0001564590-22-015075 | 10-Q | XBRL Viewer

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Document and Entity Information - shares	3 Months Ended
Document and Entity Information - shares	Mar. 31, 2022	Apr. 14, 2022
Cover [Abstract]
Entity Registrant Name	PLUS THERAPEUTICS, INC.
Entity Central Index Key	0001095981
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Emerging Growth Company	false
Entity Small Business	true
Entity Common Stock, Shares Outstanding		22,197,635
Document Fiscal Year Focus	2022
Document Fiscal Period Focus	Q1
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2022
Document Quarterly Report	true
Document Transition Report	false
Entity Interactive Data Current	Yes
Entity Current Reporting Status	Yes
Entity Shell Company	false
Trading Symbol	PSTV
Title of 12(b) Security	Common Stock, par value $0.001
Security Exchange Name	NASDAQ
Entity File Number	001-34375
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

CONDENSED BALANCE SHEETS (UNAUDITED) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Current assets:
Cash and cash equivalents	$ 21,239	$ 18,400
Other current assets	865	1,324
Total current assets	22,104	19,724
Property and equipment, net	1,558	1,477
Operating lease right-use-of assets	316	341
Goodwill	372	372
Intangible assets, net	150	51
Other assets	16	16
Total assets	24,516	21,981
Current liabilities:
Accounts payable and accrued expenses	3,203	4,151
Operating lease liability	110	111
Term loan obligation, current	1,608	1,608
Total current liabilities	4,921	5,870
Noncurrent operating lease liability	235	269
Term loan obligation	4,718	5,005
Warrant liability		1
Total liabilities	9,874	11,145
Commitments and contingencies (Note 7)
Stockholders’ equity:
Preferred stock, $0.001 par value; 5,000,000 shares authorized; 1,952 shares issued and outstanding at March 31, 2022 and December 31, 2021
Common stock, $0.001 par value; 100,000,000 shares authorized; 22,197,635 and 15,510,025 issued and outstanding at March 31, 2022 and December 31, 2021, respectively	22	16
Additional paid-in capital	465,646	457,730
Accumulated deficit	(451,026)	(446,910)
Total stockholders’ equity	14,642	10,836
Total liabilities and stockholders’ equity	$ 24,516	$ 21,981

CONDENSED BALANCE SHEETS (UNAUDITED) (Parenthetical) - $ / shares	Mar. 31, 2022	Dec. 31, 2021
Stockholders’ equity:
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred stock, shares authorized (in shares)	5,000,000	5,000,000
Preferred stock, shares issued (in shares)	1,952	1,952
Preferred stock, shares outstanding (in shares)	1,952	1,952
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	100,000,000	100,000,000
Common stock, shares issued (in shares)	22,197,635	15,510,025
Common stock, shares outstanding (in shares)	22,197,635	15,510,025

CONDENSED STATEMENTS OF OPERATIONS (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Operating expenses:
Research and development	$ 1,785	$ 1,127
General and administrative	2,141	1,352
Total operating expenses	3,926	2,479
Operating loss	(3,926)	(2,479)
Other income (expense):
Interest income	7	4
Interest expense	(198)	(247)
Change in fair value of liability instruments	1	2
Total other expense	(190)	(241)
Net loss	$ (4,116)	$ (2,720)
Net loss per share, basic and diluted	$ (0.19)	$ (0.33)
Basic and diluted weighted average shares used in calculating net loss per share attributable to common stockholders	21,507,061	8,267,901

CONDENSED STATEMENTS OF STOCKHOLDERS' EQUITY (UNAUDITED) - USD ($) $ in Thousands	Total	Convertible Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]
Balance at Dec. 31, 2020	$ 3,031		$ 7	$ 436,535	$ (433,511)
Balance (in shares) at Dec. 31, 2020		1,954	6,749,028
Stock-based compensation	107			107
Sale of common stock, net	7,078		$ 2	7,076
Sale of common stock, net, shares			2,534,879
Conversion of Series B Convertible Preferred Stock into common stock (share)		(2)	118
Issuance of common stock for exercise of warrants	2,017		$ 1	2,016
Issuance of common stock for exercise of warrants (in shares)			896,500
Net loss	(2,720)				(2,720)
Balance at Mar. 31, 2021	9,513		$ 10	445,734	(436,231)
Balance (in shares) at Mar. 31, 2021		1,952	10,180,525
Balance at Dec. 31, 2020	3,031		$ 7	436,535	(433,511)
Balance (in shares) at Dec. 31, 2020		1,954	6,749,028
Balance at Dec. 31, 2021	10,836		$ 16	457,730	(446,910)
Balance (in shares) at Dec. 31, 2021		1,952	15,510,025
Stock-based compensation	180			180
Sale of common stock, net	7,742		$ 6	7,736
Sale of common stock, net, shares			6,687,610
Net loss	(4,116)				(4,116)
Balance at Mar. 31, 2022	$ 14,642		$ 22	$ 465,646	$ (451,026)
Balance (in shares) at Mar. 31, 2022		1,952	22,197,635

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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- Definition

Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.

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CONDENSED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021
Cash flows used in operating activities:
Net loss	$ (4,116)	$ (2,720)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	147	88
Amortization of deferred financing costs and debt discount	115	151
Change in fair value of liability instruments	(1)	(2)
Share-based compensation expense	180	107
Non-cash lease expense	(10)	1
Increases (decreases) in cash caused by changes in operating assets and liabilities:
Other current assets	459	(170)
Accounts payable and accrued expenses	(650)	(461)
Net cash used in operating activities	(3,876)	(3,006)
Cash flows used in investing activities:
Purchases of property and equipment	(210)	(84)
Purchase of intangible assets	(117)	0
In process research and development acquired	(250)	0
Net cash used in investing activities	(577)	(84)
Cash flows from financing activities:
Principal payments of long-term obligations	(402)	0
Payment of financing lease liability	0	(6)
Proceeds from exercise of warrants	0	2,017
Proceeds from sale of common stock	7,694	7,180
Net cash provided by financing activities	7,292	9,191
Net increase in cash and cash equivalents	2,839	6,101
Cash and cash equivalents at beginning of period	18,400	8,346	$ 8,346
Cash and cash equivalents at end of period	21,239	14,447	$ 18,400
Cash paid during period for:
Interest	87	96
Supplemental schedule of non-cash investing and financing activities:
Unpaid offering cost	$ 171	$ 102

Basis of Presentation and New Accounting Standards

3 Months Ended

Mar. 31, 2022

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 as of March 31, 2022 and for the three months ended March 31, 2022 and 2021 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, 2021 has been derived from the audited financial statements at December 31, 2021, 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., and its subsidiaries (collectively, the “Company”) have been included. Operating results for the three months ended March 31, 2022 are not necessarily indicative of the results that may be expected for the year ending December 31, 2022. 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, 2021, filed with the Securities and Exchange Commission on February 24, 2022.

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 aren’t 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 is permitted beginning in 2019. The Company plans to adopt the new guidance on January 1, 2023, and it does not expect that adoption of this standard will have a material impact on its financial statements and related disclosures.

Use of Estimates

3 Months Ended

Mar. 31, 2022

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 revenue and expenses during the reporting period. The Company’s most significant estimates and critical accounting policies involve reviewing assets for impairment, and determining the assumptions used in measuring stock-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.

Liquidity

3 Months Ended

Mar. 31, 2022

Liquidity [Abstract]

Liquidity

The Company incurred net losses of $4.1 million for the three months ended March 31, 2022. The Company had an accumulated deficit of $451.0 million as of March 31, 2022. Additionally, the Company used net cash of $3.9 million to fund its operating activities for the three months ended March 31, 2022.

As disclosed in more detail in Note 9, the Company had entered into various financing agreements, and raised capital by issuing its common stock. The Company believes its current cash and cash equivalents will be sufficient to fund its operations for at least the next 12 months from the date these financial statements are issued.

The Company continues to seek additional capital through strategic transactions and from other financing alternatives. 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.

Fair Value Measurements

3 Months Ended

Mar. 31, 2022

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.

Certain warrants issued in an underwritten public offering in September 2019 (“Series U Warrants”) are classified as liability instruments. The Company estimated the fair value of the Series U Warrants with the Black Scholes model. Because some of the inputs to the Company’s valuation model are either not observable or are not derived principally from or corroborated by observable market data by correlation or other means, the warrant liability is classified as Level 3 in the fair value hierarchy.

The Series U Warrants will be marked to market as of each balance sheet date until they are exercised or upon expiration, with the changes in fair value recorded as non-operating income or loss in the statements of operations. As of March 31, 2022, the fair value of the Series U Warrants was immaterial, and the change in the fair value of liability classified Series U Warrants during the three months ended March 31, 2022 and 2021 was not material.

Term Loan Obligations

3 Months Ended

Mar. 31, 2022

Debt Disclosure [Abstract]

Term Loan Obligations

5.	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 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 is required to make interest only payments through May 1, 2021 and thereafter it 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. 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 its respective fair value was recorded as a discount to the debt.

From September 2017 to March 2019, the Company entered into a total of seven amendments to the Term Loan which, amongst 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, and increased the minimum liquidity covenant level to $2.0 million.

On March 29, 2020, the Company entered into the Ninth Amendment of the Loan and Security Agreement (the “Ninth Amendment”), pursuant to which Oxford agreed to defer 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. The principal repayment start date was further deferred to November 1, 2021. In addition, pursuant to the Ninth Amendment, on April 1, 2020, the Company made a $5.0 million paydown of principal upon execution of the Ninth Amendment and $0.3 million of related final payment. In addition, an amendment fee of $1.0 million will be payable in connection with the Ninth Amendment at the earlier of the maturity date, acceleration of the loans and the making of certain prepayments. All other major terms remained consistent.

Under authoritative guidance, the Ninth Amendment does not meet the criteria to be accounted for as a troubled debt restructuring. In addition, the Company performed a quantitative analysis and determined that the terms of the new debt and original debt instrument are not substantially different. Accordingly, the Ninth Amendment is accounted for as debt modification. A new effective interest rate that equates the revised cash flows to the carrying amount of the original debt is computed and applied prospectively.

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 million. As of March 31, 2022, there was $3.6 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 each of the three months ended March 31, 2022 and 2021 was $0.2 million. Interest expense is calculated using the effective interest method; therefore it is inclusive of non-cash amortization in the amount of $0.1 million for each of the three months ended March 31, 2022 and 2021, 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, 2022, the Company has not received any notification or indication from Oxford to invoke the material adverse change clause.

- 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.

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-Name Accounting Standards Codification
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+ Details

Loss per Share

3 Months Ended

Mar. 31, 2022

Earnings Per Share [Abstract]

Loss per Share

6.	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,
	2022		2021
Outstanding stock options		1,170,873		691,263
Preferred stock		422,867		422,867
Outstanding warrants		2,141,378		2,141,378
Total		3,735,118		3,255,508

Commitments and Contingencies

3 Months Ended

Mar. 31, 2022

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

7.	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. Right-of-use assets for finance leases are recorded within property and equipment, net in the condensed balance sheets. Leases with an initial term of 12 months or less are not recorded on the condensed balance sheets. Instead, the Company recognizes lease expense for these leases on a straight-line basis over the lease term in the condensed statements of operations.

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. On March 1, 2021, the Company entered into a lease agreement for office space in Charlottesville, Virginia (the “Charlottesville Lease”). The Charlottesville Lease has a term of 12 months and is renewable for four additional one-year periods. The minimum lease payment is $30,000 for the first twelve months, subject to a 3% annual increase if and when the lease is renewed. The lease commencement date is April 1, 2021 and currently expires on March 31, 2023. The Company measured the operating lease right-of-use asset and related lease liability related to the Charlottesville Lease as of the lease commencement date. In addition, the Company has entered into leases for certain equipment under various operating and finance leases. During 2021, contractual terms of all finance leases had expired and the Company did not have any right-of-use assets or lease liabilities relating to finance leases as of March 31, 2022. 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.

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 condensed balance sheets. As of March 31, 2022, weighted average discount rate used to measure operating lease liabilities and the operating leases remaining term were 9.0% and 2.82%, respectively.

The table below summarizes the Company’s lease costs from its unaudited condensed statement of operations, and cash payments from its unaudited condensed statement of cash flows during the three months ended March 31, 2022 and 2021 (in thousands):

	Three Months Ended March 31,
		2022		2021
Lease expense:
Operating lease expense	$	45	$	50
Finance lease expense:
Depreciation of right-of-use assets	—		4
Total lease expense	$	45	$	54

Cash payment information:
Operating cash used for operating leases	$	45	$	49
Financing cash used for finance leases	$	—	$	6
Total cash paid for amounts included in the measurement of lease liabilities	$	45	$	55

Total rent expenses for the three months ended March 31, 2022 and 2021 were $60,000 and $50,000, respectively, 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 March 31, 2022 are as follows (in thousands):


	Operating Leases
Remainder of 2022	$	114
2023	137
2024	113
2025	18
Thereafter		—
Total minimum lease payments	$	382
Less: amount representing interest		(37	)
Present value of obligations under leases		345
Less: current portion		(110	)
Noncurrent lease obligations	$	235

Services Agreement and Statement of Work with Medidata

On March 31, 2022, the Company and Medidata Solutions, Inc. (“Medidata”) entered into a Statement of Work (the “SOW”), pursuant to which Medidata will build a Sythetic 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 NanoLiposome (¹⁸⁶RNL) in recurrent glioblastoma (“GBM”). The SOW is governed under the terms of a services agreement (the “Services Agreement”), dated November 5, 2021.

The SOW has a term of six (6) months. The Company will pay Medidata $1.45 million in managed services fees and a contingent managed services fee of $150,000 if the U.S. Food & Drug Administration approves a path forward for the Company to use the SCA in its clinical trial of ¹⁸⁶RNL for treatment of GBM. The SOW may only be terminated for a material breach by either party or if the clinical study’s authorization or approval is withdrawn by a regulatory agency.

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 RNL-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 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, 2022, the Company did not have any clinical research study obligations.

Legal proceedings

On June 22, 2021, the Company was named as a defendant in an action brought by Lorem Vascular, Pte. Ltd. (“Lorem”) in the District Court for the District of Delaware. The complaint alleges false representations were made to Lorem regarding the manufacturing facility in the United Kingdom (the “UK Facility”) that Lorem purchased from the Company under the Asset and Equity Purchase Agreement, dated March 29, 2019, between the Company and Lorem (the “Lorem Agreement”). Lorem also claims that false representations were made regarding the UK Facility’s certification to sell and distribute devices in the European Union and export such devices to China. In connection with these allegations, Lorem claims entitlement to at least $6,000,000 in compensatory damages and operational costs and expenses (collectively, the “Lorem Claim”). The Company believes that the Lorem Claim is without merit and is vigorously defending the case. No liability was accrued as of March 31, 2022.

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.

License Agreement

3 Months Ended

Mar. 31, 2022

License Agreement [Abstract]

License Agreement

8.	License Agreements

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 The University of Texas Health Science Center at San Antonio, 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 condensed statement of operations for the year ended December 31, 2021. The upfront payment 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.

On May 7, 2020, all closing conditions under the NanoTx License Agreement were satisfied and the Company paid an upfront cash payment and issued 230,769 shares of its common stock to NanoTx. Cash and the fair value of common stock issued is recorded as in-process research and development expenses, pursuant to authoritative literature for asset acquisition, in the statement of operations and comprehensive loss for the year ended December 31, 2020.

Stockholders' Equity

3 Months Ended

Mar. 31, 2022

Stockholders Equity Note [Abstract]

Stockholders' Equity

9.	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 of Directors is authorized to designate the terms and conditions of any preferred stock the Company issues without further action by the common stockholders. On September 21, 2021, Series A 3.6% Convertible Preferred Stock was eliminated. There were no shares of Series A 3.6% Convertible Preferred Stock immediately prior to September 21, 2021, or December 31, 2020. There were 1,014 shares of Series B Convertible Preferred Stock outstanding as of March 31, 2022 and December 31, 2021. There were 938 shares of Series C Preferred Stock outstanding as of March 31, 2022 and December 31, 2021.

As of March 31, 2022, there were 938 outstanding shares of Series C Preferred Stock that can be converted into an aggregate of 416,889 shares of common stock, and 1,014 shares of Series B Convertible Preferred Stock that can be converted into an aggregate of 5,978 shares of common stock.

Warrants

On September 25, 2019, the Company completed an underwritten public offering. The Company issued 289,000 shares of its common stock, along with pre-funded warrants to purchase 2,711,000 shares of its common stock and Series U Warrants to purchase 3,450,000 shares of its common stock at $5.00 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 75,000 shares of its common stock at $6.25 per share with a term of 5 years from the issuance date, in the form of Series U Warrants (the “Representative Warrants”).

In accordance with authoritative guidance, the pre-funded warrants are classified as equity. The Series U Warrants and the Representative Warrants were initially classified at issuance as liabilities due to a contingent obligation for the Company to settle the Series U Warrants with cash upon certain change in control events. In 2020, all but 2,500 Series U Warrants were amended and met the requirements to be classified within stockholder’s equity.

As of March 31, 2022, there were 2,141,000 outstanding Series U Warrants which can be exercised into an aggregate of 2,141,000 shares of common stock.

Common Stock

Lincoln Park Purchase Agreement

On September 30, 2020, the Company entered into 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.

On June 16, 2020, the Company received stockholder approval to permit issuances of the Company’s common stock (including the issuance of more than 19.99% of the Company’s common stock) to Lincoln Park pursuant to the 2020 Purchase Agreement. Based on the closing price of the Company’s common stock of $1.05 per share on March 16, 2020, the maximum number of shares the Company could issue and sell under the 2020 Purchase Agreement is approximately 23.8 million shares. Accordingly, the Company requested and received stockholder approval for the issuance of up to 23.8 million shares of the Company’s common stock under the 2020 Purchase Agreement. The Company would seek additional stockholder approval before issuing more than 23.8 million shares.

Lincoln Park had no right to require the Company to sell any shares of common stock to Lincoln Park, but Lincoln Park was obligated to make purchases as the Company directs, subject to certain conditions.

Actual sales of shares of common stock to Lincoln Park under the 2020 Purchase Agreement depended 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 2020 Purchase Agreement to the Company depended on the frequency and prices at which the Company sold shares of its stock to Lincoln Park.

During the year ended December 31, 2021, the Company issued 5,685,186 shares of its common stock under the 2020 Purchase Agreement for net proceeds of approximately $12.5 million. During the three months ended March 31, 2022, the Company issued 5,665,000 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.

At-the-market Issuances

On January 14, 2022, the Company entered into an Equity Distribution Agreement (the “2022 Distribution Agreement”) with Canaccord Genuity LLC ( “Canaccord”), pursuant to which the Company may issue and sell, from time to time, shares of its common stock having an aggregate offering price of up to $5,000,000 shares (the “Shares”), with Canaccord acting as an agent for sales. Canaccord will use its commercially reasonable efforts to sell the Shares requested by the Company to be sold on its behalf. The Company has no obligation to sell any of the Shares. The Company may instruct Canaccord not to sell the Shares if the sales cannot be effected at or above the price designated by the Company from time to time and the Company may at any time suspend sales pursuant to the 2022 Distribution Agreement. During the three months ended March 31, 2022, the Company issued 1,022,610 shares under the 2022 Distribution Agreement for net proceeds of approximately $0.7 million.

On October 23, 2020, the Company entered into an Equity Distribution Agreement (the “2020 Distribution Agreement”) with Canaccord. The Company had no obligation to sell any of the ATM Shares and it could instruct Canaccord not to sell the ATM Shares if the sales could not be effected at or above the price the Company designated from time to time and the Company could at any time suspend sales pursuant to the 2020 Distribution Agreement.

During the year ended December 31, 2021, the Company issued 2,179,193 shares under the 2020 Distribution Agreement for net proceeds of $6.3 million. The 2020 Distribution Agreement has been terminated.

- Definition

The entire disclosure for shareholders' equity comprised of portions attributable to the parent entity and noncontrolling interest, including other comprehensive income. Includes, but is not limited to, balances of common stock, preferred stock, additional paid-in capital, other capital and retained earnings, accumulated balance for each classification of other comprehensive income and amount of comprehensive income.

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-URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e23780-122690

+ Details

Stock-based Compensation

3 Months Ended

Mar. 31, 2022

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-based Compensation

10.	Stock-based Compensation

On February 6, 2020, the Company amended the Company’s 2015 New Employee Incentive Plan (the “2015 Plan”) to increase the total number of shares of common stock reserved for issuance under the plan by 250,000 shares. Awards may only be granted under the 2015 Plan 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, 2022, there were 90,389 shares of common stock remaining and available for future issuances under the 2015 Plan.

On June 16, 2020, the stockholders of the Company approved the Company’s 2020 Stock Incentive Plan (the “2020 Plan”), which replaced the Company’s 2014 Equity Incentive Plan. The 2020 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. The 2020 Plan provides for the issuance of up to 550,000 shares of common stock, and the number of shares available for issuance will be 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). On May 17, 2021, the stockholders of the Company approved an amendment and restatement to the 2020 Plan to increase the total number of shares of common stock reserved for issuance under the 2020 Plan by 1,000,000 shares. As of March 31, 2022, there were 640,212 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 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, 2022 is as follows:

	Options			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (years)		Aggregate Intrinsic Value
Balance as of December 31, 2021		1,170,890		$	5.01		9.00
Granted		—		$	—
Cancelled/forfeited		(17	)	$	24,706.00
Balance as of March 31, 2022		1,170,873		$	4.65		8.75	$	-
Vested and expected to vest at March 31, 2022		460,217		$	8.04		8.47	$	-
Exercisable at March 31, 2022		1,102,117		$	4.74		8.73	$	-

As of March 31, 2022, the total compensation cost related to non-vested stock options not yet recognized for all the Company’s plans is approximately $1.3 million, which is expected to be recognized as a result of vesting under service conditions over a weighted average period of 2.79 years.

COVID-19 Pandemic and CARES Act

3 Months Ended

Mar. 31, 2022

Risks And Uncertainties [Abstract]

COVID-19 Pandemic and CARES Act

11. COVID-19 Pandemic and CARES Act

A novel strain of coronavirus (COVID-19) was declared a global pandemic by the World Health Organization in March 2020. COVID-19 has presented substantial public health and economic challenges and is affecting economies, financial markets and business operations around the world. While the Company has implemented additional health and safety precautions and protocols in response to the pandemic and government guidelines, the Company has not experienced a significant impact on its business and operations. However, the Company may experience disruptions that could adversely impact its business operations as well as its preclinical studies and clinical trials. The Company considered the impacts of COVID-19 on the assumptions and estimates used to prepare its financial statements and determined that there were no material adverse impacts on the Company’s results of operations and financial position at March 31, 2022. The full extent to which the COVID-19 pandemic will directly or indirectly impact its business, results of operations and financial condition, will depend on future developments that are highly uncertain, including as a result of new information that may emerge concerning COVID-19 and the actions taken to contain or treat it, as well as the economic impact on local, regional, national and international markets.

In response to the COVID-19 pandemic, the CARES Act was signed into law on March 27, 2020. The CARES Act, among other things, includes tax provisions relating to refundable payroll tax credits, deferment of employer’s social security payments, net operating loss utilization and carryback periods, modifications to the net interest deduction limitations and technical corrections to tax depreciation methods for qualified improvement property. The CARES Act had no material impact on the Company’s income tax provision for the year ended December 31, 2021 or the three months ended March 31, 2022. The Company continues to evaluate the impact of the CARES Act on its financial position, results of operations and cash flows.

Basis of Presentation and New Accounting Standards (Policies)	3 Months Ended
	Mar. 31, 2022
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Basis of Presentation and New Accounting Standards	The accompanying unaudited condensed financial statements as of March 31, 2022 and for the three months ended March 31, 2022 and 2021 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, 2021 has been derived from the audited financial statements at December 31, 2021, 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., and its subsidiaries (collectively, the “Company”) have been included. Operating results for the three months ended March 31, 2022 are not necessarily indicative of the results that may be expected for the year ending December 31, 2022. 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, 2021, filed with the Securities and Exchange Commission on February 24, 2022.
Recently Issued Accounting Pronouncements	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 aren’t 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 is permitted beginning in 2019. The Company plans to adopt the new guidance on January 1, 2023, and it does not expect that adoption of this standard will have a material impact on its financial statements and related disclosures.
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 Company’s most significant estimates and critical accounting policies involve reviewing assets for impairment, and determining the assumptions used in measuring stock-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.

- Definition

The entire disclosure for change in accounting principle. Includes, but is not limited to, nature, reason, and method of adopting amendment to accounting standards or other change in accounting principle.

+ References

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+ Details

Loss per Share (Tables)

3 Months Ended

Mar. 31, 2022

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,
	2022		2021
Outstanding stock options		1,170,873		691,263
Preferred stock		422,867		422,867
Outstanding warrants		2,141,378		2,141,378
Total		3,735,118		3,255,508

Commitments and Contingencies (Tables)

3 Months Ended

Mar. 31, 2022

Commitments And Contingencies Disclosure [Abstract]

Summary of Lease Costs

	Three Months Ended March 31,
		2022		2021
Lease expense:
Operating lease expense	$	45	$	50
Finance lease expense:
Depreciation of right-of-use assets	—		4
Total lease expense	$	45	$	54

Cash payment information:
Operating cash used for operating leases	$	45	$	49
Financing cash used for finance leases	$	—	$	6
Total cash paid for amounts included in the measurement of lease liabilities	$	45	$	55

Summary of Future Minimum Annual Lease Payments under Operating Leases

The Company’s future minimum annual lease payments under operating leases at March 31, 2022 are as follows (in thousands):


	Operating Leases
Remainder of 2022	$	114
2023	137
2024	113
2025	18
Thereafter		—
Total minimum lease payments	$	382
Less: amount representing interest		(37	)
Present value of obligations under leases		345
Less: current portion		(110	)
Noncurrent lease obligations	$	235

Stock-based Compensation (Tables)

3 Months Ended

Mar. 31, 2022

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock Option Activity

A summary of activity for the three months ended March 31, 2022 is as follows:

	Options			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (years)		Aggregate Intrinsic Value
Balance as of December 31, 2021		1,170,890		$	5.01		9.00
Granted		—		$	—
Cancelled/forfeited		(17	)	$	24,706.00
Balance as of March 31, 2022		1,170,873		$	4.65		8.75	$	-
Vested and expected to vest at March 31, 2022		460,217		$	8.04		8.47	$	-
Exercisable at March 31, 2022		1,102,117		$	4.74		8.73	$	-

Liquidity - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021
Liquidity [Abstract]
Net loss	$ (4,116)	$ (2,720)
Accumulated deficit	(451,026)		$ (446,910)
Net cash used in operating activities	$ (3,876)	$ (3,006)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References

+ Details

- Definition

The cumulative amount of the reporting entity's undistributed earnings or deficit.

+ References + Details

Fair Value Measurements - Additional Information (Details)	3 Months Ended
	Mar. 31, 2022
Series U Warrants [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Warrants issued, underwritten public offering, period	2019-09

Term Loan Obligations (Details) - USD ($)				3 Months Ended
Term Loan Obligations (Details) - USD ($)	Mar. 29, 2020	Mar. 28, 2020	May 29, 2015	Mar. 31, 2022	Mar. 31, 2021
Debt Instrument [Line Items]
Interest expense				$ 198,000	$ 247,000
Non-cash amortization				$ 100,000	$ 100,000
LIBOR [Member] \| Interest Rate Floor [Member]
Debt Instrument [Line Items]
Basis variable rate			1.00%
Term Loan [Member]
Debt Instrument [Line Items]
Origination Date				May 29, 2015
Original Loan Amount			$ 17,700,000
Basis variable rate			7.95%
Maturity date	Jun. 01, 2024	Sep. 01, 2021	May 01, 2021
Fees amount associated with loan			$ 3,200,000	$ 3,200,000
Number of shares callable by warrants (in shares)			188
Warrant exercise price (in dollars per share)			$ 5,175
Date from which warrants are exercisable				Nov. 30, 2015
Warrant expiration date				May 29, 2025
Minimum liquidity covenant				$ 2,000,000.0
Debt principal repayment start date	May 01, 2021	May 01, 2020
Prepayments of term loan	$ 5,000,000.0
Related final payment	300,000
Amendment fee	$ 1,000,000.0
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] \| 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				$ 3,600,000

Loss per Share - Schedule of Antidilutive Securities Excluded from Diluted Income (Loss) per Share (Details) - shares	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
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)	3,735,118	3,255,508
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)	1,170,873	691,263
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)	422,867	422,867
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)	2,141,378	2,141,378

Commitments and Contingencies - Additional Information (Details) - USD ($)					3 Months Ended
	Mar. 31, 2022	Jun. 22, 2021	Mar. 01, 2021	Jan. 08, 2021	Mar. 31, 2022	Mar. 31, 2021
Recorded Unconditional Purchase Obligation [Line Items]
Operating leases expiry year					2025
Minimum lease payment	$ 382,000				$ 382,000
Operating lease, weighted average discount rate	9.00%				9.00%
Operating lease, weighted average remaining term	2 years 9 months 25 days				2 years 9 months 25 days
Rent expense					$ 60,000	$ 50,000
Lorem Vascular Pte Ltd [Member]
Recorded Unconditional Purchase Obligation [Line Items]
Compensatory damages and operational		$ 6,000,000
Pre-clinical Research Study Obligations [Member]
Recorded Unconditional Purchase Obligation [Line Items]
Contractual obligation	$ 0				0
Pre-clinical Research Study Obligations [Member] \| Lorem Vascular Pte Ltd [Member]
Recorded Unconditional Purchase Obligation [Line Items]
Litigation settlement, expense					0
Charlottesville [Member]
Recorded Unconditional Purchase Obligation [Line Items]
Lessee, operating lease, term of contract			12 months
Lessee, operating lease, existence of option to extend [true false]			true
Lessee, operating lease, option to extend			The Charlottesville Lease has a term of 12 months and is renewable for four additional one-year periods.
Lessee, operating lease, renewal term			1 year
Minimum lease payment			$ 30,000
Annual increase in lease rate			3.00%
Lease commencement date			Apr. 01, 2021
Operating leases, expiration date			Mar. 31, 2023
Medidata Solutions, Inc. [Member] \| Services Agreement [Member]
Recorded Unconditional Purchase Obligation [Line Items]
Statement of work, term	6 months
Managed services fees payable	$ 1,450,000				1,450,000
Contingent managed services fee	$ 150,000				$ 150,000
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.

Commitments and Contingencies - Summary of Lease Costs (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Lease expense:
Operating lease expense	$ 45	$ 50
Finance lease expense:
Depreciation of right-of-use assets		4
Total lease expense	45	54
Cash payment information:
Operating cash used for operating leases	45	49
Financing cash used for finance leases	0	6
Total cash paid for amounts included in the measurement of lease liabilities	$ 45	$ 55

Commitments and Contingencies - Summary of Future Minimum Annual Lease Payments under Operating Leases (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Operating Leases
Remainder of 2022	$ 114
2023	137
2024	113
2025	18
Total minimum lease payments	382
Less: amount representing interest	(37)
Present value of obligations under leases	345
Less: current portion	(110)	$ (111)
Noncurrent operating lease liability	$ 235	$ 269

License Agreement - Additional Information (Details) - shares	Mar. 31, 2022	Dec. 31, 2021	May 07, 2020
Common stock, shares issued (in shares)	22,197,635	15,510,025
NanoTx License Agreement [Member]
Common stock, shares issued (in shares)			230,769

Stockholders' Equity - Preferred Stock (Details) - $ / shares			3 Months Ended	12 Months Ended
	Sep. 21, 2021	Sep. 20, 2021	Mar. 31, 2022	Dec. 31, 2020	Dec. 31, 2021
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares)			5,000,000		5,000,000
Preferred stock, par value (in dollars per share)			$ 0.001		$ 0.001
Preferred stock, shares outstanding (in shares)			1,952		1,952
Series A Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares outstanding (in shares)		0		0
Convertible preferred stock		3.60%		3.60%
Series A 3.6% Convertible preferred stock eliminated date	Sep. 21, 2021
Series B Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares outstanding (in shares)			1,014		1,014
Series B Convertible Preferred Stock [Member] \| 2018 Rights Offering [Member]
Class of Stock [Line Items]
Preferred stock, shares outstanding (in shares)			1,014
Number of preferred stock, shares converted			5,978
Series C Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares outstanding (in shares)			938		938
Series C Convertible Preferred Stock [Member] \| 2018 Rights Offering [Member]
Class of Stock [Line Items]
Preferred stock, shares outstanding (in shares)			938
Number of preferred stock, shares converted			416,889

Stockholders' Equity - Warrants (Details) - $ / shares	3 Months Ended
Stockholders' Equity - Warrants (Details) - $ / shares	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2020
Series U Warrants [Member]
Class of Stock [Line Items]
Common stock issued (in shares)	3,450,000
Sale of Stock, Price Per Share	$ 5.00
Warrants expected term	5 years
Representative Warrants [Member]
Class of Stock [Line Items]
Number of securities called by warrant or right	75,000
Warrant exercise price (in dollars per share)	$ 6.25
Common Stock [Member]
Class of Stock [Line Items]
Common stock issued (in shares)	6,687,610	2,534,879
Warrants expected term	5 years
Outstanding Warrants [Member] \| Series U Warrants [Member]
Class of Stock [Line Items]
Number of securities called by warrant or right			2,500
Class of warrant outstanding	2,141,000
Number of warrants exercised	2,141,000
Common Class A [Member]
Class of Stock [Line Items]
Common stock issued (in shares)	289,000
Common Class B [Member]
Class of Stock [Line Items]
Number of securities called by warrant or right	2,711,000

Stockholders' Equity - Common stock (Details) - USD ($)			3 Months Ended		9 Months Ended	12 Months Ended
Stockholders' Equity - Common stock (Details) - USD ($)	Jan. 14, 2022	Jun. 16, 2020	Mar. 31, 2022	Mar. 31, 2021	Sep. 30, 2020	Dec. 31, 2021	Apr. 01, 2022	Oct. 23, 2020
Class of Stock [Line Items]
Sale of common stock, net			$ 7,742,000	$ 7,078,000
Proceeds from sale of common stock			7,694,000	7,180,000
Common Stock [Member]
Class of Stock [Line Items]
Sale of common stock, net			$ 6,000	$ 2,000
Common stock issued (in shares)			6,687,610	2,534,879
2020 Purchase Agreement [Member] \| Lincoln Park Capital Fund, LLC [Member] \| Common Stock [Member]
Class of Stock [Line Items]
Sale of common stock, net					$ 25,000,000.0
Common stock issued (in shares)			5,665,000			5,685,186
Period exercisable from the date of issuance					36 months
Closing price per share		$ 1.05
Proceeds from sale of common stock			$ 7,000,000.0			$ 12,500,000
2020 Purchase Agreement [Member] \| Lincoln Park Capital Fund, LLC [Member] \| Common Stock [Member] \| Subsequent Event [Member]
Class of Stock [Line Items]
Remaining common stock issued and sold (in shares)							0
2020 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%
2020 Purchase Agreement [Member] \| Lincoln Park Capital Fund, LLC [Member] \| Common Stock [Member] \| Maximum [Member]
Class of Stock [Line Items]
Common stock issued (in shares)		23,800,000
Sales 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							$ 0
Sales 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	$ 5,000,000
2022 Distribution Agreement [Member] \| Canaccord Genuity LLC [Member] \| Common Stock [Member]
Class of Stock [Line Items]
Common stock issued (in shares)			1,022,610
Proceeds from sale of common stock			$ 700,000
2020 Distribution Agreement [Member] \| Canaccord Genuity LLC [Member] \| Common Stock [Member]
Class of Stock [Line Items]
Common stock issued (in shares)						2,179,193
Proceeds from sale of common stock						$ 6,300,000

Stock-based Compensation - Additional Information (Details) - USD ($) $ in Millions		3 Months Ended
	Jun. 16, 2020	Mar. 31, 2022	May 17, 2021	Feb. 06, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option contractual term		10 years
Stock option vesting percentage		25.00%
Total unamortized compensation cost related to outstanding unvested stock options and restricted stock awards		$ 1.3
Weighted average period for recognition of cost		2 years 9 months 14 days
Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option vesting period		2 years
Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option vesting period		4 years
Stock Based Compensation2015 Equity Incentive Plan1
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Securities remaining and available for future issuances (in shares)		90,389
Increased in the total number of shares of common stock reserved for issuance				250,000
2020 Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Securities remaining and available for future issuances (in shares)		640,212
Increased in the total number of shares of common stock reserved for issuance			1,000,000
Common stock shares to be issued (in shares)	550,000

Stock-based Compensation - Stock Option Activity (Details) - $ / shares	3 Months Ended	12 Months Ended
	Mar. 31, 2022	Dec. 31, 2021
Options [Roll Forward]
Balance as of December 31, 2021 (in shares)	1,170,873	1,170,890
Cancelled/forfeited (in shares)	(17)
Vested and expected to vest at March 31, 2022 (in shares)	460,217
Exercisable at March 31, 2022 (in shares)	1,102,117
Weighted Average Exercise Price [Roll Forward]
Balance as of December 31, 2021 (in dollars per share)	$ 4.65	$ 5.01
Cancelled/forfeited (in dollars per share)	24,706.00
Vested and expected to vest at March 31, 2022 (in dollars per share)	8.04
Exercisable at March 31, 2022 (in dollars per share)	$ 4.74
Weighted Average Remaining Contractual Life (years) [Roll Forward]
Balance as of December 31, 2021	8 years 9 months	9 years
Vested and expected to vest at March 31, 2022	8 years 5 months 19 days
Exercisable at March 31, 2022	8 years 8 months 23 days