Regulus Therapeutics (RGLS) 10-K 21 Mar 24 2023 FY Annual report Financial data

Cover

Cover - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Mar. 15, 2024	Jun. 30, 2023
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2023
Current Fiscal Year End Date	--12-31
Document Transition Report	false
Entity File Number	001-35670
Entity Registrant Name	Regulus Therapeutics Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	26-4738379
Entity Address, Address Line One	4224 Campus Point Court, Suite 210
Entity Address, City or Town	San Diego
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	92121
City Area Code	858
Local Phone Number	202-6300
Title of 12(b) Security	Common Stock, par value $0.001 per share
Trading Symbol	RGLS
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	false
Document Financial Statement Error Correction	false
Entity Shell Company	false
Entity Public Float			$ 27.4
Entity Common Stock, Shares Outstanding		65,465,251
Documents Incorporated by Reference	Portions of the registrant’s definitive proxy statement to be filed with the Securities and Exchange Commission pursuant on Schedule 14A in connection with the registrant’s 2024 Annual Meeting of Stockholders, which will be filed subsequent to the date hereof, are incorporated by reference into Part III of this Form 10-K. Such proxy statement will be filed with the Securities and Exchange Commission no later than April 29, 2024.
Amendment Flag	false
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	FY
Entity Central Index Key	0001505512

Audit Information

Audit Information	12 Months Ended
	Dec. 31, 2023
Audit Information [Abstract]
Auditor Name	Ernst & Young LLP
Auditor Location	San Diego, California
Auditor Firm ID	42

Balance Sheets

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Current assets:
Cash and cash equivalents	$ 23,767	$ 24,228
Short-term investments	0	14,932
Restricted cash	62	62
Prepaid materials, net	3,010	3,010
Prepaid expenses and other current assets	1,340	1,847
Total current assets	28,179	44,079
Property and equipment, net	1,061	536
Intangibles, net	33	62
Right of use asset	1,477	2,039
Total assets	30,750	46,716
Current liabilities:
Accounts payable	204	175
Accrued liabilities	691	961
Accrued research and development expenses	921	1,252
Accrued compensation	2,979	2,205
Current portion of term loan, less debt issuance costs	1,334	4,511
Other current liabilities	2,379	2,553
Total current liabilities	8,508	11,657
Operating lease liability, less current portion	1,055	1,768
Total liabilities	9,563	13,425
Commitments and Contingencies
Stockholders’ equity (deficit):
Common stock, $0.001 par value; 300,000,000 shares authorized at December 31, 2023 and 2022; 20,222,672 and 16,840,261 shares issued and outstanding at December 31, 2023 and 2022, respectively	20	17
Additional paid-in capital	534,375	516,457
Accumulated other comprehensive loss	0	(12)
Accumulated deficit	(513,213)	(483,176)
Total stockholders’ equity	21,187	33,291
Total liabilities and stockholders’ equity	30,750	46,716
Class A-1 Convertible Preferred Stock
Stockholders’ equity (deficit):
Convertible preferred stock	0	0
Class A-2 Convertible Preferred Stock
Stockholders’ equity (deficit):
Convertible preferred stock	1	1
Class A-3 Convertible Preferred Stock
Stockholders’ equity (deficit):
Convertible preferred stock	0	0
Class A-4 convertible preferred stock outstanding (as-converted)
Stockholders’ equity (deficit):
Convertible preferred stock	4	4
Class A-5 convertible preferred stock outstanding (as-converted)
Stockholders’ equity (deficit):
Convertible preferred stock	$ 0	$ 0

Balance Sheets (Parenthetical)

Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2023	Dec. 31, 2022
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	300,000,000	300,000,000
Common stock, shares issued (shares)	20,222,672	16,840,261
Common stock, shares outstanding (in shares)	20,222,672	16,840,261
Class A-1 Convertible Preferred Stock
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred stock, shares authorized (in shares)	256,700	256,700
Preferred stock, shares issued (in shares)	256,700	256,700
Preferred stock, shares outstanding (in shares)	256,700	256,700
Class A-2 Convertible Preferred Stock
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred stock, shares authorized (in shares)	1,330,832	1,330,832
Preferred stock, shares issued (in shares)	1,330,832	1,330,832
Preferred stock, shares outstanding (in shares)	1,330,832	1,330,832
Class A-3 Convertible Preferred Stock
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred stock, shares authorized (in shares)	258,707	258,707
Preferred stock, shares issued (in shares)	258,707	258,707
Preferred stock, shares outstanding (in shares)	258,707	258,707
Class A-4 convertible preferred stock outstanding (as-converted)
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred stock, shares authorized (in shares)	3,725,720	3,725,720
Preferred stock, shares issued (in shares)	3,725,720	3,725,720
Preferred stock, shares outstanding (in shares)	3,725,720	3,725,720
Class A-5 convertible preferred stock outstanding (as-converted)
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred stock, shares authorized (in shares)	140,827	0
Preferred stock, shares issued (in shares)	140,827	0
Preferred stock, shares outstanding (in shares)	140,827	0

Statements of Operations and Co

Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Operating expenses:
Research and development	$ 21,152	$ 18,410
General and administrative	9,957	9,829
Total operating expenses	31,109	28,239
Loss from operations	(31,109)	(28,239)
Other income (expense):
Interest and other income	1,677	605
Interest and other expense	(604)	(688)
Loss before income taxes	(30,036)	(28,322)
Income tax expense	(1)	(1)
Net loss	(30,037)	(28,323)
Other comprehensive loss:
Unrealized loss on short-term investments, net	0	(12)
Comprehensive loss	$ (30,037)	$ (28,335)
Net loss per share, basic (in dollars per share)	$ 1.58	$ 1.86
Net loss per share, diluted (in dollars per share)	$ 1.58	$ 1.86
Weighted average shares used to compute basic net loss per share (in shares)	18,960,401	15,259,958
Weighted average shares used to compute diluted net loss per share (in shares)	18,960,401	15,259,958

Statements of Stockholders' Equ

Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Preferred stock Convertible Preferred Stock	Common stock	Additional paid-in capital	Accumulated other comprehensive loss	Accumulated deficit
Beginning balance, preferred stock (in shares) at Dec. 31, 2021		5,571,959
Beginning balance, common stock (in shares) at Dec. 31, 2021			14,597,118
Beginning balance at Dec. 31, 2021	$ 54,958	$ 5	$ 15	$ 509,791	$ 0	$ (454,853)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon vesting of restricted stock units (in shares)			36,300
Stock-based compensation expense	2,177			2,177
Issuance of common stock under Employee Stock Purchase Plan (in shares)			1,743
Issuance of common stock under Employee Stock Purchase Plan	3			3
Issuance of common stock through ATM (in shares)			2,205,100
Issuance of common stock through ATM	4,488		$ 2	4,486
Unrealized gain (loss) on short-term investments	(12)				(12)
Net loss	$ (28,323)					(28,323)
Ending balance, preferred stock (in shares) at Dec. 31, 2022		5,571,959
Ending balance, common stock (in shares) at Dec. 31, 2022	16,840,261		16,840,261
Ending balance at Dec. 31, 2022	$ 33,291	$ 5	$ 17	516,457	(12)	(483,176)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation expense	2,801			2,801
Issuance of common stock and preferred stock from private placement, net of offering costs (in shares)		140,827	2,615,536
Issuance of common stock and preferred stock from private placement, net of offering costs	14,003		$ 2	14,001
Issuance of common stock under Employee Stock Purchase Plan (in shares)			41,683
Issuance of common stock under Employee Stock Purchase Plan	45			45
Issuance of common stock through ATM (in shares)			725,192
Issuance of common stock through ATM	1,072		$ 1	1,071
Unrealized gain (loss) on short-term investments	12				12
Net loss	$ (30,037)					(30,037)
Ending balance, preferred stock (in shares) at Dec. 31, 2023		5,712,786
Ending balance, common stock (in shares) at Dec. 31, 2023	20,222,672		20,222,672
Ending balance at Dec. 31, 2023	$ 21,187	$ 5	$ 20	$ 534,375	$ 0	$ (513,213)

Statements of Cash Flows

Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Operating activities
Net loss	$ (30,037)	$ (28,323)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization expense	227	122
Stock-based compensation	2,801	2,177
Amortization of premiums and accretion of discounts on investments, net	(157)	(185)
Other	141	133
Change in operating assets and liabilities:
Prepaid expenses and other assets	507	(67)
Accounts payable	29	(110)
Accrued liabilities	(396)	140
Accrued research and development expenses	(331)	442
Accrued compensation	773	189
Operating lease right-of-use assets and liabilities, net	(87)	(64)
Other liabilities	(238)	20
Net cash used in operating activities	(26,768)	(25,526)
Investing activities
Purchases of short-term investments	(4,899)	(32,759)
Sales and maturities of short-term investments	20,000	18,000
Purchases of property and equipment	(613)	(361)
Sales of property and equipment	3	0
Net cash provided by (used in) investing activities	14,491	(15,120)
Financing activities
Proceeds from issuance of securities through private placement, net of issuance costs	14,003	0
Proceeds from issuance of common stock, net	1,117	4,491
Principal payments on term loan	(3,304)	0
Net cash provided by financing activities	11,816	4,491
Net decrease in cash, cash equivalents and restricted cash	(461)	(36,155)
Cash, cash equivalents and restricted cash at beginning of period	24,290	60,445
Cash, cash equivalents and restricted cash at end of period	23,829	24,290
Reconciliation of cash, cash equivalents and restricted cash
Cash and cash equivalents	23,767	24,228
Restricted cash	62	62
Total cash, cash equivalents and restricted cash	23,829	24,290
Supplemental disclosure of cash flow information
Interest paid	(447)	(491)
Income taxes paid	(1)	(1)
Supplemental disclosure of non-cash investing and financing activities
Purchases of property and equipment included in accrued liabilities	$ 126	$ 6

The Business, Basis of Presenta

The Business, Basis of Presentation and Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2023
Accounting Policies [Abstract]
The Business, Basis of Presentation and Summary of Significant Accounting Policies	The Business, Basis of Presentation and Summary of Significant Accounting Policies We are a clinical-stage biopharmaceutical company focused on discovering and developing first-in-class drugs targeting micro RNAs to treat diseases with significant unmet medical need. We were formed in 2007 when Alnylam Pharmaceuticals, Inc. ("Alnylam") and Ionis Pharmaceuticals, Inc. ("Ionis") contributed significant intellectual property, know-how and financial and human capital to pursue the development of drugs targeting micro RNAs pursuant to a license and collaboration agreement. Basis of Presentation On June 24, 2022, we filed a Certificate of Amendment of Amended and Restated Certificate of Incorporation with the Secretary of State of the state of Delaware to effect a 1-for-10 reverse stock split of our issued and outstanding common stock. The primary purpose of the reverse stock split was to raise the per share trading price of our common stock to seek to maintain the listing of our common stock on The Nasdaq Capital Market. At the effective time of the reverse stock split, 5:00 p.m. on June 28, 2022, each 10 shares of our issued and outstanding common stock were automatically combined and converted into one issued and outstanding share of common stock. All of our stock options, restricted stock units ("RSUs") and warrants outstanding immediately prior to the reverse stock split, as well as the conversion ratio of our outstanding convertible preferred stock, were proportionately adjusted. All issued and outstanding common stock, options exercisable for common stock, restricted stock units, common stock issuable upon conversion of outstanding convertible preferred stock, warrants a nd per share amounts contained in these financial statements have been retrospectively adjusted. Liquidity Through December 31, 2023, we have principally been financed through net proceeds received from the sale of our common stock and other equity securities, debt financings, up-front payments and milestones received from collaboration agreements, totaling $557.7 million. As of December 31, 2023, we had approximately $23.8 million of cash and cash equivalents. Based on our operating plans, we believe our cash and cash equivalents will be sufficient to fund our planned operations for at least 12 months following the issuance of these financial statements. As of December 31, 2023, w e are in compliance with all loan agreement covenants. We intend to seek additional capital through equity and/or debt financings, collaborative or other funding arrangements with partners or through other sources of financing. Should we seek additional financing from outside sources, we may not be able to raise such financing on terms acceptable to us or at all. If we are unable to raise additional capital when required or on acceptable terms, we may be required to scale back or discontinue the advancement of product candidates, reduce headcount, file for bankruptcy, reorganize, merge with another entity, or cease operations. If we are unable to raise additional capital when needed, we may have to liquidate our assets, and in doing so might realize significantly less for those assets than the values at which they are carried on our financial statements. Stockholders may lose all or part of their investment in our common stock. Use of Estimates Our financial statements are prepared in accordance with GAAP, which requires us to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses and the disclosure of contingent assets and liabilities in our financial statements and accompanying notes. An estimated loss contingency is accrued in our financial statements if it is probable that a liability has been incurred and the amount of the loss can be reasonably estimated. Although these estimates are based on our knowledge of current events and actions we may undertake in the future, actual results may ultimately differ from these estimates and assumptions. Stock-Based Compensation We account for stock-based compensation expense related to stock options granted to employees and members of our board of directors by estimating the fair value of each stock option on the date of grant using the Black-Scholes option pricing model. We recognize stock-based compensation expense using the accelerated multiple-option approach. Under the accelerated multiple-option approach (also known as the graded-vesting method), we recognize compensation expense over the requisite service period for each separately vesting tranche of the award as though the award was in substance multiple awards, resulting in accelerated expense recognition over the vesting period. For performance-based awards granted to employees (i) the fair value of the award is determined on the grant date, (ii) we assess the probability of the individual milestones under the award being achieved and (iii) the fair value of the shares subject to the milestone is expensed over the implicit service period commencing once management believes the performance criteria is probable of being met. We account for restricted stock units by determining the fair value of each restricted stock unit based on the closing market price of our common stock on the date of grant. We recognize stock-based compensation expense using the accelerated multiple-option approach over the requisite service periods of the awards. Clinical Trial and Preclinical Study Accruals We make estimates of our accrued expenses for clinical trial and preclinical study activities as of each balance sheet date in our financial statements based on the facts and circumstances known to us at that time. These accruals are based upon estimates of costs incurred and fees that may be associated with services provided by clinical trial investigational sites and CROs and for other clinical trial-related activities. Payments under certain contracts with such parties depend on factors such as successful enrollment of patients, site initiation and progression through the various stages of our clinical trials. In accruing for these services, we estimate the time period over which services will be performed and the level of effort to be expended in each period. If possible, we obtain information regarding unbilled services directly from these service providers. However, we may be required to estimate these services based on other information available to us. If we underestimate or overestimate the activities or fees associated with a study or service at a given point in time, adjustments to research and development expenses may be necessary in future periods. Historically, our estimated accrued liabilities have approximated actual expense incurred. Subsequent changes in estimates may result in a material change in our accruals. Prepaid Materials We capitalize the purchase of certain raw materials and related supplies for use in the manufacturing of drug product in our preclinical and clinical development programs, as we have determined that these materials have alternative future use. We can use these raw materials and related supplies in multiple clinical drug products, and therefore have future use independent of the development status of any particular program until it is utilized in the manufacturing process. We expense the cost of materials when used. We periodically review these capitalized materials for continued alternative future use and write down the asset to its net realizable value in the period in which an impairment is identified. Research and Development Research and development costs are expensed as incurred and consist of costs associated with research activities supporting our drug discovery efforts, compensation and related benefits, non-cash stock-based compensation, license fees, laboratory supplies and associated overhead and facility costs. Income Taxes Income taxes are accounted for under the asset and liability method. This approach requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of the differences between the tax basis of assets or liabilities and their carrying amounts in the financial statements using the enacted tax rates and laws that are anticipated to be in effect when the differences are expected to reverse. We provide a valuation allowance against net deferred tax assets if it is more likely than not that these items will either expire before we are able to realize their benefit or if future deductibility is uncertain. In accordance with the accounting standards for uncertain tax positions, we evaluate the recognition threshold and measurement attribute criteria for the financial statement recognition and measurement of tax positions taken or expected to be taken in a tax return. For those benefits to be recognized, a tax position must be more likely than not to be sustained upon examination by taxing authorities. Cash and Cash Equivalents We classify time deposits and other investments that are highly liquid and have maturities of 90 days or less at the date of purchase as cash equivalents. The carrying amounts approximate fair value due to the short maturities of these instruments. Concentrations of Credit Risk Financial instruments that potentially subject us to significant concentrations of credit risk consist primarily of cash equivalents an d short-term investments. We maintain deposits in federally insured financial institutions in excess of federally insured limits. We have not experienced any material losses in such accounts and believe we are not exposed to significant risk. We have invested our excess cash primarily in money market funds and U.S. Treasury securities. Additionally, we adhere to established guidelines regarding approved investments and maturities of investments, which are designed to preserve their principal value and maintain liquidity. Property and Equipment We carry our property and equipment at cost, which consists of lab equipment, computer equipment and software, furniture and fixtures and leasehold improvements. Property and equipment is depreciated using the straight-line method over the estimated useful lives (generally three Impairment of Long-Lived Assets We regularly review the carrying amount of our property, equipment and intangible assets to determine whether indicators of impairment may exist which warrant adjustments to carrying values or estimated useful lives. If indications of impairment exist, projected future undiscounted cash flows associated with the asset are compared to the carrying amount to determine whether the asset’s value is recoverable. If the carrying value of the asset exceeds such projected undiscounted cash flows, the asset will be written down to its estimated fair value. No impairment charges were recorded during the years ended December 31, 2023 or 2022. Segment Reporting Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision-maker in making decisions regarding resource allocation and assessing performance. To date, we have viewed our operations and managed our business as one segment operating primarily within the United States. Comprehensive Loss Comprehensive loss is defined as the change in equity during a period from transactions and other events and/or circumstances from non-owner sources. Our only component of other comprehensive loss is unrealized loss on available-for-sale securities. Comprehensive loss has been reflected in the statements of operations and comprehensive loss and as a separate component in the statements of stockholders’ equity for all periods presented. Leases At the inception of a contractual arrangement, we determine 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. For operating leases with an initial term greater than 12 months, we recognize operating lease right of use assets ("ROU assets") and operating lease liabilities based on the present value of lease payments over the lease term at the commencement date. Operating lease ROU assets are comprised of the lease liability plus any lease payments made and excludes lease incentives. Lease terms include options to renew or terminate the lease when we are reasonably certain that the renewal option will be exercised or when it is reasonably certain that the termination option will not be exercised. For our operating leases, we generally cannot determine the interest rate implicit in the lease, in which case we use our incremental borrowing rate as the discount rate for the lease. We estimate our incremental borrowing rate for our operating leases based on what we would normally pay to borrow on a collateralized basis over a similar term for an amount equal to the lease payments. Operating lease expense is recognized on a straight-line basis over the lease term. Leases with a lease term of 12 months or less at inception are not recorded on the balance sheet. Instead, we recognize lease expense for these leases on a straight-line basis over the lease term. Our lease agreements do not contain any material variable lease payments, residual value guarantees or restrictive covenants. Certain leases require us to pay taxes, insurance, utilities, and maintenance costs for the building, which do not represent lease components. We elected to not separate lease and non-lease components. Fair Value of Financial Instruments We follow ASC 820-10 issued by the FASB with respect to fair value reporting for financial assets and liabilities. The guidance defines fair value, provides guidance for measuring fair value and requires certain disclosures. The guidance does not apply to measurements related to share-based payments. The guidance discusses valuation techniques such as the market approach (comparable market prices), the income approach (present value of future income or cash flow), and the cost approach (cost to replace the service capacity of an asset or replacement cost). The guidance establishes a fair value hierarchy that prioritizes the inputs to valuation techniques used to measure fair value into three broad levels. Our financial instruments consist of cash and cash equivalents, contract and other receivables, accounts payable, accrued liabilities, and our Term Loan. Fair value estimates of these instruments are made at each reporting period end based on relevant market information. These estimates may be subjective in nature and involve uncertainties and matters of significant judgment and therefore cannot be determined with precision. The carrying amount of cash, cash equivalents, contract and other receivables, accounts payable, and accrued liabilities are generally considered to be representative of their respective fair values because of the short-term nature of those instruments. We believe that the fair value of the Term Loan approximates its carrying value. Recent Accounting Pronouncements In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments . Subsequently, in November 2018, the FASB issued ASU 2018-19, Codification Improvements to Topic 326, Financial Instruments-Credit Losses . ASU 2016-13 requires entities to measure all expected credit losses for most financial assets held at the reporting date based on an expected loss model which includes historical experience, current conditions, an d reasonable and supportable forecasts. ASU 2016-13 also requires enhanced disclosures to help financial statement users better understand significant estimates and judgments used in estimating credit losses. This ASU is effective for smaller reporting companies for fiscal years beginning after Decem ber 15, 2022, with early adoption permitted. The adoption of this guidance had no impact on our financial statements and disclosures. In March 2020, the FASB issued ASU No. 2020-04, Reference Rate Reform (Topic 848), which provides guidance around reference rate reform initiatives to identify alternative reference rates that are more observable or transaction-based and less susceptible to manipulation in response to concerns about structural risks of interbank offered rates and the risk of cessation of the London Interbank Offered Rate ("LIBOR"). The amendments in the ASU provide option expedients and exceptions for applying GAAP to contracts, hedging relationships and other transactions affected by reference rate reform and apply only if such contracts, hedging relationships and other transactions that reference LIBOR or another reference rate are expected to be discontinued because of reference rate reform. On December 21, 2022, the FASB issued ASU No. 2022-06, Reference Rate Reform (Topic 848): Deferral of the Sunset Date of Topic 848,

Net Loss Per Share

Net Loss Per Share	12 Months Ended
	Dec. 31, 2023
Earnings Per Share [Abstract]
Net Loss Per Share	Net Loss Per Share Basic net loss per share is calculated by dividing net loss by the weighted average number of common shares outstanding for the period, without consideration for common stock equivalents. Diluted net loss per share is calculated by dividing net loss by the weighted-average number of common share equiva lents outstanding for the period determined using the treasury stock method or if-converted method. Dilutive common stock equivalents are comprised of stock options, restricted stock units, warrants and convertible preferred stock outstanding. For all periods presented, there is no difference in the number of shares used to calculate basic and diluted net loss per share. Potentially dilutive securities not included in the calculation of diluted net loss per common share, because to do so would be anti-dilutive, were (in common stock equivalent sh

Investments

Investments	12 Months Ended
	Dec. 31, 2023
Investments, Debt and Equity Securities [Abstract]
Investments	Investments Historically, we have invested our excess cash primarily in debt instruments of financial institutions, corporations, U.S. government-sponsored agencies and the U.S. Treasury. We generally hold our investments to maturity and do not sell our investments before we have recovered our amortized cost basis. As of December 31, 2023, our cash balance was comprised entirely of cash and cash equivalents (money market funds) and there was no unrealized gain or loss. Unrealized Maturity (in years) Amortized cost Gains Losses Estimated fair value As of December 31, 2022 U.S. Treasury securities 1 or less $ 14,944 $ — $ (12) $ 14,932 $ 14,944 $ — $ (12) $ 14,932

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2023
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements We have certain financial assets recorded at fair value which have been classified as Level 1, 2, or 3 within the fair value hierarchy as described in the accounting standards for fair value measurements. Accounting standards define fair value as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants as of the measurement date. Market participants are buyers and sellers in the principal market that are (i) independent, (ii) knowledgeable, (iii) able to transact, and (iv) willing to transact. The accounting standards provide an established hierarchy for inputs used in measuring fair value that maximizes the use of observable inputs and minimizes the use of unobservable inputs by requiring that the most observable inputs be used when available. Observable inputs are inputs that market participants would use in valuing the asset or liability and are developed based on market data obtained from independent sources. Unobservable inputs are inputs that reflect our assumptions about the factors that market participants would use in valuing the asset or liability. The accounting standards prioritize the inputs used in measuring the fair value into the following hierarchy: • Level 1 includes financial instruments for which quoted market prices for identical instruments are available in active markets. • Level 2 includes financial instruments for which there are inputs other than quoted prices included within Level 1 that are observable for the instrument such as quoted prices for similar instruments in active markets, quoted prices for identical or similar instruments in markets with insufficient volume or infrequent transactions (less active markets) or model-driven valuations in which significant inputs are observable or can be derived principally from, or corroborated by, observable market data. • Level 3 includes financial instruments for which fair value is derived from valuation techniques in which one or more significant inputs are unobservable, including management’s own assumptions. Financial Assets Measured at Fair Value The following table presents our fair value hierarchy for assets measured at fair value on a recurring basis as of December 31, 2023 and 2022 (in thousands): Fair value as of December 31, 2023 Total Level 1 Level 2 Level 3 Cash equivalents: Money market funds $ 23,084 $ 23,084 $ — $ — $ 23,084 $ 23,084 $ — $ — Fair value as of December 31, 2022 Total Level 1 Level 2 Level 3 Cash equivalents: Money market funds $ 21,490 $ 21,490 $ — $ — U.S. Treasury securities 14,932 14,932 — — $ 36,422 $ 36,422 $ — $ — We obtain pricing information from quoted market prices or quotes from brokers/dealers. We have historically determined the fair value of our investment securities using standard observable inputs, including reported trades, broker/dealer quotes, bids and/or offers.

Collaborations

Collaborations	12 Months Ended
	Dec. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Collaborations	Collaborations Sanofi In February 2014, we and Sanofi entered into a second amended and restated collaboration and license agreement (the “Sanofi Agreement”) to discover, develop and commercialize micro RNA therapeutics to focus on specific orphan disease and oncology targets. Under the terms of the Sanofi Agreement, Sanofi had opt-in rights to our clinical fibrosis program targeting miR-21 for the treatment of Alport syndrome (which rights were relinquished by Sanofi in November 2018), our preclinical program targeting miR-21 for oncology indications, and our preclinical program targeting miR-221/222 for HCC. We were responsible for developing each of these programs to proof-of-concept, at which time Sanofi had an exclusive option on each program. We were eligible to receive royalties on micro RNA therapeutic products commercialized by Sanofi and would have had have the right to co-promote these products relating to our preclinical program targeting miR-221/222.

Property and Equipment, net

Property and Equipment, net	12 Months Ended
	Dec. 31, 2023
Property, Plant and Equipment [Abstract]
Property and Equipment, net	Property and Equipment, net The following table summarizes our major classes of property and equipment (in thousands): December 31, 2023 2022 Laboratory equipment $ 4,687 $ 4,359 Computer equipment and software 367 470 Furniture and fixtures 12 12 Leasehold improvements 115 115 Construction in progress 126 — 5,307 4,956 Less accumulated depreciation and amortization (4,246) (4,420) Property and equipment, net $ 1,061 $ 536 Depreciation and amortization of property and equipme nt of $0.2 million and $0.1 million was recorded for the years ended December 31, 2023 and 2022, respectively.

Intangible Assets, net

Intangible Assets, net	12 Months Ended
	Dec. 31, 2023
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible Assets, net	Intangible Assets, net The following table summarizes our major classes of intangible assets (in thousands): December 31, 2023 2022 Patents $ 133 $ 183 Accumulated amortization - Patents (100) (121) Intangibles, net $ 33 $ 62 Intangible asset amortization of less than $0.1 million was recorded for the years ended December 31, 2023 and 2022. Amortization of intangible assets over the next five years is expected to be less than $0.1 million per year. The weighted-average period over which the amortization remaining at December 31, 2023 is expected to be recognized is approximately 11.4 years.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2023
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies License Agreements We have license agreements with third parties that require us to make annual license maintenance payments and future payments upon the success of licensed products that include milestones and/or royalties. Minimum future payments over the next five years are not material. Litigation From time to time, we may be involved in various lawsuits, legal proceedings, or claims that arise in the ordinary course of business. We believe there are no claims or actions pending against us at December 31, 2023 which will have, individually, or in the aggregate, a material adverse effect on our business, liquidity, financial position or results of operations. Litigation, however, is subject to inherent uncertainties, and an adverse result in such matters may arise from time to time that may harm our business.

Debt

Debt	12 Months Ended
	Dec. 31, 2023
Debt Disclosure [Abstract]
Debt	Debt Term Loan On June 17, 2016, we entered into a loan and security agreement ("Loan Agreement") with Oxford Finance, LLC, (the "Lender"), pursuant to which we received $20.0 million in proceeds, net of debt issuance costs, on June 22, 2016 (the "Term Loan"). The outstanding Term Loan bore interest at a floating per annum rate equal to (i) 8.51% plus (ii) the greater of (a) the 30 day U.S. Dollar LIBOR rate reported in The Wall Street Journal on the last business day of the month that immediately precedes the month in which the interest will accrue and (b) 0.44%. In June 2023, we entered into an amendment to the Loan Agreement (the “Rate Amendment”) pursuant to which, effective July 1, 2023, the Term Loan bears interest at a floating per annum rate equal to the greater of (a) 8.95% and (b) the sum of (i) the 1-month CME Term Secured Overnight Financing Rate ("SOFR") reference rate on the last business day of the month that immediately precedes the month in which the interest will accrue, (ii) 0.10% and (iii) 8.51%. Under the original Loan Agreement, we were required to make interest-only payments through June 1, 2018, followed by 24 equal monthly payments of principal and unpaid accrued interest. The Loan Agreement was amended ten times between October 2017 through August 2020. On December 31, 2021, we entered into an eleventh amendment to the Loan Agreement (the "Eleventh Amendment"). Under the terms of the Eleventh Amendment, the maturity date for the Term Loan was extended to May 1, 2024. In addition, under the Eleventh Amendment, our required monthly payments to the Lender were comprised of interest only through and including December 1, 2022. The Eleventh Amendment also provides that we are required to maintain a minimum cash balance of $5.0 million. As consideration for the Lender’s entry into the Eleventh Amendment, we made a payment of $0.3 million to the Lender. We used the proceeds from the Term Loan solely for working capital and to fund our general business requirements. Our obligations under the Loan Agreement are secured by a first priority security interest in substantially all of our current and future assets, other than our intellectual property, for which the Lender currently has a positive lien. We have also agreed not to encumber our intellectual property assets, except as permitted by the Loan Agreement. The Loan Agreement includes customary events of default, including instances of a material adverse change in our operations, that may require prepayment of the outstanding Term Loan. We are in compliance with all Loan Agreement covenants as of the date of the filing of this Form 10-K. As of December 31, 2023, $1.4 million of principal was outstanding under the Term Loan. An additional $1.3 million is also payable at the conclusion of the Term Loan (the related $1.3 million accrued liability balance is presented in other current liabilities on our balance sheet as of December 31, 2023) . We had less than $0.1 million of debt issuance costs outstanding as of December 31, 2023, which are being accreted to interest expense over the life of the T erm Loan using an effective interest rate of 8.98%. The exit fees are being accrued over the life of the Term Loan through interest expense. All amounts that remained payable under the Term Loan as of December 31, 2023 are due in 2024.

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2023
Equity [Abstract]
Stockholders’ Equity	Stockholders’ Equity Common Stock As of December 31, 2023, there were 20,222,672 shares of common stock outstanding. Each share of common stock is entitled to one vote. The holders of the common stock are also entitled to receive dividends whenever funds are legally available and when declared by our Board of Directors. Reverse Stock Split On June 24, 2022, we filed a Certificate of Amendment of Amended and Restated Certificate of Incorporation with the Secretary of State of the state of Delaware to effect a 1-for-10 reverse stock split of our issued and outstanding common stock. The primary purpose of the reverse stock split was to raise the per share trading price of our common stock to seek to maintain the listing of our common stock on The Nasdaq Capital Market. At the effective time of the reverse stock split, 5:00 p.m. on June 28, 2022, each 10 shares of our issued and outstanding common stock were automatically combined and converted into one issued and outstanding share of common stock. All of our stock options, RSUs and warrants outstanding immediately prior to the reverse stock split, as well as the conversion ratio of our outstanding convertible preferred stock, were proportionately adjusted. All issued and outstanding common stock, options exercisable for common stock, restricted stock units, common stock issuable upon conversion of outstanding convertible preferred stock, warrants a nd per share amounts contained in these financial statements have been retrospectively adjusted. 2019 Equity Incentive Plan On June 15, 2019 the Company's board of directors approved, and on August 1, 2019 the Company's stockholders approved, the Company's 2019 Equity Incentive Plan (the "2019 Plan"). The 2019 Plan is the successor to and continuation of the Company's 2012 Equity Incentive Plan. The number of shares authorized for issuance under the 2019 Plan may be increased by (a) the shares subject to outstanding stock awards granted under the Company’s 2009 Equity Incentive Plan (the "2009 Plan") and the Company’s 2012 Equity Incentive Plan (together with the 2009 Plan, the "Prior Plans") that on or after the effective date of the 2019 Plan (i) expire or terminate for any reason prior to exercise or settlement; (ii) are forfeited because of the failure to meet a contingency or condition required to vest such shares or otherwise return to the Company, or (iii) are reacquired, withheld (or not issued) to satisfy a tax withholding obligation in connection with an award or to satisfy the purchase price or exercise price of a stock award. No further grants will be made under the Prior Plans. In addition, on January 22, 2020, an additional 416,686 shares of common stock became available for issuance under the 2019 Plan pursuant to the second closing under our May 2019 securities purchase agreement. Further, on January 1 st of each year, for a period of not more than ten years, beginning on January 1, 2021 and continuing through January 1, 2029, the number of shares authorized for issuance under the 2019 Plan will increase by 5.0% of the total number of shares of our capital stock outstanding on December 31 of the preceding calendar year, or a lesser number of shares as may be determined by our Board of Directors. In addition, on June 13, 2023, our stockholders approved an amendment to the 2019 Plan, which authorized an additional 5,000,000 shares of common stock available for issuance thereunder. As of December 31, 2023, 819,459 shares of common stock were available for new equity award grants under the 2019 Plan and 7,348,950 shares of common stock were reserved for issuance pursuant to equity awards outstanding under the 2019 Plan as of December 31, 2023. 2021 Inducement Plan On November 23, 2021, our Board of Directors adopted the 2021 Inducement Plan (the “Inducement Plan”), which became effective immediately. Stockholder approval of the Inducement Plan was not required pursuant to Rule 5635I(4) of the Nasdaq Listing Rules. The Inducement Plan initially reserved 200,000 shares of common stock and provides for the grant of non-qualified stock options that was used exclusively for grants to individuals that were not previously employees or directors of the Company, as an inducement material to the individual’s entry into employment with the Company. The authorized number of shares available for grant under the Inducement Plan was subsequently increased in October 2022 to 540,000 shares in the aggregate, and subsequently increased again in December 2023 to 1,030,000 shares in the aggregate. Under the Inducement Plan, options are granted with varying vesting terms, but typically vested over four years, with 25% of the total grant vesting on the first anniversary of the effective date of the option grant and the remaining grant vesting monthly thereafter over the following 36 months. As of December 31, 2023, 500,000 shares of common stock were reserved for future issuance under the Inducement Plan and 530,000 shares of common stock were reserved for future issuance pursuant to equity awards outstanding under the Inducement Plan. 2022 Employee Stock Purchase Plan In June 2022, our stockholders approved and we adopted the 2022 Employee Stock Purchase Plan (the “2022 Purchase Plan”), which enables participants to contribute up to 15% of such participant’s eligible compensation during a defined rolling six-month periods to purchase our common stock. The purchase price of common stock under the 2022 Purchase Plan will be the lesser of: (i) 85% of the fair market value of our common stock at the inception of the enrollment period or (ii) 85% of the fair market value of our common stock at the applicable purchase date. The 2022 Purchase Plan supersedes the 2012 Employee Stock Purchase Plan, and no further offerings will be made under the 2012 Employee Stock Purchase Plan. As of December 31, 2023, a maximum of 137,424 shares of our common stock were reserved for future issuance and have been authorized for purchase under the 2022 Purchase Plan. 2023 Private Placement of Common Stock and Non-Voting Preferred Stock On April 13, 2023, we entered into a Securities Purchase Agreement (the “April 2023 SPA”) with certain institutional and other accredited investors (the “2023 Purchasers”), pursuant to which we agreed to sell and issue shares of our common stock and shares of our newly designated non-voting convertible preferred stock in a private placement transaction (the "2023 PIPE"). At the closing under the April 2023 SPA that occurred on April 13, 2023 (the “2023 Closing”), we sold and issued to the 2023 Purchasers (i) 2,615,536 shares of common stock at a purchase price of $0.9001 per share, and (ii) 140,827 shares of non-voting Class A-5 convertible preferred stock, in lieu of shares of common stock, at a price of $90.01 per share. Total gross proceeds from the 2023 Closing were approximately $15.0 million. Each share of non-voting Class A-5 convertible preferred stock is convertible into 100 shares of common stock, subject to certain beneficial ownership conversion limitations. An aggregate of 222,198 shares of common stock were purchased for $0.2 million by a director of the Company at the 2023 Closing. We evaluated the non-voting Class A-5 convertible preferred stock sold in the 2023 PIPE under ASC 480, Distinguishing Liabilities from Equity, and ASC 815, Derivatives and Hedging, and determined permanent equity treatment was appropriate for these freestanding financial instruments and there were no embedded features that required bifurcation. Additional Outstanding Non-Voting Preferred Stock and Warrants In May 2019, we sold and issued (i) 973,045 shares of common stock (ii) 415,898 shares of non-voting Class A-1 convertible preferred stock and (iii) accompanying warrants to purchase an aggregate of 1,388,943 shares of common stock. Each share of non-voting Class A-1 convertible preferred stock is convertible into one share of common stock, subject to certain beneficial ownership conversion limitations. The warrants are exercisable for a period of five years following the date of issuance and have an exercise price of $10.80 per share, subject to proportional adjustments in the event of stock splits or combinations or similar events. The warrants are also exercisable on a net exercise "cashless" basis. In December 2019, we sold and issued 3,288,390 shares of non-voting Class A-2 convertible preferred stock and accompanying warrants to purchase an aggregate of 3,288,390 shares of common stock. Each share of non-voting Class A-2 convertible preferred stock is convertible into one share of common stock, subject to certain beneficial ownership conversion limitations. The warrants will be exercisable for a period of five years following the date of issuance and have an exercise price of $6.66 per share, subject to proportional adjustments in the event of stock splits or combinations or similar events. The warrants are also exercisable on a net exercise “cashless” basis. In December 2020, we sold and issued (i) 2,434,152 shares of common stock (ii) 272,970 shares of non-voting Class A-3 convertible preferred stock and (iii) accompanying warrants to purchase an aggregate of 2,030,341 shares of common stock. Each share of non-voting Class A-3 convertible preferred stock is convertible into one share of common stock, subject to certain beneficial ownership conversion limitations. The warrants are exercisable for a period of five years following the date of issuance and have an exercise price of $7.46 per share, subject to proportional adjustments in the event of stock splits or combinations or similar events. The warrants are also exercisable on a net exercise "cashless" basis. In November 2021, we sold and issued (i) 5,892,335 shares of common stock and (ii) 3,725,720 shares of non-voting Class A-4 convertible preferred stock. Each share of non-voting Class A-4 convertible preferred stock is convertible into one share of common stock, subject to certain beneficial ownership conversion limitations. ATM Offering On December 12, 2018, we entered into a Common Stock Sales Agreement (the “Stock Sales Agreement”) with H.C. Wainwright & Co., LLC (“HCW”), pursuant to which we may sell and issue shares of our common stock from time to time through HCW, as our sales agent (the “ATM Offering”). We have no obligation to sell any shares of common stock in the ATM Offering, and may, at any time, suspend offers under the Stock Sales Agreement or terminate the Stock Sales Agreement. Subject to the terms and conditions of the Stock Sales Agreement, HCW will use its commercially reasonable efforts to sell shares of our common stock from time to time based upon our instructions (including any price, time or size limits or other parameters or conditions that we may impose, subject to certain restrictions). We pay HCW a commission of 3.0% of the gross sales price of any shares sold under the Stock Sales Agreement. On August 10, 2021, we increased the amount of common stock available for sale in the ATM Offering under the Stock Sales Agreement to $50.0 million. A total of 725,192 shares were sold and settled for net proceeds of $1.1 million (net of less than $0.1 million in offering costs) under the ATM Offering during the year ended December 31, 2023. A total of 2,205,100 shares were sold and settled for proceeds of $4.5 million (net of $0.1 million in offering costs) under the ATM Offering during the year ended December 31, 2022. At December 31, 2023, approximately $44.2 million remained eligible to be sold in the ATM Offering, subject to compliance with the rules applicable to sales on Form S-3. Shares Reserved for Future Issuance The following shares of common stock were reserved for future issuance as of December 31, 2023 (in thousands): Class A-1 convertible preferred stock outstanding (as-converted) 257 Class A-2 convertible preferred stock outstanding (as-converted) 1,331 Class A-3 convertible preferred stock outstanding (as-converted) 259 Class A-4 convertible preferred stock outstanding (as-converted) 3,726 Class A-5 convertible preferred stock outstanding (as-converted) 14,083 Warrants to purchase Common Stock 6,186 Common stock options outstanding 6,188 RSUs outstanding 1,161 Common stock available for future grant under the 2019 Equity Incentive Plan 819 Common stock available for future grant under the 2021 Inducement Plan 500 2022 Employee Stock Purchase Plan 137 Total common shares reserved for future issuance 34,647 The following table summarizes our stock option activity under all equity incentive plans for the year ended December 31, 2023 (shares and aggregate intrinsic value in thousands): Number of Weighted Weighted average remaining contractual term Aggregate intrinsic value Stock Options outstanding at December 31, 2022 1,372 $ 7.53 Granted 5,008 $ 1.39 Exercised — $ — Canceled/forfeited/expired (192) $ 6.79 Stock Options outstanding at December 31, 2023 6,188 $ 2.58 9.0 $ 7 Vested and exercisable at December 31, 2023 1,510 $ 5.34 7.9 $ — The weighted average grant date fair value per share of employee stock options granted during the years ended December 31, 2023 and 2022 was $1.09 and $2.00, respectively. There were no stock option exercises during the years ended December 31, 2023 or 2022. The total compensation cost related to stock options not yet recognized was $3.6 million as of December 31, 2023. The weighted-average period over which this expense is expected to be recognized is approximately 1.6 years. The following table summarizes our RSU activity under all equity incentive plans for the year ended December 31, 2023 (shares and aggregate intrinsic value in thousands): Number of Weighted Weighted average remaining contractual term Aggregate intrinsic value RSUs outstanding at December 31, 2022 70 $ 2.57 Granted 1,161 $ 1.38 Vested — $ — Canceled/forfeited/expired (70) $ 2.57 RSUs outstanding at December 31, 2023 1,161 $ 1.38 2.4 $ 1,486 The total compensation cost related to non-vested RSUs not yet recognized was $1.4 million as of December 31, 2023. The weighted-average period over which this expense is expected to be recognized is approximately 2.4 years. Stock-Based Compensation The following table summarizes the weighted average assumptions used to estimate the fair value of stock options and performance stock awards granted to employees under our 2019 Equity Incentive Plan, 2021 Inducement Plan and the shares purchasable under our Employee Stock Purchase Plans during the periods presented: Year ended December 31, 2023 2022 Stock options Risk-free interest rate 4.1 % 2.0 % Volatility 96.4 % 96.1 % Dividend yield — — Expected term (years) 6.1 6.1 Employee stock purchase plan shares Risk-free interest rate 4.9 % 1.6 % Volatility 89.1 % 104.7 % Dividend yield — — Expected term (years) 0.5 0.5 Risk-free interest rate - The risk-free interest rate assumption was based on observed interest rates appropriate for the expected term of the stock option grants. Expected dividend yield - The expected dividend yield assumption was based on the fact that we have never paid cash dividends and have no present intention to pay cash dividends. Expected volatility - The expected volatility assumption was based on the historical volatility of the trading price of our common stock. Expected term - The expected term represents the period of time that options are expected to be outstanding. Because we do not have sufficient historical exercise behavior data, we determine the expected life using the simplified method, which was an average of the contractual term of the option and its ordinary vesting period. Forfeitures - We account for forfeitures as they occur. The following table summarizes the allocation of our stock-based compensation expense for all stock awards during the periods presented (in thousands): Year ended December 31, 2023 2022 Research and development $ 1,029 $ 594 General and administrative 1,772 1,583 Total $ 2,801 $ 2,177

Defined Contribution Plan

Defined Contribution Plan	12 Months Ended
	Dec. 31, 2023
Retirement Benefits [Abstract]
Defined Contribution Plan	Defined Contribution Plan In 2009, we established an employee 401(k) salary deferral plan (“401(k) Plan”) covering all eligible employees. Active employees who are at least 18 years old and are not otherwise disqualified under the terms of the 401(k) Plan are eligible to participate. Employees may contribute up to 50% of their compensation per year (subject to a maximum limit prescribed by federal tax law). Under the 401(k) Plan, we may elect to match a discretionary percentage of employee contribution s. We elected to match 50% of employees’ contributions up to 6% of the employees’ eligible salary for the periods presented. We made matching contributions of $0.2 million and $0.1 million for the years ended December 31, 2023 and 2022, respectively.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2023
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The following table summarizes the components of our income ta x expense (i n thousands): Year ended December 31, 2023 2022 Current: Federal $ — $ — State 1 1 1 1 Deferred: Federal — — State — — — — Income tax expense $ 1 $ 1 The following is a reconciliation of the expected statutory federal income tax provision to our actual income tax provision (in thousands): Year ended December 31, 2023 2022 Expected income tax benefit at federal statutory tax rate $ (6,308) $ (5,948) State income taxes, net of federal benefit 1 1 Tax credits (3,022) (1,850) Change in valuation allowance 8,081 7,327 Return to provision adjustments 39 (18) Stock compensation 755 236 Reserve for uncertain tax positions 453 253 Other 2 — Income tax expense $ 1 $ 1 The following table summarizes the significant components of our deferred tax assets and liabilities (in thousands): December 31, 2023 2022 Deferred tax assets: Net operating loss carryovers $ 92,609 $ 90,085 Research and development and other tax credits 39,532 36,963 Intangibles and property and equipment basis difference 255 496 Section 174 research and development 6,546 3,302 Stock compensation expense 619 818 Lease liability 371 508 Other 748 597 Total deferred tax assets 140,680 132,769 Total deferred tax liabilities (444) (611) Gross deferred tax asset 140,236 132,158 Valuation allowance (140,236) (132,158) Net deferred tax asset $ — $ — For all periods presented, we have determined that it is more likely than not that our deferred tax asset will not be realized. Accordingly, we have recorded a valuation allowance to offset the net deferred tax asset of $140.2 million. The valuation allowance increased by approximately $8.1 million and $7.3 million during the years ended December 31, 2023 and 2022, respectively. As of December 31, 2023, we had NOL carryforwards for U.S. federal and California state tax purposes of $389.5 million and $146.3 million, respectively, portions of which begin to expire in 2030 and 2033, respectively. Our federal NOL carryforwards generated in tax years beginning after December 31, 2017 of $126.1 million will carry forward indefinitely. As of December 31, 2023, we also had federal research and development tax credits, orphan drug credits and California research and development tax credit carryforwards of $13.0 million, $24.3 million and $10.5 million, respectively. The federal research and development tax credit carryforwards will begin to expire in 2029 and the federal orphan drug credits will begin to expire in 2034. The California research and development tax credit carryforwards are available indefinitely. Pursuant to Sections 382 and 383, use of the Company’s net operating loss and credit carryforwards may be limited if a cumulative change in ownership of more than 50% (by value) occurs within a three-year period. The Company has not performed an analysis through December 31, 2023 to determine whether its net operating loss and research and development credit carryforwards are subject to annual limitation under Sections 382 or 383 of the Code, and these financial statements do not contain any adjustment relating to such potential limitations. However, if the Company experienced an ownership change that resulted in an annual limitation on the Company’s net operating loss carryforwards under Section 382 of the Code there would be no material impact to the Company’s financial statements. The following table summarizes the changes in the amount of our unrecognized tax benefits (in thousands): Year Ended December 31, 2023 2022 Beginning balance of unrecognized tax benefits $ 5,785 $ 16,953 Increase (decrease) for prior year tax positions 16 (11,463) Increase for current year tax positions 449 295 Total $ 6,250 $ 5,785 Included in unrecognized tax benefits of $6.3 million at December 31, 2023 was $5.4 million of tax benefits that, if recognized, would reduce our annual effective tax rate, subject to valuation allowance. We do not expect that there will be a significant change in the unrecognized tax benefits over the next 12 months. We are subject to taxation in the United States and state jurisdictions where applicable. Our tax years for 2010 and forward are subject to examination by the U.S. and California tax authorities due to carryforward of unutilized net operating losses and research and development credits. It is our practice to recognize interest and/or penalties related to income tax matters in income tax expense. For the years ended December 31, 2023 and 2022, we have not recognized any interest or penalties related to income taxes.

Leases

Leases	12 Months Ended
	Dec. 31, 2023
Leases [Abstract]
Leases	Leases On February 11, 2021, we entered into a lease agreement (the "Campus Point Lease") with ARE-SD Region No. 61, LLC (as successor in interest to ARE-SD Region No. 58, LLC) ("Campus Point Landlord"), for the lease of approximately 13,438 square feet of rentable area located at 4224 Campus Point Court, Suite 210, San Diego, California, 92121 (the "Campus Point Premises"). The commencement date of the Campus Point Lease was April 15, 2021. However, for accounting purposes the lease commencement date was February 11, 2021. We are using the Campus Point Premises as our principal executive offices and as a laboratory for research and development. The term of the Campus Point Lease (“Campus Point Initial Term”) is 60 months, ending April 30, 2026. The aggregate base rent due over the initial term of the Campus Point Lease is approximately $3.8 million. We are also responsible for the payment of additional amounts to cover our share of the annual operating expenses of the building, the annual tax expenses of the building and the utilities costs for the building. Under the Campus Point Lease, we were required to maintain a deposit of $61,591 in a specially designated bank account, which we recorded as restricted cash on our balance sheet at December 31, 2023 and 2022. The table below summarizes our lease liabilities and corresponding ROU assets as of December 31, 2023 and 2022 (in thousands): Year Ended December 31, 2023 2022 Assets Operating $ 1,477 $ 2,039 Total ROU assets $ 1,477 $ 2,039 Liabilities Current: Operating $ 713 $ 649 Long-term: Operating 1,055 1,768 Total lease liabilities $ 1,768 $ 2,417 The table below summarizes our lease costs from our statement of operations and cash payments from our statement of cash flows during the years ended December 31, 2023 and 2022 (in thousands): Year Ended December 31, 2023 2022 Lease cost: Operating lease cost $ 690 $ 690 Cash payment information: Operating cash used for operating leases $ 776 $ 754 Total cash paid for amounts included in the measurement of lease liabilities $ 776 $ 754 The table below summarizes other non-cash information under our operating lease obligations as of December 31, 2023 and 2022 (in thousands, except years and rates): Year Ended December 31, 2023 2022 Supplemental non-cash information: Operating lease liabilities arising from obtaining right-of-use assets $ — $ — Weighted-average remaining lease term (years) - operating leases 2.3 3.3 Weighted-average discount rate - operating leases 6.0 % 6.0 % We did not have any finance lease obligations as of December 31, 2023 or 2022. Our future lease payments under our operating lease at December 31, 2023 are as follows (in thousands): Operating Leases 2024 800 2025 824 2026 277 2027 — 2028 — Total operating lease payments $ 1,901 Less: amount representing interest (133) Present value of obligations under operating lease 1,768 Less: current portion (713) Long-term operating lease obligations $ 1,055

Subsequent Event

Subsequent Event	12 Months Ended
	Dec. 31, 2023
Subsequent Events [Abstract]
Subsequent Event	Subsequent Event Private Placement of Common Stock and Non-Voting Preferred Stock On March 11, 2024, the Company entered into a Securities Purchase Agreement (the “Purchase Agreement”) with certain institutional and other accredited investors (the “Purchasers”), pursuant to which the Company sold and issued (i) 45,108,667 shares of common stock, at a purchase price of $1.60 per share and (ii) 173,915 shares of the Company’s newly designated non-voting Class A-6 convertible preferred stock, par value $0.001 per share (the “Class A-6 Convertible Preferred Stock”), in lieu of shares of Common Stock, at a purchase price of $160.00 per share in a private placement transaction (the “Private Placement”), which closed on March 14, 2024. The Company received net proceeds of approximately $94.0 million in the Private Placement after deducting placement agent and financial advisor fees and other expenses. Stelios Papadopoulos, Ph.D., the Company’s Chairman of the Board of Directors, is a Purchaser under the Purchase Agreement and purchased 250,000 shares of Common Stock. Each share of Class A-6 Convertible Preferred Stock is convertible into 100 shares of common stock, subject to certain beneficial ownership conversion limitations. In the event of the Company’s liquidation, dissolution or winding up, holders of Class A-6 Convertible Preferred Stock will participate pari passu with any distribution of proceeds to holders of common stock, holders of the Company’s Class A-1 convertible preferred stock, holders of the Company’s Class A-2 convertible preferred stock, holders of the Company’s Class A-3 convertible preferred stock, holders of the Company’s Class A-4 convertible preferred stock, holders of the Company’s Class A-5 convertible preferred stock and the holders of any series of class of the Company’s preferred stock or other capital stock thereafter created ranking on its terms on parity with the Class A-6 Convertible Preferred Stock or the common stock. Holders of Class A-6 Convertible Preferred Stock are entitled to receive dividends on shares of Class A-6 Convertible Preferred Stock equal (on an as converted to common stock basis) to, and in the same form as, dividends actually paid on the common stock. Shares of Class A-6 Convertible Preferred Stock generally have no voting rights, except as required by law.

Pay vs Performance Disclosure

Pay vs Performance Disclosure - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Pay vs Performance Disclosure
Net loss	$ (30,037)	$ (28,323)

Insider Trading Arrangements

Insider Trading Arrangements	3 Months Ended
	Dec. 31, 2023
Trading Arrangements, by Individual
Rule 10b5-1 Arrangement Adopted	false
Non-Rule 10b5-1 Arrangement Adopted	false
Rule 10b5-1 Arrangement Terminated	false
Non-Rule 10b5-1 Arrangement Terminated	false

The Business, Basis of Presen_2

The Business, Basis of Presentation and Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2023
Accounting Policies [Abstract]
Use of Estimates	Use of Estimates Our financial statements are prepared in accordance with GAAP, which requires us to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses and the disclosure of contingent assets and liabilities in our financial statements and accompanying notes. An estimated loss contingency is accrued in our financial statements if it is probable that a liability has been incurred and the amount of the loss can be reasonably estimated. Although these estimates are based on our knowledge of current events and actions we may undertake in the future, actual results may ultimately differ from these estimates and assumptions.
Stock-Based Compensation	Stock-Based Compensation We account for stock-based compensation expense related to stock options granted to employees and members of our board of directors by estimating the fair value of each stock option on the date of grant using the Black-Scholes option pricing model. We recognize stock-based compensation expense using the accelerated multiple-option approach. Under the accelerated multiple-option approach (also known as the graded-vesting method), we recognize compensation expense over the requisite service period for each separately vesting tranche of the award as though the award was in substance multiple awards, resulting in accelerated expense recognition over the vesting period. For performance-based awards granted to employees (i) the fair value of the award is determined on the grant date, (ii) we assess the probability of the individual milestones under the award being achieved and (iii) the fair value of the shares subject to the milestone is expensed over the implicit service period commencing once management believes the performance criteria is probable of being met. We account for restricted stock units by determining the fair value of each restricted stock unit based on the closing market price of our common stock on the date of grant. We recognize stock-based compensation expense using the accelerated multiple-option approach over the requisite service periods of the awards.
Clinical Trial and Preclinical Study Accruals	Clinical Trial and Preclinical Study Accruals We make estimates of our accrued expenses for clinical trial and preclinical study activities as of each balance sheet date in our financial statements based on the facts and circumstances known to us at that time. These accruals are based upon estimates of costs incurred and fees that may be associated with services provided by clinical trial investigational sites and CROs and for other clinical trial-related activities. Payments under certain contracts with such parties depend on factors such as successful enrollment of patients, site initiation and progression through the various stages of our clinical trials. In accruing for these services, we estimate the time period over which services will be performed and the level of effort to be expended in each period. If possible, we obtain information regarding unbilled services directly from these service providers. However, we may be required to estimate these services based on other information available to us. If we underestimate or overestimate the activities or fees associated with a study or service at a given point in time, adjustments to research and development expenses may be necessary in future periods. Historically, our estimated accrued liabilities have approximated actual expense incurred. Subsequent changes in estimates may result in a material change in our accruals.
Prepaid Materials	Prepaid Materials
Research and Development	Research and Development
Income Taxes	Income Taxes Income taxes are accounted for under the asset and liability method. This approach requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of the differences between the tax basis of assets or liabilities and their carrying amounts in the financial statements using the enacted tax rates and laws that are anticipated to be in effect when the differences are expected to reverse. We provide a valuation allowance against net deferred tax assets if it is more likely than not that these items will either expire before we are able to realize their benefit or if future deductibility is uncertain.
Cash and Cash Equivalents	Cash and Cash Equivalents
Concentration of Credit Risk	Concentrations of Credit Risk Financial instruments that potentially subject us to significant concentrations of credit risk consist primarily of cash equivalents an d short-term investments. We maintain deposits in federally insured financial institutions in excess of federally insured limits. We have not experienced any material losses in such accounts and believe we are not exposed to significant risk. We have invested our excess cash primarily in money market funds and U.S. Treasury securities. Additionally, we adhere to established guidelines regarding approved investments and maturities of investments, which are designed to preserve their principal value and maintain liquidity.
Property and Equipment	Property and Equipment three
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets
Segment Reporting	Segment Reporting
Comprehensive Loss	Comprehensive Loss Comprehensive loss is defined as the change in equity during a period from transactions and other events and/or circumstances from non-owner sources. Our only component of other comprehensive loss is unrealized loss on available-for-sale securities. Comprehensive loss has been reflected in the statements of operations and comprehensive loss and as a separate component in the statements of stockholders’ equity for all periods presented.
Leases	Leases At the inception of a contractual arrangement, we determine 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. For operating leases with an initial term greater than 12 months, we recognize operating lease right of use assets ("ROU assets") and operating lease liabilities based on the present value of lease payments over the lease term at the commencement date. Operating lease ROU assets are comprised of the lease liability plus any lease payments made and excludes lease incentives. Lease terms include options to renew or terminate the lease when we are reasonably certain that the renewal option will be exercised or when it is reasonably certain that the termination option will not be exercised. For our operating leases, we generally cannot determine the interest rate implicit in the lease, in which case we use our incremental borrowing rate as the discount rate for the lease. We estimate our incremental borrowing rate for our operating leases based on what we would normally pay to borrow on a collateralized basis over a similar term for an amount equal to the lease payments. Operating lease expense is recognized on a straight-line basis over the lease term. Leases with a lease term of 12 months or less at inception are not recorded on the balance sheet. Instead, we recognize lease expense for these leases on a straight-line basis over the lease term. Our lease agreements do not contain any material variable lease payments, residual value guarantees or restrictive covenants. Certain leases require us to pay taxes, insurance, utilities, and maintenance costs for the building, which do not represent lease components. We elected to not separate lease and non-lease components.
Fair Value of Financial Instruments	Fair Value of Financial Instruments We follow ASC 820-10 issued by the FASB with respect to fair value reporting for financial assets and liabilities. The guidance defines fair value, provides guidance for measuring fair value and requires certain disclosures. The guidance does not apply to measurements related to share-based payments. The guidance discusses valuation techniques such as the market approach (comparable market prices), the income approach (present value of future income or cash flow), and the cost approach (cost to replace the service capacity of an asset or replacement cost). The guidance establishes a fair value hierarchy that prioritizes the inputs to valuation techniques used to measure fair value into three broad levels. Our financial instruments consist of cash and cash equivalents, contract and other receivables, accounts payable, accrued liabilities, and our Term Loan. Fair value estimates of these instruments are made at each reporting period end based on relevant market information. These estimates may be subjective in nature and involve uncertainties and matters of significant judgment and therefore cannot be determined with precision. The carrying amount of cash, cash equivalents, contract and other receivables, accounts payable, and accrued liabilities are generally considered to be representative of their respective fair values because of the short-term nature of those instruments. We believe that the fair value of the Term Loan approximates its carrying value.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments . Subsequently, in November 2018, the FASB issued ASU 2018-19, Codification Improvements to Topic 326, Financial Instruments-Credit Losses . ASU 2016-13 requires entities to measure all expected credit losses for most financial assets held at the reporting date based on an expected loss model which includes historical experience, current conditions, an d reasonable and supportable forecasts. ASU 2016-13 also requires enhanced disclosures to help financial statement users better understand significant estimates and judgments used in estimating credit losses. This ASU is effective for smaller reporting companies for fiscal years beginning after Decem ber 15, 2022, with early adoption permitted. The adoption of this guidance had no impact on our financial statements and disclosures. In March 2020, the FASB issued ASU No. 2020-04, Reference Rate Reform (Topic 848), which provides guidance around reference rate reform initiatives to identify alternative reference rates that are more observable or transaction-based and less susceptible to manipulation in response to concerns about structural risks of interbank offered rates and the risk of cessation of the London Interbank Offered Rate ("LIBOR"). The amendments in the ASU provide option expedients and exceptions for applying GAAP to contracts, hedging relationships and other transactions affected by reference rate reform and apply only if such contracts, hedging relationships and other transactions that reference LIBOR or another reference rate are expected to be discontinued because of reference rate reform. On December 21, 2022, the FASB issued ASU No. 2022-06, Reference Rate Reform (Topic 848): Deferral of the Sunset Date of Topic 848,
Fair Value Measurements	Fair Value Measurements We have certain financial assets recorded at fair value which have been classified as Level 1, 2, or 3 within the fair value hierarchy as described in the accounting standards for fair value measurements. Accounting standards define fair value as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants as of the measurement date. Market participants are buyers and sellers in the principal market that are (i) independent, (ii) knowledgeable, (iii) able to transact, and (iv) willing to transact. The accounting standards provide an established hierarchy for inputs used in measuring fair value that maximizes the use of observable inputs and minimizes the use of unobservable inputs by requiring that the most observable inputs be used when available. Observable inputs are inputs that market participants would use in valuing the asset or liability and are developed based on market data obtained from independent sources. Unobservable inputs are inputs that reflect our assumptions about the factors that market participants would use in valuing the asset or liability. The accounting standards prioritize the inputs used in measuring the fair value into the following hierarchy: • Level 1 includes financial instruments for which quoted market prices for identical instruments are available in active markets. • Level 2 includes financial instruments for which there are inputs other than quoted prices included within Level 1 that are observable for the instrument such as quoted prices for similar instruments in active markets, quoted prices for identical or similar instruments in markets with insufficient volume or infrequent transactions (less active markets) or model-driven valuations in which significant inputs are observable or can be derived principally from, or corroborated by, observable market data. • Level 3 includes financial instruments for which fair value is derived from valuation techniques in which one or more significant inputs are unobservable, including management’s own assumptions.

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2023
Investments, Debt and Equity Securities [Abstract]
Schedule of Debt Securities	Unrealized Maturity (in years) Amortized cost Gains Losses Estimated fair value As of December 31, 2022 U.S. Treasury securities 1 or less $ 14,944 $ — $ (12) $ 14,932 $ 14,944 $ — $ (12) $ 14,932

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2023
Fair Value Disclosures [Abstract]
Fair Value Hierarchy for Assets	The following table presents our fair value hierarchy for assets measured at fair value on a recurring basis as of December 31, 2023 and 2022 (in thousands): Fair value as of December 31, 2023 Total Level 1 Level 2 Level 3 Cash equivalents: Money market funds $ 23,084 $ 23,084 $ — $ — $ 23,084 $ 23,084 $ — $ — Fair value as of December 31, 2022 Total Level 1 Level 2 Level 3 Cash equivalents: Money market funds $ 21,490 $ 21,490 $ — $ — U.S. Treasury securities 14,932 14,932 — — $ 36,422 $ 36,422 $ — $ —

Property and Equipment, net (Ta

Property and Equipment, net (Tables)	12 Months Ended
	Dec. 31, 2023
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment	The following table summarizes our major classes of property and equipment (in thousands): December 31, 2023 2022 Laboratory equipment $ 4,687 $ 4,359 Computer equipment and software 367 470 Furniture and fixtures 12 12 Leasehold improvements 115 115 Construction in progress 126 — 5,307 4,956 Less accumulated depreciation and amortization (4,246) (4,420) Property and equipment, net $ 1,061 $ 536

Intangible Assets, net (Tables)

Intangible Assets, net (Tables)	12 Months Ended
	Dec. 31, 2023
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Finite-Lived Intangible Assets	The following table summarizes our major classes of intangible assets (in thousands): December 31, 2023 2022 Patents $ 133 $ 183 Accumulated amortization - Patents (100) (121) Intangibles, net $ 33 $ 62

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2023
Equity [Abstract]
Common Stock Reserved for Future Issuance	The following shares of common stock were reserved for future issuance as of December 31, 2023 (in thousands): Class A-1 convertible preferred stock outstanding (as-converted) 257 Class A-2 convertible preferred stock outstanding (as-converted) 1,331 Class A-3 convertible preferred stock outstanding (as-converted) 259 Class A-4 convertible preferred stock outstanding (as-converted) 3,726 Class A-5 convertible preferred stock outstanding (as-converted) 14,083 Warrants to purchase Common Stock 6,186 Common stock options outstanding 6,188 RSUs outstanding 1,161 Common stock available for future grant under the 2019 Equity Incentive Plan 819 Common stock available for future grant under the 2021 Inducement Plan 500 2022 Employee Stock Purchase Plan 137 Total common shares reserved for future issuance 34,647
Stock Option Activity	The following table summarizes our stock option activity under all equity incentive plans for the year ended December 31, 2023 (shares and aggregate intrinsic value in thousands): Number of Weighted Weighted average remaining contractual term Aggregate intrinsic value Stock Options outstanding at December 31, 2022 1,372 $ 7.53 Granted 5,008 $ 1.39 Exercised — $ — Canceled/forfeited/expired (192) $ 6.79 Stock Options outstanding at December 31, 2023 6,188 $ 2.58 9.0 $ 7 Vested and exercisable at December 31, 2023 1,510 $ 5.34 7.9 $ —
Schedule of Restricted Stock Units Activity	The following table summarizes our RSU activity under all equity incentive plans for the year ended December 31, 2023 (shares and aggregate intrinsic value in thousands): Number of Weighted Weighted average remaining contractual term Aggregate intrinsic value RSUs outstanding at December 31, 2022 70 $ 2.57 Granted 1,161 $ 1.38 Vested — $ — Canceled/forfeited/expired (70) $ 2.57 RSUs outstanding at December 31, 2023 1,161 $ 1.38 2.4 $ 1,486
Assumptions Used to Estimate Fair Value of Stock Options and Performance Stock and Employee Stock Purchase Plan	The following table summarizes the weighted average assumptions used to estimate the fair value of stock options and performance stock awards granted to employees under our 2019 Equity Incentive Plan, 2021 Inducement Plan and the shares purchasable under our Employee Stock Purchase Plans during the periods presented: Year ended December 31, 2023 2022 Stock options Risk-free interest rate 4.1 % 2.0 % Volatility 96.4 % 96.1 % Dividend yield — — Expected term (years) 6.1 6.1 Employee stock purchase plan shares Risk-free interest rate 4.9 % 1.6 % Volatility 89.1 % 104.7 % Dividend yield — — Expected term (years) 0.5 0.5
Stock-Based Compensation	The following table summarizes the allocation of our stock-based compensation expense for all stock awards during the periods presented (in thousands): Year ended December 31, 2023 2022 Research and development $ 1,029 $ 594 General and administrative 1,772 1,583 Total $ 2,801 $ 2,177

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2023
Income Tax Disclosure [Abstract]
Schedule of Components of Income Tax Expense	The following table summarizes the components of our income ta x expense (i n thousands): Year ended December 31, 2023 2022 Current: Federal $ — $ — State 1 1 1 1 Deferred: Federal — — State — — — — Income tax expense $ 1 $ 1
Schedule of Effective Income Tax Rate Reconciliation	The following is a reconciliation of the expected statutory federal income tax provision to our actual income tax provision (in thousands): Year ended December 31, 2023 2022 Expected income tax benefit at federal statutory tax rate $ (6,308) $ (5,948) State income taxes, net of federal benefit 1 1 Tax credits (3,022) (1,850) Change in valuation allowance 8,081 7,327 Return to provision adjustments 39 (18) Stock compensation 755 236 Reserve for uncertain tax positions 453 253 Other 2 — Income tax expense $ 1 $ 1
Schedule of Deferred Tax Assets and Liabilities	The following table summarizes the significant components of our deferred tax assets and liabilities (in thousands): December 31, 2023 2022 Deferred tax assets: Net operating loss carryovers $ 92,609 $ 90,085 Research and development and other tax credits 39,532 36,963 Intangibles and property and equipment basis difference 255 496 Section 174 research and development 6,546 3,302 Stock compensation expense 619 818 Lease liability 371 508 Other 748 597 Total deferred tax assets 140,680 132,769 Total deferred tax liabilities (444) (611) Gross deferred tax asset 140,236 132,158 Valuation allowance (140,236) (132,158) Net deferred tax asset $ — $ —
Summary of Income Tax Contingencies	The following table summarizes the changes in the amount of our unrecognized tax benefits (in thousands): Year Ended December 31, 2023 2022 Beginning balance of unrecognized tax benefits $ 5,785 $ 16,953 Increase (decrease) for prior year tax positions 16 (11,463) Increase for current year tax positions 449 295 Total $ 6,250 $ 5,785

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2023
Leases [Abstract]
Schedule of Lease Liabilities and ROU Assets	The table below summarizes our lease liabilities and corresponding ROU assets as of December 31, 2023 and 2022 (in thousands): Year Ended December 31, 2023 2022 Assets Operating $ 1,477 $ 2,039 Total ROU assets $ 1,477 $ 2,039 Liabilities Current: Operating $ 713 $ 649 Long-term: Operating 1,055 1,768 Total lease liabilities $ 1,768 $ 2,417
Schedule of Lease Cost	The table below summarizes our lease costs from our statement of operations and cash payments from our statement of cash flows during the years ended December 31, 2023 and 2022 (in thousands): Year Ended December 31, 2023 2022 Lease cost: Operating lease cost $ 690 $ 690 Cash payment information: Operating cash used for operating leases $ 776 $ 754 Total cash paid for amounts included in the measurement of lease liabilities $ 776 $ 754 The table below summarizes other non-cash information under our operating lease obligations as of December 31, 2023 and 2022 (in thousands, except years and rates): Year Ended December 31, 2023 2022 Supplemental non-cash information: Operating lease liabilities arising from obtaining right-of-use assets $ — $ — Weighted-average remaining lease term (years) - operating leases 2.3 3.3 Weighted-average discount rate - operating leases 6.0 % 6.0 %
Schedule of Operating Lease, Liability, Maturity	Our future lease payments under our operating lease at December 31, 2023 are as follows (in thousands): Operating Leases 2024 800 2025 824 2026 277 2027 — 2028 — Total operating lease payments $ 1,901 Less: amount representing interest (133) Present value of obligations under operating lease 1,768 Less: current portion (713) Long-term operating lease obligations $ 1,055

The Business, Basis of Presen_3

The Business, Basis of Presentation and Summary of Significant Accounting Policies - Liquidity (Details) $ in Millions		12 Months Ended
	Jun. 24, 2022	Dec. 31, 2023 USD ($)
Accounting Policies [Abstract]
Reverse stock split ratio	0.1
Financing from sale proceeds, debt, and agreement revenues		$ 557.7
Cash, cash equivalents and short-term investments		$ 23.8

The Business, Basis of Presen_4

The Business, Basis of Presentation and Summary of Significant Accounting Policies - Property and Equipment (Details)	Dec. 31, 2023
Minimum
Property, Plant and Equipment [Line Items]
Useful life, in years	3 years
Maximum
Property, Plant and Equipment [Line Items]
Useful life, in years	5 years

The Business, Basis of Presen_5

The Business, Basis of Presentation and Summary of Significant Accounting Policies - Impairment of Long-Lived Assets (Details) - USD ($)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Accounting Policies [Abstract]
Asset impairment charges	$ 0	$ 0

The Business, Basis of Presen_6

The Business, Basis of Presentation and Summary of Significant Accounting Policies - Segment Reporting (Details)	12 Months Ended
	Dec. 31, 2023 segment
Accounting Policies [Abstract]
Number of operating segments	1

Net Loss Per Share (Details)

Net Loss Per Share (Details) - shares	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Earnings Per Share [Abstract]
Potentially dilutive securities not included in calculation of diluted net loss per share (in shares)	29,216,399	13,200,906

Investments (Details)

Investments (Details)	12 Months Ended
	Dec. 31, 2023 USD ($)
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 14,944,000
Unrealized gains	0
Unrealized losses	(12,000)
Estimated fair value	14,932,000
Unrealized gain (loss)	0
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	14,944,000
Unrealized gains	0
Unrealized losses	(12,000)
Estimated fair value	$ 14,932,000

Fair Value Measurements (Detail

Fair Value Measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	$ 23,084	$ 36,422
Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	23,084	21,490
U.S. Treasury securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure		14,932
Fair Value, Inputs, Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	23,084	36,422
Fair Value, Inputs, Level 1 | Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	23,084	21,490
Fair Value, Inputs, Level 1 | U.S. Treasury securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure		14,932
Fair Value, Inputs, Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	0	0
Fair Value, Inputs, Level 2 | Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	0	0
Fair Value, Inputs, Level 2 | U.S. Treasury securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure		0
Fair Value, Inputs, Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	0	0
Fair Value, Inputs, Level 3 | Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure	$ 0	0
Fair Value, Inputs, Level 3 | U.S. Treasury securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, fair value disclosure		$ 0

Collaborations (Details)

Collaborations (Details)	Jan. 06, 2023
Sanofi
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Period for termination	30 days

Property and Equipment, net - M

Property and Equipment, net - Major Classes of Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 5,307	$ 4,956
Less accumulated depreciation and amortization	(4,246)	(4,420)
Property and equipment, net	1,061	536
Laboratory equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	4,687	4,359
Computer equipment and software
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	367	470
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	12	12
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	115	115
Construction in progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 126	$ 0

Property and Equipment, net - N

Property and Equipment, net - Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Property, Plant and Equipment [Abstract]
Depreciation and amortization of property and equipment	$ 0.2	$ 0.1

Intangible Assets, net - Major

Intangible Assets, net - Major Classes of Intangible Assets (Details) - Patents - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	$ 133	$ 183
Accumulated amortization - Patents	(100)	(121)
Intangibles, net	$ 33	$ 62

Intangible Assets, net - Narrat

Intangible Assets, net - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible asset amortization (less than)	$ 0.1	$ 0.1
Finite-lived intangible assets, amortization expense, next 12 months (less than $0.1 million)	0.1
Finite-lived intangible assets, amortization expense, year two (less than $0.1 million)	0.1
Finite-lived intangible assets, amortization expense, year three (less than $0.1 million)	0.1
Finite-lived intangible assets, amortization expense, year four (less than $0.1 million)	0.1
Finite-lived intangible assets, amortization expense, year five (less than $0.1 million)	$ 0.1
Remaining amortization period	11 years 4 months 24 days

Debt (Details)

Debt (Details) $ in Millions			12 Months Ended	32 Months Ended
	Jun. 22, 2016 USD ($)	Jun. 17, 2016 payment	Dec. 31, 2023 USD ($)	May 31, 2020 amendment
Debt Instrument [Line Items]
Number of times agreement was amended | amendment				10
Term Loan
Debt Instrument [Line Items]
Proceeds from borrowing under term loan	$ 20
Stated interest rate, percentage		8.51%
Number of monthly payments | payment		24
Long-term debt			$ 1.4
Other long-term liabilities			1.3
Deferred finance costs			$ 0.1
Debt instrument effective rate			8.98%
Term Loan | Other Current Liabilities
Debt Instrument [Line Items]
Fee amount			$ 1.3
Term Loan | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate, percentage		0.44%
Term Loan | SOFR | Option One
Debt Instrument [Line Items]
Basis spread on variable rate, percentage			8.95%
Term Loan | SOFR | Option Two, First Contributor
Debt Instrument [Line Items]
Basis spread on variable rate, percentage			0.10%
Term Loan | SOFR | Option Two, Second Contributor
Debt Instrument [Line Items]
Basis spread on variable rate, percentage			8.51%
Eleventh Amendment
Debt Instrument [Line Items]
Cash collateral for borrowed securities			$ 5
Fee amount			$ 0.3

Stockholders' Equity - Narrativ

Stockholders' Equity - Narrative (Details) $ / shares in Units, $ in Millions

1 Months Ended

Jun. 13, 2023 shares

Apr. 13, 2023 USD ($) $ / shares shares

Jun. 24, 2022

Nov. 23, 2021 shares

Jan. 22, 2020 shares

Jun. 15, 2019 shares

Jun. 30, 2022

Nov. 30, 2021 shares

Dec. 31, 2020 $ / shares shares

Dec. 31, 2019 $ / shares shares

May 31, 2019 $ / shares shares

Dec. 31, 2023 vote shares

Dec. 31, 2022 shares

Oct. 31, 2022 shares

Dec. 31, 2021 shares

Subsidiary, Sale of Stock [Line Items]

Common stock, shares outstanding (in shares)

20,222,672

16,840,261

Common stock voting rights per share | vote

1

Reverse stock split ratio

0.1

Number of years for increase of authorized shares

10 years

Total common shares reserved for future issuance (in shares)

34,647,000

Common stock

Subsidiary, Sale of Stock [Line Items]

Common stock, shares outstanding (in shares)

20,222,672

16,840,261

14,597,118

2023 Closing

Subsidiary, Sale of Stock [Line Items]

Consideration received on transaction | $

$ 15

2023 Closing | Common stock

Subsidiary, Sale of Stock [Line Items]

Number of shares issued (in shares)

2,615,536

Sale of stock (in dollars per share) | $ / shares

$ 0.9001

Convertible preferred stock, shares issued upon conversion (in shares)

100

Private Placement

Subsidiary, Sale of Stock [Line Items]

Number of shares issued (in shares)

5,892,335

2,434,152

973,045

Nonvoting convertible preferred stock issued (in shares)

415,898

Number of securities called by warrants (in shares)

2,030,341

Warrant exercise period

5 years

Class A-5 convertible preferred stock outstanding (as-converted) | 2023 Closing

Subsidiary, Sale of Stock [Line Items]

Number of shares issued (in shares)

140,827

Sale of stock (in dollars per share) | $ / shares

$ 90.01

Class A-1 Convertible Preferred Stock | Private Placement

Subsidiary, Sale of Stock [Line Items]

Number of securities called by warrants (in shares)

1,388,943

Preferred Class A | Private Placement

Subsidiary, Sale of Stock [Line Items]

Sale of stock (in dollars per share) | $ / shares

$ 10.80

Convertible preferred stock, shares issued upon conversion (in shares)

1

Class A-2 Convertible Preferred Stock | Milestone Closing

Subsidiary, Sale of Stock [Line Items]

Number of shares issued (in shares)

3,288,390

Convertible preferred stock, shares issued upon conversion (in shares)

1

Number of securities called by warrants (in shares)

3,288,390

Warrant exercise period

5 years

Warrant exercise price (in dollars per share) | $ / shares

$ 6.66

Class A-3 Convertible Preferred Stock | Private Placement

Subsidiary, Sale of Stock [Line Items]

Convertible preferred stock, shares issued upon conversion (in shares)

1

1

Nonvoting convertible preferred stock issued (in shares)

3,725,720

272,970

Warrant exercise period

5 years

Warrant exercise price (in dollars per share) | $ / shares

$ 7.46

Director | 2023 Closing

Subsidiary, Sale of Stock [Line Items]

Number of shares issued (in shares)

222,198

Director | Class A-4 convertible preferred stock outstanding (as-converted) | 2023 Closing

Subsidiary, Sale of Stock [Line Items]

Consideration received on transaction | $

$ 0.2

2019 Plan

Subsidiary, Sale of Stock [Line Items]

Additional shares available (in shares)

5,000,000

416,686

Yearly authorized increase

5%

Shares available for future grant (in shares)

819,459

Total common shares reserved for future issuance (in shares)

7,348,950

Prior Plans

Subsidiary, Sale of Stock [Line Items]

Granted (in shares)

0

Inducement Plan

Subsidiary, Sale of Stock [Line Items]

Shares available for future grant (in shares)

500,000

Total common shares reserved for future issuance (in shares)

200,000

1,030,000

540,000

Vesting period

4 years

Inducement Plan | Pursuant to Equity Awards Outstanding

Subsidiary, Sale of Stock [Line Items]

Total common shares reserved for future issuance (in shares)

530,000

Inducement Plan | First anniversary of the effective date

Subsidiary, Sale of Stock [Line Items]

Award vesting percentage

25%

Inducement Plan | Remaining 75%, vesting monthly

Subsidiary, Sale of Stock [Line Items]

Vesting period for remaining vesting rights

36 months

2022 Employee Stock Purchase Plan | 2022 Employee Stock Purchase Plan

Subsidiary, Sale of Stock [Line Items]

Share-based compensation arrangement by share-based payment award, discount from market price, purchase date

15%

Share-based compensation arrangement by share-based payment award, purchase price of common stock, percent

85%

Share-based compensation arrangement by share-based payment award, number of shares authorized (in shares)

137,424

Stockholders' Equity - ATM Offe

Stockholders' Equity - ATM Offering (Details) - At The Moment - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Aug. 10, 2021	Dec. 12, 2018
Class of Stock [Line Items]
Commission fee rate				3%
Amount eligible to be drawn down under the ATM	$ 44.2		$ 50
Number of shares issued (in shares)	725,192	2,205,100
Consideration received on transaction	$ 1.1	$ 4.5
Payments for commissions	$ 0.1	$ 0.1

Stockholders' Equity - Shares R

Stockholders' Equity - Shares Reserved for Future Issuance (Details) shares in Thousands	Dec. 31, 2023 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock options outstanding (in shares)	6,188
Total common shares reserved for future issuance (in shares)	34,647
Common stock available for future grant under the 2019 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future grant (in shares)	819
Common stock available for future grant under the 2021 Inducement Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future grant (in shares)	500
2022 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future grant (in shares)	137
RSUs outstanding
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
RSUs outstanding (in shares)	1,161
Warrants to purchase Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Warrants outstanding (in shares)	6,186
Class A-1 convertible preferred stock outstanding (as-converted)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Preferred stock, shares outstanding (in shares)	257
Class A-2 convertible preferred stock outstanding (as-converted)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Preferred stock, shares outstanding (in shares)	1,331
Class A-3 convertible preferred stock outstanding (as-converted)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Preferred stock, shares outstanding (in shares)	259
Class A-4 convertible preferred stock outstanding (as-converted)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Preferred stock, shares outstanding (in shares)	3,726
Class A-5 convertible preferred stock outstanding (as-converted)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Preferred stock, shares outstanding (in shares)	14,083

Stockholders' Equity - Stock Op

Stockholders' Equity - Stock Option Activity (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2023 USD ($) $ / shares shares
Number of options
Options outstanding, ending period (in shares)	6,188,000
Stock options
Number of options
Options outstanding, beginning period (in shares)	1,372,000
Granted (in shares)	5,008,000
Exercised (options) or Vested (RSUs) (in shares)	0
Canceled/forfeited/expired (in shares)	(192,000)
Options outstanding, ending period (in shares)	6,188,000
Vested and Exercisable (in shares)	1,510,000
Weighted average exercise price
Options outstanding, beginning period (in dollars per share) | $ / shares	$ 7.53
Granted (in dollars per share) | $ / shares	1.39
Exercised (options) or Vested (RSUs) (in dollars per share) | $ / shares	0
Canceled/forfeited/expired (in dollars per share) | $ / shares	6.79
Options outstanding, ending period (in dollars per share) | $ / shares	2.58
Exercisable (in dollars per share) | $ / shares	$ 5.34
Weighted average remaining contractual term
Options outstanding	9 years
Exercisable	7 years 10 months 24 days
Aggregate intrinsic value
Options outstanding | $	$ 7
Exercisable | $	$ 0

Stockholders' Equity - Share Ac

Stockholders' Equity - Share Activity and Employee Stock Purchase Plan Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Stock options
Class of Stock [Line Items]
Weighted average grant date fair value of employee stock options (in dollars per share)	$ 1.09	$ 2
Issuance of common stock upon exercise of options (in shares)	0
Compensation cost related to non-vested awards not yet recognized	$ 3.6
Weighted-average recognition period of compensation cost related to non-vested awards not yet recognized	1 year 7 months 6 days
RSUs outstanding
Class of Stock [Line Items]
Compensation cost related to non-vested awards not yet recognized	$ 1.4
Weighted-average recognition period of compensation cost related to non-vested awards not yet recognized	2 years 4 months 24 days

Stockholders' Equity - RSU Acti

Stockholders' Equity - RSU Activity (Details) - RSUs outstanding $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2023 USD ($) $ / shares shares
Number of options
RSUs outstanding, beginning balance (in shares) | shares	70
Granted (in shares) | shares	1,161
Vested (in shares) | shares	0
Canceled/forfeited/expired (in shares) | shares	(70)
RSUs outstanding, ending balance (in shares) | shares	1,161
Weighted average grant date fair value
RSUs outstanding, beginning balance (in dollars per share) | $ / shares	$ 2.57
Granted (in dollars per share) | $ / shares	1.38
Vested (in dollars per share) | $ / shares	0
Forfeited (in dollars per share) | $ / shares	2.57
RSUs outstanding, ending balance (in dollars per share) | $ / shares	$ 1.38
Weighted average remaining contractual term	2 years 4 months 24 days
Aggregate intrinsic value | $	$ 1,486

Stockholders' Equity - Assumpti

Stockholders' Equity - Assumptions Used to Estimate Fair Value of Stock Options and Performance Stock and Employee Stock Purchase Plan (Details)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Stock options
Weighted average assumptions
Risk-free interest rate	4.10%	2%
Volatility	96.40%	96.10%
Dividend yield	0%	0%
Expected term (years)	6 years 1 month 6 days	6 years 1 month 6 days
Employee stock purchase plan shares
Weighted average assumptions
Risk-free interest rate	4.90%	1.60%
Volatility	89.10%	104.70%
Dividend yield	0%	0%
Expected term (years)	6 months	6 months

Stockholders' Equity - Schedule

Stockholders' Equity - Schedule of Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expenses	$ 2,801	$ 2,177
Research and development
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expenses	1,029	594
General and administrative
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expenses	$ 1,772	$ 1,583

Defined Contribution Plan (Deta

Defined Contribution Plan (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Retirement Benefits [Abstract]
Defined contribution minimum age requirement of eligible employees	18 years
Maximum 401 (K) plan contribution rates as percentage of employees compensation	50%
Rate of contribution to 401(K) plan as a percentage of employees contribution	50%
Percentage of employee's earnings the company may contribute to the 401(k) plan	6%
Employer contribution to 401(k) plan	$ 0.2	$ 0.1

Income Taxes - Components of In

Income Taxes - Components of Income Tax Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Current:
Federal	$ 0	$ 0
State	1	1
Current income tax expense (benefit), total	1	1
Deferred:
Federal	0	0
State	0	0
Deferred income tax expense (benefit), total	0	0
Income tax expense	$ 1	$ 1

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Expected Statutory Federal Income Tax Provision to Actual Income Tax Provision (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Expected income tax benefit at federal statutory tax rate	$ (6,308)	$ (5,948)
State income taxes, net of federal benefit	1	1
Tax credits	(3,022)	(1,850)
Change in valuation allowance	8,081	7,327
Return to provision adjustments	39	(18)
Stock compensation	755	236
Reserve for uncertain tax positions	453	253
Other	2	0
Income tax expense	$ 1	$ 1

Income Taxes - Components of De

Income Taxes - Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Deferred tax assets:
Net operating loss carryovers	$ 92,609	$ 90,085
Research and development and other tax credits	39,532	36,963
Intangibles and property and equipment basis difference	255	496
Section 174 research and development	6,546	3,302
Stock compensation expense	619	818
Lease liability	371	508
Other	748	597
Total deferred tax assets	140,680	132,769
Total deferred tax liabilities	(444)	(611)
Gross deferred tax asset	140,236	132,158
Valuation allowance	(140,236)	(132,158)
Net deferred tax asset	$ 0	$ 0

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Operating Loss Carryforwards [Line Items]
Valuation allowance	$ 140,236,000	$ 132,158,000
Valuation allowance increased amount	8,100,000	7,300,000
Net operating loss carryovers	92,609,000	90,085,000
Unrecognized tax benefits	6,250,000	5,785,000	$ 16,953,000
Unrecognized tax benefits that would impact effective tax rate	5,400,000
Income tax penalties and interest accrued	0	$ 0
Internal Revenue Service (IRS)
Operating Loss Carryforwards [Line Items]
Net operating loss carryovers	389,500,000
Internal Revenue Service (IRS) | Research Tax Credit Carryforward
Operating Loss Carryforwards [Line Items]
Research and development tax credits carryforward	13,000,000
Internal Revenue Service (IRS) | Orphan Drug Tax Credit Carryforward
Operating Loss Carryforwards [Line Items]
Research and development tax credits carryforward	24,300,000
California
Operating Loss Carryforwards [Line Items]
Net operating loss carryovers	146,300,000
California | Research Tax Credit Carryforward
Operating Loss Carryforwards [Line Items]
Research and development tax credits carryforward	10,500,000
After 2017, No Expiration | Internal Revenue Service (IRS)
Operating Loss Carryforwards [Line Items]
Net operating loss carryovers	$ 126,100,000

Income Taxes - Changes in Amoun

Income Taxes - Changes in Amount of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning balance of unrecognized tax benefits	$ 5,785	$ 16,953
Increase for prior year tax positions	16
Decrease for prior year tax positions		(11,463)
Increase for current year tax positions	449	295
Total	$ 6,250	$ 5,785

Leases - Narrative (Details)

Leases - Narrative (Details)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Feb. 11, 2021 USD ($) ft²
Lessee, Lease, Description [Line Items]
Operating lease payments due	$ 1,901,000
Restricted cash	62,000	$ 62,000
Finance lease liability	0	0
Campus Point Lease
Lessee, Lease, Description [Line Items]
Rented area | ft²			13,438
Term of lease			60 months
Operating lease payments due			$ 3,800,000
Restricted cash	$ 61,591	$ 61,591

Leases - Lease Asset and Liabil

Leases - Lease Asset and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Assets
Operating	$ 1,477	$ 2,039
Current:
Operating	713	649
Long-term:
Operating	1,055	1,768
Total lease liabilities	$ 1,768	$ 2,417
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	Other current liabilities	Other current liabilities
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	Operating	Operating

Leases - Lease Cost (Details)

Leases - Lease Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Leases [Abstract]
Operating lease cost	$ 690	$ 690
Cash payment information:
Cash payment information:	776	754
Total cash paid for amounts included in the measurement of lease liabilities	776	754
Operating lease liabilities arising from obtaining right-of-use assets	$ 0	$ 0
Weighted-average remaining lease term (years) - operating leases	2 years 3 months 18 days	3 years 3 months 18 days
Weighted-average discount rate - operating leases	6%	6%

Leases - Future Lease Payments

Leases - Future Lease Payments Under Operating and Finance Leases (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Operating Leases
2024	$ 800
2025	824
2026	277
2027	0
2028	0
Total operating lease payments	1,901
Less: amount representing interest	(133)
Total lease liabilities	1,768	$ 2,417
Less: current portion	(713)	(649)
Long-term operating lease obligations	$ 1,055	$ 1,768

Subsequent Event (Details)

Subsequent Event (Details) - USD ($) $ / shares in Units, $ in Millions	Mar. 11, 2024	Apr. 13, 2023
2023 Closing
Subsequent Event [Line Items]
Consideration received on transaction		$ 15
Common stock | 2023 Closing
Subsequent Event [Line Items]
Number of shares issued (in shares)		2,615,536
Sale of stock (in dollars per share)		$ 0.9001
Subsequent Event
Subsequent Event [Line Items]
Sale of stock (in dollars per share)	$ 160
Consideration received on transaction	$ 94
Subsequent Event | Board of Directors Chairman
Subsequent Event [Line Items]
Number of shares issued (in shares)	250,000
Subsequent Event | Class A-6 Convertible Preferred Stock
Subsequent Event [Line Items]
Preferred stock, convertible (in shares)	100
Subsequent Event | 2023 Closing | Class A-6 Convertible Preferred Stock
Subsequent Event [Line Items]
Number of shares issued (in shares)	173,915
Sale of stock (in dollars per share)	$ 0.001
Subsequent Event | Common stock | 2023 Closing
Subsequent Event [Line Items]
Number of shares issued (in shares)	45,108,667
Sale of stock (in dollars per share)	$ 1.60