Annexon (ANNX) 10-Q 13 Nov 23 2023 Q3 Quarterly report Financial data

Cover Page

Cover Page - shares	9 Months Ended
	Sep. 30, 2023	Nov. 09, 2023
Document Information [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Transition Report	false
Amendment Flag	false
Document Period End Date	Sep. 30, 2023
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	Q3
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		53,238,985
Prefunded Warrant [Member]
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		22,113,062

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Current assets:
Cash and cash equivalents	$ 133,159	$ 140,020
Short-term investments	32,112	102,637
Prepaid expenses and other current assets	3,898	5,441
Total current assets	169,169	248,098
Restricted cash	1,032	1,032
Property and equipment, net	15,310	16,838
Operating lease right-of-use assets	18,305	19,128
Total assets	203,816	285,096
Current liabilities:
Accounts payable	4,809	7,416
Accrued liabilities	10,490	13,448
Operating lease liabilities, current	1,701	1,316
Other current liabilities	170	180
Total current liabilities	17,170	22,360
Operating lease liabilities, non-current	29,807	31,542
Total liabilities	46,977	53,902
Commitments and contingencies (Note 5)
Stockholders’ equity:
Common stock	53	48
Additional paid-in capital	701,517	669,780
Accumulated other comprehensive loss	(123)	(372)
Accumulated deficit	(544,608)	(438,262)
Total stockholders’ equity	156,839	231,194
Total liabilities and stockholders’ equity	$ 203,816	$ 285,096

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Operating expenses:
Research and development	$ 27,893	$ 27,862	$ 90,489	$ 83,966
General and administrative	6,888	8,207	23,225	24,938
Total operating expenses	34,781	36,069	113,714	108,904
Loss from operations	(34,781)	(36,069)	(113,714)	(108,904)
Interest and other income, net	2,299	1,015	7,368	1,340
Net loss	$ (32,482)	$ (35,054)	$ (106,346)	$ (107,564)
Net loss per share, basic	$ (0.43)	$ (0.51)	$ (1.42)	$ (2.21)
Net loss per share, diluted	$ (0.43)	$ (0.51)	$ (1.42)	$ (2.21)
Weighted-average shares used in computing net loss per share, basic	75,342,495	68,652,859	74,815,274	48,710,433
Weighted-average shares used in computing net loss per share, diluted	75,342,495	68,652,859	74,815,274	48,710,433

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Statement of Comprehensive Income [Abstract]
Net loss	$ (32,482)	$ (35,054)	$ (106,346)	$ (107,564)
Other comprehensive gain (loss):
Foreign currency translation adjustment	(2)	(4)	(8)	(7)
Unrealized gain (loss) on available-for-sale securities	50	37	257	(219)
Comprehensive loss	$ (32,434)	$ (35,021)	$ (106,097)	$ (107,790)

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
Beginning Balance, Values at Dec. 31, 2021	$ 231,909	$ 39	$ 528,365	$ (180)	$ (296,315)
Beginning Balance, Shares at Dec. 31, 2021		38,560,854
Stock-based compensation	4,252		4,252
Exercise of stock options	19		19
Exercise of stock options, Shares		2,711
Foreign currency translation adjustment	3			3
Unrealized gain (loss) on available-for-sale securities	(196)			(196)
Net loss	(35,373)				(35,373)
Ending Balance, Values at Mar. 31, 2022	200,614	$ 39	532,636	(373)	(331,688)
Ending Balance, Shares at Mar. 31, 2022		38,563,565
Beginning Balance, Values at Dec. 31, 2021	231,909	$ 39	528,365	(180)	(296,315)
Beginning Balance, Shares at Dec. 31, 2021		38,560,854
Foreign currency translation adjustment	(7)
Unrealized gain (loss) on available-for-sale securities	(219)
Net loss	(107,564)
Ending Balance, Values at Sep. 30, 2022	260,602	$ 48	664,839	(406)	(403,879)
Ending Balance, Shares at Sep. 30, 2022		47,626,503
Beginning Balance, Values at Mar. 31, 2022	200,614	$ 39	532,636	(373)	(331,688)
Beginning Balance, Shares at Mar. 31, 2022		38,563,565
Stock-based compensation	4,520		4,520
Issuance of common stock per Employee Stock Purchase Plan puchase	113		113
Issuance of common stock per Employee Stock Purchase Plan purchase, Shares		40,341
Foreign currency translation adjustment	(6)			(6)
Unrealized gain (loss) on available-for-sale securities	(60)			(60)
Net loss	(37,137)				(37,137)
Ending Balance, Values at Jun. 30, 2022	168,044	$ 39	537,269	(439)	(368,825)
Ending Balance, Shares at Jun. 30, 2022		38,603,906
Stock-based compensation	4,911		4,911
Exercise of stock options	45		45
Exercise of stock options, Shares		8,763
Issuance of common stock to a related party, net of issuance costs, Value	122,623	$ 9	122,614
Issuance of common stock to a related party, net of issuance costs, Shares		9,013,834
Foreign currency translation adjustment	(4)			(4)
Unrealized gain (loss) on available-for-sale securities	37			37
Net loss	(35,054)				(35,054)
Ending Balance, Values at Sep. 30, 2022	260,602	$ 48	664,839	(406)	(403,879)
Ending Balance, Shares at Sep. 30, 2022		47,626,503
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
Stock-based compensation	4,607		4,607
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 to a related party, net of issuance costs, Value	17,470	$ 2	17,468
Issuance of common stock to a related party, net of issuance costs, Shares		2,646,458
Restricted stock vested in the period		73,058
Foreign currency translation adjustment	(5)			(5)
Unrealized gain (loss) on available-for-sale securities	160			160
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, 2022	$ 231,194	$ 48	669,780	(372)	(438,262)
Beginning Balance, Shares at Dec. 31, 2022		47,722,995
Exercise of stock options, Shares	55,888
Foreign currency translation adjustment	$ (8)
Unrealized gain (loss) on available-for-sale securities	257
Net loss	(106,346)
Ending Balance, Values at Sep. 30, 2023	156,839	$ 53	701,517	(123)	(544,608)
Ending Balance, Shares at Sep. 30, 2023		53,238,985
Beginning Balance, Values at Mar. 31, 2023	214,861	$ 53	691,963	(217)	(476,938)
Beginning Balance, Shares at Mar. 31, 2023		53,080,673
Stock-based compensation	4,660		4,660
Exercise of stock options, Shares		283
Issuance of common stock per Employee Stock Purchase Plan puchase	345		345
Issuance of common stock per Employee Stock Purchase Plan purchase, Shares		70,155
Foreign currency translation adjustment	(1)			(1)
Unrealized gain (loss) on available-for-sale securities	47			47
Net loss	(35,188)				(35,188)
Ending Balance, Values at Jun. 30, 2023	184,724	$ 53	696,968	(171)	(512,126)
Ending Balance, Shares at Jun. 30, 2023		53,151,111
Stock-based compensation	4,549		4,549
Restricted stock vested in the period		87,874
Foreign currency translation adjustment	(2)			(2)
Unrealized gain (loss) on available-for-sale securities	50			50
Net loss	(32,482)				(32,482)
Ending Balance, Values at Sep. 30, 2023	$ 156,839	$ 53	$ 701,517	$ (123)	$ (544,608)
Ending Balance, Shares at Sep. 30, 2023		53,238,985

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Sep. 30, 2022
Stock issuance cost for related party	$ 525	$ 9,500

Condensed Consolidated Statem_5

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Operating activities:
Net loss	$ (106,346)	$ (107,564)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	1,610	1,576
Accretion of (discount) premium on available-for-sale securities	(2,290)	490
Stock-based compensation	13,816	13,683
Reduction in the carrying amount of right-of-use assets	823	717
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	1,543	(280)
Other assets		(204)
Accounts payable	(2,503)	(777)
Accrued liabilities	(2,958)	4,041
Operating lease liabilities	(1,350)	(883)
Other current liabilities	(10)	31
Net cash used in operating activities	(97,665)	(89,170)
Investing activities:
Purchases of property and equipment	(186)	(6,244)
Purchases of available-for-sale securities	(83,612)	(46,364)
Proceeds from maturities of available-for-sale securities	156,684	154,666
Net cash provided by investing activities	72,886	102,058
Financing activities:
Proceeds from the exercise of common stock options	111	64
Proceeds from Employee Stock Purchase Plan purchases	345	113
Proceeds from the issuance of common stock to a related party, net of commissions	17,545
Payment of common stock issuance costs	(75)
Proceeds from the issuance of common stock, pre-funded warrants and common warrants in private placement		120,976
Proceeds from the issuance of common stock, pre-funded warrants and common warrants in private placement - related party		9,500
Payment of common stock, pre-funded warrants and common warrants issuance costs related to private placement		(7,853)
Net cash provided by financing activities	17,926	122,800
(Decrease) increase in cash, cash equivalents and restricted cash	(6,853)	135,688
Effect of exchange rate changes on cash, cash equivalents and restricted cash	(8)	(7)
Cash, cash equivalents and restricted cash
Beginning of period	141,052	76,009
End of period	134,191	211,690
Supplemental disclosure of cash flow information:
Cash paid for amounts included in the measurement of lease liability	3,525	3,028
Non-cash investing and financing activities:
Purchases of property and equipment included in accounts payable and accrued liabilities	$ 6	$ 118

Organization

Organization	9 Months Ended
	Sep. 30, 2023
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 September 30, 2023, had an accumulated deficit of $ 544.6 million and cash and cash equivalents and short-term investments of $ 165.3 million. The Company has historically funded its operations through the issuance of shares of its redeemable convertible preferred stock, 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	9 Months Ended
	Sep. 30, 2023
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 September 30, 2023, the condensed consolidated statements of operations, comprehensive loss, stockholders’ equity for the three and nine months ended September 30, 2023 and 2022 and the condensed consolidated statements of cash flows for the nine months ended September 30, 2023 and 2022 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 and nine month periods are also unaudited. The results of operations for the three and nine months ended September 30, 2023 are not necessarily indicative of the results to be expected for the year ending December 31, 2023 or for any other future annual or interim period. The condensed consolidated balance sheet as of December 31, 2022 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, 2022, filed with the SEC on March 6, 2023. 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, stock options, income taxes, 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 2022 Form 10-K filed on March 6, 2023 for a detailed description of significant accounting policies. There have been no significant changes to the Company’s accounting policies as disclosed in its 2022 Form 10-K. Recently Adopted Accounting Pronouncements The Company has implemented all new accounting pronouncements, which are expected to have a material impact on its condensed consolidated financial statements and does not believe that there are any other new pronouncements that have been issued that might have a material impact on its financial position or results of operations. Recently Issued Accounting Pronouncements The Company does not believe that any recently issued effective pronouncements, or pronouncements issued but not yet effective, if adopted, would have a material impact on its financial position or results of operations.

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2023
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): September 30, 2023 Valuation Amortized Gross Gross Aggregate Assets: Cash equivalents: Money market funds Level 1 $ 102,471 $ — $ — $ 102,471 Commercial paper Level 2 29,679 — ( 12 ) 29,667 Short-term investments: Government bonds Level 2 32,146 1 ( 35 ) 32,112 Total assets $ 164,296 $ 1 $ ( 47 ) $ 164,250 December 31, 2022 Valuation Amortized Gross Gross Aggregate Assets: Cash equivalents: Money market funds Level 1 $ 139,450 $ — $ — $ 139,450 Short-term investments: Commercial paper Level 2 42,467 — ( 82 ) 42,385 Corporate bonds Level 2 35,638 14 — 35,652 Government bonds Level 2 24,804 — ( 204 ) 24,600 Total assets $ 242,359 $ 14 $ ( 286 ) $ 242,087 For the three and nine months ended September 30, 2023 and 2022, the Company recognized no material realized gains or losses on financial instruments.

Balance Sheet Components

Balance Sheet Components	9 Months Ended
	Sep. 30, 2023
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 September 30, 2023 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): September 30, December 31, Cash $ 1,021 $ 570 Cash equivalents 132,138 139,450 Cash and cash equivalents 133,159 140,020 Restricted cash 1,032 1,032 Cash, cash equivalents and restricted cash $ 134,191 $ 141,052 Prepaid Expenses and Other Current Assets Prepaid expenses and other current assets consisted of the following (in thousands): September 30, December 31, Prepaid research and development costs $ 1,956 $ 3,805 Prepaid insurance 1,027 755 Other prepaid expenses 902 678 Other current assets 13 203 Total prepaid expenses and other current assets $ 3,898 $ 5,441 Property and Equipment, Net Property and equipment, net, consisted of the following (in thousands): September 30, December 31, Leasehold improvements $ 17,246 $ 17,231 Laboratory equipment 1,832 1,764 Furniture and fixtures 692 692 Computer equipment and software 33 34 Total property and equipment, gross 19,803 19,721 Less: accumulated depreciation ( 4,493 ) ( 2,883 ) Total property and equipment, net $ 15,310 $ 16,838 The Company recognized depreciation for property and equipment of $ 0.5 million for each of the three months ended September 30, 2023 and 2022, and $ 1.6 million for each of the nine months ended September 30, 2023 and 2022. Accrued Liabilities Accrued liabilities consisted of the following (in thousands): September 30, December 31, Accrued research and development expenses $ 5,266 $ 7,720 Accrued compensation 4,864 5,299 Accrued professional services 281 394 Other accrued expenses 79 35 Total accrued liabilities $ 10,490 $ 13,448

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2023
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 for two years starting from December 2021 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 $ 0.6 million and $ 0.5 million for the three months ended September 30, 2023 and 2022, respectively, and $ 1.7 million and $ 1.6 million for the nine months ended September 30, 2023 and 2022, respectively. As of September 30, 2023, the operating lease right-of-use assets were $ 18.3 million and lease liabilities were $ 31.5 million on the condensed consolidated balance sheet. The weighted-average remaining lease term is 8.1 years. The weighted-average incremental borrowing rate used to measure the operating lease liability is 8.4 %. Operating lease costs were $ 0.5 million for each of the three months ended September 30, 2023 and 2022, and $ 1.6 million and $ 1.5 million for the nine months ended September 30, 2023 and 2022, respectively. Variable lease payments were $ 0.4 million and $ 0.1 million for the three months ended September 30, 2023 and 2022, respectively, and $ 1.1 million and $ 0.7 million for the nine months ended September 30, 2023 and 2022, respectively. Future minimum lease payments and related lease liabilities as of September 30, 2023, were as follows: (in thousands) 2023 (remaining three months) 811 2024 4,893 2025 5,065 2026 5,242 2027 and thereafter 28,025 Total undiscounted lease payments 44,036 Less: Imputed interest ( 12,528 ) Total $ 31,508 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 September 30, 2023, 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	9 Months Ended
	Sep. 30, 2023
Stockholders' Equity Note [Abstract]
Stockholders' Equity	6. Stockholders’ Equity Private Placement On July 11, 2022, the Company sold in a private placement 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 . The private placement resulted in net proceeds of $ 122.5 million, after deducting placement agent fees and expenses. An entity related to one of the Company’s directors participated in the private placement and purchased 2,453,988 shares of common stock and accompanying common warrants to purchase up to an additional 613,497 shares of common stock for an aggregate price of approximately $ 9.5 million. During the first quarter of 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 September 30, 2023, there were 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 outstanding. At-the-Market Offering In August 2021, the Company entered into a sales agreement with Cowen and Company LLC, or Cowen, as sales agent, pursuant to which the Company may issue and sell shares of its common stock for an aggregate maximum offering price of $ 100.0 million under an at-the-market offering program, or 2021 ATM program. Cowen is entitled to compensation up to 3 % of the aggregate gross proceeds for the common stock sold through the 2021 ATM program. During the first quarter of 2023, the Company sold 2,646,458 shares of common stock to an entity related to one of the Company’s directors at a price of $ 6.80 per share under the 2021 ATM program for aggregate gross proceeds of approximately $ 18.0 million. Net proceeds were approximately $ 17.5 million after commissions of approximately $ 0.5 million were paid to Cowen as sales agent. Common Stock The Company has reserved the following shares of common stock for issuance as follows: September 30, December 31, Stock options issued and outstanding 9,335,202 8,152,093 Stock options reserved for 2020 Incentive Award Plan 1,743,618 478,109 Unvested restricted stock units outstanding 533,917 550,236 Common stock reserved for 2021 ATM program 2,619,471 5,265,929 Common stock reserved for Employee Stock Purchase 1,401,331 994,257 Common stock reserved for 2022 Employment Inducement 870,400 1,610,600 Common stock reserved for pre-funded warrants 22,113,649 24,696,206 Common stock reserved for common warrants 8,427,508 8,427,508 Total common stock reserved 47,045,096 50,174,938

Equity Incentive Plans

Equity Incentive Plans	9 Months Ended
	Sep. 30, 2023
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 September 30, 2023, there were 1,743,618 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 2,000,000 shares of common stock were reserved for issuance under the Inducement Plan. As of September 30, 2023, there were 870,400 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, 2022 8,152,093 $ 11.92 8.09 $ 3,329 Stock options granted 2,418,450 $ 5.39 Stock options exercised ( 55,888 ) $ 1.99 Stock options forfeited ( 1,179,453 ) $ 10.61 Balances as of September 30, 2023 9,335,202 $ 10.45 7.73 $ 188 Exercisable as of September 30, 2023 4,784,184 $ 12.14 6.74 $ 183 The total intrinsic value of options exercised was insignificant during the three months ended September 30, 2023 and 2022, and $ 0.3 million and $ 5,000 for the nine months ended September 30, 2023 and 2022, respectively. 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 nine months ended September 30, 2023 and 2022 was $ 3.87 and $ 3.93 per share, respectively. As of September 30, 2023, the total unrecognized stock-based compensation cost related to outstanding unvested stock options was $ 27.7 million, which the Company expects to recognize over an estimated weighted-average period of 3.1 years. Restricted Stock Units RSUs are equity awards that entitle the holder to receive 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 2020 Plan and related information is as follows: Number of Shares Weighted-Average Grant Date Fair Value Per Share Unvested as of December 31, 2022 550,236 $ 5.58 Granted 249,615 5.95 Vested ( 160,932 ) 5.29 Cancelled ( 105,002 ) 5.58 Unvested as of September 30, 2023 533,917 $ 5.84 As of September 30, 2023, unrecognized stock-based compensation expense related to outstanding unvested RSUs was $ 2.4 million, which is expected to be recognized over a weighted-average period of 2.9 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 the least of 360,086 shares, 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 September 30, 2023, 1,401,331 shares were available for future purchase. The ESPP generally provides for six-month consecutive offering periods beginning on May 15 th and November 15 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 and nine months ended September 30, 2023. The stock-based compensation expense related to the ESPP was $ 0.1 million and $ 29,000 for the three months ended September 30, 2023 and 2022, respectively, and $ 0.2 million and $ 0.1 million for the nine months ended September 30, 2023 and 2022, respectively. Stock-Based Compensation Expense The total stock-based compensation expense related to all equity-based awards recognized in the period was as follows (in thousands): Three Months Ended Nine Months Ended 2023 2022 2023 2022 Research and development $ 2,243 $ 2,433 $ 6,801 $ 6,509 General and administrative 2,306 2,478 7,015 7,174 Total stock-based compensation expense $ 4,549 $ 4,911 $ 13,816 $ 13,683 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. The fair value of each award issued was estimated on the date of grant using the Black-Scholes option pricing model with the following assumptions: Three Months Ended Nine Months Ended 2023 2022 2023 2022 Expected term (in years) 6.08 5.77 - 6.08 5.50 - 6.08 5.50 - 6.08 Expected volatility 81.80 % - 81.90 % 81.70 % - 82.70 % 81.20 % - 82.40 % 78.80 % - 82.80 % Risk-free interest rate 4.20 % - 4.42 % 2.92 % - 3.61 % 3.47 % - 4.42 % 1.49 % - 3.61 % Dividend yield — — — —

Net Loss Per Share

Net Loss Per Share	9 Months Ended
	Sep. 30, 2023
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 the pre-funded warrants to purchase up to 22,113,062 shares of common stock issued in connection with the private placement 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 Nine Months Ended 2023 2022 2023 2022 Stock options to purchase common stock 9,335,202 8,178,136 9,335,202 8,178,136 Shares subject to Employee Stock Purchase Plan 97,880 110,897 97,880 110,897 Unvested restricted stock units 533,917 591,608 533,917 591,608 Common warrants 8,427,508 8,427,508 8,427,508 8,427,508 Total 18,394,507 17,308,149 18,394,507 17,308,149

Basis of Presentation and Sig_2

Basis of Presentation and Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2023
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 September 30, 2023, the condensed consolidated statements of operations, comprehensive loss, stockholders’ equity for the three and nine months ended September 30, 2023 and 2022 and the condensed consolidated statements of cash flows for the nine months ended September 30, 2023 and 2022 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 and nine month periods are also unaudited. The results of operations for the three and nine months ended September 30, 2023 are not necessarily indicative of the results to be expected for the year ending December 31, 2023 or for any other future annual or interim period. The condensed consolidated balance sheet as of December 31, 2022 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, 2022, filed with the SEC on March 6, 2023.
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, stock options, income taxes, 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 Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements The Company has implemented all new accounting pronouncements, which are expected to have a material impact on its condensed consolidated financial statements and does not believe that there are any other new pronouncements that have been issued that might have a material impact on its financial position or results of operations.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements The Company does not believe that any recently issued effective pronouncements, or pronouncements issued but not yet effective, if adopted, would have a material impact on its financial position or results of operations.

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2023
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): September 30, 2023 Valuation Amortized Gross Gross Aggregate Assets: Cash equivalents: Money market funds Level 1 $ 102,471 $ — $ — $ 102,471 Commercial paper Level 2 29,679 — ( 12 ) 29,667 Short-term investments: Government bonds Level 2 32,146 1 ( 35 ) 32,112 Total assets $ 164,296 $ 1 $ ( 47 ) $ 164,250 December 31, 2022 Valuation Amortized Gross Gross Aggregate Assets: Cash equivalents: Money market funds Level 1 $ 139,450 $ — $ — $ 139,450 Short-term investments: Commercial paper Level 2 42,467 — ( 82 ) 42,385 Corporate bonds Level 2 35,638 14 — 35,652 Government bonds Level 2 24,804 — ( 204 ) 24,600 Total assets $ 242,359 $ 14 $ ( 286 ) $ 242,087

Balance Sheet Components (Table

Balance Sheet Components (Tables)	9 Months Ended
	Sep. 30, 2023
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): September 30, December 31, Cash $ 1,021 $ 570 Cash equivalents 132,138 139,450 Cash and cash equivalents 133,159 140,020 Restricted cash 1,032 1,032 Cash, cash equivalents and restricted cash $ 134,191 $ 141,052
Summary of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets consisted of the following (in thousands): September 30, December 31, Prepaid research and development costs $ 1,956 $ 3,805 Prepaid insurance 1,027 755 Other prepaid expenses 902 678 Other current assets 13 203 Total prepaid expenses and other current assets $ 3,898 $ 5,441
Summary of Property and Equipment, Net	Property and equipment, net, consisted of the following (in thousands): September 30, December 31, Leasehold improvements $ 17,246 $ 17,231 Laboratory equipment 1,832 1,764 Furniture and fixtures 692 692 Computer equipment and software 33 34 Total property and equipment, gross 19,803 19,721 Less: accumulated depreciation ( 4,493 ) ( 2,883 ) Total property and equipment, net $ 15,310 $ 16,838
Summary of Accrued Liabilities	Accrued liabilities consisted of the following (in thousands): September 30, December 31, Accrued research and development expenses $ 5,266 $ 7,720 Accrued compensation 4,864 5,299 Accrued professional services 281 394 Other accrued expenses 79 35 Total accrued liabilities $ 10,490 $ 13,448

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2023
Commitments and Contingencies Disclosure [Abstract]
Future Minimum Lease Payments and Related Lease Liabilities	Future minimum lease payments and related lease liabilities as of September 30, 2023, were as follows: (in thousands) 2023 (remaining three months) 811 2024 4,893 2025 5,065 2026 5,242 2027 and thereafter 28,025 Total undiscounted lease payments 44,036 Less: Imputed interest ( 12,528 ) Total $ 31,508

Stockholder's Equity (Tables)

Stockholder's Equity (Tables)	9 Months Ended
	Sep. 30, 2023
Stockholders' Equity Note [Abstract]
Schedule of Common Stock Reserved for Future Issuance	The Company has reserved the following shares of common stock for issuance as follows: September 30, December 31, Stock options issued and outstanding 9,335,202 8,152,093 Stock options reserved for 2020 Incentive Award Plan 1,743,618 478,109 Unvested restricted stock units outstanding 533,917 550,236 Common stock reserved for 2021 ATM program 2,619,471 5,265,929 Common stock reserved for Employee Stock Purchase 1,401,331 994,257 Common stock reserved for 2022 Employment Inducement 870,400 1,610,600 Common stock reserved for pre-funded warrants 22,113,649 24,696,206 Common stock reserved for common warrants 8,427,508 8,427,508 Total common stock reserved 47,045,096 50,174,938

Equity Incentive Plans (Tables)

Equity Incentive Plans (Tables)	9 Months Ended
	Sep. 30, 2023
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, 2022 8,152,093 $ 11.92 8.09 $ 3,329 Stock options granted 2,418,450 $ 5.39 Stock options exercised ( 55,888 ) $ 1.99 Stock options forfeited ( 1,179,453 ) $ 10.61 Balances as of September 30, 2023 9,335,202 $ 10.45 7.73 $ 188 Exercisable as of September 30, 2023 4,784,184 $ 12.14 6.74 $ 183
Summary of RSU Activity under 2020 Plan	A summary of RSU activity under the 2020 Plan and related information is as follows: Number of Shares Weighted-Average Grant Date Fair Value Per Share Unvested as of December 31, 2022 550,236 $ 5.58 Granted 249,615 5.95 Vested ( 160,932 ) 5.29 Cancelled ( 105,002 ) 5.58 Unvested as of September 30, 2023 533,917 $ 5.84
Summary of Stock-Based Compensation Expense	The total stock-based compensation expense related to all equity-based awards recognized in the period was as follows (in thousands): Three Months Ended Nine Months Ended 2023 2022 2023 2022 Research and development $ 2,243 $ 2,433 $ 6,801 $ 6,509 General and administrative 2,306 2,478 7,015 7,174 Total stock-based compensation expense $ 4,549 $ 4,911 $ 13,816 $ 13,683
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 award issued was estimated on the date of grant using the Black-Scholes option pricing model with the following assumptions: Three Months Ended Nine Months Ended 2023 2022 2023 2022 Expected term (in years) 6.08 5.77 - 6.08 5.50 - 6.08 5.50 - 6.08 Expected volatility 81.80 % - 81.90 % 81.70 % - 82.70 % 81.20 % - 82.40 % 78.80 % - 82.80 % Risk-free interest rate 4.20 % - 4.42 % 2.92 % - 3.61 % 3.47 % - 4.42 % 1.49 % - 3.61 % Dividend yield — — — —

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	9 Months Ended
	Sep. 30, 2023
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 Nine Months Ended 2023 2022 2023 2022 Stock options to purchase common stock 9,335,202 8,178,136 9,335,202 8,178,136 Shares subject to Employee Stock Purchase Plan 97,880 110,897 97,880 110,897 Unvested restricted stock units 533,917 591,608 533,917 591,608 Common warrants 8,427,508 8,427,508 8,427,508 8,427,508 Total 18,394,507 17,308,149 18,394,507 17,308,149

Organization - Additional Infor

Organization - Additional Information (Detail) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Accounting Policies [Abstract]
Accumulated deficit	$ 544,608	$ 438,262
Cash and cash equivalents and short-term investments	$ 165,300

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on a Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 164,296	$ 242,359
Gross Unrealized Holding Gains	1	14
Gross Unrealized Holding Losses	(47)	(286)
Aggregate Fair Value	164,250	242,087
Level 1 [Member] | Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	102,471	139,450
Aggregate Fair Value	102,471	139,450
Level 2 [Member] | Commercial Paper [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	29,679	42,467
Gross Unrealized Holding Losses	(12)	(82)
Aggregate Fair Value	29,667	42,385
Level 2 [Member] | Government Bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	32,146	24,804
Gross Unrealized Holding Gains	1
Gross Unrealized Holding Losses	(35)	(204)
Aggregate Fair Value	$ 32,112	24,600
Level 2 [Member] | Corporate Bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost		35,638
Gross Unrealized Holding Gains		14
Aggregate Fair Value		$ 35,652

Balance Sheet Components - Sche

Balance Sheet Components - Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Cash, Cash Equivalents, Restricted Cash, and Restricted Cash Equivalents [Abstract]
Cash	$ 1,021	$ 570
Cash equivalents	132,138	139,450
Cash and cash equivalents	133,159	140,020
Restricted cash	1,032	1,032
Cash, cash equivalents and restricted cash	$ 134,191	$ 141,052

Balance Sheet Components - Summ

Balance Sheet Components - Summary of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Prepaid Expense and Other Assets, Current [Abstract]
Prepaid research and development costs	$ 1,956	$ 3,805
Prepaid insurance	1,027	755
Other prepaid expenses	902	678
Other current assets	13	203
Total prepaid expenses and other current assets	$ 3,898	$ 5,441

Balance Sheet Components - Su_2

Balance Sheet Components - Summary of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Property Plant And Equipment [Line Items]
Total property and equipment, gross	$ 19,803	$ 19,721
Less: accumulated depreciation	(4,493)	(2,883)
Total property and equipment, net	15,310	16,838
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	17,246	17,231
Laboratory Equipment [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	1,832	1,764
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	$ 34

Balance Sheet Components - Addi

Balance Sheet Components - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Prepaid Expense and Other Assets, Current [Abstract]
Depreciation expense	$ 0.5	$ 0.5	$ 1.6	$ 1.6

Balance Sheet Components - Su_3

Balance Sheet Components - Summary of Accrued Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Accrued Liabilities, Current [Abstract]
Accrued research and development expenses	$ 5,266	$ 7,720
Accrued compensation	4,864	5,299
Accrued professional services	281	394
Other accrued expenses	79	35
Total accrued liabilities	$ 10,490	$ 13,448

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)		3 Months Ended		9 Months Ended
	Nov. 01, 2021 USD ($)	Sep. 30, 2023 USD ($) Claim	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($) Claim	Sep. 30, 2022 USD ($)	Dec. 31, 2022 USD ($)
Gain Contingencies [Line Items]
Aggregate sublease payment receivable	$ 3,400,000
Sublease income		$ 600,000	$ 500,000	$ 1,700,000	$ 1,600,000
Operating lease right-of-use asset		18,305,000		18,305,000		$ 19,128,000
Lease liabilities		$ 31,508,000		$ 31,508,000
Weighted average incremental borrowing rate		8.40%		8.40%
Operating lease costs		$ 500,000	500,000	$ 1,600,000	1,500,000
Variable lease payments		400,000	$ 100,000	1,100,000	$ 700,000
Loss contingency accrual		$ 0		$ 0
Indemnification Agreement [Member]
Gain Contingencies [Line Items]
Loss contingency pending claims | Claim		0		0
Brisbane, California [Member]
Gain Contingencies [Line Items]
Operating lease, term of contract		10 years		10 years
Operating lease, expiration month and year				2031-10
Operating lease, renewal term		10 years		10 years
Operating lease right-of-use asset		$ 18,300,000		$ 18,300,000
Lease liabilities		$ 31,500,000		$ 31,500,000
Weighted average remaining lease term		8 years 1 month 6 days		8 years 1 month 6 days

Commitments and Contingencies_2

Commitments and Contingencies - Future Minimum Lease Payments and Related Lease Liabilities (Detail) $ in Thousands	Sep. 30, 2023 USD ($)
Commitments and Contingencies Disclosure [Abstract]
2023 (remaining three months)	$ 811
2024	4,893
2025	5,065
2026	5,242
2027 and thereafter	28,025
Total undiscounted lease payments	44,036
Less: Imputed interest	(12,528)
Lease liabilities	$ 31,508

Stockholder's Equity - Schedule

Stockholder's Equity - Schedule of Common Stock Reserved for Future Issuance (Detail) - shares	Sep. 30, 2023	Dec. 31, 2022
Class Of Stock [Line Items]
Common stock reserved	9,335,202	8,152,093
Common stock reserved	1,743,618
Common stock reserved	47,045,096	50,174,938
Stock Options Issued and Outstanding [Member]
Class Of Stock [Line Items]
Common stock reserved	9,335,202	8,152,093
Stock Options Reserved for 2020 Incentive Award Plan [Member]
Class Of Stock [Line Items]
Common stock reserved	1,743,618	478,109
Unvested Restricted Stock Units Outstanding [Member]
Class Of Stock [Line Items]
Common stock reserved	533,917	550,236
Common Stock Reserved for Employee Stock Purchase Plan [Member]
Class Of Stock [Line Items]
Common stock reserved	1,401,331	994,257
2022 Employment Inducement Award Plan [Member]
Class Of Stock [Line Items]
Common stock reserved	870,400	1,610,600
Pre-Funded Warrants [Member]
Class Of Stock [Line Items]
Common stock reserved	22,113,649	24,696,206
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	2,619,471	5,265,929

Stockholder's Equity - Addition

Stockholder's Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended	3 Months Ended		9 Months Ended
	Jul. 11, 2022	Aug. 31, 2021	Mar. 31, 2023	Sep. 30, 2022	Sep. 30, 2023	Dec. 31, 2022
Class Of Stock [Line Items]
Net proceeds from private placement	$ 122,500
Common stock					$ 53	$ 48
Net proceeds from sale of common stock					17,545
Payment of issuance costs					$ 75
Common Stock [Member]
Class Of Stock [Line Items]
Sale of shares			2,646,458	9,013,834
Common stock shares issued for exercise of warrants			2,582,557
Pre-funded Warrants [Member]
Class Of Stock [Line Items]
Purchase of shares			2,583,144
Offering price per share	$ 3.87025
Pre-funded Warrants [Member] | Maximum [Member]
Class Of Stock [Line Items]
Purchase of shares	24,696,206				22,113,062
Purchase price per share	$ 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				2,453,988
Additional common shares purchased					613,497
Exchange value					$ 9,500
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
Shares issued price per share			$ 6.8
Gross proceeds from sale of common stock			$ 18,000
Net proceeds from sale of common stock			17,500
Payment of issuance costs			$ 500

Equity Incentive Plans - Additi

Equity Incentive Plans - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022	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	1,743,618		1,743,618
Intrinsic value of options exercised			$ 300,000	$ 5,000
Weighted- average exercise price per share, Stock options granted			$ 5.39
Unrecognized stock-based compensation cost expected period for recognition			3 years 1 month 6 days
Common stock for future issuance	47,045,096		47,045,096		50,174,938
Total stock-based compensation expense	$ 4,549,000	$ 4,911,000	$ 13,816,000	$ 13,683,000
Employees [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted- average exercise price per share, Stock options granted			$ 3.87	$ 3.93
2022 Employment Inducement Award Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options available for future grant	870,400		870,400			2,000,000
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	$ 27,700,000		$ 27,700,000
Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation cost	$ 2,400,000		$ 2,400,000
Unrecognized stock-based compensation cost expected period for recognition			2 years 10 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%		1%
Share purchases funded through payroll deductions maximum percentage of employee's eligible compensation	15%		15%
Common stock for future issuance	1,401,331		1,401,331		994,257
Maximum number of shares to be issued in accordance with plan as a proportion of outstanding capital stock	360,086		360,086
Percentage of shares of common stock outstanding on last day of immediately preceding fiscal year			1%
Total stock-based compensation expense	$ 100,000	$ 29,000	$ 200,000	$ 100,000

Equity Incentive Plans - Summar

Equity Incentive Plans - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2023	Dec. 31, 2022
Share-Based Payment Arrangement [Abstract]
Number of shares, Beginning balance	8,152,093
Number of shares, Stock options granted	2,418,450
Number of shares, Stock options exercised	(55,888)
Number of shares, Stock options forfeited	(1,179,453)
Number of shares, Ending balance	9,335,202	8,152,093
Number of shares, Exercisable	4,784,184
Weighted- average exercise price per share, Beginning balance	$ 11.92
Weighted- average exercise price per share, Stock options granted	5.39
Weighted- average exercise price per share, Stock options exercised	1.99
Weighted average exercise price per share, Stock options forfeited	10.61
Weighted- average exercise price per share, Ending balance	10.45	$ 11.92
Weighted- average exercise price per share, Exercisable	$ 12.14
Weighted- average remaining contractual term	7 years 8 months 23 days	8 years 1 month 2 days
Weighted- average remaining contractual term, Exercisable	6 years 8 months 26 days
Aggregate intrinsic value	$ 188	$ 3,329
Aggregate intrinsic value, Exercisable	$ 183

Equity Incentive Plans - Summ_2

Equity Incentive Plans - Summary of RSU Activity under 2020 Plan (Detail)	9 Months Ended
	Sep. 30, 2023 $ / shares shares
Share-Based Payment Arrangement [Abstract]
Number of Shares, Unvested as of December 31, 2022 | shares	550,236
Number of Shares, Granted | shares	249,615
Number of Shares, Vested | shares	(160,932)
Number of Shares, Cancelled | shares	(105,002)
Number of Shares, Unvested as of September 30, 2023 | shares	533,917
Weighted-Average Grant Date Fair Value Per Share, Granted Unvested as of December 31, 2022 | $ / shares	$ 5.58
Weighted-Average Grant Date Fair Value Per Share, Granted | $ / shares	5.95
Weighted-Average Grant Date Fair Value Per Share, Vested | $ / shares	5.29
Weighted-Average Grant Date Fair Value Per Share, Cancelled | $ / shares	5.58
Weighted-Average Grant Date Fair Value Per Share, Granted Unvested as of September 30, 2023 | $ / shares	$ 5.84

Equity Incentive Plans - Summ_3

Equity Incentive Plans - Summary of Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation expense	$ 4,549	$ 4,911	$ 13,816	$ 13,683
Research and Development [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation expense	2,243	2,433	6,801	6,509
General and Administrative [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total stock-based compensation expense	$ 2,306	$ 2,478	$ 7,015	$ 7,174

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		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	6 years 29 days
Expected volatility, Minimum	81.80%	81.70%	81.20%	78.80%
Expected volatility, Maximum	81.90%	82.70%	82.40%	82.80%
Risk-free interest rate, Minimum	4.20%	2.92%	3.47%	1.49%
Risk-free interest rate, Maximum	4.42%	3.61%	4.42%	3.61%
Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)		5 years 9 months 7 days	5 years 6 months	5 years 6 months
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	6 years 29 days

Net Loss Per Share - Additional

Net Loss Per Share - Additional Information (Detail) - Pre-funded Warrants [Member] - shares	Sep. 30, 2023	Mar. 31, 2023	Jul. 11, 2022
Class of Warrant or Right [Line Items]
Warrants exercised		2,583,144
Maximum [Member]
Class of Warrant or Right [Line Items]
Warrants exercised	22,113,062		24,696,206

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		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the computation of diluted net loss per share	18,394,507	17,308,149	18,394,507	17,308,149
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	97,880	110,897	97,880	110,897
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	9,335,202	8,178,136	9,335,202	8,178,136
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	533,917	591,608	533,917	591,608
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	8,427,508	8,427,508