Annexon (ANNX) 10-Q 17 May 21 2021 Q1 Quarterly report Financial data

Cover Page

Cover Page - shares	3 Months Ended
	Mar. 31, 2021	Apr. 30, 2021
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Transition Report	false
Amendment Flag	false
Document Period End Date	Mar. 31, 2021
Document Fiscal Year Focus	2021
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	180 Kimball Way
Entity Address, Address Line Two	Suite 200
Entity Address, City or Town	South San Francisco
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94080
City Area Code	650
Local Phone Number	822-5500
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		38,258,687

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 206,669	$ 268,565
Short-term investments	119,985	82,641
Prepaid expenses and other current assets	3,868	2,805
Total current assets	330,522	354,011
Restricted cash	1,166
Property and equipment, less accumulated depreciation of $2,509 and $1,971 as of March 31, 2021 and December 31, 2020, respectively	1,429	1,935
Total assets	333,117	355,946
Current liabilities:
Accounts payable	4,518	3,734
Accrued liabilities	5,779	6,497
Deferred rent, current	398	391
Total current liabilities	10,695	10,622
Deferred rent	946	1,046
Total liabilities	11,641	11,668
Commitments and contingencies (Note 5)
Stockholders’ Equity:
Common stock	38	38
Additional paid-in capital	513,539	510,309
Accumulated other comprehensive loss	(103)	(77)
Accumulated deficit	(191,998)	(165,992)
Total stockholders’ equity	321,476	344,278
Total liabilities and stockholders’ equity	$ 333,117	$ 355,946

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Statement Of Financial Position [Abstract]
Accumulated depreciation on property plant and equipment	$ 2,509	$ 1,971

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Operating expenses:
Research and development	$ 20,696	$ 10,217
General and administrative	5,452	2,239
Total operating expenses	26,148	12,456
Loss from operations	(26,148)	(12,456)
Other income, net	142	115
Net loss	(26,006)	(12,341)
Accretion on redeemable convertible preferred stock		(279)
Net loss attributable to common stockholders	$ (26,006)	$ (12,620)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.68)	$ (29.10)
Weighted-average shares used in computing net loss per share attributable to common stockholders, basic and diluted	38,163,062	433,749

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Comprehensive Loss - USD ($)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (26,006,000)	$ (12,341,000)
Other comprehensive loss:
Foreign currency translation adjustment	(1,000)	(11,000)
Unrealized loss on available for sale securities	(25,000)
Comprehensive loss	$ (26,032,000)	$ (12,352,000)

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($)	Total	Redeemable Convertible Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
Beginning Balance, Values at Dec. 31, 2019		$ 143,984,000
Beginning Balance, Shares at Dec. 31, 2019		111,748,065
Beginning Balance, Shares at Dec. 31, 2019			433,749
Beginning Balance, Values at Dec. 31, 2019	$ (100,454,000)		$ 4,000	$ 2,202,000	$ (80,000)	$ (102,580,000)
Accretion on redeemable convertible preferred stock	(279,000)			(279,000)
Accretion on redeemable convertible preferred stock		$ 279,000
Stock-based compensation	$ 663,000			663,000
Stock option exercises, Shares	0
Foreign currency translation adjustment	$ (11,000)				(11,000)
Net loss	(12,341,000)					(12,341,000)
Ending Balance, Values at Mar. 31, 2020		$ 144,263,000
Ending Balance, Shares at Mar. 31, 2020		111,748,065
Ending Balance, Values at Mar. 31, 2020	(112,422,000)		$ 4,000	2,586,000	(91,000)	(114,921,000)
Ending Balance, Shares at Mar. 31, 2020			433,749
Beginning Balance, Shares at Dec. 31, 2020			38,157,618
Beginning Balance, Values at Dec. 31, 2020	344,278,000		$ 38,000	510,309,000	(77,000)	(165,992,000)
Stock-based compensation	2,962,000			2,962,000
Stock option exercises	$ 268,000			268,000
Stock option exercises, Shares	74,930		74,930
Foreign currency translation adjustment	$ (1,000)				(1,000)
Unrealized loss on available for sale securities	(25,000)				(25,000)
Net loss	(26,006,000)					(26,006,000)
Ending Balance, Values at Mar. 31, 2021	$ 321,476,000		$ 38,000	$ 513,539,000	$ (103,000)	$ (191,998,000)
Ending Balance, Shares at Mar. 31, 2021			38,232,548

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Operating activities:
Net loss	$ (26,006)	$ (12,341)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	538	131
Accretion of discount on available-for-sale securities	180
Stock-based compensation	2,962	663
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(1,063)	314
Accounts payable	784	797
Accrued liabilities	(718)	523
Deferred rent	(93)	(88)
Net cash used in operating activities	(23,416)	(10,001)
Investing activities:
Purchases of property and equipment	(32)
Purchases of available-for-sale securities	(42,549)
Proceeds from maturities of available-for-sale securities	5,000
Net cash used in investing activities	(37,581)
Financing activities:
Deferred offering cost payments		(571)
Proceeds from the exercise common stock options	268
Net cash provided by (used in) financing activities	268	(571)
Net decrease in cash, cash equivalents and restricted cash	(60,729)	(10,572)
Effect of exchange rate changes on cash, cash equivalents and restricted cash	(1)	(11)
Cash, cash equivalents and restricted cash
Beginning of period	268,565	43,931
End of period	$ 207,835	33,348
Non-cash investing and financing activities:
Accretion on redeemable convertible preferred stock		279
Deferred offering costs included in accounts payable and accrued liabilities		$ 723

Organization

Organization	3 Months Ended
	Mar. 31, 2021
Accounting Policies [Abstract]
Organization	1. Organization Annexon, Inc., or the Company, is a clinical-stage biopharmaceutical company targeting C1q and initiating molecules of the classical complement pathway to develop transformative therapies for autoimmune and neurodegenerative disorders of the body, brain, and eye. The Company is located in South San Francisco, 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. The Subsidiary is also engaged in research and development activities in support of its parent company. Initial Public Offering On July 23 14,750,000 2,139,403 262.4 Reverse Stock Split On July 17, 2020, the Company’s board of directors approved an amendment to the Company’s certificate of incorporation to effect a reverse split of shares of the Company’s common stock on a one-for-8.81 basis, or the Reverse Stock Split. The number of authorized shares and the par values of the common stock and redeemable convertible preferred stock were not adjusted as a result of the Reverse Stock Split. In connection with the Reverse Stock Split, the conversion ratio for the Company’s outstanding redeemable convertible preferred stock was proportionately adjusted such that the common stock issuable upon conversion of such preferred stock was decreased in proportion to the Reverse Stock Split. All references to common stock and options to purchase common stock share data, per share data and related information contained in the unaudited condensed consolidated financial statements have been retroactively adjusted to reflect the effect of the Reverse Stock Split for all periods presented. 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, 2021, had an accumulated deficit of $192.0 million The Company has historically funded its operations through the issuance of shares of its redeemable convertible preferred stock and common stock. Based on projected activities, management projects that cash on hand is sufficient to support operations for at least the next 12 months following issuance of these condensed consolidated financial statements. Management expects to continue to incur losses and negative cash flows from operations for at least the next several years.

Basis of Presentation and Signi

Basis of Presentation and Significant Accounting Policies	3 Months Ended
	Mar. 31, 2021
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, 2021 March 31, 2021 March 31, 2021 March 31, 2021 March 31, 2021 Use of Estimates The preparation of financial statements in conformity with 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. On an ongoing basis, management evaluates its estimates, including but not limited to stock options, Principles of Consolidation The 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. 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 includes amounts invested in money market funds, are stated at fair value. Restricted cash as of March 31, 2021 relates to a letter 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: March 31, 2021 December 31, 2020 Cash $ 610 $ 597 Cash equivalents 206,059 267,968 Cash and cash equivalents 206,669 268,565 Restricted cash 1,166 — Cash, cash equivalents and restricted cash $ 207,835 $ 268,565 Short-Term Investments Short-term investments have been classified as available-for-sale and are carried at estimated fair value as determined based upon quoted market prices or pricing models for similar securities. The Company determines the appropriate classification of its investments in debt securities at the time of purchase. Available-for-sale securities with original maturities beyond three months at the date of purchase are classified as current based on their availability for use in current operations. Unrealized gains and losses are excluded from earnings and are reported as a component of other comprehensive loss. The Company periodically evaluates whether declines in fair values of its marketable securities below their book value are other-than-temporary. This evaluation consists of several qualitative and quantitative factors regarding the severity and duration of the unrealized loss as well as the Company’s ability and intent to hold the marketable security until a forecasted recovery occurs. Additionally, the Company assesses whether it has plans to sell the security or it is more likely than not it will be required to sell any marketable securities before recovery of its amortized cost basis. Impairment assessments are made at the individual security level each reporting period. When the fair value of an available-for- sale security is less than its cost at the balance sheet date, a determination is made as to whether the impairment is other-than-temporary and, if it is other-than-temporary, an impairment loss is recognized in other income, net, equal to the difference between the investment’s amortized cost and fair value at such date. The cost of investments sold is based on the specific-identification method. Interest on marketable securities is included in other income, net. Foreign Currencies The Company’s reporting currency is the U.S. dollar. The functional currency of the Company’s subsidiary located in Australia is the Australian Dollar. Balance sheets prepared in the functional currencies are translated to the reporting currency at exchange rates in effect at the end of the accounting period, except for stockholders’ equity accounts, which are translated at rates in effect when these balances were originally recorded. Revenue and expense accounts are translated using a weighted-average rate during the year. The resulting foreign currency translation adjustments are recorded as a separate component of accumulated other comprehensive loss in the accompanying condensed consolidated balance sheets. Foreign currency translation losses for the three months ended March 31, 2021 and 2020 Gains and losses resulting from exchange rate changes on transactions denominated in a currency other than the local currency are included in earnings as incurred. Research and Development Expense Research and development expenses consist primarily of direct and indirect costs incurred for the development of the Company’s product candidates. Direct expenses include (i) preclinical and clinical outside service costs associated with discovery, preclinical and clinical testing of the Company’s product candidates; (ii) professional services agreements with third-party contract organizations, investigative clinical trial sites and consultants that conduct research and development activities on the Company’s behalf; (iii) contract manufacturing costs to produce clinical trial materials; and (iv) laboratory supplies and materials. Indirect expenses include (A) compensation and personnel-related expenses (including stock-based compensation); (B) allocated expenses for facilities and depreciation; and (C) other indirect costs. Research and development costs are expensed as incurred. Payments made to third parties are under agreements that are generally cancelable by the Company. Advance payments for research and development activities are deferred as prepaid expenses. The prepaid amounts are expensed as the related services are performed. The Company estimates preclinical studies and clinical trial expenses based on the services performed pursuant to contracts with research institutions and clinical research organizations that conduct and manage preclinical studies and clinical trials on the Company’s behalf. In accruing service fees, the Company estimates the period over which services will be performed and the level of effort to be expended in each period. These estimates are based on the Company’s communications with the third-party service providers and on information available at each balance sheet date. If the actual timing of the performance of services or the level of effort varies significantly from the estimate, the Company will adjust the accrual accordingly to reflect the best information available at the time of the financial statement issuance. The Company has not experienced any material differences between accrued costs and actual costs incurred since its inception. Stock-Based Compensation The Company accounts for stock-based compensation arrangements using a fair value method which requires the recognition of compensation expense for costs related to all stock-based payments, including stock options. The fair value method requires the Company to estimate the fair value of stock-based payment awards to employees on the date of grant using the Black-Scholes option pricing model. The Company grants certain employees performance-based stock options. For awards that include performance conditions, no compensation cost is recognized until the performance goals are probable of being met, at which time the cumulative compensation expense from the service inception date would be recognized. Stock-based compensation costs are based on the fair value of the underlying option calculated using the Black-Scholes option pricing model and recognized as expense on a straight-line basis (for all but performance based awards for which the accelerated method is used) over the requisite service period, which is the vesting period. Determining the appropriate fair value model and related assumptions requires judgment, including estimating the fair value of the underlying common stock, expected term, expected stock price volatility, risk-free interest rate and dividend yield. The Company accounts for forfeitures as they occur. Net Loss Per Share Attributable to Common Stockholders Basic net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of shares of common stock outstanding for the period, without consideration for potential dilutive shares of common stock. As the Company was in a loss position for all periods presented, basic net loss per share attributable to common stockholders is the same as diluted net loss per share attributable to common stockholders because the effects of potentially dilutive securities are antidilutive. Concentration of Credit Risk Financial instruments that potentially subject the Company to concentrations of credit risk consist primarily of cash and cash equivalents and short-term investments. The Company’s cash and Management Recently Adopted Accounting Pronouncements In August 2018, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, No. 2018-15, Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement that is a Service Contract (Topic 350) Recently Issued Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842), , Leases In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments Codification Improvements to Topic 326, Financial Instrument —Credit Losses Codification Improvements to Topic 326, Financial Instruments—Credit Losses, Derivatives and Hedging (Topic 815), and Financial Instruments (Topic 825), Codification Improvements to Financial Instruments is effective for fiscal years, and interim periods within those years, beginning after December 15, 2019 for public business entities, excluding smaller reporting companies.

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2021
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 and liabilities measured at fair value on a recurring basis by level within the fair value hierarchy (in thousands): March 31, 2021 Valuation Hierarchy Amortized Cost Gross Unrealized Holding Gains Gross Unrealized Holding Losses Aggregate Fair Value Assets: Cash equivalents: Money market funds Level 1 $ 206,059 $ — $ — $ 206,059 Short-term investments: Commercial paper Level 2 69,944 — — 69,944 Corporate debt Level 2 46,002 — (30 ) 45,972 Government bond Level 2 4,069 — — 4,069 Total assets $ 326,074 $ — $ (30 ) $ 326,044 December 31, 2020 Valuation Hierarchy Amortized Cost Gross Unrealized Holding Gains Gross Unrealized Holding Losses Aggregate Fair Value Assets: Cash equivalents: Money market funds Level 1 $ 267,968 $ — $ — $ 267,968 Short-term investments: Commercial paper Level 2 59,930 — — 59,930 Corporate bonds Level 2 22,717 1 (7 ) 22,711 Total assets $ 350,615 $ 1 $ (7 ) $ 350,609 For the three months ended March 31, 2021 and 2020

Balance Sheet Components

Balance Sheet Components	3 Months Ended
	Mar. 31, 2021
Disclosure Of Balance Sheet Components [Abstract]
Balance Sheet Components	4. Balance Sheet Components Prepaid Expenses and Other Current Assets Prepaid expenses and other current assets consisted of the following (in thousands): March 31, 2021 December 31, 2020 Prepaid research and development costs $ 2,319 $ 1,039 Prepaid insurance 720 1,236 Prepaid rent 451 — Other prepaid expenses 340 527 Other receivables 38 3 Total prepaid expenses and other current assets $ 3,868 $ 2,805 Accrued Liabilities Accrued liabilities consisted of the following (in thousands): March 31, 2021 December 31, 2020 Accrued research and development expenses $ 3,789 $ 3,260 Accrued compensation 1,184 2,543 Accrued professional services 298 524 Other accrued expenses 508 170 Total accrued liabilities $ 5,779 $ 6,497

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2021
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	5. Commitments and Contingencies Leases The Company leases its offices and laboratory in South San Francisco, California, or the South San Francisco Lease, under a 7-year noncancelable lease agreement that ends in June 2024 with a 5-year renewal option. In December 2020, the Company entered into an agreement to lease office and laboratory space in Brisbane, California, or the Brisbane Lease, with an affiliate of its current landlord. The term of the Brisbane Lease is expected to begin in November 2021 and end in October 2031, with an option by the Company to extend for an additional ten years. Concurrent with the execution of the Brisbane Lease, the Company entered into an agreement to terminate its existing lease in South San Francisco, California effective upon the commencement of the Brisbane Lease. The base rent will be abated for three months and the landlord will provide the Company with a tenant improvement allowance of up to $10.8 million. In connection with the Brisbane Lease, the Company is required to maintain a letter of credit for the benefit of the landlord in the amount of $1.0 million, which was delivered in January 2021 and is included in restricted cash on the condensed consolidated balance sheet. The total rent expense was $90,000 and $88,000 for each of the three months ended March 31, 2021 and 2020, respectively. License and Other Agreements In November 2011, the Company entered into an exclusive licensing agreement, or the Stanford Agreement, with The Board of Trustees of the Leland Stanford Junior University, or Stanford, whereby the Company was granted an exclusive, worldwide, royalty-bearing, sublicensable license, under certain patent rights, or the Licensed Patents, to make, use, offer for sale, sell, import and otherwise commercialize products covered by the Licensed Patents for human or animal diseases, disorders or conditions. Under the Stanford Agreement, the Company made an upfront payment and is obligated to pay Stanford annual license maintenance fees, potential future milestone payments totaling up to $500,000, and royalty payments at a rate equal to a low single-digit percentage of worldwide net sales of licensed products. The Company did not achieve any milestones or make any milestone payments for the three months ended March 31, 2021 and 2020, respectively. In December 2016, the Company entered into a Sponsored Research Agreement with a not-for-profit entity to perform research on multiple sclerosis. The Sponsored Research Agreement was amended in March 2019. Under the terms of the Sponsored Research Agreement, as amended, the Company may receive up to $651,000 in funding. If, within 15 years of the end of the Sponsored Research Agreement, the Company files a marketing authorization application for a product treating multiple sclerosis, the Company will be obligated to pay milestone payments up to four times the amounts received under the Sponsored Research Agreement. The Company has received $590,000 in funding to date, including $135,000 received during the three months ended March 31, 2021, which was recorded as other income. No income was recognized for the three months ended March 31, 2020. 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 not paid any claims or been required to defend any action related to its indemnification obligations. As of March 31, 2021, the Company did not 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, 2021
Stockholders Equity Note [Abstract]
Stockholders' Equity	6. Stockholders’ Equity Common Stock The Company has reserved the following shares of common stock for issuance as follows: March 31, 2021 December 31, 2020 Options issued and outstanding 5,373,215 3,909,873 Options available for future grant 2,695,979 2,707,947 Reserved for employee stock purchase plan 741,662 360,086 Total common stock reserved 8,810,856 6,977,906

Equity Incentive Plan

Equity Incentive Plan	3 Months Ended
	Mar. 31, 2021
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Equity Incentive Plan	7. Equity Incentive Plan 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 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 . 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, commencing on January 1, 2021 and ending in 2030, 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 the 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, 2021, there were 2,695,979 shares available for issuance under the 2020 Plan. Stock option activity under the 2011 Plan and the 2020 Plan was as follows: Shares Available for Grant Number of Shares Weighted- Average Exercise Price Per Share Weighted- Average Remaining Contractual Term (in years) Aggregate Intrinsic Value (in thousands) Balances as of December 31, 2020 2,707,947 3,909,873 $ 10.78 8.40 $ 56,128 Additional shares authorized 1,526,304 — Stock options granted (1,612,600 ) 1,612,600 $ 29.81 Stock options exercised — (74,930 ) $ 3.57 Stock options forfeited 74,328 (74,328 ) $ 18.80 Balances as of March 31, 2021 2,695,979 5,373,215 $ 16.48 8.69 $ 64,399 Exercisable as of March 31, 2021 1,486,521 $ 6.14 7.01 $ 32,325 The total intrinsic value of options exercised during the three months ended March 31, 2021 was $1.1 million. There were no options exercised during the three months ended March 31, 2020. 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. 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, beginning on January 1, 2021, 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 March 31, 2021, 741,662 Stock-Based Compensation Expense The total stock-based compensation expense recognized for options granted was as follows (in thousands): Three Months Ended March 31, 2021 2020 Research and development $ 1,546 $ 325 General and administrative 1,416 338 Total stock-based compensation expense $ 2,962 $ 663 As of March 31, 2021, the total unrecognized stock-based compensation cost related to outstanding unvested stock options that are expected to vest was $57.3 million, which the Company expects to recognize over an estimated weighted-average period of 3.29 years. 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— Historically, for all periods prior to the IPO in July 2020, fair values of the shares of common stock underlying the Company’s share-based awards were estimated on each grant date by the Company’s board of directors. The board of directors considered, among other things, valuations of the Company’s common stock which were prepared by an independent third- party valuation firm in accordance with the guidance provided by the American Institute of Certified Public Accountants 2013 Practice Aid, Valuation of Privately-Held-Company Equity Securities Issued as Compensation . After the completion of the IPO in July 2020, 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 Company’s expected term represents the period that the Company’s stock-based awards are expected to be outstanding. The Company’s historical share option exercise information is limited due to a lack of sufficient data points, and did not provide a reasonable basis upon which to estimate an expected term. The expected term for option grants is therefore determined using the simplified method (based on the mid-point between the vesting date and the end of the contractual term). Expected Volatility —Because the Company does not have sufficient trading history for its common stock, the expected volatility was estimated based on the average volatility for comparable publicly traded life science 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 the life cycle, or area of specialty. The Company will continue to apply this process until a sufficient amount of historical information regarding the volatility of its own stock price becomes available and to align with the Company’s expected term. 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 award issued was estimated on the date of grant using the Black-Scholes option pricing model with the following assumptions: Three Months Ended March 31, 2021 2020 Expected term (in years) 4.33 - 6.02 5.00 - 6.08 Expected volatility 90% 85% Risk-free interest rate 0.35%-1.02% 0.64% - 1.45% Dividend yield — —

Income Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2021
Income Tax Disclosure [Abstract]
Income Taxes	8. Income Taxes For the three months ended March 31, 2021 and 2020, the Company incurred insignificant amounts for an income tax provision. The U.S. federal and California deferred tax assets generated from the Company’s net operating losses have been fully reserved, as the Company believes it is not more likely than not that the benefit will be realized.

Net Loss Per Share Attributable

Net Loss Per Share Attributable to Common Stockholders	3 Months Ended
	Mar. 31, 2021
Earnings Per Share [Abstract]
Net Loss Per Share Attributable to Common Stockholders	9. Net Loss Per Share Attributable to Common Stockholders 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 March 31, 2021 2020 Stock options to purchase common stock 5,373,215 2,136,390 Shares subject to employee stock purchase plan 4,393 — Redeemable convertible preferred stock on an as-converted basis — 12,684,214 Total 5,377,608 14,820,604

Basis of Presentation and Sig_2

Basis of Presentation and Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2021
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, 2021 March 31, 2021 March 31, 2021 March 31, 2021 March 31, 2021
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with 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. On an ongoing basis, management evaluates its estimates, including but not limited to stock options,
Principles of Consolidation	Principles of Consolidation The 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.
Cash, Cash Equivalents and Restricted Cash	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 includes amounts invested in money market funds, are stated at fair value. Restricted cash as of March 31, 2021 relates to a letter 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: March 31, 2021 December 31, 2020 Cash $ 610 $ 597 Cash equivalents 206,059 267,968 Cash and cash equivalents 206,669 268,565 Restricted cash 1,166 — Cash, cash equivalents and restricted cash $ 207,835 $ 268,565
Short-Term Investments	Short-Term Investments Short-term investments have been classified as available-for-sale and are carried at estimated fair value as determined based upon quoted market prices or pricing models for similar securities. The Company determines the appropriate classification of its investments in debt securities at the time of purchase. Available-for-sale securities with original maturities beyond three months at the date of purchase are classified as current based on their availability for use in current operations. Unrealized gains and losses are excluded from earnings and are reported as a component of other comprehensive loss. The Company periodically evaluates whether declines in fair values of its marketable securities below their book value are other-than-temporary. This evaluation consists of several qualitative and quantitative factors regarding the severity and duration of the unrealized loss as well as the Company’s ability and intent to hold the marketable security until a forecasted recovery occurs. Additionally, the Company assesses whether it has plans to sell the security or it is more likely than not it will be required to sell any marketable securities before recovery of its amortized cost basis. Impairment assessments are made at the individual security level each reporting period. When the fair value of an available-for- sale security is less than its cost at the balance sheet date, a determination is made as to whether the impairment is other-than-temporary and, if it is other-than-temporary, an impairment loss is recognized in other income, net, equal to the difference between the investment’s amortized cost and fair value at such date. The cost of investments sold is based on the specific-identification method. Interest on marketable securities is included in other income, net.
Foreign Currencies	Foreign Currencies The Company’s reporting currency is the U.S. dollar. The functional currency of the Company’s subsidiary located in Australia is the Australian Dollar. Balance sheets prepared in the functional currencies are translated to the reporting currency at exchange rates in effect at the end of the accounting period, except for stockholders’ equity accounts, which are translated at rates in effect when these balances were originally recorded. Revenue and expense accounts are translated using a weighted-average rate during the year. The resulting foreign currency translation adjustments are recorded as a separate component of accumulated other comprehensive loss in the accompanying condensed consolidated balance sheets. Foreign currency translation losses for the three months ended March 31, 2021 and 2020 Gains and losses resulting from exchange rate changes on transactions denominated in a currency other than the local currency are included in earnings as incurred.
Research and Development Expense	Research and Development Expense Research and development expenses consist primarily of direct and indirect costs incurred for the development of the Company’s product candidates. Direct expenses include (i) preclinical and clinical outside service costs associated with discovery, preclinical and clinical testing of the Company’s product candidates; (ii) professional services agreements with third-party contract organizations, investigative clinical trial sites and consultants that conduct research and development activities on the Company’s behalf; (iii) contract manufacturing costs to produce clinical trial materials; and (iv) laboratory supplies and materials. Indirect expenses include (A) compensation and personnel-related expenses (including stock-based compensation); (B) allocated expenses for facilities and depreciation; and (C) other indirect costs. Research and development costs are expensed as incurred. Payments made to third parties are under agreements that are generally cancelable by the Company. Advance payments for research and development activities are deferred as prepaid expenses. The prepaid amounts are expensed as the related services are performed. The Company estimates preclinical studies and clinical trial expenses based on the services performed pursuant to contracts with research institutions and clinical research organizations that conduct and manage preclinical studies and clinical trials on the Company’s behalf. In accruing service fees, the Company estimates the period over which services will be performed and the level of effort to be expended in each period. These estimates are based on the Company’s communications with the third-party service providers and on information available at each balance sheet date. If the actual timing of the performance of services or the level of effort varies significantly from the estimate, the Company will adjust the accrual accordingly to reflect the best information available at the time of the financial statement issuance. The Company has not experienced any material differences between accrued costs and actual costs incurred since its inception.
Stock-Based Compensation	Stock-Based Compensation The Company accounts for stock-based compensation arrangements using a fair value method which requires the recognition of compensation expense for costs related to all stock-based payments, including stock options. The fair value method requires the Company to estimate the fair value of stock-based payment awards to employees on the date of grant using the Black-Scholes option pricing model. The Company grants certain employees performance-based stock options. For awards that include performance conditions, no compensation cost is recognized until the performance goals are probable of being met, at which time the cumulative compensation expense from the service inception date would be recognized. Stock-based compensation costs are based on the fair value of the underlying option calculated using the Black-Scholes option pricing model and recognized as expense on a straight-line basis (for all but performance based awards for which the accelerated method is used) over the requisite service period, which is the vesting period. Determining the appropriate fair value model and related assumptions requires judgment, including estimating the fair value of the underlying common stock, expected term, expected stock price volatility, risk-free interest rate and dividend yield. The Company accounts for forfeitures as they occur.
Net Loss Per Share Attributable to Common Stockholders	Net Loss Per Share Attributable to Common Stockholders Basic net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of shares of common stock outstanding for the period, without consideration for potential dilutive shares of common stock. As the Company was in a loss position for all periods presented, basic net loss per share attributable to common stockholders is the same as diluted net loss per share attributable to common stockholders because the effects of potentially dilutive securities are antidilutive.
Concentration of Credit Risk	Concentration of Credit Risk Financial instruments that potentially subject the Company to concentrations of credit risk consist primarily of cash and cash equivalents and short-term investments. The Company’s cash and Management
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements In August 2018, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, No. 2018-15, Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement that is a Service Contract (Topic 350)
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842), , Leases In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments Codification Improvements to Topic 326, Financial Instrument —Credit Losses Codification Improvements to Topic 326, Financial Instruments—Credit Losses, Derivatives and Hedging (Topic 815), and Financial Instruments (Topic 825), Codification Improvements to Financial Instruments is effective for fiscal years, and interim periods within those years, beginning after December 15, 2019 for public business entities, excluding smaller reporting companies.

Basis of Presentation and Sig_3

Basis of Presentation and Significant Accounting Policies (Tables)	3 Months Ended
	Mar. 31, 2021
Accounting Policies [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: March 31, 2021 December 31, 2020 Cash $ 610 $ 597 Cash equivalents 206,059 267,968 Cash and cash equivalents 206,669 268,565 Restricted cash 1,166 — Cash, cash equivalents and restricted cash $ 207,835 $ 268,565

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2021
Fair Value Disclosures [Abstract]
Summary of Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis	The following tables summarize the fair value of the Company’s financial assets and liabilities measured at fair value on a recurring basis by level within the fair value hierarchy (in thousands): March 31, 2021 Valuation Hierarchy Amortized Cost Gross Unrealized Holding Gains Gross Unrealized Holding Losses Aggregate Fair Value Assets: Cash equivalents: Money market funds Level 1 $ 206,059 $ — $ — $ 206,059 Short-term investments: Commercial paper Level 2 69,944 — — 69,944 Corporate debt Level 2 46,002 — (30 ) 45,972 Government bond Level 2 4,069 — — 4,069 Total assets $ 326,074 $ — $ (30 ) $ 326,044 December 31, 2020 Valuation Hierarchy Amortized Cost Gross Unrealized Holding Gains Gross Unrealized Holding Losses Aggregate Fair Value Assets: Cash equivalents: Money market funds Level 1 $ 267,968 $ — $ — $ 267,968 Short-term investments: Commercial paper Level 2 59,930 — — 59,930 Corporate bonds Level 2 22,717 1 (7 ) 22,711 Total assets $ 350,615 $ 1 $ (7 ) $ 350,609

Balance Sheet Components (Table

Balance Sheet Components (Tables)	3 Months Ended
	Mar. 31, 2021
Disclosure Of Balance Sheet Components [Abstract]
Summary of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets consisted of the following (in thousands): March 31, 2021 December 31, 2020 Prepaid research and development costs $ 2,319 $ 1,039 Prepaid insurance 720 1,236 Prepaid rent 451 — Other prepaid expenses 340 527 Other receivables 38 3 Total prepaid expenses and other current assets $ 3,868 $ 2,805
Summary of Accrued Liabilities	Accrued liabilities consisted of the following (in thousands): March 31, 2021 December 31, 2020 Accrued research and development expenses $ 3,789 $ 3,260 Accrued compensation 1,184 2,543 Accrued professional services 298 524 Other accrued expenses 508 170 Total accrued liabilities $ 5,779 $ 6,497

Stockholder's Equity (Tables)

Stockholder's Equity (Tables)	3 Months Ended
	Mar. 31, 2021
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: March 31, 2021 December 31, 2020 Options issued and outstanding 5,373,215 3,909,873 Options available for future grant 2,695,979 2,707,947 Reserved for employee stock purchase plan 741,662 360,086 Total common stock reserved 8,810,856 6,977,906

Equity Incentive Plan (Tables)

Equity Incentive Plan (Tables)	3 Months Ended
	Mar. 31, 2021
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Stock Option Activity	Stock option activity under the 2011 Plan and the 2020 Plan was as follows: Shares Available for Grant Number of Shares Weighted- Average Exercise Price Per Share Weighted- Average Remaining Contractual Term (in years) Aggregate Intrinsic Value (in thousands) Balances as of December 31, 2020 2,707,947 3,909,873 $ 10.78 8.40 $ 56,128 Additional shares authorized 1,526,304 — Stock options granted (1,612,600 ) 1,612,600 $ 29.81 Stock options exercised — (74,930 ) $ 3.57 Stock options forfeited 74,328 (74,328 ) $ 18.80 Balances as of March 31, 2021 2,695,979 5,373,215 $ 16.48 8.69 $ 64,399 Exercisable as of March 31, 2021 1,486,521 $ 6.14 7.01 $ 32,325
Summary of Stock-Based Compensation Expense	The total stock-based compensation expense recognized for options granted was as follows (in thousands): Three Months Ended March 31, 2021 2020 Research and development $ 1,546 $ 325 General and administrative 1,416 338 Total stock-based compensation expense $ 2,962 $ 663
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 March 31, 2021 2020 Expected term (in years) 4.33 - 6.02 5.00 - 6.08 Expected volatility 90% 85% Risk-free interest rate 0.35%-1.02% 0.64% - 1.45% Dividend yield — —

Net Loss Per Share Attributab_2

Net Loss Per Share Attributable to Common Stockholders (Tables)	3 Months Ended
	Mar. 31, 2021
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 March 31, 2021 2020 Stock options to purchase common stock 5,373,215 2,136,390 Shares subject to employee stock purchase plan 4,393 — Redeemable convertible preferred stock on an as-converted basis — 12,684,214 Total 5,377,608 14,820,604

Organization - Additional Infor

Organization - Additional Information (Detail) $ / shares in Units, $ in Thousands	Aug. 04, 2020shares	Jul. 24, 2020USD ($)$ / sharesshares	Jul. 17, 2020	Mar. 31, 2020USD ($)	Mar. 31, 2021USD ($)	Dec. 31, 2020USD ($)
Proceeds from issuance of common stock upon initial public offering, net of underwriting discounts and commissions		$ 262,400
Payment of deferred offering costs				$ 571
Redeemable convertible preferred stock, shares issued upon conversion | shares		20,824,938
Accumulated deficit					$ (191,998)	$ (165,992)
Cash and cash equivalents and short-term investments					$ 326,700
Common Stock [Member]
Description of reverse stock split			one-for-8.81 basis
Reverse stock split, conversion ratio			0.113507378
IPO [Member]
Shares issued during the period | shares		14,750,000
Shares issued price per share | $ / shares		$ 17
Payments for underwriting expense		$ 20,100
Payment of deferred offering costs		$ 4,600
Over-Allotment Option [Member]
Shares issued during the period | shares	2,139,403

Basis of Presentation and Sig_4

Basis of Presentation and Significant Accounting Policies - Schedule of Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents [Abstract]
Cash	$ 610	$ 597
Cash equivalents	206,059	267,968
Cash and cash equivalents	206,669	268,565
Restricted cash	1,166
Cash, cash equivalents and restricted cash	$ 207,835	$ 268,565

Basis of Presentation and Sig_5

Basis of Presentation and Significant Accounting Policies - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Basis Of Presentation And Significant Accounting Policies [Line Items]
Foreign currency translation losses	$ 1,000	$ 11,000
ASU 2018-15 [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Change in accounting principle, ASU adopted	true
Change in accounting principle, ASU, adoption date	Jan. 1, 2021
Change in accounting principle, ASU, Immaterial effect	true

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 326,074	$ 350,615
Gross Unrealized Holding Gains		1
Gross Unrealized Holding Losses	(30)	(7)
Aggregate Fair Value	326,044	350,609
Level 1 [Member] | Money Market Funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	206,059	267,968
Aggregate Fair Value	206,059	267,968
Level 2 [Member] | Commercial Paper [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	69,944	59,930
Aggregate Fair Value	69,944	59,930
Level 2 [Member] | Corporate Debt [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	46,002
Gross Unrealized Holding Losses	(30)
Aggregate Fair Value	45,972
Level 2 [Member] | Government Bond [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	4,069
Aggregate Fair Value	$ 4,069
Level 2 [Member] | Corporate Bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost		22,717
Gross Unrealized Holding Gains		1
Gross Unrealized Holding Losses		(7)
Aggregate Fair Value		$ 22,711

Balance Sheet Components - Summ

Balance Sheet Components - Summary of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Prepaid Expense And Other Assets Current [Abstract]
Prepaid research and development costs	$ 2,319	$ 1,039
Prepaid insurance	720	1,236
Prepaid rent	451
Other prepaid expenses	340	527
Other receivables	38	3
Total prepaid expenses and other current assets	$ 3,868	$ 2,805

Balance Sheet Components - Su_2

Balance Sheet Components - Summary of Accrued Liabilities (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Accrued Liabilities Current [Abstract]
Accrued research and development expenses	$ 3,789	$ 3,260
Accrued compensation	1,184	2,543
Accrued professional services	298	524
Other accrued expenses	508	170
Total accrued liabilities	$ 5,779	$ 6,497

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	1 Months Ended			3 Months Ended
	Dec. 31, 2020USD ($)	Mar. 31, 2019USD ($)	Nov. 30, 2011USD ($)	Mar. 31, 2021USD ($)Claim	Mar. 31, 2020USD ($)
Gain Contingencies [Line Items]
Income (loss)				$ (26,006,000)	$ (12,341,000)
Loss contingency accrual				$ 0
Indemnification Agreement [Member]
Gain Contingencies [Line Items]
Loss contingency pending claims | Claim				0
Stanford Agreement [Member] | Research and Development Arrangement [Member]
Gain Contingencies [Line Items]
Sponsor research agreements, commitment amount			$ 500,000
Sponsored Research Agreement [Member]
Gain Contingencies [Line Items]
Maximum threshold period allowed for filing market authorization		15 years
Proceeds from funding				$ 135,000
Income (loss)				0
Sponsored Research Agreement [Member] | Other Income [Member]
Gain Contingencies [Line Items]
Proceeds from funding				$ 590,000
Sponsored Research Agreement [Member] | Research and Development Arrangement [Member]
Gain Contingencies [Line Items]
Maximum expected research funding		$ 651,000
South San Francisco, California [Member]
Gain Contingencies [Line Items]
Operating lease, term of contract				7 years
Operating lease, expiration month and year				Jun. 30, 2024
Operating lease, renewal term				5 years
Brisbane, California [Member]
Gain Contingencies [Line Items]
Operating lease, not yet commenced beginning month and year	2021-11
Operating lease, not yet commenced expiration month and year	2031-10
Operating lease, not yet commenced option to extend [true false]	true
Operating lease, not yet commenced extended additional term	10 years
Operating lease, base rent abatement period	3 months
Letter of credit	$ 1,000
Brisbane, California [Member] | Maximum [Member]
Gain Contingencies [Line Items]
Tenant improvement allowance receivable	$ 10,800

Stockholder's Equity - Schedule

Stockholder's Equity - Schedule of Common Stock Reserved for Future Issuance (Detail) - shares	Mar. 31, 2021	Dec. 31, 2020
Class of Stock [Line Items]
Options issued and outstanding	5,373,215	3,909,873
Options available for future grant	2,695,979	2,707,947
Common stock reserved	8,810,856	6,977,906
Reserved for Employee Stock Purchase Plan [Member]
Class of Stock [Line Items]
Common stock reserved	741,662	360,086
Options Issued and Outstanding [Member]
Class of Stock [Line Items]
Options issued and outstanding	5,373,215	3,909,873
Options Available for Future Grant [Member]
Class of Stock [Line Items]
Options available for future grant	2,695,979	2,707,947

Equity Incentive Plan - Additio

Equity Incentive Plan - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
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	4.00%
Stock option grants description	Awards granted under the 2020 Plan expire no later than ten years from the date of grant. For the 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	2,695,979		2,707,947
Intrinsic value of options exercised	$ 1.1
Stock option exercises, Shares	74,930	0
Common stock for future issuance	8,810,856		6,977,906
Unrecognized stock-based compensation cost	$ 57.3
Unrecognized stock-based compensation cost expected period for recognition	3 years 3 months 14 days
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	100.00%
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	85.00%
Share purchases funded through payroll deductions minimum percentage of employee’s eligible compensation	1.00%
Share purchases funded through payroll deductions maximum percentage of employee’s eligible compensation	15.00%
Common stock for future issuance	741,662		360,086
Percentage of shares of common stock outstanding on last day of immediately preceding fiscal year	1.00%
Employee Stock Purchase Plan [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock for future issuance	360,086

Equity Incentive Plan - Summary

Equity Incentive Plan - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Shares available for grant, Beginning balance	2,707,947
Shares available for grant, Additional shares authorized	1,526,304
Shares available for grant, Stock options granted	(1,612,600)
Shares available for grant, Stock options exercised	(74,930)	0
Shares available for grant, Stock options forfeited	74,328
Shares available for grant, Ending balance	2,695,979		2,707,947
Shares available for grant, Exercisable	1,486,521
Number of shares, Beginning balance	3,909,873
Number of shares, Stock options granted	1,612,600
Number of shares, Stock options forfeited	(74,328)
Number of shares, Ending balance	5,373,215		3,909,873
Weighted- average exercise price per share, Beginning balance	$ 10.78
Weighted- average exercise price per share, Stock options granted	29.81
Weighted- average exercise price per share, Stock options exercised	3.57
Weighted average exercise price per share, Stock options forfeited	18.80
Weighted- average exercise price per share, Ending balance	16.48		$ 10.78
Weighted- average exercise price per share, Exercisable	$ 6.14
Weighted- average remaining contractual term	8 years 8 months 8 days		8 years 4 months 24 days
Weighted- average remaining contractual term, Exercisable	7 years 3 days
Aggregate intrinsic value	$ 64,399		$ 56,128
Aggregate intrinsic value, Exercisable	$ 32,325

Equity Incentive Plan - Summa_2

Equity Incentive Plan - Summary of Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	$ 2,962	$ 663
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	1,546	325
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	$ 1,416	$ 338

Equity Incentive Plan - Summa_3

Equity Incentive Plan - 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, 2021	Mar. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility	90.00%	85.00%
Risk-free interest rate, Minimum	0.35%	0.64%
Risk-free interest rate, Maximum	1.02%	1.45%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	4 years 3 months 29 days	5 years
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years 7 days	6 years 29 days

Net Loss Per Share Attributab_3

Net Loss Per Share Attributable to Common Stockholders - Summary of Outstanding Potentially Dilutive Shares Excluded From Calculation of Diluted Net Loss Per Share (Detail) - shares	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from the computation of diluted net loss per share	5,377,608	14,820,604
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	4,393
Redeemable Convertible Preferred Stock on an as-converted Basis [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		12,684,214
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	5,373,215	2,136,390