Plus Therapeutics (PSTV) 10-Q 22 Apr 21 2021 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2021	Apr. 09, 2021
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		21,106,217
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q1
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2021
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

CONSOLIDATED CONDENSED BALANCE

CONSOLIDATED CONDENSED BALANCE SHEETS (UNAUDITED) - USD ($)	Mar. 31, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 14,447,000	$ 8,346,000
Other current assets	999,000	829,000
Total current assets	15,446,000	9,175,000
Property and equipment, net	1,825,000	1,820,000
Operating lease right-of-use assets	600,000	636,000
Goodwill	372,000	372,000
Intangible assets, net	77,000	86,000
Other assets	16,000	16,000
Total assets	18,336,000	12,105,000
Current liabilities:
Accounts payable and accrued expenses	1,716,000	2,081,000
Operating lease liability	113,000	123,000
Term loan obligations, net of discount	6,486,000	6,335,000
Total current liabilities	8,315,000	8,539,000
Noncurrent operating lease liability	503,000	528,000
Warrant liability	5,000	7,000
Total liabilities	8,823,000	9,074,000
Commitments and contingencies (Note 7)
Stockholders’ equity:
Preferred stock, $0.001 par value; 5,000,000 shares authorized; 1,952 and 1,954 shares issued and outstanding at March 31, 2021 and December 31, 2020, respectively
Common stock, $0.001 par value; 100,000,000 shares authorized; 10,180,525 and 6,749,028 shares issued and outstanding at March 31, 2021 and December 31, 2020, respectively	10,000	7,000
Additional paid-in capital	445,734,000	436,535,000
Accumulated deficit	(436,231,000)	(433,511,000)
Total stockholders’ equity	9,513,000	3,031,000
Total liabilities and stockholders’ equity	$ 18,336,000	$ 12,105,000

CONSOLIDATED CONDENSED BALANC_2

CONSOLIDATED CONDENSED BALANCE SHEETS (UNAUDITED) (Parenthetical) - $ / shares	Mar. 31, 2021	Dec. 31, 2020
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	1,954
Preferred stock, shares outstanding (in shares)	1,952	1,954
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)	10,180,525	6,749,028
Common stock, shares outstanding (in shares)	10,180,525	6,749,028

CONSOLIDATED CONDENSED STATEMEN

CONSOLIDATED CONDENSED STATEMENTS OF OPERATIONS (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Development revenues:
Government contracts and other		$ 118
Operating expenses:
Research and development	$ 1,127	941
General and administrative	1,352	1,618
Total operating expenses	2,479	2,559
Loss from operations	(2,479)	(2,441)
Other income (expense):
Interest income	4	36
Interest expense	(247)	(349)
Change in fair value of warrants	2	1,667
Total other income (expense)	(241)	1,354
Net loss	$ (2,720)	$ (1,087)
Net loss per share, basic and diluted	$ (0.33)	$ (0.28)
Basic and diluted weighted average shares used in calculating net loss per share attributable to common stockholders	8,267,901	3,880,588

CONSOLIDATED CONDENSED STATEM_2

CONSOLIDATED CONDENSED STATEMENTS OF STOCKHOLDERS' EQUITY (UNAUDITED) - USD ($) $ in Thousands	Total	Convertible Preferred Stock [Member]	Convertible Preferred Stock [Member]Series B Convertible Preferred Stock [Member]	Common Stock [Member]	Common Stock [Member]Series B Convertible Preferred Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]
Balance at Dec. 31, 2019	$ 1,160			$ 4		$ 426,426	$ (425,270)
Balance (in shares) at Dec. 31, 2019		1,959,000		3,880,588,000
Stock-based compensation	12					12
Net loss	(1,087)						(1,087)
Balance at Mar. 31, 2020	85			$ 4		426,438	(426,357)
Balance (in shares) at Mar. 31, 2020		1,959,000		3,880,588,000
Balance at Dec. 31, 2019	1,160			$ 4		426,426	(425,270)
Balance (in shares) at Dec. 31, 2019		1,959,000		3,880,588,000
Balance at Dec. 31, 2020	$ 3,031			$ 7		436,535	(433,511)
Balance (in shares) at Dec. 31, 2020	6,749,028	1,954,000		6,749,028,000
Stock-based compensation	$ 107					107
Sale of common stock, net	7,078			$ 2		7,076
Sale of common stock, net (in shares)				2,534,879,000
Conversion of Convertible Preferred Stock into common stock (share)			(2,000)		118,000
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	10,180,525	1,952,000		10,180,525,000

CONSOLIDATED CONDENSED STATEM_3

CONSOLIDATED CONDENSED STATEMENTS OF STOCKHOLDERS' EQUITY (UNAUDITED) (Parenthetical) $ in Millions	3 Months Ended
	Mar. 31, 2021USD ($)
Statement Of Stockholders Equity [Abstract]
Offering cost, net	$ 0.1

CONSOLIDATED CONDENSED STATEM_4

CONSOLIDATED CONDENSED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Cash flows used in operating activities:
Net loss	$ (2,720)	$ (1,087)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	88	94
Amortization of deferred financing costs and debt discount	151	122
Noncash lease expenses	1	4
Change in fair value of warrants	(2)	(1,667)
Stock-based compensation expense	107	12
Increases (decreases) in cash caused by changes in operating assets and liabilities:
Accounts receivable	0	191
Other current assets	(170)	405
Other assets	0	14
Accounts payable and accrued expenses	(461)	410
Net cash used in operating activities	(3,006)	(1,502)
Cash flows provided by (used in) investing activities:
Purchases of property and equipment	(84)	(11)
Net cash used in investing activities	(84)	(11)
Cash flows used in financing activities:
Payment of financing lease liability	(6)	(18)
Proceeds from exercise of warrants	2,017	0
Proceeds from sale of common stock, net	7,180	0
Net cash provided by (used in) financing activities	9,191	(18)
Net increase (decrease) in cash and cash equivalents	6,101	(1,531)
Cash and cash equivalents at beginning of period	8,346	17,592	$ 17,592
Cash and cash equivalents at end of period	14,447	16,061	$ 8,346
Cash paid during period for:
Interest	96	227
Supplemental schedule of non-cash investing and financing activities:
Unpaid offering costs	$ 102	$ 0

Basis of Presentation and New A

Basis of Presentation and New Accounting Standards	3 Months Ended
	Mar. 31, 2021
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 consolidated condensed financial statements as of March 31, 2021 and for the three months ended March 31, 2021 and 2020 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 consolidated condensed balance sheet at December 31, 2020 has been derived from the audited financial statements at December 31, 2020, 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, 2021 are not necessarily indicative of the results that may be expected for the year ending December 31, 2021. These financial statements should be read in conjunction with the consolidated financial statements and notes therein included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2020, filed with the Securities and Exchange Commission on February 22, 2021. 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 consolidated financial statements and related disclosures. In August 2020, the FASB issued ​ASU No. 2020-06​, ​Debt - Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging - Contracts in Entity’s Own Equity (Subtopic 815-40)​ (“ASU 2020-06”). ​ASU 2020-06 eliminates the beneficial conversion and cash conversion accounting models for convertible instruments. It also amends the accounting for certain contracts in an entity’s own equity that are currently accounted for as derivatives because of specific settlement provisions. In addition, ASU 2020-06 modifies how particular convertible instruments and certain contracts that may be settled in cash or shares impact the diluted EPS computation. The amendments in ASU 2020-06 are effective for smaller reporting companies as defined by the SEC for fiscal years beginning after December 15, 2023, including interim periods within those fiscal years. Early adoption is permitted, but no earlier than fiscal years beginning after December 15, 2020. The Company adopted ASU 2020-06 as of January 1, 2021, which did not have an impact on its consolidated financial statements. Recently Adopted Accounting Pronouncement In December 2019, the FASB issued ASU No. 2019-12, Income Taxes, Simplifying the Accounting for Income Taxes (“ASU 2019-12”). The new guidance eliminates certain exceptions related to the approach for intraperiod tax allocation, the methodology for calculating income taxes in an interim period and the recognition of deferred tax liabilities for outside basis differences. It also clarifies and simplifies other aspects of the accounting for income taxes. ASU 2019-12 became effective for the Company on January 1, 2021 . Adoption of ASU 2019-12 did not have a material impact on the Company’s consolidated financial statements.

Use of Estimates

Use of Estimates	3 Months Ended
	Mar. 31, 2021
Use Of Estimates [Abstract]
Use of Estimates	2. Use of Estimates The preparation of consolidated 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, determining the assumptions used in measuring stock-based compensation expense and valuing warrants. Actual results could differ from these estimates. Management’s estimates and assumptions are reviewed regularly, and the effects of revisions are reflected in the consolidated financial statements in the periods they are determined to be necessary.

Liquidity and Going Concern

Liquidity and Going Concern	3 Months Ended
	Mar. 31, 2021
Liquidity [Abstract]
Liquidity and Going Concern	3. Liquidity and Going Concern The Company incurred net losses of $2.7 million for the three months ended March 31, 2021. The Company had an accumulated deficit of $436.2 million as of March 31, 2021. Additionally, the Company used net cash of $3.0 million to fund its operating activities for the three months ended March 31, 2021. T he Company continues to seek additional capital through strategic transactions and from other financing alternatives. Without additional capital, the Company’s current working capital will not provide adequate funding to make debt repayments or support its 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. On October 23, 2020, the Company entered into an Equity Distribution Agreement (the “Distribution Agreement”) with Canaccord Genuity LLC (“Canaccord”) pursuant to which it may issue and sell, from time to time, shares of its common stock having an aggregate offering price of up to $10,000,000 (the “ATM Shares”), depending on market demand, with Canaccord acting as an agent for sales. Sales of the ATM Shares may be made by any method permitted by law deemed to be an “at the market offering” as defined in Rule 415(a)(4) of the Securities Act of 1933, as amended (the “Securities Act”), including, without limitation, sales made directly on or through the NASDAQ Capital Market. Canaccord will use its commercially reasonable efforts to sell the ATM Shares we request to be sold on our behalf, consistent with Canaccord’s normal trading and sales practices, under the terms and subject to the conditions set forth in the Distribution Agreement. The Company does not have an obligation to sell any of the ATM Shares. The Company may instruct Canaccord not to sell the ATM Shares if the sales cannot be effected at or above the price we designate from time to time and we may at any time suspend sales pursuant to the Distribution Agreement. During the year ended December 31, 2020, the Company issued 1,616,331 shares under the Distribution Agreement for net proceeds of approximately $3.2 million. During the three months ended March 31, 2021, it issued 1,137,193 shares under the Distribution Agreement for net proceeds of $3.2 million. On September 30, 2020, the Company entered into a purchase agreement (the “2020 Purchase Agreement”) and a registration rights agreement with Lincoln Park Capital Fund, LLC (“Lincoln Park”), pursuant to which Lincoln Park committed to purchase up to $25.0 million of its common stock. Under the terms and subject to the conditions of the 2020 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 $25.0 million of its common stock. Such sales of common stock by us, if any, will be subject to certain limitations, and may occur from time to time, at our sole discretion, over the 36-month period commencing November 6, 2020, subject to satisfaction of certain conditions. The net proceeds under the 2020 Purchase Agreement will depend on the frequency and prices at which the Company sells shares of its common stock to Lincoln Park. During the year ended December 31, 2020, the Company issued 353,113 shares, excluding 180,701 shares issued as commitment fee, under the 2020 Purchase Agreement for net proceeds of approximately $0.7 million. During the three months ended March 31, 2021, the Company issued 1,397,686 shares of its common stock under the 2020 Purchase Agreement for net proceeds of $3.9 million. Should the Company fail to raise additional cash from outside sources, this would have a material adverse impact on its operations . The accompanying consolidated 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 concer n.

Fair Value

Fair Value	3 Months Ended
	Mar. 31, 2021
Fair Value Disclosures [Abstract]
Fair Value	4 . Fair 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. Warrants issued in an underwritten public offering in September 2019 (“Series U Warrants”) are classified as liability instruments. 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 Company estimated the fair value of the Series U Warrants with the Black Scholes model. The Series U Warrants will be March 31, 2021 December 31, 2020 Expected term 3.5 years 3.75 years Common stock market price $ 2.40 $ 2.02 Risk-free interest rate 0.48 % 0.24 % Expected volatility 146 % 149 % Resulting fair value (per warrant) $ 1.81 $ 1.56 The following tables summarizes the change in Level 3 warrant liability value (in thousands): Three Months Ended Warrant liability March 31, 2021 March 31, 2020 Beginning balance $ 7 $ 6,929 Change in fair value (2 ) (1,667 ) Ending balance $ 5 $ 5,262

Term Loan Obligations

Term Loan Obligations	3 Months Ended
	Mar. 31, 2021
Debt Disclosure [Abstract]
Term Loan Obligations	5 . On May 29, 2015, the Company entered into 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 September 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 (“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 September 1, 2024. 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. After giving effect to this payment, $4.3 million of principal remains outstanding under the Loan Agreement. In addition, an amendment fee of $1.0 million will be payable in connection with the 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, 2021, there was $4.3 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 months ended March 31, 2021 and 2020 was $0.2 million and $0.3 million, respectively. Interest expense is calculated using the effective interest method; therefore it is inclusive of non-cash amortization in the amount of $0.1 million and $0.1 million for the three months ended March 31, 2021 and 2020 , 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, 2021, the Company has not received any notification or indication from Oxford to invoke the material adverse change clause. However, due to the Company’s current cash flow position and the substantial doubt about its ability to continue as a going concern, the entire principal amount of the Term Loan is presented as short-term. The Company will continue to evaluate the debt classification on a quarterly basis and evaluate for reclassification in the future should its financial condition improve.

Loss per Share

Loss per Share	3 Months Ended
	Mar. 31, 2021
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 Three Months Ended March 31, 2021 2020 Outstanding stock options 691,263 87,741 Outstanding warrants 2,141,378 3,637,000 Preferred stocks 422,867 298,000 Total 3,255,508 4,022,741

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2021
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. 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 financing leases are recorded within property and equipment, net in the balance sheet. Leases with an initial term of 12 months or less are not recorded on the balance sheet. Instead, the Company recognizes lease expense for these leases on a straight-line basis over the lease term. The Company leases laboratory, office and storage facilities in San Antonio, Texas, under operating lease agreements that expire in 2028. 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”). In addition, the Company leases certain equipment under various operating and finance leases. The 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 table below summarizes the Company’s lease liabilities and corresponding right-of-use assets (in thousands, except years and rates): March 31, 2021 Assets Operating $ 600 Financing 3 Total leased assets $ 603 Liabilities Current: Operating $ 113 Financing 3 Noncurrent: Operating 503 Financing — Total lease liabilities $ 619 Weighted-average remaining lease term (years) - operating leases 6.29 Weighted-average remaining lease term (years) - finance leases 0.17 Weighted-average discount rate - operating leases 7.8 % Weighted-average discount rate - finance leases 5.0 % The table below summarizes the Company’s lease costs from its unaudited consolidated condensed statement of operations, and cash payments from its unaudited consolidated condensed statement of cash flows during the three months ended March 31, 2021 and 2020 (in thousands): Three Months Ended March 31, 2021 March 31, 2020 Lease expense: Operating lease expense $ 50 $ 55 Finance lease expense: Depreciation of right-of-use assets 4 32 Interest expense on lease liabilities — 2 Total lease expense $ 54 $ 89 Cash payment information: Operating cash used for operating leases $ 49 $ 51 Financing cash used for financing leases 6 18 Total cash paid for amounts included in the measurement of lease liabilities $ 55 $ 69 Total rent expenses for the three months ended March 31, 2021 and 2020 was $50,000 and $95,000, 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 and financing leases at March 31, 2021 are as follows (in thousands): Financing Leases Operating Leases Remaining 2021 $ 3 $ 134 2022 — 123 2023 — 100 2024 — 106 2025 — 108 Thereafter — 233 Total minimum lease payments $ 3 $ 804 Less: amount representing interest — (188 ) Present value of obligations under leases $ 3 $ 616 Less: current portion — (113 ) Noncurrent lease obligations $ 3 $ 503 The Charlottesville Lease has a term of 12 months and is renewable for four additional one-year periods. Piramal Master Services Agreement O n January 8, 2021 will be performed at Piramal’s facility located in Lexington, Kentucky. The parties contemplate that the MSA will lead to clinical and commercial supply agreements between the Company and Piramal. 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 , the Company did not have any clinical research study obligations. 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.

NanoTx License Agreement

NanoTx License Agreement	3 Months Ended
	Mar. 31, 2021
License Agreement [Abstract]
NanoTx License Agreement	8 . 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 payment of $400,000 in cash and issued 230,769 shares of its common stock to NanoTx. Cash and the fair value of common stock issued totaled $781,000 and is recorded as in-process research and development expenses, pursuant to authoritative literature for asset acquisition, in the consolidated statement of operations and comprehensive loss for the year ended December 31, 2020. Pursuant to the terms of the NanoTx License Agreement, the Company may be required to pay up to $136.5 million in development and sales milestone payments and a tiered single-digit royalty on U.S. and European sales.

Stockholders' Equity

Stockholders' Equity	3 Months Ended
	Mar. 31, 2021
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. There were no shares of Series A 3.6% Convertible Preferred Stock outstanding as of March 31, 2021 or December 31, 2020. There were 1,014 and 1,016 shares of Series B Convertible Preferred Stock outstanding as of March 31, 2021 and December 31, 2020, respectively. There were 938 shares of Series C Preferred Stock outstanding as of each of March 31, 2021 and December 31, 2020. As of March 31, 2021, 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 April 27, 2018, the Company registered and distributed to holders of its common stock and Series B Convertible Preferred Stock, at no charge, non-transferable subscription rights to purchase up to an aggregate of 20,000 units for $1,000. Each unit consisted of one share of Series C Preferred Stock and 1,050 warrants (“Series T Warrants”). As previously reported, all Series T Warrants expired unexercised in January 2021. 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 are 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. Between April and September 2020, the Company entered into revised warrant agreements with the holders of 3,447,500 Series U Warrants (the “Warrant Amendments”). In return for reducing the strike price of the warrants to $2.25 per share, the warrant holders agreed to amend the settlement provisions upon a fundamental transaction such that the warrants would meet the requirements to be classified within stockholders’ equity. In September 2020, the Company entered into revised warrant agreements for the Representative Warrants that reduced the strike price of the warrants to $2.81 per share, and the warrant holders agreed to amend the settlement provisions upon a fundamental transaction such that the Representative Warrants would meet the requirements to be classified within stockholders’ equity. Accordingly, approximately $4.5 million of warrant liability was reclassified to stockholders’ equity on the respective effective date of the Warrant Amendments. In addition, approximately $0.7 million of other income representing change in the fair value of amended warrants from April 1, 2020 to the respective effective date of the Warrant Amendments is recorded in the consolidated statement of operations for the year ended December 31, 2020. As of March 31, 2021, 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 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 has the right, but not the obligation, to sell to Lincoln Park, and Lincoln Park is obligated to purchase up to $25.0 million of the Company’s common stock. The 2020 Purchase Agreement provides that the number of shares the Company may sell to Lincoln Park on any single business day in a regular purchase is 50,000, but that amount may be increased up to 100,000 shares, depending upon the market price of the Company’s common stock at the time of sale and subject to a maximum limit of $500,000 per regular purchase. The purchase price per share for each such regular purchase will be based on prevailing market prices of the Company’s common stock immediately preceding the time of sale as computed under the 2020 Purchase Agreement. In addition to regular purchases, the Company may also direct Lincoln Park to purchase other amounts as accelerated purchases or as additional accelerated purchases if the closing sale price of the common stock exceeds certain threshold prices as set forth in the 2020 Purchase 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 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. Actual sales of shares of common stock to Lincoln Park under the 2020 Purchase Agreement will 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 2020 Purchase Agreement to the Company will depend on the frequency and prices at which the Company sells shares of its stock to Lincoln Park. During the year ended December 31, 2020, the Company issued 353,113 shares, excluding 180,701 shares issued as a commitment fee, of common stock under the 2020 Purchase Agreement for total net proceeds of approximately $0.7 million. During the three months ended March 31, 2021, the Company issued 1,397,686 shares of its common stock under the 2020 Purchase Agreement for net proceeds of approximately $3.9 million. At-the-market Issuances On October 23, 2020, the Company entered into the Distribution Agreement with Canaccord Genuity LLC (“Canaccord”), pursuant to which the Company may issue and sell, from time to time, ATM Shares, depending on market demand, with Canaccord acting as an agent for sales. The Company has no obligation to sell any of the ATM Shares. The Company may instruct Canaccord not to sell the ATM Shares if the sales cannot be effected at or above the price the Company designates from time to time and the Company may at any time suspend sales pursuant to the Distribution Agreement. During the year ended December 31, 2020, the Company issued 1,616,331 shares under the Distribution Agreement for net proceeds of approximately $3.2 million. During the three months ended March 31, 2021, the Company issued 1,137,193 shares under the Distribution Agreement for net proceeds of $3.2 million

Stock-based Compensation

Stock-based Compensation	3 Months Ended
	Mar. 31, 2021
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-based Compensation	1 0 . 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, 2021 there were 210,389 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). As of March 31, 2021, there were no 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, and have a contractual term of 10 years. A summary of activity for the three months ended March 31, 2021 is as follows: Options Weighted Average Exercise Price Aggregate Intrinsic Value (in $,000) Outstanding as of December 31, 2020 531,336 $ 10.01 Granted 159,939 $ 3.64 Cancelled/forfeited (12 ) $ 41,300.00 Outstanding as of March 31, 2021 691,263 $ 7.82 $ 149 Vested as of March 31, 2021 159,140 $ 24.91 $ 42 Vested and expected to be vested as of March 31, 2021 636,442 $ 8.14 $ 140 As of March 31, 2021, the total compensation cost related to non-vested stock options not yet recognized for all the Company’s plans is approximately $1.1 million, which is expected to be recognized as a result of vesting under service conditions over a weighted average period of 3.14 years.

COVID-19 Pandemic and CARES Act

COVID-19 Pandemic and CARES Act	3 Months Ended
	Mar. 31, 2021
Risks And Uncertainties [Abstract]
COVID-19 Pandemic and CARES Act	11. COVID-19 Pandemic and CARES Act The COVID-19 pandemic has presented substantial public health and economic challenges and is affecting economies, financial markets and business operations around the world. International and U.S. governmental authorities in impacted regions have taken action in an effort to slow the spread of COVID-19, including issuing varying forms of “stay-at-home” orders, and restricting business functions outside of one’s home. In response, the Company put restrictions on employee travel and working from its executive offices with many employees continuing their work remotely. 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 is currently continuing the clinical trials it has underway in sites across the U.S., and, although there has been no significant impact to date, the Company expects that COVID-19 precautions may directly or indirectly impact the timeline for some of its clinical trials. Some of the Company’s clinical trial sites, including those located in areas severely impacted by the pandemic, have placed new patient enrollment into clinical trials on hold or, for patients traveling from out-of-state, have implemented a 14-day self-quarantine before appointments. In addition, some clinical trial sites have imposed limited accessibility to conduct clinical monitoring and training on-site. Although there are vaccines available, the ability to obtain a vaccine or know when herd immunity will be met, is difficult to anticipate. The Company considered the impacts of COVID-19 on the assumptions and estimates used to prepare its consolidated financial statements and determined that there were no material adverse impacts on the Company’s results of operations and financial position at March 31, 2021. 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 Coronavirus Aid, Relief and Economic Security Act (“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 (QIP). The CARES Act had no material impact on the Company’s income tax provision for the year ended December 31, 2020 or the three months ended March 31, 2021. The Company continues to evaluate the impact of the CARES Act on its financial position, results of operations and cash flows.

Subsequent Events

Subsequent Events	3 Months Ended
	Mar. 31, 2021
Subsequent Events [Abstract]
Subsequent Events	12. Subsequent Events During the period from April 1, 2021 through the date of the filing of this Quarterly Report on Form 10-Q, the Company issued 55,000 shares of its common stock under the Purchase Agreement with Lincoln Park for net proceeds of approximately $124,000.

Basis of Presentation and New_2

Basis of Presentation and New Accounting Standards (Policies)	3 Months Ended
	Mar. 31, 2021
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Basis of Presentation and New Accounting Standards	The accompanying unaudited consolidated condensed financial statements as of March 31, 2021 and for the three months ended March 31, 2021 and 2020 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 consolidated condensed balance sheet at December 31, 2020 has been derived from the audited financial statements at December 31, 2020, 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, 2021 are not necessarily indicative of the results that may be expected for the year ending December 31, 2021. These financial statements should be read in conjunction with the consolidated financial statements and notes therein included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2020, filed with the Securities and Exchange Commission on February 22, 2021.
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 consolidated financial statements and related disclosures. In August 2020, the FASB issued ​ASU No. 2020-06​, ​Debt - Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging - Contracts in Entity’s Own Equity (Subtopic 815-40)​ (“ASU 2020-06”). ​ASU 2020-06 eliminates the beneficial conversion and cash conversion accounting models for convertible instruments. It also amends the accounting for certain contracts in an entity’s own equity that are currently accounted for as derivatives because of specific settlement provisions. In addition, ASU 2020-06 modifies how particular convertible instruments and certain contracts that may be settled in cash or shares impact the diluted EPS computation. The amendments in ASU 2020-06 are effective for smaller reporting companies as defined by the SEC for fiscal years beginning after December 15, 2023, including interim periods within those fiscal years. Early adoption is permitted, but no earlier than fiscal years beginning after December 15, 2020. The Company adopted ASU 2020-06 as of January 1, 2021, which did not have an impact on its consolidated financial statements.
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncement In December 2019, the FASB issued ASU No. 2019-12, Income Taxes, Simplifying the Accounting for Income Taxes (“ASU 2019-12”). The new guidance eliminates certain exceptions related to the approach for intraperiod tax allocation, the methodology for calculating income taxes in an interim period and the recognition of deferred tax liabilities for outside basis differences. It also clarifies and simplifies other aspects of the accounting for income taxes. ASU 2019-12 became effective for the Company on January 1, 2021 . Adoption of ASU 2019-12 did not have a material impact on the Company’s consolidated financial statements.
Use of Estimates	The preparation of consolidated 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, determining the assumptions used in measuring stock-based compensation expense and valuing warrants. Actual results could differ from these estimates. Management’s estimates and assumptions are reviewed regularly, and the effects of revisions are reflected in the consolidated financial statements in the periods they are determined to be necessary.

Fair Value (Tables)

Fair Value (Tables)	3 Months Ended
	Mar. 31, 2021
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Summary of Change in Level 3 Warrant Liability Value	The following tables summarizes the change in Level 3 warrant liability value (in thousands): Three Months Ended Warrant liability March 31, 2021 March 31, 2020 Beginning balance $ 7 $ 6,929 Change in fair value (2 ) (1,667 ) Ending balance $ 5 $ 5,262
Series U Warrants [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Schedule of Estimated Fair Value Determined Using Option Pricing Model Assumptions	The Company estimated the fair value of the Series U Warrants with the Black Scholes model. The Series U Warrants will be March 31, 2021 December 31, 2020 Expected term 3.5 years 3.75 years Common stock market price $ 2.40 $ 2.02 Risk-free interest rate 0.48 % 0.24 % Expected volatility 146 % 149 % Resulting fair value (per warrant) $ 1.81 $ 1.56

Loss per Share (Tables)

Loss per Share (Tables)	3 Months Ended
	Mar. 31, 2021
Earnings Per Share [Abstract]
Schedule of Antidilutive Securities Excluded from Diluted Income (Loss) per Share	The following were excluded from the diluted Three Months Ended March 31, 2021 2020 Outstanding stock options 691,263 87,741 Outstanding warrants 2,141,378 3,637,000 Preferred stocks 422,867 298,000 Total 3,255,508 4,022,741

Commitments and Contingencies (

Commitments and Contingencies (Tables)	3 Months Ended
	Mar. 31, 2021
Commitments And Contingencies Disclosure [Abstract]
Summary of Lease Liabilities and Right-of-Use Assets	The table below summarizes the Company’s lease liabilities and corresponding right-of-use assets (in thousands, except years and rates): March 31, 2021 Assets Operating $ 600 Financing 3 Total leased assets $ 603 Liabilities Current: Operating $ 113 Financing 3 Noncurrent: Operating 503 Financing — Total lease liabilities $ 619 Weighted-average remaining lease term (years) - operating leases 6.29 Weighted-average remaining lease term (years) - finance leases 0.17 Weighted-average discount rate - operating leases 7.8 % Weighted-average discount rate - finance leases 5.0 %
Summary of Lease Costs	The table below summarizes the Company’s lease costs from its unaudited consolidated condensed statement of operations, and cash payments from its unaudited consolidated condensed statement of cash flows during the three months ended March 31, 2021 and 2020 (in thousands): Three Months Ended March 31, 2021 March 31, 2020 Lease expense: Operating lease expense $ 50 $ 55 Finance lease expense: Depreciation of right-of-use assets 4 32 Interest expense on lease liabilities — 2 Total lease expense $ 54 $ 89 Cash payment information: Operating cash used for operating leases $ 49 $ 51 Financing cash used for financing leases 6 18 Total cash paid for amounts included in the measurement of lease liabilities $ 55 $ 69
Summary of Future Minimum Annual Lease Payments under Operating and Financing Leases	Total rent expenses for the three months ended March 31, 2021 and 2020 was $50,000 and $95,000, 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 and financing leases at March 31, 2021 are as follows (in thousands): Financing Leases Operating Leases Remaining 2021 $ 3 $ 134 2022 — 123 2023 — 100 2024 — 106 2025 — 108 Thereafter — 233 Total minimum lease payments $ 3 $ 804 Less: amount representing interest — (188 ) Present value of obligations under leases $ 3 $ 616 Less: current portion — (113 ) Noncurrent lease obligations $ 3 $ 503

Stock-based Compensation (Table

Stock-based Compensation (Tables)	3 Months Ended
	Mar. 31, 2021
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock Option Activity	A summary of activity for the three months ended March 31, 2021 is as follows: Options Weighted Average Exercise Price Aggregate Intrinsic Value (in $,000) Outstanding as of December 31, 2020 531,336 $ 10.01 Granted 159,939 $ 3.64 Cancelled/forfeited (12 ) $ 41,300.00 Outstanding as of March 31, 2021 691,263 $ 7.82 $ 149 Vested as of March 31, 2021 159,140 $ 24.91 $ 42 Vested and expected to be vested as of March 31, 2021 636,442 $ 8.14 $ 140

Liquidity and Going Concern - A

Liquidity and Going Concern - Additional Information (Details) - USD ($)	Oct. 23, 2020	Sep. 30, 2020	Mar. 31, 2021	Mar. 31, 2020	Sep. 30, 2020	Dec. 31, 2020
Liquidity [Line Items]
Net loss			$ (2,720,000)	$ (1,087,000)
Accumulated deficit			(436,231,000)			$ (433,511,000)
Net cash used in operating activities			$ (3,006,000)	(1,502,000)
Substantial doubt about going concern, description			These factors raise substantial doubt about the Company’s ability to continue as a going concern.
Proceeds from sale of common stock			$ 7,180,000	$ 0
Sale of common stock, net			$ 7,078,000
Common Stock [Member]
Liquidity [Line Items]
Common stock issued (in shares)			2,534,879,000
Sale of common stock, net			$ 2,000
Underwriter [Member] | Common Stock [Member]
Liquidity [Line Items]
Proceeds from sale of common stock			$ 3,200,000			$ 3,200,000
Common stock issued (in shares)			1,137,193			1,616,331
Canaccord Genuity LLC [Member] | Common Stock [Member]
Liquidity [Line Items]
Proceeds from sale of common stock	$ 10,000,000		$ 3,200,000			$ 3,200,000
Remaining availability under financing facility	$ 0
Common stock issued (in shares)			1,137,193			1,616,331
Lincoln Park Capital Fund, LLC [Member] | Common Stock [Member] | 2020 Repurchase Agreement [Member]
Liquidity [Line Items]
Proceeds from sale of common stock			$ 3,900,000			$ 700,000
Common stock issued (in shares)			1,397,686			353,113
Sale of common stock, net		$ 25,000,000			$ 25,000,000
Period exercisable from the date of issuance		36 months			36 months

Fair Value - Schedule of Estima

Fair Value - Schedule of Estimated Fair Value Determined Using Option Pricing Model Assumptions (Details) - Series U Warrants [Member] - $ / shares	Mar. 31, 2021	Dec. 31, 2020
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Warrants expected term	5 years
Level 3 [Member] | Expected Term [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Warrants expected term	3 years 6 months	3 years 9 months
Level 3 [Member] | Common Stock Market Price [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Warrants measurement input	2.40	2.02
Level 3 [Member] | Risk-free Interest Rate [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Warrants measurement input	0.48	0.24
Level 3 [Member] | Expected Volatility [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Warrants measurement input	146	149
Level 3 [Member] | Resulting Fair Value (Per Warrant) [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Warrants measurement input	1.81	1.56

Fair Value - Summary of Change

Fair Value - Summary of Change in Level 3 Warrant Liability Value (Details) - Warrant Liability [Member] - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Warrant liability
Beginning balance	$ 7	$ 6,929
Change in fair value	(2)	(1,667)
Ending balance	$ 5	$ 5,262

Term Loan Obligations (Details)

Term Loan Obligations (Details) - USD ($)	Mar. 29, 2020	Mar. 28, 2020	May 29, 2015	Mar. 31, 2021	Mar. 31, 2020	Sep. 30, 2020
Debt Instrument [Line Items]
Warrant exercise price (in dollars per share)						$ 2.25
Interest expense				$ 247,000	$ 349,000
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	Sep. 1, 2024	Sep. 1, 2021	Sep. 1, 2024
Fees amount associated with loan			$ 3,200,000	$ 3,200,000
Number of securities called by warrant or right			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
Prepayments of term loan	$ 5,000,000		$ 3,100,000
Minimum liquidity covenant			$ 2,000
Debt principal repayment start date	May 1, 2021	May 1, 2020
Related final payment	$ 300,000
Term loan, remaining amount	4,300,000
Amendment fee	$ 1,000,000
Debt Instrument, Covenant Compliance				the Company was in compliance with all of the debt covenants under the Loan and Security Agreement.
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				$ 4,300,000
Long Term Debt Outstanding Threshold Amount				$ 3,000,000

Loss per Share - Schedule of An

Loss per Share - Schedule of Antidilutive Securities Excluded from Diluted Income (Loss) per Share (Details) - shares	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
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)	3,255,508	4,022,741
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)	691,263	87,741
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	3,637,000
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	298,000

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details) - USD ($)	Jan. 08, 2021	Mar. 31, 2021	Mar. 31, 2020
Recorded Unconditional Purchase Obligation [Line Items]
Rent expense		$ 50,000	$ 95,000
Minimum lease payment		804,000
Pre-clinical Research Study Obligations [Member]
Recorded Unconditional Purchase Obligation [Line Items]
Contractual obligation		$ 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. 1, 2021
Piramal Pharma Solutions, Inc. [Member]
Recorded Unconditional Purchase Obligation [Line Items]
Master service agreement date	Jan. 8, 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.
San Antonio, Texas [Member] | Office Laboratory and Storage Space [Member]
Recorded Unconditional Purchase Obligation [Line Items]
Operating leases expiry year		2028

Commitments and Contingencies_2

Commitments and Contingencies - Summary of Lease Liabilities and Right-of-Use Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Assets
Operating	$ 600	$ 636
Financing	3
Total leased assets	603
Liabilities
Operating lease liabilities, current	113	123
Financing lease liabilities, current	3
Operating lease liabilities. noncurrent	503	$ 528
Financing lease liabilities, noncurrent	3
Total lease liabilities	$ 619
Weighted-average remaining lease term (years) - operating leases	6 years 3 months 14 days
Weighted-average remaining lease term (years) - finance leases	2 months 1 day
Weighted-average discount rate - operating leases	7.80%
Weighted-average discount rate - finance leases	5.00%

Commitments and Contingencies_3

Commitments and Contingencies - Summary of Lease Costs (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Lease expense:
Operating lease expense	$ 50	$ 55
Finance lease expense:
Depreciation of right-of-use assets	4	32
Interest expense on lease liabilities		2
Total lease expense	54	89
Cash payment information:
Operating cash used for operating leases	49	51
Financing cash used for financing leases	6	18
Total cash paid for amounts included in the measurement of lease liabilities	$ 55	$ 69

Commitments and Contingencies_4

Commitments and Contingencies - Summary of Future Minimum Annual Lease Payments under Operating and Financing Leases (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Financing Leases
Remaining 2021	$ 3
Total minimum lease payments	3
Present value of obligations under leases	3
Less: current portion	(3)
Financing lease liabilities, noncurrent	3
Operating Leases
Remaining 2021	134
2022	123
2023	100
2024	106
2025	108
Thereafter	233
Total minimum lease payments	804
Less: amount representing interest	(188)
Present value of obligations under leases	616
Less: current portion	(113)	$ (123)
Noncurrent operating lease liability	$ 503	$ 528

NanoTx License Agreement - Addi

NanoTx License Agreement - Additional Information (Details) - USD ($)	May 07, 2020	Mar. 31, 2021	Dec. 31, 2020
Common stock, shares issued (in shares)		10,180,525	6,749,028
Common stock, $0.001 par value; 100,000,000 shares authorized; 10,180,525 and 6,749,028 shares issued and outstanding at March 31, 2021 and December 31, 2020, respectively	$ 781,000	$ 10,000	$ 7,000
Nano Tx Licenses Agreement [Member]
License fee payment in cash	$ 400,000
Common stock, shares issued (in shares)	230,769
Nano Tx Licenses Agreement [Member] | Development And Sales Milestone [Member]
Milestone payments	$ 136,500,000

Stockholders' Equity - Preferre

Stockholders' Equity - Preferred Stock (Details) - $ / shares	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2019
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)	$ 0.001	$ 0.001	$ 0.001
Preferred stock, shares outstanding (in shares)	1,952	1,954
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 B Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares outstanding (in shares)	1,014	1,016
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

Stockholders' Equity - Warrants (Details) - USD ($)	Apr. 27, 2018	Mar. 31, 2021	Mar. 31, 2020	Sep. 30, 2020	Dec. 31, 2020
Class of Stock [Line Items]
Warrant exercise price (in dollars per share)				$ 2.25
Warrant liability		$ 5,000			$ 7,000
Change in fair value of warrants		$ (2,000)	$ (1,667,000)
Outstanding Warrants [Member]
Class of Stock [Line Items]
Warrant liability				$ 4,500,000
Change in fair value of warrants				$ 700,000
Series U Warrants [Member]
Class of Stock [Line Items]
Common stock issued (in shares)		3,450,000
Sale of Stock, Price Per Share		$ 5
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
Warrant exercise price decrease				$ 2.81
Common Stock [Member]
Class of Stock [Line Items]
Common stock issued (in shares)		2,534,879,000
Warrants expected term		5 years
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
2018 Rights Offering [Member]
Class of Stock [Line Items]
Number of securities called by warrant or right	1
2018 Rights Offering [Member] | Outstanding Warrants [Member]
Class of Stock [Line Items]
Number of securities called by warrant or right	1,050
Warrant exercise price (in dollars per share)	$ 1,000
2018 Rights Offering [Member] | Common Stock and Series B Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Charge on Non Transferable Subscription Rights	$ 0
2018 Rights Offering [Member] | Series C Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Number of securities called by warrant or right	20,000
Series U Warrants [Member] | Outstanding Warrants [Member]
Class of Stock [Line Items]
Number of securities called by warrant or right				3,447,500
Class of warrant outstanding		2,141,000
Number of warrants exercised		2,141,000

Stockholders' Equity - Common s

Stockholders' Equity - Common stock (Details) - USD ($)	Oct. 23, 2020	Sep. 30, 2020	Jun. 16, 2020	Mar. 31, 2021	Mar. 31, 2020	Sep. 30, 2020	Dec. 31, 2020
Class of Stock [Line Items]
Sale of common stock, net				$ 7,078,000
Proceeds from sale of common stock, net				7,180,000	$ 0
Common Stock [Member]
Class of Stock [Line Items]
Sale of common stock, net				$ 2,000
Common stock issued (in shares)				2,534,879,000
Lincoln Park Capital Fund, LLC [Member] | 2020 Repurchase Agreement [Member] | Common Stock [Member]
Class of Stock [Line Items]
Sale of common stock, net		$ 25,000,000				$ 25,000,000
Common stock issued (in shares)				1,397,686			353,113
Period exercisable from the date of issuance		36 months				36 months
Closing price per share			$ 1.05
Proceeds from sale of common stock, net				$ 3,900,000			$ 700,000
Lincoln Park Capital Fund, LLC [Member] | 2020 Repurchase Agreement [Member] | Common Stock [Member] | Commitment Fee
Class of Stock [Line Items]
Common stock issued (in shares)							180,701
Lincoln Park Capital Fund, LLC [Member] | 2020 Repurchase Agreement [Member] | Common Stock [Member] | Minimum [Member]
Class of Stock [Line Items]
Percentage issuance of common stock			19.99%
Lincoln Park Capital Fund, LLC [Member] | 2020 Repurchase Agreement [Member] | Common Stock [Member] | Minimum [Member] | Single Regular Purchase [Member]
Class of Stock [Line Items]
Sale of common stock, net		$ 500,000
Common stock issued (in shares)		50,000
Lincoln Park Capital Fund, LLC [Member] | 2020 Repurchase Agreement [Member] | Common Stock [Member] | Maximum [Member]
Class of Stock [Line Items]
Common stock issued (in shares)			23,800,000
Lincoln Park Capital Fund, LLC [Member] | 2020 Repurchase Agreement [Member] | Common Stock [Member] | Maximum [Member] | Single Regular Purchase [Member]
Class of Stock [Line Items]
Common stock issued (in shares)		100,000
Closing price per share		$ 0.25				$ 0.25
Canaccord Genuity LLC [Member] | Common Stock [Member]
Class of Stock [Line Items]
Common stock issued (in shares)				1,137,193			1,616,331
Proceeds from sale of common stock, net	$ 10,000,000			$ 3,200,000			$ 3,200,000
Remaining availability under financing facility	0
Canaccord Genuity LLC [Member] | Sales Agreement | Common Stock [Member] | At The Market Offering Program
Class of Stock [Line Items]
Remaining availability under financing facility	$ 0

Stock-based Compensation - Addi

Stock-based Compensation - Additional Information (Details) - USD ($)	Jun. 16, 2020	Mar. 31, 2021	Feb. 06, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock shares to be issued (in shares)		159,939
Total unamortized compensation cost related to outstanding unvested stock options and restricted stock awards		$ 1,100
Weighted average period for recognition of cost		3 years 1 month 20 days
2015 Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Increased in the total number of shares of common stock reserved for issuance			250,000
Securities remaining and available for future issuances (in shares)		210,389
Securities remaining and available for future issuances (in shares)		210,389
2020 Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Securities remaining and available for future issuances (in shares)		0
Common stock shares to be issued (in shares)	550,000
Securities remaining and available for future issuances (in shares)		0

Stock-based Compensation - Stoc

Stock-based Compensation - Stock Option Activity (Details)	3 Months Ended
	Mar. 31, 2021USD ($)$ / sharesshares
Options [Roll Forward]
Balance as of December 31, 2020 (in share) | shares	531,336
Granted (in share) | shares	159,939
Cancelled/forfeited (in share) | shares	(12)
Balance as of March 31, 2021 (in share) | shares	691,263
Vested as of March 31, 2021 | shares	159,140
Vested and expected to be vested as of March 31, 2021 | shares	636,442
Weighted Average Exercise Price [Roll Forward]
Balance as of December 31, 2020 (in dollars per share) | $ / shares	$ 10.01
Granted (in dollars per share) | $ / shares	3.64
Cancelled/forfeited (in dollars per share) | $ / shares	41,300
Balance as of March 31, 2021 (in dollars per share) | $ / shares	7.82
Vested as of March 31, 2021 | $ / shares	24.91
Vested and expected to be vested as of March 31, 2021 | $ / shares	$ 8.14
Aggregate Intrinsic Value [Roll Forward]
Balance as of March 31, 2021 | $	$ 149
Vested as of March 31, 2021 | $	42
Vested and expected to be vested as of March 31, 2021 | $	$ 140

Subsequent Events - Additional

Subsequent Events - Additional Information (Details) - USD ($)	Apr. 01, 2021	Mar. 31, 2021	Mar. 31, 2020
Subsequent Event [Line Items]
Proceeds from sale of common stock, net		$ 7,180,000	$ 0
Common Stock [Member]
Subsequent Event [Line Items]
Common stock issued (in shares)		2,534,879,000
Subsequent Event | Common Stock [Member] | Lincoln Park Capital Fund, LLC [Member] | Purchase Agreement [Member]
Subsequent Event [Line Items]
Common stock issued (in shares)	55,000
Proceeds from sale of common stock, net	$ 124,000