,448,990	$	—	$	—

	Fair Value Measurements Using
As of December 31, 2022	Fair Value		Level 1		Level 2		Level 3
Money market	$	17,573,584	$	17,573,584	$	—	$	—

Nonfinancial Assets and Liabilities

The Company applies fair value techniques on a

non-recurring

basis, if and when necessary, associated with: (1) valuing potential impairment losses related to goodwill which are accounted for pursuant to the authoritative guidance for intangibles—goodwill and other; and (2) valuing potential impairment losses related to long-lived assets which are accounted for pursuant to the authoritative guidance for property, plant and equipment.

Commitments and Contingencies - Summary of Lease Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Lease expense:
Operating lease expense	$ 141	$ 159
Total lease expense	141	159
Cash payment information:
Operating cash used for operating leases	141	159
Total cash paid for amounts included in the measurement of lease liabilities	$ 141	$ 159

CONDENSED STATEMENTS OF OPERATIONS (UNAUDITED) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022
Operating expenses:
Research and development	$ 2,763,000	$ 2,983,000	$ 9,690,000	$ 9,698,000
General and administrative	2,213,000	2,245,000	8,544,000	10,238,000
Total operating expenses	4,976,000	5,228,000	18,234,000	19,936,000
Operating loss	(3,299,000)	(4,722,000)	(13,321,000)	(19,712,000)
Other income (expense):
Interest income	72,000	51,000	400,000	147,000
Interest expense	(34,000)	(134,000)	(395,000)	(711,000)
Change in fair value of liability instruments			0	1,000
Total other expense	38,000	(83,000)	5,000	(563,000)
Net loss	$ (3,261,000)	$ (4,805,000)	$ (13,316,000)	$ (20,275,000)
Net loss per share, basic	$ (0.75)	$ (2.07)	$ (4.24)	$ (11.58)
Net loss per share, diluted	$ (0.75)	$ (2.07)	$ (4.24)	$ (11.58)
Basic weighted average shares used in calculating net loss per share attributable to common stockholders	4,321,731	2,320,017	3,140,925	1,750,350
Diluted weighted average shares used in calculating net loss per share attributable to common stockholders	4,321,731	2,320,017	3,140,925	1,750,350
Grant [Member]
Revenue	$ 1,677,000	$ 506,000	$ 4,913,000	$ 224,000

Stock-based Compensation - Summary of Share-Based Compensation Recognized (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation expense	$ 146	$ 140	$ 569	$ 606
Research and Development [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation expense			66	87
General and Administrative [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share-based compensation expense			$ 503	$ 519

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 35
-Paragraph 1D
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480483/718-10-35-1D

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 35
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480483/718-10-35-3

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480429/718-10-50-2

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480429/718-10-50-2

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)(3)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480429/718-10-50-2

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(i)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480429/718-10-50-2

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(ii)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480429/718-10-50-2

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iii)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480429/718-10-50-2

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iv)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480429/718-10-50-2

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

Basis of Presentation and New Accounting Standards (Policies)	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation and New Accounting Standards	1. Basis of Presentation and New Accounting Standards The accompanying unaudited condensed financial statements for the three months ended March 31, 2024 and 2023 have been prepared in accordance with accounting principles generally accepted in the United States of America for interim financial information. Accordingly, they do not include all of the information and footnotes required by accounting principles generally accepted in the United States of America for annual financial statements. The condensed balance sheet at December 31, 2023 has been derived from the audited financial statements at December 31, 2023, but does not include all of the information and footnotes required by accounting principles generally accepted in the United States of America for complete financial statements. In the opinion of management, all adjustments (consisting of normal recurring adjustments) considered necessary for a fair presentation of the financial position and results of operations of Plus Therapeutics, Inc. (the “Company”) have been included. Operating results for the three months ended March 31, 2024 are not necessarily indicative of the results that may be expected for the year ending December 31, 2024. These financial statements should be read in conjunction with the financial statements and notes therein included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2023, filed with the Securities and Exchange Commission on March 5, 2024.
Amendments to Certificate of Incorporation and Reverse Stock Split	Amendments to Certificate of Incorporation and Reverse Stock Split At the Annual Meeting of Stockholders of the Company held on April 20, 2023 (the “Annual Meeting”), the stockholders of the Company approved an amendment to the Company’s Amended and Restated Certificate of Incorporation (the “Charter”) to implement a reverse stock split of the Company’s then issued and outstanding common stock, par value $0.001 per share, with the ratio to be determined by the Board of Directors (the “Board”) of the Company, within a range of not less than 1-for-3 and not greater than 1-for-15 . Subsequently, on April 21, 2023, the Board determined to fix the ratio for the reverse stock split at 1-for-15 , without any change to its par value (the “Reverse Stock Split”). On April 27, 2023, following stockholder and Board approval, the Company filed a Certificate of Amendment to its Charter (the “Amendment”), with the Secretary of State of the State of Delaware to effectuate the Reverse Stock Split. The Amendment became effective on May 1, 2023. Upon effectiveness of the Reverse Stock Split, the number of shares of the Company’s common stock (x) issued and outstanding decreased from approximately 37.4 million shares to approximately 2.5 million shares; (y) reserved for issuance upon exercise of outstanding warrants and options decreased from approximately 2.0 million shares to approximately 0.1 million shares, and (z) reserved but unallocated under the Company’s current equity incentive plans decreased from approximately 3.0 million shares of common stock to approximately 0.2 million shares of common stock. The Company’s common stock began trading on the Nasdaq Capital Market (“Nasdaq”) on a post-split basis on May 1, 2023. The Company’s 5,000,000 shares of authorized Preferred Stock were not affected by the Reverse Stock Split. No fractional shares were issued in connection with the Reverse Stock Split, and accordingly, the outstanding number of shares post Reverse Stock Split was adjusted down by approximately 1,310 (post-effect of the Reverse Stock Split) shares. Proportional adjustments for the Reverse Stock Split were made to the Company’s outstanding stock options, warrants and equity incentive plans for the period ended March 31, 2023 as presented in the condensed financial statements in this Quarterly Report on Form 10-Q. The Company’s financial statements, and all references thereto have been retroactively adjusted to reflect the Reverse Stock Split unless specifically stated otherwise.
Grant Revenue Recognition	Grant Revenue Recognition In applying the provisions of Accounting Standards Codification (“ASC”) Topic 606, Revenue from Contracts with Customers (“ASC 606”), the Company has determined that government grants are out of the scope of ASC 606 because the funding entities do not meet the definition of a “customer”, as defined by ASC 606, as the Company does not consider there to be a transfer of control of goods or services. With respect to each grant, the Company determines if it has a collaboration in accordance with ASC Topic 808, Collaborative Arrangements (“ASC 808”). For grants outside the scope of ASC 808, the Company applies International Accounting Standards No. 20 (“IAS 20”), Accounting for Government Grants and Disclosure of Government Assistance, by analogy, and revenue is recognized when the Company incurs expenses related to the grant for the amount the Company is entitled to under the provisions of the contract. The Company also considers the guidance in ASC Topic 730, Research and Development, which requires an assessment, at the inception of each grant, of whether each grant agreement is a liability. If the Company is obligated to repay funds received regardless of the outcome of the related research and development activities, then the Company is required to estimate and recognize that liability. Alternatively, if the Company is not required to repay the funds, then payments received are recorded as revenue or contra-expense as the expenses are incurred. Deferred grant liability represents grant funds received or receivable for which the allowable expenses have not yet been incurred as of the balance sheet date.	Grant Revenue Recognition In applying the provisions of Accounting Standards Codification (“ASC”) Topic 606, Revenue from Contracts with Customers (“ASC 606”), the Company has determined that government grants are out of the scope of ASC 606 because the funding entities do not meet the definition of a “customer,” as defined by ASC 606, as there is not considered to be a transfer of control of goods or services. With respect to the grant, the Company determines if it is a collaboration arrangement in accordance with ASC Topic 808, Collaborative Arrangements (“ASC 808”). For grants outside the scope of ASC 808, the Company applies International Accounting Standards No. 20, Accounting for Government Grants and Disclosure of Government Assistance, by analogy, and revenue is recognized when the Company incurs expenses related to the grant for the amount the Company is entitled to under the provisions of the contract. The Company also considers the guidance in ASC Topic 730, Research and Development (“ASC 730”), which requires an assessment, at the inception of the grant, of whether the agreement is a liability. If the Company is obligated to repay funds received regardless of the outcome of the related research and development activities, then the Company is required to estimate and recognize that liability. Alternatively, if the Company is not required to repay the funds, then payments received are recorded as revenue or contra-expense as the expenses are incurred. Deferred grant liability represents grant funds received or receivable for which the allowable expenses have not yet been incurred as of the balance sheet date.
Available-for-Sale Securities	Available-for-Sale Securities The Company’s available-for-sale securities consist of U.S. government and agency securities. The Company classifies its marketable securities as available-for-sale and records such assets at estimated fair value in the condensed balance sheets, with unrealized gains and losses, if any, reported as a component of other comprehensive income (loss) within the condensed statements of operations and comprehensive income/loss and as a separate component of stockholders’ equity. Realized gains and losses are calculated on the specific identification method and recorded as interest income (loss). At each balance sheet date, the Company assesses available-for-sale securities in an unrealized loss position to determine whether the decline in fair value below amortized cost is a result of credit losses or other factors, whether the Company expects to recover the amortized cost of the security, the Company’s intent to sell and if it is more likely than not that the Company will be required to sell the securities before the recovery of amortized cost. The Company records changes in allowance for expected credit loss in other income (expense). As of March 31, 2024, there were no available-for-sale securities in an unrealized loss position, and there has been no allowance for expected credit losses recorded during any of the periods presented. Any premium arising at purchase is amortized to the earliest call date and any discount arising at purchase is accreted to maturity. Accretion of discounts are recorded in interest income in the condensed statements of operations and comprehensive income/loss. During the three months ended March 31, 2024, the unrealized gain on the Company’s available-for-sale securities was less than $1,000, and not presented separately in the condensed statement of operations and comprehensive income/loss.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In December 2023, the Financial Accounting Standards Board (the “FASB”) issued Accounting Standards Update (ASU) No. 2023-09 Income Taxes (Topic 740): Improvements to Income Tax Disclosure. This ASU includes amendments that further enhance income tax disclosures, primarily through standardization and disaggregation of rate reconciliation categories and income taxes paid by jurisdiction. The ASU is effective for years beginning after December 15, 2024, but early adoption is permitted. This ASU should be applied on a prospective basis, although retrospective application is permitted. Management is currently evaluating the impact of the changes required by the new standard on the Company’s financial statements and related disclosures. In November 2023, the FASB issued Accounting Standard Update (ASU) No. 2023-07, Segment Reporting (Topic 280): Improvements to Reportable Segment Disclosures. The new standard is intended to improve annual and interim reportable segment disclosure requirements regardless of number of reporting units, primarily through enhanced disclosures of significant expenses. The amendment requires public entities to disclose significant segment expenses that are regularly provided to the C ODM and included within each reported measure of segment profit and loss. This update is effective for fiscal years beginning after December 15, 2023, and interim periods within those fiscal years starting after December 15, 2024. This ASU must be applied retrospectively to all prior periods presented. Management is currently evaluating the impact of the changes required by the new standard on the Company’s financial statements and related disclosures .	Recently Issued Accounting Pronouncements In June 2016, the FASB issued ASU 2016-13, Financial Instruments—Credit Losses (Topic 326), Measurement of Credit Losses on Financial Instruments. The standard amends the impairment model by requiring entities to use a forward-looking approach based on expected losses to estimate credit losses for most financial assets and certain other instruments that are not measured at fair value through net income. For available-for-sale debt securities, entities will be required to recognize an allowance for credit losses rather than a reduction in carrying value of the asset. Entities will no longer be permitted to consider the length of time that fair value has been less than amortized cost when evaluating when credit losses should be recognized. This new guidance is effective in the first quarter of 2023 for calendar-year SEC filers that are smaller reporting companies as of the one-time determination date. Early adoption was permitted beginning in 2019. The Company adopted the new guidance as of January 1, 2023, which did not have a material impact on its financial statements and related disclosures.

- Definition

Disclosure of accounting policy for revenue from contract with customer.

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+ Details

Stockholders' Equity	3 Months Ended	12 Months Ended
Stockholders' Equity	Mar. 31, 2024	Dec. 31, 2023
Stockholders Equity Note [Abstract]
Stockholders' Equity	10. Stockholders’ Equity Preferred Stock The Company has authorized 5,000,000 shares of preferred stock, par value $0.001 per share. The Company’s Board is authorized to designate the terms and conditions of any preferred stock the Company issues without further action by the common stockholders. Series F Preferred Stock On March 3, 2023, the Company filed a certificate of designation (the “Certificate of Designation”) with the Secretary of State of the State of Delaware, effective as of the time of filing, designating the rights, preferences, privileges and restrictions of the Series F Preferred Stock, with the total authorization of one (1) share of Series F Preferred Stock. The Certificate of Designation provided that the share of Series F Preferred Stock would have 50,000,000 votes per share of Series F Preferred Stock and would vote together with the Company’s common stock, as a single class exclusively with respect to any proposal to amend the Company’s Charter to effect the Reverse Stock Split. On March 3, 2023, the Company entered into a subscription and investment representation agreement with Richard J. Hawkins, chairman of the board of the Company, who is an accredited investor (the “Series F Preferred Stock Purchaser”), pursuant to which the Company agreed to issue and sell one (1) share of the Company’s Series F Preferred Stock, par value $0.001 per share, to the Series F Preferred Stock Purchaser for $1,000 in cash. The sale closed on March 3, 2023. At the Company’s 2023 annual meeting, the Series F Preferred Stock was voted, without action by the holder, on the proposal to approve the Reverse Stock Split in the same proportion as shares of common stock voted to approve the Reverse Stock Split. The Series F Preferred Stock otherwise had no voting rights except as otherwise required by the General Corporation Law of the State of Delaware. The Series F Preferred Stock was not convertible into, or exchangeable for, shares of any other class or series of stock or other securities of the Company. The Series F Preferred Stock had no rights with respect to any distribution of assets of the Company, including upon a liquidation, bankruptcy, reorganization, merger, acquisition, sale, dissolution or winding up of the Company, whether voluntarily or involuntarily. The holder of the Series F Preferred Stock was not entitled to receive dividends of any kind. The outstanding share of Series F Preferred Stock was redeemed in whole, automatically effective upon the approval by the Company’s stockholders of the Reverse Stock Split. Upon such redemption, the holder of the Series F Preferred Stock received consideration of $1,000 in cash. Series B and C Preferred Stock As of March 31, 2024, there were 938 outstanding shares of Series C Preferred Stock that can be converted into an aggregate of 27,792 shares of common stock, and 1,014 shares of Series B Convertible Preferred Stock that can be converted into an aggregate of 398 shares of common stock. Warrants On September 25, 2019, the Company completed an underwritten public offering. The Company issued 19,266 shares of its common stock, along with pre-funded warrants to purchase 180,733 shares of its common stock and Series U Warrants to purchase 230,000 shares of its common stock. The Series U Warrants have a term of five years from the issuance date. In addition, the Company issued warrants to H.C. Wainwright & Co., LLC, as representatives of the underwriters, to purchase 5,000 shares of its common stock with a term of five years from the issuance date, in the form of Series U Warrants (the “Representative Warrants”). As of March 31, 2024, there were 142,733 outstanding Series U Warrants and Representative Warrants which can be exercised into an aggregate of 142,733 shares of common stock at a weighted average exercise price of $34.10 per share. Common Stock Lincoln Park Purchase Agreement On August 2, 2022, the Company entered into a purchase agreement (the “2022 Purchase Agreement”) and registration rights agreement pursuant to which Lincoln Park Capital Fund (“Lincoln Park”) committed to purchase up to $50.0 million of the Company’s common stock. Under the terms and subject to the conditions of the 2022 Purchase Agreement, the Company has the right, but not the obligation, to sell to Lincoln Park, and Lincoln Park is obligated to purchase up to $50.0 million of the Company’s common stock. Such sales of common stock by the Company are subject to certain limitations, and can occur from time to time, at the Company’s sole discretion, over the 36-month period commencing on August 17, 2022, subject to the satisfaction of certain conditions. On May 16, 2022, the Company received stockholder approval for purposes of the Nasdaq listing rules to permit issuances of up to 57.5 million shares of the Company’s common stock (including the issuance of more than 19.99% of the Company’s common stock) to Lincoln Park, and it was pursuant to that approval that the Company entered into the 2022 Purchase Agreement. Upon execution of the 2022 Purchase Agreement, the Company paid $0.1 million in cash as the initial commitment fee, and issued 32,846 shares as the initial commitment shares, to Lincoln Park as consideration for its irrevocable commitment to purchase shares of the Company’s common stock at its direction under the 2022 Purchase Agreement. The Company has agreed to pay an additional commitment fee, which it may elect to pay in cash and/or shares of its common stock, upon receipt of $25.0 million aggregate gross proceeds from sales of common stock to Lincoln Park under the 2022 Purchase Agreement. On August 17, 2022, a registration statement (the “First Registration Statement”) was declared effective to cover the resale of up to 633,333 shares of the Company’s common stock comprised of (i) the 32,846 initial commitment shares, and (ii) up to 600,486 that the Company has reserved for issuance and sale to Lincoln Park under the 2022 Purchase Agreement from time to time from and after the date of the prospectus. The Company sold approximately 527,166 shares under the First Registration Statement. On August 18, 2023, a second registration statement (the “Second Registration Statement”) was declared effective to cover the resale of up to an additional 1,500,000 shares of the Company’s common stock that the Company reserved for issuance and sale to Lincoln Park under the 2022 Purchase Agreement from time to time. The Company sold 150,000 shares under the Second Registration Statement. The Company cannot sell more shares than registered under the Second Registration Statement under the 2022 Purchase Agreement without registering additional shares. Actual sales of shares of common stock to Lincoln Park under the 2022 Purchase Agreement depend on a variety of factors to be determined by the Company from time to time, including, among others, market conditions, the trading price of the common stock and determinations by the Company as to the appropriate sources of funding for the Company and its operations. The net proceeds under the 2022 Purchase Agreement to the Company depend on the frequency and prices at which the Company sells shares of its stock to Lincoln Park. During the period from August 17, 2022 to December 31, 2022, the Company issued 266,666 shares under the 2022 Purchase Agreement for net proceeds of approximately $3.2 million. The Company issued 410,500 shares under the 2022 Purchase Agreement for net proceeds of approximately $1.0 million from January 1, 2023 to December 31, 2023. The Company did not issue any common stock under the 2022 Purchase Agreement during the three months ended March 31, 2024. At-the-market Issuances On September 9, 2022, the Company entered into the September 2022 Distribution Agreement with Canaccord, pursuant to which the Company could issue and sell, from time to time, shares of its common stock having an aggregate offering price of up to $5,000,000, depending on market demand, with Canaccord acting as an agent for sales. During the period from September 9, 2022 to December 31, 2022, the Company issued 68,758 shares of its common stock under the September 2022 Distribution Agreement for net proceeds of approximately $0.6 million. From January 1, 2023 through December 31, 2023, the Company issued 1,819,993 shares under the September 2022 Distribution Agreement for net proceeds of approximately $4.3 million. The Company has reached the capacity for sales of shares under the September 2022 Distribution Agreement and the agreement has terminated. The Company was obligated to pay Canaccord a commission of up to 3.0% of the gross proceeds from the sale of its common stock under the September 2022 Distribution Agreement. The Company also agreed to reimburse Canaccord for its reasonable documented out-of-pocket expenses, including fees and disbursements of its counsel, in the amount of $50,000. In addition, the Company agreed to provide customary indemnification rights to Canaccord. On January 14, 2022, the Company entered into the January 2022 Distribution Agreement with Canaccord, pursuant to which the Company could issue and sell, from time to time, shares of its common stock having an aggregate offering price of up to $5,000,000, with Canaccord acting as an agent for sales. The Company had no obligation to sell any of the Company’s shares and it could instruct Canaccord not to sell any shares if the sales could not be effected at or above the price designated by the Company from time to time and the Company could at any time suspend sales pursuant to the January 2022 Distribution Agreement. During the year ended December 31, 2023, the Company issued 460,151 shares under the January 2022 Distribution Agreement for net proceeds of approximately $4.8 million. The January 2022 Distribution Agreement has been terminated after all available registered shares were fully utilized. Share Repurchase Program and Treasury Stock On October 31, 2023, the Company announced that its Board has approved a share repurchase program (the “Share Repurchase Program”), with authorization to repurchase up to $500,000 of the outstanding shares of the Company’s common stock. The Company funded repurchases under the Share Repurchase Program with available cash. During the year ended December 31, 2023, the Company purchased 78,559 of its common stock for approximately $0.1 million as treasury stock. The Company purchased 179,866 of its common shares for approximately $0.4 million as treasury stock during the three months ended March 31, 2024. As of March 31, 2024, no amount remained authorized for repurchase.	12. Stockholders’ Equity Preferred Stock The Company has authorized 5,000,000 shares of preferred stock, par value $0.001 per share. The Board is authorized to designate the terms and conditions of any preferred stock the Company issues without further action by the common stockholders. Series F Preferred Stock On March 3, 2023, the Company filed a certificate of designation (the “Certificate of Designation”) with the Secretary of State of the State of Delaware, effective as of the time of filing, designating the rights, preferences, privileges and restrictions of the Series F Preferred Stock, with the total authorization of one (1) share of Series F Preferred Stock. The Certificate of Designation provided that the share of Series F Preferred Stock will have 50,000,000 votes per share of Series F Preferred Stock and will vote together with the Company’s common stock, $0.001 par value (the “Common Stock”) as a single class exclusively with respect to any proposal to amend the Company’s Charter to effect a reverse stock split of the Common Stock (the “Reverse Stock Split”). On March 3, 2023, the Company entered into a Subscription and Investment Representation Agreement (the “Subscription Agreement”) with Richard J. Hawkins, Chairman of the Board, who is an accredited investor (the “Purchaser”), pursuant to which the Company agreed to issue and sell one (1) share of the Company’s Series F Preferred Stock, par value $0.001 per share (the “Preferred Stock”), to the Purchaser for $1,000 in cash. The sale closed on March 3, 2023. At the Company’s annual meeting of stockholders held on April 20, 2023, the Series F Preferred Stock was voted, without action by the holder, on the proposal to approve the Reverse Stock Split in the same proportion as shares of Common Stock voted to approve the Reverse Stock Split. The Series F Preferred Stock otherwise had no voting rights except as otherwise required by the General Corporation Law of the State of Delaware. The Series F Preferred Stock was not convertible into, or exchangeable for, shares of any other class or series of stock or other securities of the Company. The Series F Preferred Stock had no rights with respect to any distribution of assets of the Company, including upon a liquidation, bankruptcy, reorganization, merger, acquisition, sale, dissolution or winding up of the Company, whether voluntarily or involuntarily. The holder of the Series F Preferred Stock was not entitled to receive dividends of any kind. The outstanding share of Series F Preferred Stock was redeemed in whole, automatically effective upon the approval by the Company’s stockholders of a Reverse Stock Split. Upon such redemption, the holder of the Series F Preferred Stock received consideration of $1,000 in cash. Series B and C Preferred Stock As of December 31, 2023, there were 938 outstanding shares of Series C Preferred Stock that can be converted into an aggregate of 27,792 shares of common stock, and 1,014 shares of Series B Convertible Preferred Stock that can be converted into an aggregate of 398 shares of common stock. Warrants On September 25, 2019, the Company completed an underwritten public offering. The Company issued 19,266 shares of its common stock, along with pre-funded warrants to purchase 180,733 shares of its common stock and Series U Warrants to purchase 230,000 shares of its common stock at $75.00 per share. The Series U Warrants have a term of five years from the issuance date. In addition, the Company issued warrants to H.C. Wainwright & Co., LLC, as representatives of the underwriters, to purchase 5,000 shares of its common stock at $93.75 per share with a term of 5 years from the issuance date, in the form of Series U Warrants (the “Representative Warrants”). As of December 31, 2023, there were 142,733 outstanding Series U Warrants which can be exercised into an aggregate of 142,733 shares of common stock at the weighted average exercise price of $34.10 per share. Common Stock On August 2, 2022, the Company entered into a purchase agreement (the “2022 Purchase Agreement”) and registration rights agreement pursuant to which Lincoln Park committed to purchase up to $50.0 million of the Company’s common stock. Under the terms and subject to the conditions of the 2022 Purchase Agreement, the Company has the right, but not the obligation, to sell to Lincoln Park, and Lincoln Park is obligated to purchase up to $50.0 million of the Company’s common stock. Such sales of common stock by the Company are subject to certain limitations, and can occur from time to time, at the Company’s sole discretion, over the 36-month period commencing on August 17, 2022, subject to the satisfaction of certain conditions. Lincoln Park has no right to require the Company to sell any shares of common stock to Lincoln Park, but Lincoln Park is obligated to make purchases as the Company directs, subject to certain conditions. On May 16, 2022, the Company received stockholder approval for purposes of the Nasdaq listing rules to permit issuances of up to 57.5 million shares of the Company’s common stock (including the issuance of more than 19.99% of the Company’s common stock) to Lincoln Park, and it was pursuant to that approval that the Company entered into the 2022 Purchase Agreement. Upon execution of the 2022 Purchase Agreement, the Company paid $125,000 in cash as the initial commitment fee, and issued 32,846 shares as the initial commitment shares, to Lincoln Park as consideration for its irrevocable commitment to purchase shares of the Company’s common stock at its direction under the Purchase Agreement. The Company has agreed to pay an additional commitment fee, which it may elect to pay in cash and/or shares of its common stock, upon receipt of $25.0 million aggregate gross proceeds from sales of common stock to Lincoln Park under the 2022 Purchase Agreement. On August 17, 2022, a registration statement (the “First Registration Statement”) was declared effective to cover the resale of up to 633,333 shares of the Company’s common stock comprised of (i) the 32,846 initial commitment shares, and (ii) up to 600,486 that the Company has reserved for issuance and sale to Lincoln Park under the 2022 Purchase Agreement from time to time from and after the date of the prospectus. The Company sold approximately 527,166 shares under the First Registration Statement. On August 18, 2023, a second registration statement (the “Second Registration Statement”) was declared effective to cover the resale of up to an additional 1,500,000 shares of the Company’s common stock that the Company reserved for issuance and sale to Lincoln Park under the 2022 Purchase Agreement from time to time. We sold 150,000 shares under the Second Registration Statement. The Company cannot sell more shares than registered under the Second Registration Statement under the 2022 Purchase Agreement without registering additional shares. Actual sales of shares of common stock to Lincoln Park under the 2022 Purchase Agreement depend on a variety of factors to be determined by the Company from time to time, including, among others, market conditions, the trading price of the common stock and determinations by the Company as to the appropriate sources of funding for the Company and its operations. The net proceeds under the 2022 Purchase Agreement to the Company depend on the frequency and prices at which the Company sells shares of its stock to Lincoln Park. During the period from August 17, 2022 to December 31, 2022, the Company issued 266,666 shares under the 2022 Purchase Agreement for net proceeds of approximately $3.2 million. The Company issued 410,500 shares under the 2022 Purchase Agreement for net proceeds of approximately $1.0 million from January 1, 2023 to December 31, 2023. On September 30, 2020, the Company entered into a purchase agreement (the “2020 Purchase Agreement”) and registration rights agreement pursuant to which Lincoln Park committed to purchase up to $25.0 million of the Company’s common stock. Under the terms and subject to the conditions of the 2020 Purchase Agreement, the Company had the right, but not the obligation, to sell to Lincoln Park, and Lincoln Park was obligated to purchase up to $25.0 million of the Company’s common stock. Such sales of common stock by the Company were subject to certain limitations, and could occur from time to time, at the Company’s sole discretion, over the 36-month period commencing on November 6, 2020, subject to the satisfaction of certain conditions. During the year ended December 31, 2022, the Company issued 377,666 shares of its common stock under the 2020 Purchase Agreement for net proceeds of approximately $7.0 million. The Company no longer has any additional shares of common stock registered to sell under the 2020 Purchase Agreement and has terminated the 2020 Purchase Agreement. At-the-market Issuances On September 9, 2022, the Company entered into an Equity Distribution Agreement (the “September 2022 Distribution Agreement”) with Canaccord Genuity LLC (“Canaccord”), pursuant to which the Company could issue and sell, from time to time, shares of its common stock having an aggregate offering price of up to $5,000,000, depending on market demand, with Canaccord acting as an agent for sales. During the period from September 9, 2022 to December 31, 2022, the Company issued 68,758 shares of its common stock under the September 2022 Distribution Agreement for net proceeds of approximately $0.6 million. From January 1, 2023 through December 31, 2023, the Company issued 1,819,993 shares under the September 2022 Distribution Agreement for net proceeds of approximately $4.3 million. The Company has reached the capacity for sales of shares under the September 2022 Distribution Agreement and the agreement has terminated. The Company was obligated to pay Canaccord a commission of up to 3.0% of the gross proceeds from the sale of its common stock under the September 2022 Distribution Agreement. The Company also agreed to reimburse Canaccord for its reasonable documented out-of-pocket expenses, including fees and disbursements of its counsel, in the amount of $50,000. In addition, the Company agreed to provide customary indemnification rights to Canaccord. On January 14, 2022, the Company entered into an Equity Distribution Agreement (the “January 2022 Distribution Agreement”) with Canaccord, pursuant to which the Company could issue and sell, from time to time, shares of its common stock having an aggregate offering price of up to $5,000,000, with Canaccord acting as an agent for sales. The Company had no obligation to sell any of the Company’s shares and it could instruct Canaccord not to sell any shares if the sales could not be effected at or above the price designated by the Company from time to time and the Company could at any time suspend sales pursuant to the January 2022 Distribution Agreement. During the year ended December 31, 2022, the Company issued 460,151 shares under the January 2022 Distribution Agreement for net proceeds of approximately $4.8 million. The January 2022 Distribution Agreement has been terminated after all available registered shares were fully utilized. Share Repurchase Program and Treasury Stock On October 31, 2023, the Company announced that its Board has approved a share repurchase program (the “Share Repurchase Program”), with authorization to repurchase up to $500,000 of the outstanding shares of the Company’s common stock. The Company intends to fund any repurchases under the Share Repurchase Program with available cash. The timing and amount of any shares repurchased will be determined based on the Company’s evaluation of market conditions and other factors, including consent of the Company’s lender. Repurchases may be made from time to time on the open market through October 31, 2024. During the year ended December 31, 2023, the Company purchased 78,559 of its common shares for approximately $126,000 as treasury stock. During the period January 1, 2024 through February 26, 2024, the Company purchased 168,015 of its common shares for approximately $340,000 as treasury stock.

- Definition

The entire disclosure for equity.

+ References

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+ Details

Composition of Certain Financial Statement Captions - Accounts Payable and Accrued Expenses (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Accounts Payable and Accrued Expenses [Abstract]
Accounts payable		$ 4,758	$ 8,364
Accrued payroll and bonus		987	989
Accrued professional fees		128	147
Accrued vacation and compensation		370	325
Accrued R&D studies		388	309
Accounts payable and accrued expenses, Total	$ 6,447	$ 6,631	$ 10,134

Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on Recurring Basis (Details) - Recurring Basis [Member] - USD ($)	Mar. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 499,000
Money Market [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	176,000	$ 5,448,990	$ 17,573,584
Treasury Bills and Government Agency Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	323,000
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	499,000
Level 1 [Member] \| Money Market [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	176,000	$ 5,448,990	$ 17,573,584
Level 1 [Member] \| Treasury Bills and Government Agency Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 323,000

Liquidity and Going Concern	3 Months Ended
Liquidity and Going Concern	Mar. 31, 2024
Liquidity [Abstract]
Liquidity and Going Concern	3. Liquidity and Going Concern The Company incurred a net loss of $3.3 million for the three months ended March 31, 2024. The Company had an accumulated deficit of $483.8 million as of March 31, 2024. Additionally, the Company used net cash of $4.5 million to fund its operating activities for the three months ended March 31, 2024. To date, the Company’s operating losses have been funded primarily from outside sources of invested capital from issuance of its common and preferred stocks, proceeds from its term loan and grant funding. However, the Company has had, and will continue to have, an ongoing need to raise additional cash from outside sources to fund its future clinical development programs and other operations. There can be no assurance that the Company will be able to continue to raise additional capital in the future. The Company’s inability to raise additional cash would have a material and adverse impact on its operations and could cause the Company to default on its term loan. These factors raise substantial doubt about the Company’s ability to continue as a going concern. On May 9, 2024, the Company closed a private placement of securities of the Company for aggregate gross proceeds of approximately $7.25 million, before deducting certain expenses payable by the Company, and excluding the proceeds, if any, from the exercise of warrants issued in the private placement. See Note 12 for further information regarding the private placement. Nasdaq Listing Compliance On March 8, 2024, the Company received a letter (the “Notice”) from the Listing Qualifications staff of Nasdaq, notifying the Company that it no longer complied with the requirement under Nasdaq Listing Rule 5550(b)(1) to maintain a minimum of $2.5 million in stockholders’ equity for continued listing on Nasdaq or the alternative requirements of having a market value of listed securities of $35 million or net income from continuing operations of $500,000 in the most recently completed fiscal year or two of the last three most recently completed fiscal years (the “Alternative Standards”). The Notice states that the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2023, disclosed stockholders’ equity of ($1,348,000) as of December 31, 2023, and that, as of March 8, 2024, the Company did not meet the Alternative Standards. On April 22, 2024, the Company provided Nasdaq with its plan to achieve and sustain compliance with the stockholders’ equity requirement and requested that Nasdaq grant the Company an extension of time until September 4, 2024, to provide evidence of compliance with the stockholders’ equity requirement. Nasdaq has not yet responded to the Company’s plan, and there can be no assurance that Nasdaq will grant an extension or that the Company will be able to comply with the applicable listing standards of Nasdaq. The Company continues to seek additional capital from other financing alternatives and other sources in order to ensure adequate funding is available to allow the Company to continue research and product development activities at their current levels. If sufficient capital is not raised, the Company will at a minimum need to significantly reduce or curtail its research and development and other operations, and this would negatively affect its ability to achieve corporate growth goals. Should the Company fail to raise additional cash from outside sources, it would have a material adverse impact on its operations. The accompanying condensed financial statements have been prepared assuming the Company will continue to operate as a going concern, which contemplates the realization of assets and settlement of liabilities in the normal course of business, and do not include any adjustments to reflect the possible future effects on the recoverability and classification of assets or the amounts and classifications of liabilities that may result from uncertainty related to its ability to continue as a going concern.

Term Loan Obligations (Tables)

12 Months Ended

Dec. 31, 2023

Debt Disclosure [Abstract]

Term Loan Outstanding

Additional information relating to the then-outstanding Term Loan as of December 31, 2023 and 2022 is presented in the following table (in thousands, except interest rate):


Year ended December 31, 2023
Origination Date	Original Loan Amount		Interest Rate*			Current Monthly Payment**		Amended expiration date		Remaining Principal (Face Value)
May 2015	$	17,700		13.39	%	$	134		June 1, 2024	$	804


Year ended December 31, 2022
Origination Date	Original Loan Amount		Interest Rate*			Monthly Payment**		Amended expiration date		Remaining Principal (Face Value)
May 2015	$	17,700		8.95	%	$	134		June 1, 2024	$	2,412

*	Three month LIBOR rate with a floor of 1% plus 7.95%

**	Monthly payment reflects principal and interest

- Definition

Tabular disclosure of long-debt instruments or arrangements, including identification, terms, features, collateral requirements and other information necessary to a fair presentation. These are debt arrangements that originally required repayment more than twelve months after issuance or greater than the normal operating cycle of the entity, if longer.

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+ Details

Income Taxes

12 Months Ended

Dec. 31, 2023

Income Tax Disclosure [Abstract]

Income Taxes

10. Income Taxes

Pursuant to the Internal Revenue Code (“IRC”) of 1986, as amended, specifically IRC §382 (“Section 382”) and IRC §383, the Company’s ability to use net operating loss (“NOLs”) and R&D tax credit carry forwards (“tax attribute carry forwards”) to offset future taxable income is limited if the Company experiences a cumulative change in ownership of more than 50% within a three-year testing period. The Company’s use of federal and state NOLs and research credits could be limited further by the provisions of Section 382 depending upon the timing and amount of additional equity securities that the Company has issued or will issue. State NOL carryforwards may be similarly limited. If a change in ownership were to have occurred, NOL and tax credits carryforwards could be eliminated or restricted. If eliminated, the related asset would be removed from the deferred tax asset schedule with a corresponding reduction in the valuation allowance. Due to the existence of the valuation allowance, limitations created by ownership changes, if any, will not impact the Company’s effective tax rate.

The Company has recorded a full valuation allowance against its net deferred tax assets and due to our net losses for the years ended December 31, 2023 and 2022, there was no provision or benefit for income taxes recorded.

A reconciliation of the total income tax provision tax rate to the statutory federal income tax rates of 21% for the years ended December 31, 2023 and 2022, respectively, is as follows:


	2023			2022
Income tax expense (benefit) at federal statutory rate		(21.0	)%		(21.0	)%
Change in valuation allowance		25.5	%		22.5	%
Income tax expense (benefit) at state statutory rate		(0.2	)%		(0.2	)%
Share based compensation		1.0	%		0.9	%
NOLs expiring and adjustments to NOL		(0.1	)%		0.5	%
Research credit		(5.1	)%		(2.5	)%
Return to provision		(0.1	)%		(0.1	)%
Change in state rate		—			(0.1	)%

		0.0	%		0.0	%

The tax effects of temporary differences that give rise to significant portions of our deferred tax assets and deferred tax liabilities as of December 31, 2023 and 2022 are as follows (in thousands):


	2023			2022
Deferred tax assets:
Accrued expenses	$	269		$	262
Share based compensation		99			107
Net operating loss carryforwards		13,397			12,605
Income tax credit carryforwards		1,630			956
Property and equipment, principally due to differences in depreciation		154			89
Intangible assets		3,527			2,073
Other, net		453			53

		19,529			16,145
Valuation allowance		(19,486	)		(16,092	)

Total deferred tax assets, net of allowance		43			53


	2023			2022
Deferred tax liabilities:
Other		(43	)		(53	)

Total deferred tax liability		(43	)		(53	)

Net deferred tax assets (liability)	$	—		$	—

The Company has established a valuation allowance against its net deferred tax assets due to the uncertainty surrounding the realization of such assets. The Company periodically evaluates the recoverability of the deferred tax assets. At such time as it is determined that it is more likely than not that deferred tax assets are realizable, the valuation allowance will be reduced. The Company has recorded a full valuation allowance of $19.5 million as of December 31, 2023 as it does not believe it is more likely than not the net deferred tax assets will be realized. The Company increased its valuation allowance by approximately $3.4 million during the year ended December 31, 2023.

At December 31, 2023, the Company had federal and state tax loss carry forwards of approximately $63.2 million, and $2.6 million, respectively. The federal and state net operating loss carry forwards begin to expire in 2037 and 2038, if unused, respectively. The federal net operating loss carryover includes $59.8 million of net operating losses generated after 2017. Federal net operating losses generated from 2018 onwards carryover indefinitely and may generally be used to offset up to 80% of future taxable income. At December 31, 2023, the Company had federal tax credit carry forwards of approximately $1.9 million, before reduction for uncertain tax positions. The federal credits will begin to expire in 2039, if unused. In addition, at December 31, 2023, the Company had state tax credit carry forwards of approximately $0.2 million, before reduction for uncertain tax positions. The state credits will begin to expire in 2043, if unused.

The Company follows the provisions of income tax guidance which provides recognition criteria and a related measurement model for uncertain tax positions taken or expected to be taken in income tax returns. The guidance requires that a position taken or expected to be taken in a tax return be recognized in the financial statements when it is more likely than not that the position would be sustained upon examination by tax authorities. Tax positions that meet the more likely than not threshold are then measured using a probability weighted approach recognizing the largest amount of tax benefit that is greater than 50% likely of being realized upon ultimate settlement. The Company has not recognized any liability for uncertain tax positions as of December 31, 2023 and 2022.

Following is a tabular reconciliation of the unrecognized tax benefits activity during the years ended December 31, 2023 and 2022 (in thousands):


	2023			2022
Unrecognized Tax Benefits – Beginning	$	209		$	81
Gross decreases – tax positions in prior period		(16	)		(1	)
Gross increase – current-period tax positions		215			129

Unrecognized Tax Benefits – Ending	$	408		$	209

The unrecognized tax benefit amounts are reflected in the determination of the Company’s deferred tax assets. If recognized, none of these amounts would affect the Company’s effective tax rate, since it would be offset by an equal reduction in the deferred tax asset valuation allowance. The Company does not foresee material changes to its liability for uncertain tax benefits within the next twelve months.

The Company did not recognize interest related to unrecognized tax benefits in interest expense and penalties in operating expenses for the year ended December 31, 2023.

The Company files income tax returns with the United States and various state jurisdictions. To its knowledge, the Company is currently not under examination by the Internal Revenue Service or any other taxing authority.

With few exceptions, the Company’s tax years prior to 2020 are no longer open to examination by the taxing authority. While not open to examination, the tax attributes generated in tax years 2018 and forward remain subject to adjustment by the taxing authorities if utilized in tax years which are still open to examination.

- Definition

The entire disclosure for income taxes. Disclosures may include net deferred tax liability or asset recognized in an enterprise's statement of financial position, net change during the year in the total valuation allowance, approximate tax effect of each type of temporary difference and carryforward that gives rise to a significant portion of deferred tax liabilities and deferred tax assets, utilization of a tax carryback, and tax uncertainties information.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480990/946-20-50-13

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(h)(2))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480678/235-10-S99-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-Name Accounting Standards Codification
-Publisher FASB
-URI https://asc.fasb.org//740/tableOfContent

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 14
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482685/740-10-50-14

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 21
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482685/740-10-50-21

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 270
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482526/740-270-50-1

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 17
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482685/740-10-50-17

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SAB TOPIC 6.I.5.Q1)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479360/740-10-S99-1

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Subparagraph (SAB Topic 11.C)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479360/740-10-S99-2

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482603/740-30-50-2

+ Details

CONDENSED BALANCE SHEETS (UNAUDITED) - USD ($)	Mar. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Current assets:
Cash and cash equivalents	$ 2,901,000	$ 8,554,000	$ 18,120,000
Investments	323,000	0
Other current assets	989,000	1,280,000	3,697,000
Total current assets	4,213,000	9,834,000	21,817,000
Property and equipment, net	800,000	906,000	1,324,000
Operating lease right-use-of assets	171,000	202,000	248,000
Goodwill	372,000	372,000	372,000
Intangible assets, net	33,000	42,000	94,000
Other assets	32,000	32,000	12,000
Total assets	5,621,000	11,388,000	23,867,000
Current liabilities:
Accounts payable and accrued expenses	6,447,000	6,631,000	10,134,000
Operating lease liability	115,000	120,000	110,000
Deferred grant liability	247,000	0
Term loan obligation, current	3,590,000	3,976,000	1,608,000
Total current liabilities	10,399,000	10,727,000	11,852,000
Noncurrent operating lease liability	59,000	85,000	141,000
Term loan obligation		0	3,786,000
Deferred grant liability		1,924,000	1,643,000
Total liabilities	10,458,000	12,736,000	17,422,000
Commitments and contingencies (Note 8)
Stockholders' deficit:
Preferred stock	0	0	0
Common stock	5,000	5,000	2,000
Treasury stock	(500,000)	(126,000)	0
Additional paid-in capital	479,420,000	479,274,000	473,628,000
Accumulated deficit	(483,762,000)	(480,501,000)	(467,185,000)
Total stockholders' deficit	(4,837,000)	(1,348,000)	6,445,000
Total liabilities and stockholders' deficit	$ 5,621,000	$ 11,388,000	$ 23,867,000

Employee Benefit Plan	12 Months Ended
Employee Benefit Plan	Dec. 31, 2023
Retirement Benefits [Abstract]
Employee Benefit Plan	11. Employee Benefit Plan The Company implemented a 401(k) retirement savings and profit sharing plan (the “Plan”) effective January 1, 1999. During 2022, the Company commenced safe harbor matching contribution for up to 4% of eligible employee contributions. Total matching contribution under the Plan amounted to approximately $107,000 and $100,000 for the year ended December 31, 2023 and 2022, respectively.

CONDENSED STATEMENTS OF STOCKHOLDERS' EQUITY/(DEFICIT) (UNAUDITED) - USD ($)	Total	Series F Preferred Stock [Member]	Convertible Preferred Stock [Member]	Preferred Stock [Member]	Preferred Stock [Member] Series F Preferred Stock [Member]	Common Stock [Member]	Treasury Stock [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] Series F Preferred Stock [Member]	Accumulated Deficit [Member]
Balance at Dec. 31, 2021	$ 10,836,000					$ 1,000		$ 457,745,000		$ (446,910,000)
Balance (in shares) at Dec. 31, 2021			1,952			1,034,002
Stock-based compensation	606,000							606,000
Issue and sale of stock, net	15,278,000					$ 1,000		15,277,000
Issue and sale of share						1,206,090
Net loss	(20,275,000)									(20,275,000)
Balance at Dec. 31, 2022	6,445,000					$ 2,000		473,628,000		(467,185,000)
Balance (in shares) at Dec. 31, 2022			1,952			2,240,092
Stock-based compensation	140,000							140,000
Issue and sale of stock, net	895,000	$ 1,000						895,000	$ 1,000
Issue and sale of share					1	168,164
Net loss	(4,805,000)									(4,805,000)
Balance at Mar. 31, 2023	2,676,000					$ 2,000		474,664,000		(471,990,000)
Balance (in shares) at Mar. 31, 2023			1,952	1		2,408,256
Balance at Dec. 31, 2022	6,445,000					$ 2,000		473,628,000		(467,185,000)
Balance (in shares) at Dec. 31, 2022			1,952			2,240,092
Stock-based compensation	569,000							569,000
Issue and sale of stock, net	5,005,000					$ 3,000		5,002,000
Issue and sale of share					1	2,230,493
Redemption of Series F preferred stock (in shares)				(1)
Issuance of common stock for in process research and development	75,000							75,000
Issuance of common stock for in process research and development (in shares)						53,381
Fractional adjustment						(1,310)
Purchase of treasury stock	(126,000)						$ (126,000)
Purchase of treasury stock (in Shares)							(78,559)
Net loss	(13,316,000)									(13,316,000)
Balance at Dec. 31, 2023	(1,348,000)					$ 5,000	$ (126,000)	479,274,000		(480,501,000)
Balance (in shares) at Dec. 31, 2023			1,952			4,522,656	(78,559)
Stock-based compensation	146,000							146,000
Purchase of treasury stock	(374,000)						$ (374,000)
Purchase of treasury stock (in Shares)							(179,866)
Net loss	(3,261,000)									(3,261,000)
Balance at Mar. 31, 2024	$ (4,837,000)					$ 5,000	$ (500,000)	$ 479,420,000		$ (483,762,000)
Balance (in shares) at Mar. 31, 2024			1,952			4,522,656	(258,425)

Cover Page	3 Months Ended
Cover Page	Mar. 31, 2024
Document Information [Line Items]
Document Type	S-1
Amendment Flag	false
Entity Registrant Name	PLUS THERAPEUTICS, INC.
Entity Central Index Key	0001095981
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	33-0827593
Entity Address, Address Line One	4200 Marathon Blvd.
Entity Address, Address Line Two	Suite 200
Entity Address, City or Town	Austin
Entity Address, State or Province	TX
Entity Address, Postal Zip Code	78756
City Area Code	737
Local Phone Number	255-7194
Entity Primary SIC Number	2834
Business Contact [Member]
Document Information [Line Items]
Entity Address, Address Line One	4200 Marathon Blvd.
Entity Address, Address Line Two	Suite 200
Entity Address, City or Town	Austin
Entity Address, State or Province	TX
Entity Address, Postal Zip Code	78756
City Area Code	737
Local Phone Number	255-7194
Contact Personnel Name	Marc H. Hedrick, M.D.

Stockholders' Equity - Warrants (Details) - $ / shares	3 Months Ended		12 Months Ended
Stockholders' Equity - Warrants (Details) - $ / shares	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022
Series U Warrants [Member]
Class of Stock [Line Items]
Issue and sale of share	230,000		230,000
Sale of Stock, Price Per Share			$ 75
Warrants expected term	5 years		5 years
Representative Warrants [Member]
Class of Stock [Line Items]
Number of securities called by warrant or right	5,000		5,000
Warrant exercise price (in dollars per share)			$ 93.75
Common Stock [Member]
Class of Stock [Line Items]
Issue and sale of share		168,164	2,230,493	1,206,090
Warrants expected term			5 years
Outstanding Warrants [Member] \| Series U Warrants [Member]
Class of Stock [Line Items]
Warrant exercise price (in dollars per share)	$ 34.1		$ 34.1
Class of warrant outstanding	142,733		142,733
Number of warrants exercised	142,733		142,733
Common Class A [Member]
Class of Stock [Line Items]
Issue and sale of share	19,266		19,266
Common Class B [Member]
Class of Stock [Line Items]
Number of securities called by warrant or right	180,733		180,733

Term Loan Obligations - Term Loan Outstanding (Details) - USD ($)
$ in Thousands

3 Months Ended

12 Months Ended

Mar. 31, 2024

Dec. 31, 2023

Dec. 31, 2022

Jun. 28, 2023

May 29, 2015

Additional details relating term loan that is outstanding [Abstract]

Origination date

May 29, 2015

Term Loan [Member]

Additional details relating term loan that is outstanding [Abstract]

Origination date

May 29, 2015

Original loan amount

$ 17,700

Interest Rate

13.39%

^[1]

8.95%

^[1]

8.95%

Current Monthly Payment

^[2]

$ 134

Amended expiration date

Jun. 01, 2024

Remaining Principal (Face Value)

$ 804

$ 2,412

[1]	Three month LIBOR rate with a floor of 1% plus 7.95%
[2]	Monthly payment reflects principal and interest

Composition of Certain Financial Statement Captions (Tables)

12 Months Ended

Dec. 31, 2023

Composition Of Certain Financial Statement Captions [Abstract]

Other Current Assets

As of December 31, 2023 and 2022, other current assets were comprised of the following (in thousands):


	December 31,
	2023		2022
Prepaid services	$	644	$	2,999
Prepaid insurance		636		698

	$	1,280	$	3,697

Property and Equipment, net

As of December 31, 2023 and 2022, property and equipment, net, were comprised of the following (in thousands):


	December 31,
	2023			2022
Office and computer equipment	$	1,632		$	1,474
Leasehold improvements		1,810			1,810

		3,442			3,284
Less accumulated depreciation		(2,536	)		(1,960	)

	$	906		$	1,324

Accounts Payable and Accrued Expenses

As of December 31, 2023 and 2022, accounts payable and accrued expenses were comprised of the following (in thousands):


	December 31,
	2023		2022
Accounts payable	$	4,758	$	8,364
Accrued payroll and bonus		987		989
Accrued professional fees		128		147
Accrued vacation and compensation		370		325
Accrued R&D studies		388		309

	$	6,631	$	10,134

Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Deferred tax assets [Abstract]
Accrued expenses	$ 269	$ 262
Share based compensation	99	107
Net operating loss carryforwards	13,397	12,605
Income tax credit carryforwards	1,630	956
Property and equipment, principally due to differences in depreciation	154	89
Intangible assets	3,527	2,073
Other, net	453	53
Deferred tax assets (Total)	19,529	16,145
Valuation allowance	(19,486)	(16,092)
Total deferred tax assets, net of allowance	43	53
Deferred tax liabilities [Abstract]
Other	(43)	(53)
Total deferred tax liability	(43)	(53)
Net deferred tax assets (liability)	$ 0	$ 0

Term Loan Obligations - Additional Information (Details) - USD ($)

3 Months Ended

12 Months Ended

Jun. 28, 2023

Mar. 29, 2020

Mar. 28, 2020

May 29, 2015

Mar. 31, 2024

Mar. 31, 2023

Dec. 31, 2023

Dec. 31, 2022

Debt Instrument [Line Items]

Origination date

May 29, 2015

Interest expense

$ 34,000

$ 134,000

$ 395,000

$ 711,000

Non-cash amortization

$ 21,000

$ 100,000

$ 200,000

400,000

LIBOR [Member] | Interest Rate Floor [Member]

Debt Instrument [Line Items]

Basis variable rate

1.00%

Term Loan [Member]

Debt Instrument [Line Items]

May 29, 2015

$ 17,700,000

7.95%

Jun. 01, 2024

Sep. 01, 2021

May 01, 2021

Jun. 01, 2024

Fees amount associated with loan

$ 3,200,000

Number of shares callable by warrants (in shares)

188

Warrant exercise price (in dollars per share)

$ 5,175

Long term debt outstanding threshold amount

$ 800,000

Interest rate

8.95%

13.39%

^[1]

8.95%

^[1]

Minimum liquidity covenant

$ 2,000,000

Debt principal repayment start date

May 01, 2021

May 01, 2020

Debt instrument, covenant compliance

the Company was in compliance with all of the debt covenants under the Loan and Security Agreement.

Term Loan [Member] | Loan and Security Agreement [Member] | Sale of the Japanese Subsidiary and Certain Assets [Member]

Debt Instrument [Line Items]

Payments for principal, interest and fees

$ 3,100,000

Term Loan [Member] | SOFR [Member]

Debt Instrument [Line Items]

Basis variable rate

8.05%

Term Loan [Member] | Minimum [Member]

Debt Instrument [Line Items]

Interest rate

8.95%

Term Loan [Member] | Maximum [Member]

Debt Instrument [Line Items]

Original loan amount

$ 3,000,000

Long term debt outstanding threshold amount

$ 400,000

$ 800,000

[1]	Three month LIBOR rate with a floor of 1% plus 7.95%

Grant Revenue	3 Months Ended	12 Months Ended
Grant Revenue	Mar. 31, 2024	Dec. 31, 2023
Revenue from Contract with Customer [Abstract]
Grant Revenue	7. Grant Revenue On September 19, 2022, the Company entered into that certain CPRIT contract (“CPRIT Contract”), effective as of August 31, 2022, with CPRIT, pursuant to which CPRIT will provide the Company with a CPRIT grant (“CPRIT Grant”) over a three-year period to fund the continued development of rhenium ( 186 Re) obisbemeda for the treatment of patients with leptomeningeal metastases (“LM”). The CPRIT Grant is subject to customary CPRIT funding conditions, including, but not limited to, a matching fund requirement (one dollar for every two dollars awarded by CPRIT), revenue sharing obligations upon commercialization of rhenium ( 186 Re) obisbemeda based on specific dollar thresholds and tiered low single digit royalty rates until CPRIT receives the aggregate amount of 400% of the proceeds awarded under the CPRIT Grant, and certain reporting requirements. The CPRIT Contract will terminate on August 30, 2025, unless terminated earlier by (a) the mutual written consent of all parties to the CPRIT Contract, (b) CPRIT for an event of default by the Company, (c) CPRIT, if the funds allocated to the CPRIT Grant become legally unavailable during the term of the CPRIT Contract and CPRIT is unable to obtain additional funds for such purposes, and (d) the Company for convenience. CPRIT may require the Company to repay some or all of the disbursed CPRIT Grant proceeds (with interest not to exceed 5% annually) in the event of the early termination of the CPRIT Contract by CPRIT for an event of default by the Company or by the Company for convenience, or if the Company relocates its principal place of business outside of the state of Texas during the CPRIT Contract term or within three years after the final payment of the grant funds. The Company retains ownership over any intellectual property developed under the CPRIT Contract (each, a “Project Result”). With respect to non-commercial use of any Project Result, the Company granted to CPRIT a nonexclusive, irrevocable, royalty-free, perpetual, worldwide license with right to sublicense any necessary additional intellectual property rights to exploit all Project Results by CPRIT, other governmental entities and agencies of the State of Texas, and private or independent institutions of higher education located in Texas, for education, research and other non-commercial purposes. The Company recognized $1.7 million and $0.5 million in grant revenue from the CPRIT Contract during the three months ended March 31, 2024 and 2023, respectively.	9. Grant Revenue On September 19, 2022, the Company entered into the CPRIT Contract, effective as of August 31, 2022, with CPRIT, pursuant to which CPRIT will provide the Company with the CPRIT Grant of up to $17.6 million over a three-year period to fund the continued development of rhenium ( 186 Re) obisbemeda (previously known as 186 RNL) for the treatment of patients with leptomeningeal metastases (“LM”). The CPRIT Grant is subject to customary CPRIT funding conditions, including, but not limited to, a matching fund requirement (one dollar for every two dollars awarded by CPRIT), revenue sharing obligations upon commercialization of rhenium ( 186 Re) obisbemeda based on specific dollar thresholds and tiered low single digit royalty rates until CPRIT receives the aggregate amount of 400% of the proceeds awarded under the CPRIT Grant, and certain reporting requirements. The CPRIT Contract will terminate on August 30, 2025, unless terminated earlier by (a) the mutual written consent of all parties to the CPRIT Contract, (b) CPRIT for an event of default by the Company, (c) CPRIT, if the funds allocated to the CPRIT Grant become legally unavailable during the term of the CPRIT Contract and CPRIT is unable to obtain additional funds for such purposes, and (d) the Company for convenience. CPRIT may require the Company to repay some or all of the disbursed CPRIT Grant proceeds (with interest not to exceed 5% annually) in the event of the early termination of the CPRIT Contract by CPRIT for an event of default by the Company or by the Company for convenience, or if the Company relocates its principal place of business outside of the state of Texas during the CPRIT Contract term or within three years after the final payment of the grant funds. The Company retains ownership over any intellectual property developed under the contract (each, a “Project Result”). With respect to non-commercial use of any Project Result, the Company granted to CPRIT a nonexclusive, irrevocable, royalty-free, perpetual, worldwide license with right to sublicense any necessary additional intellectual property rights to exploit all Project Results by CPRIT, other governmental entities and agencies of the State of Texas, and private or independent institutions of higher education located in Texas, for education, research and other non-commercial purposes. The Company determined that the CPRIT Contract is not in the scope of ASC 808, ASC 958-605, or ASC 606. Applying IAS 20 by analogy, the Company recognizes proceeds received under the CPRIT Contract as grant revenue on the statement of operations when related costs are incurred. During the three months ended June 30, 2023, the Company identified eligible costs of approximately $637,000 and $168,000 that were incurred in the fourth quarter of 2022 and the three months ended March 31, 2023, respectively, that were reimbursable under the CPRIT arrangement. The Company determined that these costs should have been recorded in these respective periods as grant revenue. The Company assessed the impact of this error on the Company’s previously issued financial statements and determined that it was immaterial. As a result, an out-of-period adjustment of approximately $637,000 has been recorded as an increase in grant revenue in the year ended December 31, 2023. The Company recognized $4.9 million and $0.2 million in grant revenue from the CPRIT Contract during the year ended December 31, 2023 and 2022, respectively.

- Definition

The entire disclosure of revenue from contract with customer to transfer good or service and to transfer nonfinancial asset. Includes, but is not limited to, disaggregation of revenue, credit loss recognized from contract with customer, judgment and change in judgment related to contract with customer, and asset recognized from cost incurred to obtain or fulfill contract with customer. Excludes insurance and lease contracts.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 9
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479806/606-10-50-9

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 10
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479806/606-10-50-10

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
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-URI https://asc.fasb.org//1943274/2147479806/606-10-50-15

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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-Subparagraph (a)
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Reference 5: http://www.xbrl.org/2003/role/disclosureRef
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-Section 50
-Paragraph 12
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-URI https://asc.fasb.org//1943274/2147479806/606-10-50-12

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
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-Paragraph 12
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-URI https://asc.fasb.org//1943274/2147479806/606-10-50-12

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
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-URI https://asc.fasb.org//1943274/2147479806/606-10-50-12

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
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Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
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-Section 50
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-URI https://asc.fasb.org//1943274/2147479806/606-10-50-13

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Name Accounting Standards Codification
-Topic 606
-Publisher FASB
-URI https://asc.fasb.org//606/tableOfContent

+ Details

Stock-based Compensation

3 Months Ended

12 Months Ended

Mar. 31, 2024

Dec. 31, 2023

Share-Based Payment Arrangement [Abstract]

Stock-based Compensation

11. Stock-based Compensation

Under the Company’s 2015 New Employee Incentive Plan (the “2015 Plan”), awards may only be granted to employees who were not previously an employee or director of the Company, or following a bona fide period of

non-employment,

as a material inducement to entering into employment with the Company. As of March 31, 2024, there were 6,024 shares of common stock remaining and available for future issuances under the 2015 Plan.

The Company’s 2020 Stock Incentive Plan (the “2020 Plan”), which replaced the Company’s 2014 Equity Incentive Plan, provides for the award or sale of shares of common stock (including restricted stock), the award of stock units and stock appreciation rights, and the grant of both incentive stock options to purchase common stock to directors, officers, employees and consultants of the Company. The 2020 Plan, as amended, provides for the issuance of up to 236,667 shares of common stock, plus the number of shares available for issuance is increased to the extent that awards granted under the 2020 Plan and the Company’s 2014 Equity Incentive Plan are forfeited or expire (except as otherwise provided in the 2020 Plan). As of March 31, 2024, there were 17,582 shares remaining and available for future issuances under the 2020 Plan.

Generally, options issued under the 2020 Plan are subject to a

two-year

or four-year vesting schedule with 25% of the options vesting on the one year anniversary of the grant date followed by equal monthly installment vesting, and have a contractual term of 10 years.

A summary of activity for the three months ended March 31, 2024 is as follows:


	Options			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (years)		Aggregate Intrinsic Value
Balance as of December 31, 2023		140,109		$	37.48		8.07
Granted		163,025		$	2.03
Cancelled/forfeited		(1	)	$	289,125

Balance as of March 31, 2024		303,133		$	17.46		8.94	$	7,600

Vested and expected to vest at March 31, 2024		278,083		$	18.64		8.87	$	6,800

Exercisable at March 31, 2024		84,908		$	50.13		7.37	$	500

As of March 31, 2024, the total compensation cost related to

non-vested

stock options not yet recognized for all the Company’s plans is approximately $0.8 million, which is expected to be recognized as a result of vesting under service conditions over a weighted average period of 2.3 years.

13. Share-based Compensation

non-employment,

as a material inducement to entering into employment with the Company. As of December 31, 2023, there were 6,024 shares of common stock remaining and available for future issuances under the 2015 Plan.

The Company’s 2020 Stock Incentive Plan (the “2020 Plan”), which replaced the Company’s 2014 Equity Incentive Plan, provides for the award or sale of shares of common stock (including restricted stock), the award of stock units and stock appreciation rights, and the grant of both incentive stock options to purchase common stock to directors, officers, employees and consultants of the Company. The 2020 Plan, as amended, provides for the issuance of up to 3,550,000 shares of common stock, plus the number of shares available for issuance is increased to the extent that awards granted under the 2020 Plan and the Company’s 2014 Equity Incentive Plan are forfeited or expire (except as otherwise provided in the 2020 Plan). As of December 31, 2023, there were 180,607 shares remaining and available for future issuances under the 2020 Plan.

Generally, options issued under the 2020 Plan are subject to a

two-year

or four-year vesting schedule with 25% of the options vesting on the one year anniversary of the grant date followed by equal monthly installment vesting, and have a contractual term of 10 years.

A summary of activity for the year ended December 31, 2023 is as follows:


	Options			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (years)		Aggregate Intrinsic Value
Balance as of December 31, 2022		78,334		$	68.16		8.00
Granted		68,422			5.00
Cancelled/forfeited		(6,647	)		283.76

Balance as of December 31, 2023		140,109		$	37.48		8.07	$	7,000

Vested and expected to vest at December 31, 2023		133,810		$	38.62		8.02	$	—

Exercisable at December 31, 2023		72,843		$	60.17		7.41	$	6,000

The Company settles exercises of stock options with newly issued shares of its common stock. There were no stock options exercised in 2023 or 2022.

The estimated fair value of options, including the effect of estimated forfeitures, is recognized over the requisite service period, which is typically the vesting period of each option. The fair value of each option awarded during the years ended December 31, 2023 and 2022 was estimated on the date of grant using the Black-Scholes-Merton option valuation model based on the following weighted-average assumptions:


	December 31, 2023			December 31, 2022
Expected term		6.0 years			6.0 years
Risk-free interest rate		4.06	%		2.83	%
Expected volatility		127.0	%		123.4	%
Dividends		0	%		0	%
Resulting fair value	$	4.47		$	7.05

The weighted average risk-free interest rate represents the interest rate for treasury constant maturity instruments published by the Federal Reserve Board. If the term of available treasury constant maturity instruments is not equal to the expected term of an employee option, the Company uses the weighted average of the two Federal Reserve securities closest to the expected term of the employee option.

The dividend yield has been assumed to be zero as the Company (a) has never declared or paid any dividends and (b) does not currently anticipate paying any cash dividends on its outstanding shares of common stock in the foreseeable future.

The following table summarizes share-based compensation recognized during the years ended December 31, 2023 and 2022 in the statement of operations:


	2023		2022
Research and development	$	66	$	87
General and administrative		503		519

Total share-based compensation	$	569	$	606

As of December 31, 2023, the total compensation cost related to

non-vested

stock options and stock awards not yet recognized for all our plans is approximately $0.7 million, which is expected to be recognized as a result of vesting under service conditions over a weighted average period of 1.8 years.

Basis of Presentation and New Accounting Standards (Additional Information) (Details) $ / shares in Units, $ in Thousands				3 Months Ended	12 Months Ended
	Apr. 27, 2023	Apr. 21, 2023	Apr. 20, 2023 $ / shares	Mar. 31, 2024 USD ($) $ / shares shares	Dec. 31, 2023 $ / shares shares	May 01, 2023 shares	Apr. 30, 2023 shares	Apr. 04, 2023 $ / shares	Dec. 31, 2022 $ / shares shares
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Common stock, par value (in dollars per share) \| $ / shares			$ 0.001	$ 0.001	$ 0.001			$ 0.001	$ 0.001
Stock split conversion ratio		0.06
Reverse stock split effective date	May 01, 2023
Common stock, shares issued (in shares)				4,522,656	4,522,656	2,500,000	37,400,000		2,240,092
Preferred stock, shares authorized (in shares)				5,000,000	5,000,000				5,000,000
Common stock, reverse stock split, fractional shares issued				0	0
Stock issued during period reverse stock splits, shares				1,310	1,310
Equity Incentive Plans [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Common stock capital shares reserved for future issuance						200,000	3,000,000
Outstanding Warrants and Options [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Common stock capital shares reserved for future issuance						100,000	2,000,000
Minimum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stock split conversion ratio			0.33
Maximum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stock split conversion ratio			0.06
Unrealized gain on available-for-sale securities \| $				$ 1,000

Employee Benefit Plan - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Defined Contribution Plan Disclosure [Line Items]
Discretionary or matching contributions	$ 107,000	$ 100,000
Maximum [Member]
Defined Contribution Plan Disclosure [Line Items]
Percentage of matching contribution for employee contributions	4.00%

Loss per Share (Tables)

3 Months Ended

12 Months Ended

Mar. 31, 2024

Dec. 31, 2023

Earnings Per Share [Abstract]

Schedule of Antidilutive Securities Excluded from Diluted Income (Loss) per Share

The following were excluded from the diluted loss per share calculation for the periods presented because their effect would be anti-dilutive:


	As of March 31,
	2024		2023
Outstanding stock options		303,133		134,283
Preferred stock		28,190		28,190
Outstanding warrants		142,733		142,733

Total		474,056		305,206

The following were excluded from the diluted loss per share calculation for the periods presented because their effect would be anti-dilutive:


	For the Year Ended December 31,
	2023		2022
Outstanding stock options		140,109		78,334
Preferred stock		28,190		28,190
Outstanding warrants		142,733		142,758

Total		311,032		249,282

Stockholders' Equity - Preferred Stock (Details)		3 Months Ended	12 Months Ended
Stockholders' Equity - Preferred Stock (Details)	Mar. 03, 2023 USD ($) Votes $ / shares shares	Mar. 31, 2024 $ / shares shares	Dec. 31, 2023 $ / shares shares	Apr. 20, 2023 $ / shares	Apr. 04, 2023 $ / shares	Dec. 31, 2022 $ / shares shares
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares)		5,000,000	5,000,000			5,000,000
Preferred stock, par value (in dollars per share) \| $ / shares		$ 0.001	$ 0.001			$ 0.001
Common stock, par value (in dollars per share) \| $ / shares		$ 0.001	$ 0.001	$ 0.001	$ 0.001	$ 0.001
Preferred stock, shares outstanding (in shares)		1,952	1,952			1,952
Series F Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares)	1
Number Of Votes Per Share \| Votes	50,000,000
Common stock, par value (in dollars per share) \| $ / shares	$ 0.001
Preferred stock, voting rights	no
Rights related to distribution of assets	no
Consideration paid to preferred stock holders \| $	$ 1,000
Series F Convertible Preferred Stock [Member] \| Subscription Agreement [Member]
Class of Stock [Line Items]
Preferred stock, par value (in dollars per share) \| $ / shares	$ 0.001
Common stock, par value (in dollars per share) \| $ / shares	$ 0.001
Issue and sale of share	1
Consideration to purchaser in cash \| $	$ 1,000
Sale closed date	Mar. 03, 2023
Series B Convertible Preferred Stock [Member] \| 2018 Rights Offering [Member]
Class of Stock [Line Items]
Preferred stock, shares outstanding (in shares)		1,014	1,014
Number of preferred stock, shares converted		398	398
Series C Convertible Preferred Stock [Member] \| 2018 Rights Offering [Member]
Class of Stock [Line Items]
Preferred stock, shares outstanding (in shares)		938	938
Number of preferred stock, shares converted		27,792	27,792

Term Loan Obligations

3 Months Ended

12 Months Ended

Mar. 31, 2024

Dec. 31, 2023

Debt Disclosure [Abstract]

Term Loan Obligations

5. Term Loan Obligations

$17.7 million (the “Term Loan”), subject to the terms and conditions set forth in the Loan and Security Agreement. The Term Loan accrues interest at a floating rate of at least 8.95% per annum, comprised of a three-month LIBOR rate with a floor of 1.00% plus 7.95%. Pursuant to the Loan and Security Agreement, as amended, the Company made interest only payments through May 1, 2021, and thereafter is required to make payments of principal and accrued interest in equal monthly installments sufficient to amortize the Term Loan through June 1, 2024, the maturity date. At maturity of the Term Loan, or earlier repayment in full following voluntary prepayment or upon acceleration, the Company is required to make a final payment in an aggregate amount equal to approximately $3.2 million.

From September 2017 to March 2020, the Company entered into a total of nine amendments to the Term Loan that, among other things, extended the interest only period, required repayment of $3.1 million using the proceeds received from sale of the Company’s former UK and Japan subsidiaries in April 2019, increased the final payment, increased the final payment fee upon maturity or early repayment of the Term Loan, increased the minimum liquidity covenant level to $2.0 million and deferred the start date of principal repayment from May 1, 2020 to May 1, 2021 and extended the term of the Term Loan from September 1, 2021 to June 1, 2024.

On June 28, 2023, the Company and Oxford entered into a tenth amendment to the Loan and Security Agreement, and revised the interest rate of the Loan to the greater of: (1) 8.95%, or (2) the sum of

1-month

Secured Overnight Financing Rate and 8.05%, effective July 1, 2023.

The Term Loan, as amended, is collateralized by a security interest in substantially all of the Company’s existing and subsequently acquired assets, including its intellectual property assets, subject to certain exceptions set forth in the Loan and Security Agreement, as amended. The intellectual property asset collateral will be released upon the Company achieving a certain liquidity level when the total principal outstanding under the Loan and Security Agreement is less than $3.0 million. As of March 31, 2024, there was $0.4 million principal amount outstanding under the Term Loan, excluding the $3.2 million final payment fee, and the Company was in compliance with all of the debt covenants under the Loan and Security Agreement.

The Company’s interest expense for the three ended March 31, 2024 and 2023 was $34,000 and $0.1 million, respectively. Interest expense is calculated using the effective interest method; therefore it is inclusive of

non-cash

amortization in the amount of $21,000 and $0.1 million for the three ended March 31, 2024 and 2023, respectively, related to the amortization of the debt discount, capitalized loan costs, and accretion of final payment.

The Loan and Security Agreement, as amended, contains customary indemnification obligations and customary events of default, including, among other things, the Company’s failure to fulfill certain obligations under the Term Loan, as amended, and the occurrence of a material adverse change, which is defined as a material adverse change in the Company’s business, operations, or condition (financial or otherwise), a material impairment of the prospect of repayment of any portion of the loan. In the event of default by the Company or a declaration of material adverse change by its lender, under the Term Loan, the lender would be entitled to exercise its remedies thereunder, including the right to accelerate the debt, upon which the Company may be required to repay all amounts then outstanding under the Term Loan, which could materially harm the Company’s financial condition. As of March 31, 2024, the Company has not received any notification or indication from Oxford that it intends to invoke the material adverse change clause. See Note 12 for further information regarding the repayment of the Term Loan.

8. Term Loan Obligations

On May 29, 2015, the Company entered into the Loan and Security Agreement (the “Loan and Security Agreement”), pursuant to which Oxford Finance, LLC (“Oxford”) funded an aggregate principal amount of $17.7 million (the “Term Loan”), subject to the terms and conditions set forth in the Loan and Security Agreement. The Term Loan accrued interest at a floating rate of at least 8.95% per annum, comprised of a three-month LIBOR rate with a floor of 1.00% plus 7.95%. Pursuant to the Loan and Security Agreement, as amended, the Company was required to make interest only payments through May 1, 2021 and thereafter it was required to make payments of principal and accrued interest in equal monthly installments sufficient to amortize the Term Loan through June 1, 2024, the maturity date. At maturity of the Term Loan, or earlier repayment in full following voluntary prepayment or upon acceleration, the Company is required to make a final payment in an aggregate amount equal to approximately $3.2 million. In connection with the Term Loan, on May 29, 2015, the Company issued to Oxford warrants to purchase an aggregate of 188 shares of the Company’s common stock at an exercise price of $5,175 per share. These warrants became exercisable as of November 30, 2015 and will expire on May 29, 2025 and, following the authoritative accounting guidance, are equity classified and their respective fair value was recorded as a discount to the debt.

The Term Loan was collateralized by a security interest in substantially all of the Company’s existing and subsequently acquired assets, including its intellectual property assets, subject to certain exceptions set forth in the Loan and Security Agreement, as amended. The intellectual property asset collateral would be released upon the Company achieving a certain liquidity level when the total principal outstanding under the Loan and Security Agreement is less than $3 million. As of December 31, 2023, there was $0.8 million principal amount outstanding under the Term Loan, excluding the $3.2 million final payment fee, and the Company was in compliance with all of the debt covenants under the Loan and Security Agreement.

The Company’s interest expense for the years ended December 31, 2023 and 2022 was $0.4 million and $0.7 million, respectively. Interest expense is calculated using the effective interest method; therefore it is inclusive of

non-cash

amortization in the amount of $0.2 million and $0.4 million for the year ended December 31, 2023 and 2022, respectively, related to the amortization of the debt discount, deferred financing costs, and accretion of final payment.

The Loan and Security Agreement contains customary indemnification obligations and customary events of default, including, among other things, the Company’s failure to fulfill certain obligations under the Term Loan, as amended, and the occurrence of a material adverse change, which is defined as a material adverse change in the Company’s business, operations, or condition (financial or otherwise), a material impairment of the prospect of repayment of any portion of the loan. In the event of default by the Company or a declaration of material adverse change by its lender, under the Term Loan, the lender would be entitled to exercise its remedies thereunder, including the right to accelerate the debt, upon which the Company may be required to repay all amounts then outstanding under the Term Loan, which could materially harm the Company’s financial condition. As of December 31, 2023, the Company has not received any notification or indication from Oxford to invoke the material adverse change clause.

Additional information relating to the then-outstanding Term Loan as of December 31, 2023 and 2022 is presented in the following table (in thousands, except interest rate):


Year ended December 31, 2023
Origination Date	Original Loan Amount		Interest Rate*			Current Monthly Payment**		Amended expiration date		Remaining Principal (Face Value)
May 2015	$	17,700		13.39	%	$	134		June 1, 2024	$	804


Year ended December 31, 2022
Origination Date	Original Loan Amount		Interest Rate*			Monthly Payment**		Amended expiration date		Remaining Principal (Face Value)
May 2015	$	17,700		8.95	%	$	134		June 1, 2024	$	2,412

*	Three month LIBOR rate with a floor of 1% plus 7.95%

**	Monthly payment reflects principal and interest

- Definition

The entire disclosure for information about short-term and long-term debt arrangements, which includes amounts of borrowings under each line of credit, note payable, commercial paper issue, bonds indenture, debenture issue, own-share lending arrangements and any other contractual agreement to repay funds, and about the underlying arrangements, rationale for a classification as long-term, including repayment terms, interest rates, collateral provided, restrictions on use of assets and activities, whether or not in compliance with debt covenants, and other matters important to users of the financial statements, such as the effects of refinancing and noncompliance with debt covenants.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Subparagraph (h)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481139/470-20-50-1B

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 470
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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Reference 6: http://www.xbrl.org/2003/role/disclosureRef
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Reference 7: http://www.xbrl.org/2003/role/disclosureRef
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Reference 9: http://www.xbrl.org/2003/role/disclosureRef
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-SubTopic 20
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Reference 10: http://www.xbrl.org/2003/role/disclosureRef
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Reference 11: http://www.xbrl.org/2003/role/disclosureRef
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Reference 12: http://www.xbrl.org/2003/role/disclosureRef
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-URI https://asc.fasb.org//1943274/2147481139/470-20-50-1I

Reference 13: http://www.xbrl.org/2003/role/disclosureRef
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-Section 50
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-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481139/470-20-50-1I

+ Details

Liquidity and Going Concern - Additional Information (Details) - USD ($)		3 Months Ended		12 Months Ended
	Mar. 08, 2024	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022	May 09, 2024	Dec. 31, 2021
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net loss		$ (3,261,000)	$ (4,805,000)	$ (13,316,000)	$ (20,275,000)
Accumulated deficit		(483,762,000)		(480,501,000)	(467,185,000)
Net cash used in operating activities		(4,513,000)	(5,793,000)	(12,851,000)	(12,972,000)
Stockholders' deficit		$ (4,837,000)	2,676,000	(1,348,000)	6,445,000		$ 10,836,000
Stockholders' equity requirement	$ 2,500,000		$ 895,000	$ 5,005,000	$ 15,278,000
Net income from continuing operations	500,000
Maintain minimum market value of listed securities	$ 35,000,000
Private Placement [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Warrants And Rights Outstanding						$ 7,250,000

CONDENSED BALANCE SHEETS (UNAUDITED) (Parenthetical) - $ / shares	Mar. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Stockholders' deficit:
Treasury stock, shares	258,425	78,559
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001	$ 0.001
Preferred stock, shares authorized (in shares)	5,000,000	5,000,000	5,000,000
Preferred stock, shares issued (in shares)	1,952	1,952	1,952
Preferred stock, shares outstanding (in shares)	1,952	1,952	1,952
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	100,000,000	100,000,000	100,000,000
Common stock, shares issued (in shares)	4,522,656	4,522,656	2,240,092
Common stock, shares outstanding (in shares)	4,264,231	4,444,097	2,240,092

Basis of Presentation and New Accounting Standards	3 Months Ended
Basis of Presentation and New Accounting Standards	Mar. 31, 2024
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation and New Accounting Standards	1. Basis of Presentation and New Accounting Standards The accompanying unaudited condensed financial statements for the three months ended March 31, 2024 and 2023 have been prepared in accordance with accounting principles generally accepted in the United States of America for interim financial information. Accordingly, they do not include all of the information and footnotes required by accounting principles generally accepted in the United States of America for annual financial statements. The condensed balance sheet at December 31, 2023 has been derived from the audited financial statements at December 31, 2023, but does not include all of the information and footnotes required by accounting principles generally accepted in the United States of America for complete financial statements. In the opinion of management, all adjustments (consisting of normal recurring adjustments) considered necessary for a fair presentation of the financial position and results of operations of Plus Therapeutics, Inc. (the “Company”) have been included. Operating results for the three months ended March 31, 2024 are not necessarily indicative of the results that may be expected for the year ending December 31, 2024. These financial statements should be read in conjunction with the financial statements and notes therein included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2023, filed with the Securities and Exchange Commission on March 5, 2024. Amendments to Certificate of Incorporation and Reverse Stock Split At the Annual Meeting of Stockholders of the Company held on April 20, 2023 (the “Annual Meeting”), the stockholders of the Company approved an amendment to the Company’s Amended and Restated Certificate of Incorporation (the “Charter”) to implement a reverse stock split of the Company’s then issued and outstanding common stock, par value $0.001 per share, with the ratio to be determined by the Board of Directors (the “Board”) of the Company, within a range of not less than 1-for-3 and not greater than 1-for-15 . Subsequently, on April 21, 2023, the Board determined to fix the ratio for the reverse stock split at 1-for-15 , without any change to its par value (the “Reverse Stock Split”). On April 27, 2023, following stockholder and Board approval, the Company filed a Certificate of Amendment to its Charter (the “Amendment”), with the Secretary of State of the State of Delaware to effectuate the Reverse Stock Split. The Amendment became effective on May 1, 2023. Upon effectiveness of the Reverse Stock Split, the number of shares of the Company’s common stock (x) issued and outstanding decreased from approximately 37.4 million shares to approximately 2.5 million shares; (y) reserved for issuance upon exercise of outstanding warrants and options decreased from approximately 2.0 million shares to approximately 0.1 million shares, and (z) reserved but unallocated under the Company’s current equity incentive plans decreased from approximately 3.0 million shares of common stock to approximately 0.2 million shares of common stock. The Company’s common stock began trading on the Nasdaq Capital Market (“Nasdaq”) on a post-split basis on May 1, 2023. The Company’s 5,000,000 shares of authorized Preferred Stock were not affected by the Reverse Stock Split. No fractional shares were issued in connection with the Reverse Stock Split, and accordingly, the outstanding number of shares post Reverse Stock Split was adjusted down by approximately 1,310 (post-effect of the Reverse Stock Split) shares. Proportional adjustments for the Reverse Stock Split were made to the Company’s outstanding stock options, warrants and equity incentive plans for the period ended March 31, 2023 as presented in the condensed financial statements in this Quarterly Report on Form 10-Q. The Company’s financial statements, and all references thereto have been retroactively adjusted to reflect the Reverse Stock Split unless specifically stated otherwise. Grant Revenue Recognition In applying the provisions of Accounting Standards Codification (“ASC”) Topic 606, Revenue from Contracts with Customers (“ASC 606”), the Company has determined that government grants are out of the scope of ASC 606 because the funding entities do not meet the definition of a “customer”, as defined by ASC 606, as the Company does not consider there to be a transfer of control of goods or services. With respect to each grant, the Company determines if it has a collaboration in accordance with ASC Topic 808, Collaborative Arrangements (“ASC 808”). For grants outside the scope of ASC 808, the Company applies International Accounting Standards No. 20 (“IAS 20”), Accounting for Government Grants and Disclosure of Government Assistance, by analogy, and revenue is recognized when the Company incurs expenses related to the grant for the amount the Company is entitled to under the provisions of the contract. The Company also considers the guidance in ASC Topic 730, Research and Development, which requires an assessment, at the inception of each grant, of whether each grant agreement is a liability. If the Company is obligated to repay funds received regardless of the outcome of the related research and development activities, then the Company is required to estimate and recognize that liability. Alternatively, if the Company is not required to repay the funds, then payments received are recorded as revenue or contra-expense as the expenses are incurred. Deferred grant liability represents grant funds received or receivable for which the allowable expenses have not yet been incurred as of the balance sheet date. Available-for-Sale Securities The Company’s available-for-sale securities consist of U.S. government and agency securities. The Company classifies its marketable securities as available-for-sale and records such assets at estimated fair value in the condensed balance sheets, with unrealized gains and losses, if any, reported as a component of other comprehensive income (loss) within the condensed statements of operations and comprehensive income/loss and as a separate component of stockholders’ equity. Realized gains and losses are calculated on the specific identification method and recorded as interest income (loss). At each balance sheet date, the Company assesses available-for-sale securities in an unrealized loss position to determine whether the decline in fair value below amortized cost is a result of credit losses or other factors, whether the Company expects to recover the amortized cost of the security, the Company’s intent to sell and if it is more likely than not that the Company will be required to sell the securities before the recovery of amortized cost. The Company records changes in allowance for expected credit loss in other income (expense). As of March 31, 2024, there were no available-for-sale securities in an unrealized loss position, and there has been no allowance for expected credit losses recorded during any of the periods presented. Any premium arising at purchase is amortized to the earliest call date and any discount arising at purchase is accreted to maturity. Accretion of discounts are recorded in interest income in the condensed statements of operations and comprehensive income/loss. During the three months ended March 31, 2024, the unrealized gain on the Company’s available-for-sale securities was less than $1,000, and not presented separately in the condensed statement of operations and comprehensive income/loss. Recently Issued Accounting Pronouncements In December 2023, the Financial Accounting Standards Board (the “FASB”) issued Accounting Standards Update (ASU) No. 2023-09 Income Taxes (Topic 740): Improvements to Income Tax Disclosure. This ASU includes amendments that further enhance income tax disclosures, primarily through standardization and disaggregation of rate reconciliation categories and income taxes paid by jurisdiction. The ASU is effective for years beginning after December 15, 2024, but early adoption is permitted. This ASU should be applied on a prospective basis, although retrospective application is permitted. Management is currently evaluating the impact of the changes required by the new standard on the Company’s financial statements and related disclosures. In November 2023, the FASB issued Accounting Standard Update (ASU) No. 2023-07, Segment Reporting (Topic 280): Improvements to Reportable Segment Disclosures. The new standard is intended to improve annual and interim reportable segment disclosure requirements regardless of number of reporting units, primarily through enhanced disclosures of significant expenses. The amendment requires public entities to disclose significant segment expenses that are regularly provided to the C ODM and included within each reported measure of segment profit and loss. This update is effective for fiscal years beginning after December 15, 2023, and interim periods within those fiscal years starting after December 15, 2024. This ASU must be applied retrospectively to all prior periods presented. Management is currently evaluating the impact of the changes required by the new standard on the Company’s financial statements and related disclosures .

License Agreements	3 Months Ended	12 Months Ended
License Agreements	Mar. 31, 2024	Dec. 31, 2023
License Agreement [Abstract]
License Agreements	9. License Agreements Biocept License Agreement On September 7, 2023, the Company entered into a Non-Exclusive License and Services Agreement (the “Biocept Agreement”) with Biocept, Inc (“Biocept”), pursuant to which Biocept granted the Company a non-exclusive license to use the Biocept proprietary cell enumeration test, CNsideTM (“CNside”). In exchange for the license, the Company issued to Biocept 53,381 unregistered shares, the fair value of which was $75,000. The Biocept Agreement also provides that if Biocept fully transfers the technology to the Company, a tech transfer and validation fee of $300,000 will be payable. In addition, the Company was granted an option for an exclusive worldwide license for $1,000,000 on or before December 31, 2024, to process and perform cell enumeration testing for treatments for other patients including those on the Company’s radiotherapeutic drugs. On October 16, 2023, Biocept filed a voluntary petition for relief under the provisions of Chapter 7 of Title 11 of the United States Bankruptcy Code. See Note 12 for further information regarding the Company’s acquisition of substantially all right, title and interest in CNside. UT Health Science Center at San Antonio (“UTHSCSA”) License Agreement On December 31, 2021, the Company entered into a Patent and Know-How License Agreement (the “UTHSCSA License Agreement”) with The University of Texas Health Science Center at San Antonio (“UTHSCSA”), pursuant to which UTHSCSA granted the Company an irrevocable, perpetual, exclusive, fully paid-up license, with the right to sublicense and to make, develop, commercialize and otherwise exploit certain patents, know-how and technology related to the development of biodegradable alginate microspheres (“BAM”) containing nanoliposomes loaded with imaging and/or therapeutic payloads. NanoTx License Agreement On March 29, 2020, the Company and NanoTx, Corp. (“NanoTx”) entered into a Patent and Know-How License Agreement, pursuant to which NanoTx granted the Company an irrevocable, perpetual, exclusive, fully paid-up license, with the right to sublicense and to make, develop, commercialize and otherwise exploit certain patents, know-how and technology related to the development of radiolabeled nanoliposomes. The transaction terms included an upfront payment of $0.4 million in cash and $0.3 million in the Company’s voting stock. The transaction terms also included success-based milestone and royalty payments contingent on key clinical, regulatory and sales milestones, as well as the requirement to pay 15% of any non-dilutive monetary awards or grants received from external agencies to support product development of the nanoliposome encapsulated BMEDA-chelated radioisotope, which includes grants from CPRIT. As of March 31, 2024, the Company accrued $0.5 million of payments due to NanoTx as a result of the CPRIT grant received (see Note 7, Grant Revenue of the condensed financial statements for additional information).	7. License Agreements Biocept License Agreement On September 7, 2023, the Company entered into a Non-Exclusive License and Services Agreement (the “Biocept Agreement”) with Biocept, Inc (“Biocept”), pursuant to which Biocept granted the Company a non-exclusive license to use the Biocept proprietary cell enumeration test, CNside TM . In exchange for the license, the Company issued to Biocept 53,381 unregistered shares, the fair value of which was $75,000. The Biocept Agreement also provides that if Biocept fully transfers the technology to the Company, a tech transfer and validation fee of $300,000 will be payable. In addition, the Company was granted an option for an exclusive worldwide license for $1,000,000 on or before December 31, 2024, to process and perform cell enumeration testing for treatments for other patients including those on the Company’s radiotherapeutic drugs. On October 16, 2023, Biocept filed a voluntary petition for relief under the provisions of Chapter 7 of Title 11 of the United States Bankruptcy Code, making the full transfer of the Biocept technology to the Company unlikely. In addition, the Biocept Agreement is subject to provisions under the Bankruptcy Code. UT Health Science Center at San Antonio (“UTHSA”) License Agreement On December 31, 2021, the Company entered into a Patent and Know-How License Agreement (the “UTHSA License Agreement”) with UTHSA, pursuant to which UTHSA granted the Company an irrevocable, perpetual, exclusive, fully paid-up license, with the right to sublicense and to make, develop, commercialize and otherwise exploit certain patents, know-how and technology related to the development of biodegradable alginate microspheres (BAM) containing nanoliposomes loaded with imaging and/or therapeutic payloads. Pursuant to the UTHSA License Agreement, the Company was required to make an upfront payment, which was recorded as in-process research and development acquired in the statement of operations for the year ended December 31, 2021. The upfront payment of $0.3 million was paid in cash in January 2022. NanoTx License Agreement On March 29, 2020, the Company and NanoTx, Corp. (“NanoTx”) entered into a Patent and Know-How License Agreement (the “NanoTx License Agreement”), pursuant to which NanoTx granted the Company an irrevocable, perpetual, exclusive, fully paid-up license, with the right to sublicense and to make, develop, commercialize and otherwise exploit certain patents, know-how and technology related to the development of radiolabeled nanoliposomes. The transaction terms included an upfront payment of $0.4 million in cash and $0.3 million in the Company’s voting stock. The transaction terms also included success-based milestone and royalty payments contingent on key clinical, regulatory and sales milestones, as well as the requirement to pay 15% of any non-dilutive monetary awards or grants received from external agencies to support product development of the nanoliposome encapsulated BMEDA-chelated radioisotope, which includes grants from the Cancer Prevention & Research Institute of Texas (“CPRIT”). As of December 31, 2023, the Company accrued $0.5 million of payments due to NanoTx as a result of the CPRIT grant received (Note 9).

Loss per Share - Schedule of Antidilutive Securities Excluded from Diluted Income (Loss) per Share (Details) - shares	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Dilutive common shares excluded from the calculations of diluted loss per share (in shares)	474,056	305,206	311,032	249,282
Outstanding Stock Options [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Dilutive common shares excluded from the calculations of diluted loss per share (in shares)	303,133	134,283	140,109	78,334
Convertible Preferred Stock [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Dilutive common shares excluded from the calculations of diluted loss per share (in shares)	28,190	28,190	28,190	28,190
Outstanding Warrants [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Dilutive common shares excluded from the calculations of diluted loss per share (in shares)	142,733	142,733	142,733	142,758