Annexon (ANNX) 10-Q 13 May 24 2024 Q1 Quarterly report Financial data

Cover Page

Cover Page - shares	3 Months Ended
	Mar. 31, 2024	May 08, 2024
Document Information [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Transition Report	false
Amendment Flag	false
Document Period End Date	Mar. 31, 2024
Document Fiscal Year Focus	2024
Document Fiscal Period Focus	Q1
Trading Symbol	ANNX
Entity Registrant Name	ANNEXON, INC.
Entity Central Index Key	0001528115
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Title of 12(b) Security	Common Stock, par value $0.001 per share
Security Exchange Name	NASDAQ
Entity File Number	001-39402
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	27-5414423
Entity Address, Address Line One	1400 Sierra Point Parkway
Entity Address, Address Line Two	Bldg C
Entity Address, Address Line Three	Suite 200
Entity Address, City or Town	Brisbane
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94005
City Area Code	650
Local Phone Number	822-5500
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
Common Stock [Member]
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		92,412,866
Prefunded Warrant [Member]
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		34,283,052

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Current assets:
Cash and cash equivalents	$ 151,941	$ 225,110
Short-term investments	113,007	34,606
Prepaid expenses and other current assets	5,792	4,144
Total current assets	270,740	263,860
Restricted cash	1,032	1,032
Property and equipment, net	14,235	14,773
Operating lease right-of-use assets	17,701	18,009
Other non-current assets	361
Total assets	304,069	297,674
Current liabilities:
Accounts payable	3,972	5,487
Accrued liabilities	6,857	10,235
Operating lease liabilities, current	2,254	2,165
Other current liabilities	50	41
Total current liabilities	13,133	17,928
Operating lease liabilities, non-current	28,531	29,190
Total liabilities	41,664	47,118
Commitments and contingencies (Note 5)
Stockholders’ equity:
Common stock	90	78
Additional paid-in capital	860,092	823,029
Accumulated other comprehensive loss	(102)	(52)
Accumulated deficit	(597,675)	(572,499)
Total stockholders’ equity	262,405	250,556
Total liabilities and stockholders’ equity	$ 304,069	$ 297,674

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Operating expenses:
Research and development	$ 20,963	$ 32,345
General and administrative	7,609	8,897
Total operating expenses	28,572	41,242
Loss from operations	(28,572)	(41,242)
Interest and other income, net	3,396	2,566
Net loss	$ (25,176)	$ (38,676)
Net loss per share, basic	$ (0.21)	$ (0.52)
Net loss per share, diluted	$ (0.21)	$ (0.52)
Weighted-average shares used in computing net loss per share, basic	122,673,202	73,855,642
Weighted-average shares used in computing net loss per share, diluted	122,673,202	73,855,642

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Statement of Comprehensive Income [Abstract]
Net loss	$ (25,176)	$ (38,676)
Other comprehensive income (loss):
Foreign currency translation adjustment	(12)	(5)
Unrealized (loss) gain on available-for-sale securities	(38)	160
Comprehensive loss	$ (25,226)	$ (38,521)

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Related Party [Member]	Common Stock [Member]	Common Stock [Member] Related Party [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] Related Party [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
Beginning Balance, Values at Dec. 31, 2022	$ 231,194		$ 48		$ 669,780		$ (372)	$ (438,262)
Beginning Balance, Shares at Dec. 31, 2022			47,722,995
Exercise of stock options	111				111
Exercise of stock options, Shares			55,605
Exercise of pre-funded warrants, Value			$ 3		(3)
Exercise of pre-funded warrants, Shares			2,582,557
Issuance of common stock, net of issuance costs, Value		$ 17,470		$ 2		$ 17,468
Issuance of common stock, net of issuance costs, Shares				2,646,458
Restricted stock vested in the period			73,058
Stock-based compensation	4,607				4,607
Other comprehensive income (loss)	155						155
Net loss	(38,676)							(38,676)
Ending Balance, Values at Mar. 31, 2023	214,861		$ 53		691,963		(217)	(476,938)
Ending Balance, Shares at Mar. 31, 2023			53,080,673
Beginning Balance, Values at Dec. 31, 2023	250,556		$ 78		823,029		(52)	(572,499)
Beginning Balance, Shares at Dec. 31, 2023			78,369,099
Exercise of stock options	$ 217				217
Exercise of stock options, Shares	105,526		105,526
Exercise of pre-funded warrants, Value			$ 5		(5)
Exercise of pre-funded warrants, Shares			5,243,400
Issuance of common stock, net of issuance costs, Value	$ 32,198		$ 7		32,191
Issuance of common stock, net of issuance costs, Shares			6,639,348
Restricted stock vested in the period			124,695
Stock-based compensation	4,660				4,660
Other comprehensive income (loss)	(50)						(50)
Net loss	(25,176)							(25,176)
Ending Balance, Values at Mar. 31, 2024	$ 262,405		$ 90		$ 860,092		$ (102)	$ (597,675)
Ending Balance, Shares at Mar. 31, 2024			90,482,068

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Stock issuance costs	$ 933
Related Party [Member]
Stock issuance costs		$ 525

Condensed Consolidated Statem_5

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Operating activities:
Net loss	$ (25,176)	$ (38,676)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	538	536
Accretion of discount on available-for-sale securities	(1,137)	(902)
Stock-based compensation	4,660	4,607
Reduction in the carrying amount of right-of-use assets	308	264
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,629)	1,070
Other non-current assets	(361)	(12)
Accounts payable	(1,531)	3,035
Accrued liabilities	(3,378)	(2,771)
Operating lease liabilities	(570)	(348)
Other current liabilities	9	204
Net cash used in operating activities	(28,267)	(32,993)
Investing activities:
Purchases of property and equipment		(137)
Purchases of available-for-sale securities	(100,302)	(31,435)
Proceeds from maturities of available-for-sale securities	23,000	51,089
Net cash (used in) provided by investing activities	(77,302)	19,517
Financing activities:
Proceeds from the exercise of common stock options	198	111
Proceeds from the issuance of common stock, including related party of zero and $17,995 for the three months ended March 31, 2024 and 2023, respectively	33,131	17,995
Payment of financing costs	(917)	(525)
Net cash provided by financing activities	32,412	17,581
(Decrease) increase in cash, cash equivalents and restricted cash	(73,157)	4,105
Effect of exchange rate changes on cash, cash equivalents and restricted cash	(12)	(5)
Cash, cash equivalents and restricted cash
Beginning of period	226,142	141,052	$ 141,052
End of period	152,973	145,152	$ 226,142
Supplemental disclosure of cash flow information:
Cash paid for amounts included in the measurement of lease liability	1,216	1,175
Non-cash investing and financing activities:
Deferred offering costs included in accounts payable and accrued liabilities	$ 117
Purchases of property and equipment included in accounts payable and accrued liabilities		$ 5

Condensed Consolidated Statem_6

Condensed Consolidated Statements of Cash Flows (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Net Cash Provided by (Used in) Financing Activities [Abstract]
Proceeds from the issuance of common stock including related party	$ 0	$ 17,995

Organization

Organization	3 Months Ended
	Mar. 31, 2024
Accounting Policies [Abstract]
Organization	1. Organization Annexon, Inc., or the Company, is a clinical-stage biopharmaceutical company pioneering a new class of complement medicines for people living with devasting inflammatory-related diseases. The Company is located in Brisbane, California and was incorporated in Delaware in March 2011. The Company’s wholly-owned subsidiary, Annexon Biosciences Australia Pty Ltd, or the Subsidiary, is a proprietary limited company incorporated in 2016 and domiciled in Australia. Liquidity Since inception, the Company has been involved primarily in performing research and development activities, conducting clinical trials, hiring personnel, and raising capital to support and expand these activities. The Company has experienced losses and negative cash flows from operations since its inception and, as of March 31, 2024, had an accumulated deficit of $ 597.7 million and cash and cash equivalents and short-term investments of $ 264.9 million. The Company has historically funded its operations through the issuance of shares of its common stock and warrants. Based on projected activities, management projects that existing cash and cash equivalents and short-term investments will enable the Company to fund its operating expenses and capital expenditure requirements for at least twelve months from the date of issuance of these financial statements. The Company’s future viability beyond that point is dependent on its ability to achieve development milestones and obtain additional funding. Management expects to continue to incur losses and negative cash flows from operations for at least the next several years. There are uncertainties associated with the Company’s ability to (1) obtain additional equity or debt financing on terms that are favorable to the Company, (2) enter into collaborative agreements with strategic partners, and (3) succeed in its future operations. If the Company is not able to obtain the required funding for its operations or is not able to obtain funding on terms that are favorable to the Company, it could be forced to delay, reduce or eliminate its research and development programs and its business could be materially harmed.

Basis of Presentation and Signi

Basis of Presentation and Significant Accounting Policies	3 Months Ended
	Mar. 31, 2024
Accounting Policies [Abstract]
Basis of Presentation and Significant Accounting Policies	2. Basis of Presentation and Significant Accounting Policies Basis of Presentation The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States, or GAAP, and applicable rules and regulations of the SEC regarding interim financial reporting. The condensed consolidated balance sheet as of March 31, 2024, the condensed consolidated statements of operations, comprehensive loss, stockholders’ equity for the three months ended March 31, 2024 and 2023 and the condensed consolidated statements of cash flows for the three months ended March 31, 2024 and 2023 are unaudited. These unaudited condensed consolidated financial statements have been prepared on the same basis as the annual consolidated financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary to present fairly the Company’s consolidated financial position, results of operations and cash flows for the interim period presented. The financial data and the other financial information contained in these notes to the condensed consolidated financial statements related to the three-month periods are also unaudited. The results of operations for the three months ended March 31, 2024 are not necessarily indicative of the results to be expected for the year ending December 31, 2024 or for any other future annual or interim period. The condensed consolidated balance sheet as of December 31, 2023 included herein was derived from the audited financial statements as of that date. These unaudited condensed consolidated financial statements should be read in conjunction with the Company's audited consolidated financial statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2023, filed with the SEC on March 26, 2024. Use of Estimates The preparation of financial statements in conformity with accounting principles generally accepted in the United States of America, or GAAP, requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported expenses during the reporting period. Management evaluates its estimates, including but not limited to the fair value of investments, operating lease right-of-use assets and liabilities, valuation of deferred tax assets and uncertain tax positions (including valuation allowance), clinical trial accruals and stock-based compensation. The Company evaluates its estimates and assumptions on an ongoing basis using historical experience and other factors and adjusts those estimates and assumptions when facts and circumstances dictate. Actual results could differ from those estimates. Principles of Consolidation The condensed consolidated financial statements include the operations of Annexon, Inc. and its wholly-owned subsidiary and include the results of operations and cash flows of these entities. All intercompany balances and transactions have been eliminated in consolidation. Summary of Significant Accounting Policies Reference is made to Note 2, Summary of Significant Accounting Policies, in the Company’s 2023 Form 10-K filed on March 26, 2024 for a detailed description of significant accounting policies. There have been no significant changes to the Company’s accounting policies as disclosed in its 2023 Form 10-K. Recently Issued Accounting Pronouncements In November 2023, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) 2023-07, Segment Reporting: Improvements to Reportable Segment Disclosures , which requires disclosure of incremental segment information on an interim and annual basis. This ASU is effective for fiscal years beginning after December 15, 2023, and interim periods within fiscal periods beginning after December 15, 2024, and requires retrospective application to all prior periods presented in the financial statements. The Company adopted annual requirements under ASU 2023-07 on January 1, 2024 and plans to adopt interim requirements under ASU 2023-07 on January 1, 2025. The Company will begin including financial statement disclosures in accordance with ASU 2023-07 in its Annual Report on Form 10-K for the year ended December 31, 2024. The Company is evaluating the impact of this guidance on its financial statements and related disclosures. In December 2023, the FASB issued ASU 2023-09, Income Taxes: Improvements to Income Tax Disclosures , which requires enhanced annual disclosures regarding the rate reconciliation and income taxes paid information. ASU 2023-09 is effective for fiscal years beginning after December 15, 2024 and may be adopted on a prospective or retrospective basis. Early adoption is permitted. The Company is evaluating the impact of this guidance on its financial statements and related disclosures.

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2024
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements The Company utilizes valuation techniques that maximize the use of observable inputs and minimize the use of unobservable inputs to the extent possible. The Company determines fair value based on assumptions that market participants would use in pricing an asset or liability in the principal or most advantageous market. When considering market participant assumptions in fair value measurements, the following fair value hierarchy distinguishes between observable and unobservable inputs, which are categorized in one of the following levels: • Level 1 Inputs: Unadjusted quoted prices in active markets for identical assets or liabilities accessible to the reporting entity at the measurement date. • Level 2 Inputs: Other than quoted prices included in Level 1 inputs that are observable for the asset or liability, either directly or indirectly, for substantially the full term of the asset or liability. • Level 3 Inputs: Unobservable inputs for the asset or liability used to measure fair value to the extent that observable inputs are not available, thereby allowing for situations in which there is little, if any, market activity for the asset or liability at measurement date. On a recurring basis, the Company measures certain financial assets and liabilities at fair value. The following tables summarize the fair value of the Company’s financial assets measured at fair value on a recurring basis by level within the fair value hierarchy (in thousands): March 31, 2024 Valuation Amortized Gross Gross Aggregate Assets: Cash equivalents: Money market funds Level 1 $ 40,150 $ — $ — $ 40,150 Commercial paper Level 2 13,980 — — 13,980 Government bonds Level 2 86,360 1 ( 1 ) 86,360 Total cash equivalents 140,490 1 ( 1 ) 140,490 Short-term investments: Government bonds Level 2 113,025 1 ( 19 ) 113,007 Total short-term investments 113,025 1 ( 19 ) 113,007 $ 253,515 $ 2 $ ( 20 ) $ 253,497 December 31, 2023 Valuation Amortized Gross Gross Aggregate Assets: Cash equivalents: Money market funds Level 1 $ 143,933 $ — $ — $ 143,933 Government bonds Level 2 72,689 — — 72,689 Total cash equivalents 216,622 — — 216,622 Short-term investments: Government bonds Level 2 34,596 10 — 34,606 Total short-term investments 34,596 10 — 34,606 $ 251,218 $ 10 $ — $ 251,228 For the three months ended March 31, 2024 and 2023, the Company recognized no material realized gains or losses on financial instruments.

Balance Sheet Components

Balance Sheet Components	3 Months Ended
	Mar. 31, 2024
Disclosure Of Balance Sheet Components [Abstract]
Balance Sheet Components	4. Balance Sheet Components Cash, Cash Equivalents and Restricted Cash The Company considers all highly liquid instruments with an original maturity of three months or less at time of purchase to be cash equivalents. Cash equivalents, which include amounts invested in money market funds, are stated at fair value. Restricted cash as of March 31, 2024 relates to the letters of credit established for the Company’s office leases. The following table provides a reconciliation of cash, cash equivalents and restricted cash reported within the condensed consolidated balance sheets that sum to the total of the same amounts shown in the condensed consolidated statements of cash flows (in thousands): March 31, December 31, Cash $ 11,451 $ 8,488 Cash equivalents 140,490 216,622 Cash and cash equivalents 151,941 225,110 Restricted cash 1,032 1,032 Cash, cash equivalents and restricted cash $ 152,973 $ 226,142 Prepaid Expenses and Other Current Assets Prepaid expenses and other current assets consisted of the following (in thousands): March 31, December 31, Prepaid research and development costs $ 4,643 $ 2,617 Prepaid insurance 418 704 Other prepaid expenses 708 760 Other current assets 23 63 Total prepaid expenses and other current assets $ 5,792 $ 4,144 Property and Equipment, Net Property and equipment, net, consisted of the following (in thousands): March 31, December 31, Leasehold improvements $ 17,246 $ 17,245 Laboratory equipment 1,832 1,832 Furniture and fixtures 692 692 Computer equipment and software 33 33 Total property and equipment, gross 19,803 19,802 Less: accumulated depreciation ( 5,568 ) ( 5,029 ) Total property and equipment, net $ 14,235 $ 14,773 The Company recognized depreciation for property and equipment of $ 0.5 million for each of the three months ended March 31, 2024 and 2023. Accrued Liabilities Accrued liabilities consisted of the following (in thousands): March 31, December 31, Accrued research and development expenses $ 4,527 $ 4,027 Accrued compensation 1,643 5,607 Accrued professional services 570 501 Other accrued expenses 117 100 Total accrued liabilities $ 6,857 $ 10,235

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2024
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	5. Commitments and Contingencies Leases The Company leases its offices and laboratory in Brisbane, California, or the Brisbane Lease, under a ten-year noncancelable lease agreement that ends in October 2031 with a ten-year renewable option. In November 2021, the Company subleased unoccupied space from December 2021 through November 2023 , for aggregate sublease payments of $ 3.4 million. The sublease income, while it reduces the rent expense, is not considered in the value of the right-of-use assets or lease liabilities. The Company’s sublease income was zero and $ 0.4 million for the three months ended March 31, 2024 and 2023, respectively. As of March 31, 2024, the operating lease right-of-use assets were $ 17.7 million and lease liabilities were $ 30.8 million on the condensed consolidated balance sheet. The weighted-average remaining lease term is 7.6 years. The weighted-average incremental borrowing rate used to measure the operating lease liability is 8.4 %. Operating lease costs were $ 1.0 million and $ 0.5 million for the three months ended March 31, 2024 and 2023, respectively. Variable lease payments were $ 0.5 million and $ 0.4 million for the three months ended March 31, 2024 and 2023, respectively. Future minimum lease payments and related lease liabilities as of March 31, 2024, were as follows: (in thousands) 2024 (remaining nine months) $ 3,677 2025 5,065 2026 5,242 2027 5,425 2028 and thereafter 22,600 Total undiscounted lease payments 42,009 Less: Imputed interest ( 11,224 ) Total $ 30,785 Guarantees and Indemnifications In the normal course of business, the Company enters into agreements that contain a variety of representations and provide for general indemnification. The Company’s exposure under these agreements is unknown because it involves claims that may be made against the Company in the future. To date, the Company has no t paid any claims or been required to defend any action related to its indemnification obligations. As of March 31, 2024, the Company did no t have any material indemnification claims that were probable or reasonably possible and consequently has not recorded related liabilities.

Stockholder's Equity

Stockholder's Equity	3 Months Ended
	Mar. 31, 2024
Stockholders' Equity Note [Abstract]
Stockholders' Equity	6. Stockholders’ Equity 2023 Financing In December 2023, the Company raised net proceeds of approximately $ 117.0 million after deducting underwriting discounts and offering expenses through the sale of 25,035,000 shares of the Company’s common stock, par value $ 0.001 per share at a price of $ 2.880 per share and pre-funded warrants to purchase an aggregate of 18,379,861 shares of common stock at a price of $ 2.879 per share, which equals the per share offering price for the shares of common stock less the $ 0.001 exercise price for each pre-funded warrant. An entity related to one of the Company’s directors participated in the public offering and purchased 350,000 shares of common stock for an aggregate price of approximately $ 1.0 million. The pre-funded warrants are immediately exercisable, subject to certain beneficial ownership limitations. The warrants meet the criteria for equity classification and were therefore recorded at fair value as of the grant date as a component of stockholders’ equity within additional paid-in capital in the amount of $ 52.9 million. In February 2024, the Company issued an aggregate of 5,243,400 shares of common stock upon the cashless exercise of pre-funded warrants to purchase 5,244,444 shares of common stock. In April 2024, the Company issued an aggregate of 965,427 shares of common stock upon the exercise of pre-funded warrants. As of March 31, 2024, and as of the date of issuance of these interim condensed financial statements, pre-funded warrants to purchase up to 13,135,417 shares and 12,169,990 shares of common stock remained outstanding from the 2023 financing, respectively. 2022 Financing In July 2022, the Company raised net proceeds of approximately $ 122.5 million after deducting fees and expenses through the sale of an aggregate of 9,013,834 shares of common stock, pre-funded warrants to purchase up to 24,696,206 shares of its common stock and accompanying common warrants to purchase up to 8,427,508 shares of its common stock. The offering price per share and accompanying common warrant was $ 3.87125 per share and the offering price per pre-funded warrant and accompanying common warrant was $ 3.87025 per share, which equals the per share offering price for the shares of common stock less the $ 0.001 exercise price for each such pre-funded warrant. The pre-funded warrants remain exercisable until exercised in full. The common warrants have an exercise price of $ 5.806875 per share and expire on June 30, 2025 . Both the pre-funded and common warrants are immediately exercisable, subject to beneficial ownership limitations. The warrants meet the criteria for equity classification and were therefore recorded at fair value as of the grant date as a component of stockholders’ equity within additional paid-in capital. In March 2023, the Company issued an aggregate of 2,582,557 shares of common stock upon the cashless exercise of pre-funded warrants to purchase 2,583,144 shares of common stock. As of March 31, 2024, pre-funded warrants to purchase up to 22,113,062 shares of common stock and common warrants to purchase up to 8,427,508 shares of common stock remained outstanding from the 2022 financing. 2021 At-the-Market (ATM) Program In August 2021, the Company entered into a sales agreement with Cowen and Company LLC, or TD Cowen, as sales agent, pursuant to which the Company may issue and sell shares of its common stock for an aggregate maximum offering of $ 100.0 million under an at-the-market offering program, or 2021 ATM program. TD Cowen is entitled to compensation up to 3 % of the aggregate gross proceeds for the common stock sold through the 2021 ATM program. During 2023, the Company sold 2,646,458 shares of common stock at a price of $ 6.80 per share under the 2021 ATM program for net proceeds of approximately $ 17.5 million after deducting commissions paid to TD Cowen. During the three months ended March 31, 2024, the Company sold 6,639,348 shares of common stock under the 2021 ATM program for net proceeds of approximately $ 32.2 million. As of March 31, 2024, approximately $ 48.9 million remained available for the offer and sale of shares of common stock under the 2021 ATM program. Subsequent to March 31, 2024 and through the date of issuance of these interim condensed financial statements, net proceeds of approximately $ 6.4 million were raised as a result of the sale of shares of the Company’s common stock through the 2021 ATM program. 2024 ATM Program In March 2024, the Company entered into a sales agreement with TD Cowen, as sales agent, or 2024 ATM program, pursuant to which the Company may issue and sell shares of its common stock for an aggregate maximum offering of $ 100.0 million. TD Cowen is entitled to compensation up to 3 % of the aggregate gross proceeds for the common stock sold through the 2024 ATM program. As of March 31, 2024, the Company has not made any sales under the 2024 ATM program. Common Stock The Company reserved the following shares of common stock for issuance as follows: March 31, December 31, Stock options issued and outstanding 11,859,362 9,208,970 Stock options reserved for 2020 Incentive Award Plan 1,762,992 1,988,340 Unvested restricted stock units outstanding 915,861 495,579 Common stock reserved for 2021 ATM program 25,980,123 2,619,471 Common stock reserved for 2024 ATM program 25,000,000 — Common stock reserved for Employee Stock Purchase 2,122,071 1,338,381 Common stock reserved for 2022 Employment Inducement 1,667,300 758,084 Common stock reserved for pre-funded warrants 35,248,479 40,493,510 Common stock reserved for common warrants 8,427,508 8,427,508 Total common stock reserved 112,983,696 65,329,843

Equity Incentive Plans

Equity Incentive Plans	3 Months Ended
	Mar. 31, 2024
Share-Based Payment Arrangement [Abstract]
Equity Incentive Plans	7. Equity Incentive Plans In July 2020, the Company’s board of directors and stockholders adopted and approved the 2020 Incentive Award Plan, or the 2020 Plan, and the Employee Stock Purchase Plan, or the ESPP, which became effective in connection with the Company’s initial public offering, or the IPO. The Company may not grant any additional awards under the 2011 Equity Incentive Plan, or the 2011 Plan. The 2011 Plan will continue to govern outstanding equity awards granted thereunder. 2020 Equity Incentive Plan The number of shares of common stock reserved for issuance under the 2020 Plan automatically increases on the first day of January, in an amount equal to 4 % of the total number of shares of the Company’s capital stock outstanding on the last day of the preceding year, or a lesser number of shares determined by the Company’s board of directors. Awards granted under the 2020 Plan expire no later than ten years from the date of grant. For Incentive Stock Options, or ISOs, and Nonstatutory Stock Options, or NSOs, the option price shall not be less than 100 % of the estimated fair value on the date of grant. Options granted typically vest over a four-year period but may be granted with different vesting terms. As of March 31, 2024, there were 1,762,992 shares available for issuance under the 2020 Plan. 2022 Employment Inducement Award Plan In July 2022, the Company’s board of directors adopted the Annexon, Inc. 2022 Employment Inducement Award Plan, or the Inducement Plan, and together with the 2011 Plan and the 2020 Plan, the Plans. The Inducement Plan was adopted by the Company’s board of directors without stockholder approval pursuant to Nasdaq Marketplace Rule 5635(c)(4), or Rule 5635(c)(4). In accordance with Rule 5635(c)(4), awards made under the Inducement Plan may only be granted to newly hired employees as an inducement material to the employees entering into employment with the Company. Awards granted under the Inducement Plan expire no later than ten years from the date of grant. An aggregate of 2,850,000 shares of common stock were reserved for issuance under the Inducement Plan. As of March 31, 2024, there were 1,667,300 shares available for issuance under the Inducement Plan. Stock Options The following table presents stock option activity under the Plans for the period: Number of Weighted- Weighted- Aggregate Balances as of December 31, 2023 9,208,970 $ 10.31 7.53 $ 2,930 Stock options granted 3,121,592 $ 5.13 Stock options exercised ( 105,526 ) $ 2.06 Stock options forfeited ( 365,674 ) $ 9.15 Balances as of March 31, 2024 11,859,362 $ 9.05 7.91 $ 18,184 Exercisable as of March 31, 2024 5,469,398 $ 12.11 6.49 $ 5,860 The total intrinsic value of options exercised was $ 0.3 million for each of the three months ended March 31, 2024 and 2023. The intrinsic value is the difference between the fair value of the Company’s common stock at the time of exercise and the exercise price of the stock option. The weighted-average grant date fair value of options granted to employees during the three months ended March 31, 2024 and 2023 was $ 4.06 and $ 4.35 per share, respectively. As of March 31, 2024, the total unrecognized stock-based compensation cost related to outstanding unvested stock options was $ 29.8 million, which the Company expects to recognize over an estimated weighted-average period of 2.6 years. Restricted Stock Units RSUs are share awards that entitle the holder to receive freely tradeable shares of the Company’s common stock upon vesting. The RSUs cannot be transferred and the awards are subject to forfeiture if the holder’s employment terminates prior to the release of the vesting restrictions. The RSUs generally vest over a three-year period in equal amounts on an annual basis, provided the employee remains continuously employed with the Company. The fair value of the RSUs is equal to the closing price of the Company’s common stock on the grant date. A summary of RSU activity under the Company’s equity incentive plan and related information is as follows: Number of Shares Weighted-Average Grant Date Fair Value Per Share Unvested as of December 31, 2023 495,579 $ 5.69 Granted 602,600 5.13 Vested ( 124,695 ) 6.44 Cancelled ( 57,623 ) 5.33 Unvested as of March 31, 2024 915,861 $ 5.24 As of March 31, 2024, unrecognized stock-based compensation expense related to outstanding unvested RSUs was $ 4.3 million, which is expected to be recognized over a weighted-average period of 2.4 years. Employee Stock Purchase Plan The ESPP enables eligible employees to purchase shares of the Company’s common stock at the end of each offering period at a price equal to 85 % of the fair market value of the shares on the first business day or the last business day of the offering period, whichever is lower. Eligible employees generally include all employees. Share purchases are funded through payroll deductions of at least 1 %, and up to 15 % of an employee’s eligible compensation for each payroll period. The number of shares reserved for issuance under the ESPP increase automatically on the first day of each fiscal year, by a number equal to, 1 % of the shares of common stock outstanding on the last day of the immediately preceding fiscal year, or such number of shares determined by the Company’s board of directors. As of March 31, 2024, 2,122,071 shares were available for future purchase. The ESPP generally provides for six-month consecutive offering periods beginning on May 16 th and November 16 th of each year. The ESPP is a compensatory plan as defined by the authoritative guidance for stock compensation. As such, stock-based compensation expense has been recorded for the three months ended March 31, 2024. The stock-based compensation expense related to the ESPP was $ 0.1 million for each of the three months ended March 31, 2024 and 2023. Stock-Based Compensation Expense The total stock-based compensation expense recognized was as follows (in thousands): Three Months Ended 2024 2023 Research and development $ 2,282 $ 2,251 General and administrative 2,378 2,356 Total stock-based compensation expense $ 4,660 $ 4,607 To determine the value of stock option awards for stock-based compensation purposes, the Company uses the Black-Scholes option pricing model and the assumptions discussed below. Each of these inputs is subjective and generally requires significant judgment. Fair Value of Common Stock —The fair value of each share of underlying common stock is based on the closing price of the Company’s common stock as reported on the date of grant on the Nasdaq Global Select Market. Expected Term —The expected term represents the period that the stock-based awards are expected to be outstanding and is determined using the simplified method (based on the mid-point between the vesting date and the end of the contractual term). The Company continues using the simplified method as it does not have sufficient historical exercise data to provide a reasonable basis upon which to estimate expected term due to the limited period of time its equity shares have been publicly traded. Expected Volatility —Beginning in 2024, the expected volatility was estimated based on a weighted volatility using both the Company’s trading history for its common stock and the average volatility for comparable publicly traded life sciences companies over a period equal to the expected term of the stock option grants. Prior to 2024, the expected volatility was estimated based on the average volatility for comparable publicly traded life sciences companies over a period equal to the expected term of the stock option grants. The comparable companies were chosen based on the similar size, stage in life cycle or area of specialty. Risk-Free Interest Rate —The risk-free interest rate is based on the U.S. Treasury zero coupon issues in effect at the time of grant for periods corresponding with the expected term of the option. Dividend Yield —The Company has never paid dividends on its common stock and has no plans to pay dividends on its common stock. Therefore, the Company used an expected dividend yield of zero. The fair value of each stock option issued was estimated on the date of grant using the Black-Scholes option pricing model with the following assumptions: Three Months Ended 2024 2023 Expected term (in years) 6.02 - 6.08 6.02 - 6.08 Expected volatility 95.90 % - 96.20 % 81.20 % - 81.50 % Risk-free interest rate 4.29 % - 4.33 % 3.58 % - 4.02 % Dividend yield — —

Net Loss Per Share

Net Loss Per Share	3 Months Ended
	Mar. 31, 2024
Earnings Per Share [Abstract]
Net Loss Per Share	8. Net Loss Per Share The Company calculates basic net loss per share by dividing net loss by the weighted-average number of shares of common stock outstanding, excluding restricted common stock. The weighted-average number of shares of common stock used in the basic and diluted net loss per share calculation include pre-funded warrants to purchase up to 35,248,479 shares of common stock, as the pre-funded warrants are exercisable at any time for nominal cash consideration. The Company has generated a net loss in all periods presented, so the basic and diluted net loss per share are the same, as the inclusion of the potentially dilutive securities would be anti-dilutive. The following outstanding potentially dilutive shares have been excluded from the calculation of diluted net loss per share due to their anti-dilutive effect: Three Months Ended 2024 2023 Stock options to purchase common stock 11,859,362 9,389,106 Shares subject to Employee Stock Purchase Plan 79,748 87,983 Unvested restricted stock units 915,861 678,996 Common warrants 8,427,508 8,427,508 Total 21,282,479 18,583,593

Basis of Presentation and Sig_2

Basis of Presentation and Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2024
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States, or GAAP, and applicable rules and regulations of the SEC regarding interim financial reporting. The condensed consolidated balance sheet as of March 31, 2024, the condensed consolidated statements of operations, comprehensive loss, stockholders’ equity for the three months ended March 31, 2024 and 2023 and the condensed consolidated statements of cash flows for the three months ended March 31, 2024 and 2023 are unaudited. These unaudited condensed consolidated financial statements have been prepared on the same basis as the annual consolidated financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary to present fairly the Company’s consolidated financial position, results of operations and cash flows for the interim period presented. The financial data and the other financial information contained in these notes to the condensed consolidated financial statements related to the three-month periods are also unaudited. The results of operations for the three months ended March 31, 2024 are not necessarily indicative of the results to be expected for the year ending December 31, 2024 or for any other future annual or interim period. The condensed consolidated balance sheet as of December 31, 2023 included herein was derived from the audited financial statements as of that date. These unaudited condensed consolidated financial statements should be read in conjunction with the Company's audited consolidated financial statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2023, filed with the SEC on March 26, 2024.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with accounting principles generally accepted in the United States of America, or GAAP, requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported expenses during the reporting period. Management evaluates its estimates, including but not limited to the fair value of investments, operating lease right-of-use assets and liabilities, valuation of deferred tax assets and uncertain tax positions (including valuation allowance), clinical trial accruals and stock-based compensation. The Company evaluates its estimates and assumptions on an ongoing basis using historical experience and other factors and adjusts those estimates and assumptions when facts and circumstances dictate. Actual results could differ from those estimates.
Principles of Consolidation	Principles of Consolidation The condensed consolidated financial statements include the operations of Annexon, Inc. and its wholly-owned subsidiary and include the results of operations and cash flows of these entities. All intercompany balances and transactions have been eliminated in consolidation.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In November 2023, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) 2023-07, Segment Reporting: Improvements to Reportable Segment Disclosures , which requires disclosure of incremental segment information on an interim and annual basis. This ASU is effective for fiscal years beginning after December 15, 2023, and interim periods within fiscal periods beginning after December 15, 2024, and requires retrospective application to all prior periods presented in the financial statements. The Company adopted annual requirements under ASU 2023-07 on January 1, 2024 and plans to adopt interim requirements under ASU 2023-07 on January 1, 2025. The Company will begin including financial statement disclosures in accordance with ASU 2023-07 in its Annual Report on Form 10-K for the year ended December 31, 2024. The Company is evaluating the impact of this guidance on its financial statements and related disclosures. In December 2023, the FASB issued ASU 2023-09, Income Taxes: Improvements to Income Tax Disclosures , which requires enhanced annual disclosures regarding the rate reconciliation and income taxes paid information. ASU 2023-09 is effective for fiscal years beginning after December 15, 2024 and may be adopted on a prospective or retrospective basis. Early adoption is permitted. The Company is evaluating the impact of this guidance on its financial statements and related disclosures.

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2024
Fair Value Disclosures [Abstract]
Summary of Financial Assets Measured at Fair Value on a Recurring Basis	The following tables summarize the fair value of the Company’s financial assets measured at fair value on a recurring basis by level within the fair value hierarchy (in thousands): March 31, 2024 Valuation Amortized Gross Gross Aggregate Assets: Cash equivalents: Money market funds Level 1 $ 40,150 $ — $ — $ 40,150 Commercial paper Level 2 13,980 — — 13,980 Government bonds Level 2 86,360 1 ( 1 ) 86,360 Total cash equivalents 140,490 1 ( 1 ) 140,490 Short-term investments: Government bonds Level 2 113,025 1 ( 19 ) 113,007 Total short-term investments 113,025 1 ( 19 ) 113,007 $ 253,515 $ 2 $ ( 20 ) $ 253,497 December 31, 2023 Valuation Amortized Gross Gross Aggregate Assets: Cash equivalents: Money market funds Level 1 $ 143,933 $ — $ — $ 143,933 Government bonds Level 2 72,689 — — 72,689 Total cash equivalents 216,622 — — 216,622 Short-term investments: Government bonds Level 2 34,596 10 — 34,606 Total short-term investments 34,596 10 — 34,606 $ 251,218 $ 10 $ — $ 251,228

Balance Sheet Components (Table

Balance Sheet Components (Tables)	3 Months Ended
	Mar. 31, 2024
Disclosure Of Balance Sheet Components [Abstract]
Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash	The following table provides a reconciliation of cash, cash equivalents and restricted cash reported within the condensed consolidated balance sheets that sum to the total of the same amounts shown in the condensed consolidated statements of cash flows (in thousands): March 31, December 31, Cash $ 11,451 $ 8,488 Cash equivalents 140,490 216,622 Cash and cash equivalents 151,941 225,110 Restricted cash 1,032 1,032 Cash, cash equivalents and restricted cash $ 152,973 $ 226,142
Summary of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets consisted of the following (in thousands): March 31, December 31, Prepaid research and development costs $ 4,643 $ 2,617 Prepaid insurance 418 704 Other prepaid expenses 708 760 Other current assets 23 63 Total prepaid expenses and other current assets $ 5,792 $ 4,144
Summary of Property and Equipment, Net	Property and equipment, net, consisted of the following (in thousands): March 31, December 31, Leasehold improvements $ 17,246 $ 17,245 Laboratory equipment 1,832 1,832 Furniture and fixtures 692 692 Computer equipment and software 33 33 Total property and equipment, gross 19,803 19,802 Less: accumulated depreciation ( 5,568 ) ( 5,029 ) Total property and equipment, net $ 14,235 $ 14,773
Summary of Accrued Liabilities	Accrued liabilities consisted of the following (in thousands): March 31, December 31, Accrued research and development expenses $ 4,527 $ 4,027 Accrued compensation 1,643 5,607 Accrued professional services 570 501 Other accrued expenses 117 100 Total accrued liabilities $ 6,857 $ 10,235

Commitments and Contingencies (

Commitments and Contingencies (Tables)	3 Months Ended
	Mar. 31, 2024
Commitments and Contingencies Disclosure [Abstract]
Future Minimum Lease Payments and Related Lease Liabilities	Future minimum lease payments and related lease liabilities as of March 31, 2024, were as follows: (in thousands) 2024 (remaining nine months) $ 3,677 2025 5,065 2026 5,242 2027 5,425 2028 and thereafter 22,600 Total undiscounted lease payments 42,009 Less: Imputed interest ( 11,224 ) Total $ 30,785

Stockholder's Equity (Tables)

Stockholder's Equity (Tables)	3 Months Ended
	Mar. 31, 2024
Stockholders' Equity Note [Abstract]
Schedule of Common Stock Reserved for Future Issuance	The Company reserved the following shares of common stock for issuance as follows: March 31, December 31, Stock options issued and outstanding 11,859,362 9,208,970 Stock options reserved for 2020 Incentive Award Plan 1,762,992 1,988,340 Unvested restricted stock units outstanding 915,861 495,579 Common stock reserved for 2021 ATM program 25,980,123 2,619,471 Common stock reserved for 2024 ATM program 25,000,000 — Common stock reserved for Employee Stock Purchase 2,122,071 1,338,381 Common stock reserved for 2022 Employment Inducement 1,667,300 758,084 Common stock reserved for pre-funded warrants 35,248,479 40,493,510 Common stock reserved for common warrants 8,427,508 8,427,508 Total common stock reserved 112,983,696 65,329,843

Equity Incentive Plans (Tables)

Equity Incentive Plans (Tables)	3 Months Ended
	Mar. 31, 2024
Share-Based Payment Arrangement [Abstract]
Summary of Stock Option Activity	The following table presents stock option activity under the Plans for the period: Number of Weighted- Weighted- Aggregate Balances as of December 31, 2023 9,208,970 $ 10.31 7.53 $ 2,930 Stock options granted 3,121,592 $ 5.13 Stock options exercised ( 105,526 ) $ 2.06 Stock options forfeited ( 365,674 ) $ 9.15 Balances as of March 31, 2024 11,859,362 $ 9.05 7.91 $ 18,184 Exercisable as of March 31, 2024 5,469,398 $ 12.11 6.49 $ 5,860
Summary of RSU Activity under 2020 Plan	A summary of RSU activity under the Company’s equity incentive plan and related information is as follows: Number of Shares Weighted-Average Grant Date Fair Value Per Share Unvested as of December 31, 2023 495,579 $ 5.69 Granted 602,600 5.13 Vested ( 124,695 ) 6.44 Cancelled ( 57,623 ) 5.33 Unvested as of March 31, 2024 915,861 $ 5.24
Summary of Stock-Based Compensation Expense	The total stock-based compensation expense recognized was as follows (in thousands): Three Months Ended 2024 2023 Research and development $ 2,282 $ 2,251 General and administrative 2,378 2,356 Total stock-based compensation expense $ 4,660 $ 4,607
Summary of Fair Value of Each Award Issued Estimated on the Date of Grant Using the Black-Scholes Option Pricing Model	The fair value of each stock option issued was estimated on the date of grant using the Black-Scholes option pricing model with the following assumptions: Three Months Ended 2024 2023 Expected term (in years) 6.02 - 6.08 6.02 - 6.08 Expected volatility 95.90 % - 96.20 % 81.20 % - 81.50 % Risk-free interest rate 4.29 % - 4.33 % 3.58 % - 4.02 % Dividend yield — —

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	3 Months Ended
	Mar. 31, 2024
Earnings Per Share [Abstract]
Summary of Outstanding Potentially Dilutive Shares Excluded From Calculation of Diluted Net Loss Per Share	The following outstanding potentially dilutive shares have been excluded from the calculation of diluted net loss per share due to their anti-dilutive effect: Three Months Ended 2024 2023 Stock options to purchase common stock 11,859,362 9,389,106 Shares subject to Employee Stock Purchase Plan 79,748 87,983 Unvested restricted stock units 915,861 678,996 Common warrants 8,427,508 8,427,508 Total 21,282,479 18,583,593

Organization - Additional Infor

Organization - Additional Information (Detail) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Accounting Policies [Abstract]
Accumulated deficit	$ 597,675	$ 572,499
Cash and cash equivalents and short-term investments	$ 264,900

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on a Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 253,515	$ 251,218
Gross Unrealized Holding Gains	2	10
Gross Unrealized Holding Losses	(20)
Aggregate Fair Value	253,497	251,228
Level 1 [Member] | Cash and Cash Equivalents [Member] | Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	40,150	143,933
Aggregate Fair Value	40,150	143,933
Level 2 [Member] | Cash and Cash Equivalents [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	140,490	216,622
Gross Unrealized Holding Gains	1
Gross Unrealized Holding Losses	(1)
Aggregate Fair Value	140,490	216,622
Level 2 [Member] | Cash and Cash Equivalents [Member] | Commercial Paper [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	13,980
Aggregate Fair Value	13,980
Level 2 [Member] | Cash and Cash Equivalents [Member] | Government Bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	86,360	72,689
Gross Unrealized Holding Gains	1
Gross Unrealized Holding Losses	(1)
Aggregate Fair Value	86,360	72,689
Level 2 [Member] | Short-Term Investments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	113,025	34,596
Gross Unrealized Holding Gains	1	10
Gross Unrealized Holding Losses	(19)
Aggregate Fair Value	113,007	34,606
Level 2 [Member] | Short-Term Investments [Member] | Government Bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	113,025	34,596
Gross Unrealized Holding Gains	1	10
Gross Unrealized Holding Losses	(19)
Aggregate Fair Value	$ 113,007	$ 34,606

Balance Sheet Components - Sche

Balance Sheet Components - Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Cash, Cash Equivalents, Restricted Cash, and Restricted Cash Equivalents [Abstract]
Cash	$ 11,451	$ 8,488
Cash equivalents	140,490	216,622
Cash and cash equivalents	151,941	225,110
Restricted cash	1,032	1,032
Cash, cash equivalents and restricted cash	$ 152,973	$ 226,142

Balance Sheet Components - Summ

Balance Sheet Components - Summary of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Prepaid Expense and Other Assets, Current [Abstract]
Prepaid research and development costs	$ 4,643	$ 2,617
Prepaid insurance	418	704
Other prepaid expenses	708	760
Other current assets	23	63
Total prepaid expenses and other current assets	$ 5,792	$ 4,144

Balance Sheet Components - Su_2

Balance Sheet Components - Summary of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Property Plant And Equipment [Line Items]
Total property and equipment, gross	$ 19,803	$ 19,802
Less: accumulated depreciation	(5,568)	(5,029)
Total property and equipment, net	14,235	14,773
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	17,246	17,245
Laboratory Equipment [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	1,832	1,832
Furniture and Fixtures [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	692	692
Computer Equipment and Software [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	$ 33	$ 33

Balance Sheet Components - Addi

Balance Sheet Components - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Prepaid Expense and Other Assets, Current [Abstract]
Depreciation expense	$ 0.5	$ 0.5

Balance Sheet Components - Su_3

Balance Sheet Components - Summary of Accrued Liabilities (Detail) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Accrued Liabilities, Current [Abstract]
Accrued research and development expenses	$ 4,527	$ 4,027
Accrued compensation	1,643	5,607
Accrued professional services	570	501
Other accrued expenses	117	100
Total accrued liabilities	$ 6,857	$ 10,235

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	1 Months Ended	3 Months Ended		24 Months Ended
	Nov. 30, 2021	Mar. 31, 2024 USD ($) Claim	Mar. 31, 2023 USD ($)	Nov. 30, 2023 USD ($)	Dec. 31, 2023 USD ($)
Gain Contingencies [Line Items]
Lessee, Operating Sublease Commencement Year and Month	2021-12
Lessee operating sublease expiration year and month	2023-11
Aggregate sublease payment receivable				$ 3,400,000
Sublease income		$ 0	$ 400,000
Operating lease right-of-use asset		17,701,000			$ 18,009,000
Lease liabilities		$ 30,785,000
Weighted average incremental borrowing rate		8.40%
Operating lease costs		$ 1,000,000	500,000
Variable lease payments		500,000	$ 400,000
Loss contingency accrual		$ 0
Indemnification Agreement [Member]
Gain Contingencies [Line Items]
Loss contingency pending claims | Claim		0
Brisbane, California [Member]
Gain Contingencies [Line Items]
Operating lease, term of contract		10 years
Operating lease, expiration month and year		2031-10
Operating lease, renewal term		10 years
Operating lease right-of-use asset		$ 17,700,000
Lease liabilities		$ 30,800,000
Weighted average remaining lease term		7 years 7 months 6 days

Commitments and Contingencies_2

Commitments and Contingencies - Future Minimum Lease Payments and Related Lease Liabilities (Detail) $ in Thousands	Mar. 31, 2024 USD ($)
Commitments and Contingencies Disclosure [Abstract]
2023 (remaining three months)	$ 3,677
2024	5,425
2025	5,065
2026	5,242
2027 and thereafter	22,600
Total undiscounted lease payments	42,009
Less: Imputed interest	(11,224)
Lease liabilities	$ 30,785

Stockholder's Equity - Addition

Stockholder's Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended						3 Months Ended		12 Months Ended
	May 13, 2024	Apr. 30, 2024	Feb. 29, 2024	Dec. 31, 2023	Mar. 31, 2023	Jul. 31, 2022	Aug. 31, 2021	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Class Of Stock [Line Items]
Net proceeds from private placement						$ 122,500
Common stock				$ 78				$ 90		$ 78
Net proceeds from sale of common stock								$ 33,131	$ 17,995
Common Stock [Member]
Class Of Stock [Line Items]
Sale of shares								6,639,348
Common stock shares issued for exercise of warrants			5,243,400		2,582,557
Common Stock [Member] | Subsequent Event [Member]
Class Of Stock [Line Items]
Common stock shares issued for exercise of warrants		965,427
Pre-funded Warrants [Member]
Class Of Stock [Line Items]
Purchase of shares			5,244,444	18,379,861	2,583,144				2,583,144	18,379,861
Offering price per share				$ 2.879		$ 3.87025
Pre-funded warrants were recorded at fair value				$ 52,900
Pre-funded Warrants [Member] | Maximum [Member]
Class Of Stock [Line Items]
Purchase of shares						24,696,206		13,135,417
Purchase price per share				$ 0.001		$ 0.001				$ 0.001
Warrants or Common Warrants [Member]
Class Of Stock [Line Items]
Offering price per share						3.87125
Purchase price per share						$ 5.806875
Warrant expiration date						Jun. 30, 2025
Warrants or Common Warrants [Member] | Maximum [Member]
Class Of Stock [Line Items]
Purchase of shares						8,427,508		8,427,508
Private Placement [Member]
Class Of Stock [Line Items]
Sale of shares						9,013,834
Private Placement [Member] | Pre-funded Warrants [Member] | Maximum [Member]
Class Of Stock [Line Items]
Purchase of shares								22,113,062
Underwriting agreement [Member]
Class Of Stock [Line Items]
Sale of shares				25,035,000
Common stock, par value per share				$ 0.001						0.001
Common stock shares issued for exercise of warrants								12,169,990
Exchange value				$ 1,000
Shares issued price per share				$ 2.88						$ 2.88
Net proceeds from sale of common stock				$ 117,000
Underwriting agreement [Member] | Director [Member]
Class Of Stock [Line Items]
Sale of shares				350,000
Sales Agreement with Cowen and Company LLC [Member] | 2021 ATM Program [Member]
Class Of Stock [Line Items]
Common stock							$ 100,000
Percentage of gross proceeds of common stock to be paid							3%
Common stock, shares issued				2,646,458				6,639,348		2,646,458
Shares issued price per share				$ 6.8						$ 6.8
Net proceeds from sale of common stock								$ 32,200		$ 17,500
Remains available for offer and sales of shares of common stock								48,900
Sales Agreement with Cowen and Company LLC [Member] | 2024 ATM Program [Member]
Class Of Stock [Line Items]
Common stock								$ 100,000
Sales Agreement with Cowen and Company LLC [Member] | 2024 ATM Program [Member] | Maximum [Member]
Class Of Stock [Line Items]
Percentage of gross proceeds of common stock to be paid								3%
Cowen Sales Agent [Member] | 2021 ATM Program [Member] | Subsequent Event [Member]
Class Of Stock [Line Items]
Net proceeds from sale of common stock	$ 6,400

Stockholder's Equity - Schedule

Stockholder's Equity - Schedule of Common Stock Reserved for Future Issuance (Detail) - shares	Mar. 31, 2024	Dec. 31, 2023
Class Of Stock [Line Items]
Common stock reserved	11,859,362	9,208,970
Common stock reserved	1,762,992
Common stock reserved	112,983,696	65,329,843
Stock Options Issued and Outstanding [Member]
Class Of Stock [Line Items]
Common stock reserved	11,859,362	9,208,970
Stock Options Reserved for 2020 Incentive Award Plan [Member]
Class Of Stock [Line Items]
Common stock reserved	1,762,992	1,988,340
Unvested Restricted Stock Units Outstanding [Member]
Class Of Stock [Line Items]
Common stock reserved	915,861	495,579
Common Stock Reserved for Employee Stock Purchase Plan [Member]
Class Of Stock [Line Items]
Common stock reserved	2,122,071	1,338,381
2022 Employment Inducement Award Plan [Member]
Class Of Stock [Line Items]
Common stock reserved	1,667,300	758,084
Pre-Funded Warrants [Member]
Class Of Stock [Line Items]
Common stock reserved	35,248,479	40,493,510
Common Warrants [Member]
Class Of Stock [Line Items]
Common stock reserved	8,427,508	8,427,508
2021 ATM Program [Member]
Class Of Stock [Line Items]
Common stock reserved	25,980,123	2,619,471
2024 ATM Program [Member]
Class Of Stock [Line Items]
Common stock reserved	25,000,000

Equity Incentive Plans - Additi

Equity Incentive Plans - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Jul. 31, 2022
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of number of shares of capital stock outstanding on last day of preceding year	4%
Stock option grants description	Awards granted under the 2020 Plan expire no later than ten years from the date of grant. For Incentive Stock Options, or ISOs, and Nonstatutory Stock Options, or NSOs, the option price shall not be less than 100% of the estimated fair value on the date of grant. Options granted typically vest over a four-year period but may be granted with different vesting terms.
Option expiry period	10 years
Vesting Period	4 years
Options available for future grant	1,762,992
Intrinsic value of options exercised	$ 300	$ 300
Weighted- average exercise price per share, Stock options granted	$ 5.13
Unrecognized stock-based compensation cost expected period for recognition	2 years 7 months 6 days
Common stock for future issuance	112,983,696		65,329,843
Total stock-based compensation expense	$ 4,660	$ 4,607
Employees [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted- average exercise price per share, Stock options granted	$ 4.06	$ 4.35
2022 Employment Inducement Award Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options available for future grant	1,667,300			2,850,000
Incentive Stock Options, or ISOs, and Nonstatutory Stock Options, or NSOs [Member] | Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Purchase of Common stock at end of each offering period at price equal to percentage of fair market value of shares	100%
Stock Options Issued and Outstanding [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation cost	$ 29,800
Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation cost	$ 4,300
Unrecognized stock-based compensation cost expected period for recognition	2 years 4 months 24 days
Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Purchase of Common stock at end of each offering period at price equal to percentage of fair market value of shares	85%
Share purchases funded through payroll deductions minimum percentage of employee's eligible compensation	1%
Share purchases funded through payroll deductions maximum percentage of employee's eligible compensation	15%
Common stock for future issuance	2,122,071		1,338,381
Percentage of shares of common stock outstanding on last day of immediately preceding fiscal year	1%
Total stock-based compensation expense	$ 100	$ 100

Equity Incentive Plans - Summar

Equity Incentive Plans - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Share-Based Payment Arrangement [Abstract]
Number of shares, Beginning balance	9,208,970
Number of shares, Stock options granted	3,121,592
Number of shares, Stock options exercised	(105,526)
Number of shares, Stock options forfeited	(365,674)
Number of shares, Ending balance	11,859,362	9,208,970
Number of shares, Exercisable	5,469,398
Weighted- average exercise price per share, Beginning balance	$ 10.31
Weighted- average exercise price per share, Stock options granted	5.13
Weighted- average exercise price per share, Stock options exercised	2.06
Weighted- average exercise price per share, Stock options forfeited	9.15
Weighted- average exercise price per share, Ending balance	9.05	$ 10.31
Weighted- average exercise price per share, Exercisable	$ 12.11
Weighted- average remaining contractual term	7 years 10 months 28 days	7 years 6 months 10 days
Weighted- average remaining contractual term, Exercisable	6 years 5 months 26 days
Aggregate intrinsic value	$ 18,184	$ 2,930
Aggregate intrinsic value, Exercisable	$ 5,860

Equity Incentive Plans - Summ_2

Equity Incentive Plans - Summary of RSU Activity under 2020 Plan (Detail)	3 Months Ended
	Mar. 31, 2024 $ / shares shares
Share-Based Payment Arrangement [Abstract]
Number of Shares, Unvested as of December 31, 2023 | shares	495,579
Number of Shares, Granted | shares	602,600
Number of Shares, Vested | shares	(124,695)
Number of Shares, Cancelled | shares	(57,623)
Number of Shares, Unvested as of March 31, 2024 | shares	915,861
Weighted-Average Grant Date Fair Value Per Share, Granted Unvested as of December 31, 2023 | $ / shares	$ 5.69
Weighted-Average Grant Date Fair Value Per Share, Granted | $ / shares	5.13
Weighted-Average Grant Date Fair Value Per Share, Vested | $ / shares	6.44
Weighted-Average Grant Date Fair Value Per Share, Cancelled | $ / shares	5.33
Weighted-Average Grant Date Fair Value Per Share, Granted Unvested as of March 31, 2024 | $ / shares	$ 5.24

Equity Incentive Plans - Summ_3

Equity Incentive Plans - Summary of Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation expense	$ 4,660	$ 4,607
Research and Development [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation expense	2,282	2,251
General and Administrative [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation expense	$ 2,378	$ 2,356

Equity Incentive Plans - Summ_4

Equity Incentive Plans - Summary of Fair Value of Each Award Issued Estimated on the Date of Grant Using the Black-Scholes Option Pricing Model (Detail)	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, Minimum	95.90%	81.20%
Expected volatility, Maximum	96.20%	81.50%
Risk-free interest rate, Minimum	4.29%	3.58%
Risk-free interest rate, Maximum	4.33%	4.02%
Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	6 years 7 days	6 years 7 days
Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	6 years 29 days	6 years 29 days

Net Loss Per Share - Additional

Net Loss Per Share - Additional Information (Detail) - Pre-funded Warrants [Member] - shares	Mar. 31, 2024	Feb. 29, 2024	Dec. 31, 2023	Mar. 31, 2023	Jul. 31, 2022
Class of Warrant or Right [Line Items]
Warrants exercised		5,244,444	18,379,861	2,583,144
Maximum [Member]
Class of Warrant or Right [Line Items]
Warrants exercised	13,135,417				24,696,206
Maximum [Member] | Common Stock [Member]
Class of Warrant or Right [Line Items]
Warrants exercised	35,248,479

Net Loss Per Share - Summary of

Net Loss Per Share - Summary of Outstanding Potentially Dilutive Shares Excluded From Calculation of Diluted Net Loss Per Share (Detail) - shares	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the computation of diluted net loss per share	21,282,479	18,583,593
Shares Subject to Employee Stock Purchase Plan [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the computation of diluted net loss per share	79,748	87,983
Stock Options to Purchase Common Stock [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the computation of diluted net loss per share	11,859,362	9,389,106
Unvested Restricted Stock Units [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the computation of diluted net loss per share	915,861	678,996
Common Warrants [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the computation of diluted net loss per share	8,427,508	8,427,508