0000950170-21-004106 | 10-Q | XBRL Viewer | Silverback Therapeutics INC

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Cover Page - shares	9 Months Ended
Cover Page - shares	Sep. 30, 2021	Nov. 08, 2021
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Entity Registrant Name	Silverback Therapeutics, Inc.
Entity Central Index Key	0001671858
Document Period End Date	Sep. 30, 2021
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Document Quarterly Report	true
Document Transition Report	false
Entity Interactive Data Current	Yes
Entity Current Reporting Status	Yes
Title of 12(b) Security	Common Stock
Trading Symbol	SBTX
Security Exchange Name	NASDAQ
Entity File Number	001-39756
Entity Tax Identification Number	81-1489190
Entity Incorporation, State or Country Code	DE
Entity Address, Address Line One	500 Fairview Ave N
Entity Address, Address Line Two	Suite 600
Entity Address, City or Town	Seattle
Entity Address, State or Province	WA
Entity Address, Postal Zip Code	98109
City Area Code	206
Local Phone Number	456-2900
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		35,076,083

Condensed Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 300,660	$ 386,569
Prepaid expenses and other current assets	4,695	4,087
Total current assets	305,355	390,656
Investments	39,938	0
Restricted cash	350	350
Right-of-use asset	5,011	2,180
Property and equipment, net	1,907	1,618
Total assets	352,561	394,804
Current liabilities:
Accounts payable	1,720	2,583
Accrued expenses	12,537	5,278
Term loan payable, net	0	844
Current portion of lease liability	1,090	896
Total current liabilities	15,347	9,601
Lease liability, net of current portion	5,080	2,326
Total liabilities	20,427	11,927
Commitments and contingencies (Note 10)
Stockholders' equity:
Preferred Stock, $0.0001 par value per share; 10,000,000 shares authorized at September 30, 2021 and December 31, 2020; no shares issued and outstanding at September 30, 2021 and December 31, 2020	0	0
Common stock, $0.0001 par value per share; 200,000,000 shares authorized at September 30, 2021 and December 31, 2020, 35,067,751 and 34,801,537 shares issued and 35,037,136 and 34,701,274 shares outstanding at September 30, 2021 and December 31, 2020, respectively	4	3
Additional paid-in capital	494,916	479,608
Accumulated other comprehensive loss	(34)	0
Accumulated deficit	(162,752)	(96,734)
Total stockholders' equity	332,134	382,877
Total liabilities, and stockholders' equity	$ 352,561	$ 394,804

Condensed Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2021	Dec. 31, 2020
Statement of Financial Position [Abstract]
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock , shares authorized	10,000,000	10,000,000
Preferred stock, shares, issued	0	0
Preferred stock, shares, outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares, issued	35,067,751	34,801,537
Common stock, shares, outstanding	35,037,136	34,701,274

Condensed Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Operating expenses:
Research and development	$ 15,641	$ 6,200	$ 45,630	$ 15,740
General and administrative	7,040	1,912	20,447	4,077
Total operating expenses	22,681	8,112	66,077	19,817
Loss from operations	(22,681)	(8,112)	(66,077)	(19,817)
Interest income (expense), net	26	(4)	59	(45)
Net loss	(22,655)	(8,116)	(66,018)	(19,862)
Unrealized loss on available-for-sale securities	(34)	0	(34)	0
Comprehensive loss attributable to common stockholders	$ (22,689)	$ (8,116)	$ (66,052)	$ (19,862)
Net loss per share applicable to common stockholders, basic and diluted	$ (0.65)	$ (11.97)	$ (1.89)	$ (29.53)
Weighted-average shares used in computing net loss per share attributable to common stockholders, basic and diluted	35,001,466	678,048	34,884,656	672,531

Condensed Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income [Member]	Accumulated Deficit [Member]	Redeemable Convertible Preferred Stock [Member]	Series B Redeemable Convertible Preferred Stock [Member]	Series C Redeemable Convertible Preferred Stock [Member]
Temporary equity, Beginning balance at Dec. 31, 2019						$ 53,174
Temporary equity, Beginning balance, shares at Dec. 31, 2019						15,714,283
Beginning balance at Dec. 31, 2019	$ (58,777)		$ 5,010		$ (63,787)
Beginning balance, shares at Dec. 31, 2019		664,431
Issuance of redeemable convertible preferred stock for cash (Note 7)							$ 21,458
Issuance of redeemable convertible preferred stock for cash (Note 7), shares							10,027,666
Issuance of Series B redeemable convertible preferred stock upon conversion of convertible notes							$ 10,095
Issuance of Series B redeemable convertible preferred stock upon conversion of convertible notes, shares							4,673,388
Exercise of common stock options and vesting of early exercised common stock options	6		6
Exercise of common stock options and vesting of early exercised common stock options, shares		5,350
Stock-based compensation	47		47
Net loss and comprehensive loss	(5,279)				(5,279)
Temporary Equity, Ending balance at Mar. 31, 2020						$ 84,727
Temporary Equity, Ending balance, shares at Mar. 31, 2020						30,415,337
Ending balance at Mar. 31, 2020	(64,003)		5,063		(69,066)
Ending balance, shares at Mar. 31, 2020		669,781
Proceeds received in advance of issuance of Series B redeemable convertible preferred stock (Note 7)							$ 12,048
Exercise of common stock options and vesting of early exercised common stock options	2		2
Exercise of common stock options and vesting of early exercised common stock options, shares		1,843
Stock-based compensation	128		128
Net loss and comprehensive loss	(6,467)				(6,467)
Temporary Equity, Ending balance at Jun. 30, 2020						$ 96,775
Temporary Equity, Ending balance, shares at Jun. 30, 2020						30,415,337
Ending balance at Jun. 30, 2020	(70,340)		5,193		(75,533)
Ending balance, shares at Jun. 30, 2020		671,624
Issuance of redeemable convertible preferred stock for cash (Note 7)							$ 34,895	$ 84,897
Issuance of redeemable convertible preferred stock for cash (Note 7), shares							21,732,862	24,926,685
Exercise of common stock options and vesting of early exercised common stock options	16		16
Exercise of common stock options and vesting of early exercised common stock options, shares		12,754
Stock-based compensation	191		191
Net loss and comprehensive loss	(8,116)				(8,116)
Temporary Equity, Ending balance at Sep. 30, 2020						$ 216,567
Temporary Equity, Ending balance, shares at Sep. 30, 2020						77,074,884
Ending balance at Sep. 30, 2020	(78,249)		5,400		(83,649)
Ending balance, shares at Sep. 30, 2020		684,378
Beginning balance at Dec. 31, 2020	$ 382,877	$ 3	479,608		(96,734)
Beginning balance, shares at Dec. 31, 2020	34,701,274	34,701,274
Exercise of common stock options and vesting of early exercised common stock options	$ 254		254
Exercise of common stock options and vesting of early exercised common stock options, shares		125,930
Stock-based compensation	4,285		4,285
Net loss and comprehensive loss	(18,867)				(18,867)
Ending balance at Mar. 31, 2021	368,549	$ 3	484,147		(115,601)
Ending balance, shares at Mar. 31, 2021		34,827,204
Beginning balance at Dec. 31, 2020	$ 382,877	$ 3	479,608		(96,734)
Beginning balance, shares at Dec. 31, 2020	34,701,274	34,701,274
Exercise of common stock options and vesting of early exercised common stock options, shares	234,710
Ending balance at Sep. 30, 2021	$ 332,134	$ 4	494,916	$ (34)	(162,752)
Ending balance, shares at Sep. 30, 2021	35,037,136	35,037,136
Beginning balance at Mar. 31, 2021	$ 368,549	$ 3	484,147		(115,601)
Beginning balance, shares at Mar. 31, 2021		34,827,204
Exercise of common stock options, shares issued under the employee stock purchase plan, and vesting of early exercised common stock options	867	$ 1	866
Exercise of common stock options, shares issued under the employee stock purchase plan, and vesting of early exercised common stock options, shares		135,881
Stock-based compensation	4,730		4,730
Net loss and comprehensive loss	(24,496)				(24,496)
Ending balance at Jun. 30, 2021	349,650	$ 4	489,743		(140,097)
Ending balance, shares at Jun. 30, 2021		34,963,085
Exercise of common stock options and vesting of early exercised common stock options	154		154
Exercise of common stock options and vesting of early exercised common stock options, shares		74,051
Stock-based compensation	5,019		5,019
Net loss and comprehensive loss	(22,689)			(34)	(22,655)
Ending balance at Sep. 30, 2021	$ 332,134	$ 4	$ 494,916	$ (34)	$ (162,752)
Ending balance, shares at Sep. 30, 2021	35,037,136	35,037,136

Condensed Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Sep. 30, 2020	Mar. 31, 2020
Series B Redeemable Convertible Preferred Stock [Member]
Stock issuance costs		$ 76
Series C Redeemable Convertible Preferred Stock [Member]
Stock issuance costs	$ 53

Condensed Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
Condensed Statements of Cash Flows - USD ($) $ in Thousands	Sep. 30, 2021	Sep. 30, 2020
Cash flows from operating activities:
Net loss	$ (66,018)	$ (19,862)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	596	448
Stock-based compensation	14,034	366
Non-cash lease expense	903	797
Amortization of debt issuance costs	2	28
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(608)	(313)
Accounts payable and accrued expenses	6,422	(914)
Lease liability	(786)	(650)
Net cash used in operating activities	(45,455)	(20,100)
Cash flows from investing activities:
Purchases of available-for-sale securities	(39,971)	0
Purchase of property and equipment	(822)	(670)
Net cash used in investing activities	(40,793)	(670)
Cash flows from financing activities:
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs	0	153,401
Principal payments on term loan payable	(846)	(467)
Proceeds from exercise of common stock options and employee stock purchase plan	1,185	177
Net cash provided by financing activities	339	153,111
Change in cash, cash equivalents, and restricted cash	(85,909)	132,341
Cash, cash equivalents, and restricted cash at beginning of period	386,919	10,526
Cash, cash equivalents, and restricted cash at end of period	301,010	142,867
Supplemental disclosure of cash flow information:
Right-of-use assets and lease liabilities recognized	3,733	0
Deferred offering costs included in accounts payable and accrued liabilities	0	1,020
Issuance of Series B redeemable convertible preferred stock upon conversion of convertible notes	$ 0	$ 10,095

Nature of Business

9 Months Ended

Sep. 30, 2021

Accounting Policies [Abstract]

Nature of Business

1. Nature of Business

Silverback Therapeutics, Inc. (“Silverback” or “the Company”) is a clinical-stage biopharmaceutical company focused on leveraging its proprietary ImmunoTAC technology platform to develop systemically delivered and tissue targeted therapeutics for the treatment of cancer, chronic viral infections, and other serious diseases. The Company’s platform enables it to strategically pair proprietary linker-payloads that modulate key disease-modifying pathways with monoclonal antibodies directed at specific disease sites. The Company was formed in Seattle, Washington and incorporated in the state of Delaware on January 4, 2016.

Initial Public Offering and Related Transaction

On December 3, 2020, the Company’s registration statement on Form S-1 (File No. 333-250009) for its initial public offering of common stock (“IPO”) was declared effective by the Securities and Exchange Commission (“SEC”). On December 8, 2020, the Company issued and sold 13,225,000 shares of common stock in the IPO at a public offering price of $21.00 per share, resulting in net proceeds of $255.3 million after deducting underwriting discounts and commissions and offering expenses paid by the Company.

In connection with the IPO, all 77,074,884 shares of redeemable convertible preferred stock outstanding at the time of the IPO converted into 20,758,098 shares of the Company’s common stock.

Risks and Uncertainties

The Company is subject to a number of inherent risks which include, but are not limited to, the need to obtain adequate additional funding, possible failure of clinical trials or other events demonstrating a lack of clinical safety or efficacy of its product candidates, dependence on key personnel, reliance on third-party service providers for manufacturing drug product and conducting clinical trials, the ability to successfully secure its proprietary technology, and risks related to the regulatory approval and commercialization of a product candidate. Additionally, the development and commercialization of new drug products is highly competitive. Products or technologies developed by competitors may diminish or render obsolete the Company’s existing products under development.

Liquidity and Capital Resources

The accompanying financial statements have been prepared assuming that the Company will continue as a going concern. The Company has incurred net operating losses since its inception and had an accumulated deficit of $162.8 million as of September 30, 2021. The Company had cash, cash equivalents, restricted cash, and investments of $340.9 million as of September 30, 2021 and has not generated positive cash flows from operations. To date, the Company has funded its operations primarily through the issuance of redeemable convertible preferred stock, convertible notes, and the sale of common stock in connection with the IPO. The Company’s currently available cash, cash equivalents, restricted cash, and investments as of September 30, 2021 are sufficient to meet its anticipated cash requirements for the 12 months following the date the financial statements are issued. Management considers that there are no conditions or events, in the aggregate, that raise substantial doubt about the entity’s ability to continue as a going concern for a period of at least 12 months from the date the financial statements are issued.

Management expects operating losses to continue for the foreseeable future. There can be no assurance that the Company will ever earn revenues or achieve profitability, or if achieved, that they will be sustained on a continuing basis. In addition, the manufacturing, clinical and preclinical development activities as well as the commercialization of the Company’s products, if approved, will require significant additional financing. The Company may be unable to secure such financing when needed, or if available, such financings may be under terms that are unfavorable to the Company or the current stockholders. If the Company is unable to raise additional funds when needed, it may be required to delay, reduce the scope of, or eliminate development programs, which may adversely affect its business and operations.

Summary of Significant Accounting Policies

9 Months Ended

Sep. 30, 2021

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation

The financial statements have been prepared in conformity with U.S. generally accepted accounting principles (“U.S. GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative U.S. GAAP as found in the Accounting Standards Codification (“ASC”), and Accounting Standards Update (“ASU”), of the Financial Accounting Standards Board (“FASB”).

Unaudited Interim Condensed Financial Statements

The accompanying condensed balance sheet as of September 30, 2021, the condensed statements of operations and comprehensive loss and condensed statements of redeemable convertible preferred stock and stockholders’ equity (deficit) for the three and nine months ended September 30, 2021 and 2020, and the condensed statements of cash flows for the nine months ended September 30, 2021 and 2020, are unaudited. The balance sheet as of December 31, 2020 was derived from the audited financial statements as of and for the year ended December 31, 2020. The unaudited interim condensed financial statements have been prepared on a basis consistent with the audited annual financial statements as of and for the year ended December 31, 2020, and, in the opinion of management, reflect all adjustments, consisting solely of normal recurring adjustments, necessary for the fair presentation of the Company’s financial position as of September 30, 2021, the condensed results of its operations for the three and nine months ended September 30, 2021 and 2020, and its cash flows for the nine months ended September 30, 2021 and 2020. The financial data and other information disclosed in these notes related to the nine months ended September 30, 2021 and 2020 are also unaudited. The condensed results of operations for the three and nine months ended September 30, 2021 are not necessarily indicative of the results to be expected for the full year ending December 31, 2021 or any other period.

Use of Estimates

The preparation of the Company’s financial statements requires it to make estimates and assumptions that impact the reported amounts of assets, liabilities and expenses in the Company’s financial statements and accompanying notes. The most significant estimates in the Company’s financial statements relate to accruals for research and development expenses, valuation of equity awards, and valuation allowances for deferred tax assets. These estimates and assumptions are based on current facts, historical experience and various other factors believed to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying values of assets and liabilities and the recording of expenses that are not readily apparent from other sources. Actual results may differ materially and adversely from these estimates.

The full extent to which the coronavirus (“COVID-19”) pandemic will directly or indirectly impact the Company’s business, results of operations and financial condition, including expenses, clinical trials and research and development costs, will depend on future developments that are highly uncertain, including as a result of new information that may emerge concerning COVID-19 and the actions taken to contain or treat COVID-19, as well as the economic impact on local, regional, national and international markets. The Company has considered potential impacts arising from the COVID-19 pandemic and is not presently aware of any events or circumstances that would require the Company to update its estimates, judgments or revise the carrying value of its assets or liabilities.

Fair Value of Financial Instruments

Cash and cash equivalents, restricted cash, and investments are carried at fair value. Accounts payable and accrued expenses are carried at cost, which approximates fair value given their short-term nature. The term loan payable is carried at cost, which approximates fair value as its effective interest rate approximates current market rates.

Cash and Cash Equivalents

Cash equivalents are comprised of short-term, highly-liquid investments with maturities of 90 days or less at the date of purchase. At September 30, 2021 and December 31, 2020, the Company’s cash equivalents consisted of money market funds.

Restricted Cash

Restricted cash consists of a deposit securing a collateral letter of credit issued in connection with the Company’s facility operating lease.

The following table provides a reconciliation of cash and cash equivalents and restricted cash reported within the condensed balance sheets that sum to the amounts shown in the condensed statements of cash flows (in thousands):


	September 30, 2021		December 31, 2020
Cash and cash equivalents	$	300,660	$	386,569
Restricted cash		350		350
Total cash and cash equivalents and restricted cash	$	301,010	$	386,919

Investments

The Company invests excess cash in investment grade intermediate-term fixed income securities. These investments are included in long-term investments on the condensed balance sheets, classified as available-for-sale, and reported at fair value with unrealized gains and losses included in accumulated other comprehensive loss. Realized gains and losses on the sale of these securities are recognized in net loss.

The Company periodically evaluates whether declines in fair values of its investments 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 investment 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 investment before recovery of its amortized cost basis. Factors considered include quoted market prices, recent financial results and operating trends, implied values from any recent transactions or offers of investee securities, credit quality of debt instrument issuers, other publicly available information that may affect the value of the investments, duration and severity of the decline in value, and the Company's strategy and intentions for holding the investment.

Concentrations of Credit Risk

The Company is subject to credit risk from holding its cash and cash equivalents at a limited number of commercial banks. The Company limits its exposure to credit losses by investing in money market funds through a U.S. bank with high credit ratings and U.S. Treasury securities. Cash may consist of deposits held with banks that may at times exceed federally insured limits, however, exposure to credit risk in the event of default by the financial institution is limited to the extent of amounts recorded on the balance sheets. The Company has not experienced any losses in such accounts and management believes that the Company is not exposed to significant credit risk due to the financial position of the depository institutions in which those deposits are held.

Leases

Leases consist of the Company’s operating leases. In accordance with ASC 842, Leases, the Company determines if an arrangement is a lease at inception and evaluates each lease agreement to determine whether the lease is an operating or finance lease. For leases where the Company is the lessee, right-of-use (“ROU”) assets represent the Company’s right to use an underlying asset for the lease term and lease liabilities represent an obligation to make lease payments arising from the lease. ROU assets and lease liabilities are recognized at the lease commencement date based on the present value of lease payments over the lease term. The Company uses its incremental borrowing rate based on the information available at the commencement date in determining the present value of lease payments. Operating lease ROU assets also include any prepaid lease payments, lease incentives received, and costs which will be incurred in exiting a lease.

The Company’s leases include options to extend or terminate the leases. Periods covered by an option to extend a lease are included in the lease term when it is reasonably certain that the Company will exercise that option. Periods covered by an option to terminate a lease are included in the lease term when it is reasonably certain that the Company will not exercise that option.

Short-term leases with an initial term of 12 months or less are not recorded on the balance sheet. The Company does not have material short-term lease costs. Lease expense for minimum lease payments is recognized on a straight-line basis over the lease term. For real estate leases, the Company does not separate lease and non-lease components. The Company’s lease agreements do not contain any material residual value guarantees or material restrictive covenants.

Research and Development Expenses

All research and development costs are expensed in the period incurred. Research and development expenses consist primarily of direct and indirect costs incurred in connection with the development of the Company’s ImmunoTAC technology platform, discovery efforts, and preclinical study and clinical trial activities related to the Company’s program pipeline, including the Company’s lead product candidate, SBT6050, and other pipeline programs, including SBT6290 and SBT8230. Direct costs include expenses incurred under agreements with contract research organizations (“CROs”) and other vendors that conduct the Company’s preclinical and clinical activities, expenses associated with manufacturing the Company’s product candidates including under agreements with contract development and manufacturing organizations (“CDMOs”) and other vendors, and consulting fees. Indirect costs include personnel-related expenses, consisting of employee salaries, bonuses, benefits, and stock-based compensation expense and recruiting costs for personnel engaged in research and development activities, facility and equipment related expenses, consisting of indirect and allocated expenses for rent, depreciation, and equipment maintenance, and other unallocated research and development expenses incurred in connection with the Company’s research and development programs, including laboratory materials and supplies and license fees. Research and development expenses are charged to operating expenses as incurred when these expenditures relate to the Company’s research and development efforts and have no alternative future uses.

The Company is obligated to make upfront payments upon execution of certain research and development agreements. Advance payments, including nonrefundable amounts, for goods or services that will be used or rendered for future research and development activities are capitalized until such goods are delivered or the related services are performed, or such time when the Company does not expect the goods to be delivered or services to be performed. The Company estimates the period over which such services will be performed and the level of effort to be expended in each period. If actual timing of performance or the level of effort varies from the estimate, the Company will adjust the amounts recorded accordingly. Since inception, the Company has not experienced any material differences between accrued or prepaid costs and actual costs.

Stock-Based Compensation

The cost of employee services received in exchange for an award of an equity instrument is measured at the grant date based on the award’s estimated fair value using the Black-Scholes option pricing model. The estimated fair value of the awards is recognized into expense on a straight-line basis over the requisite service period. Stock-based compensation expense for an award with a performance condition is recognized when the achievement of such performance condition is determined to be probable. If the outcome of such performance condition is not determined to be probable or is not met, no compensation expense is recognized, and any previously recognized compensation expense is reversed. Management evaluates when the achievement of a performance condition is probable based on the expected satisfaction of the performance condition at each reporting date. Forfeitures are recognized as a reduction of stock-based compensation expense as they occur. The option plan permits, but does not require, the inclusion of early exercise provisions in individual awards. Proceeds from early option exercises are recorded as a liability until the underlying restricted shares vest. While the restricted shares have voting rights, they are not considered outstanding for accounting purposes.

Comprehensive Loss

Comprehensive loss is defined as a change in equity during a period from transactions and other events and circumstances from non-owner sources. The Company presents one continuous Statement of Operations and Comprehensive Loss. The Company’s comprehensive loss includes unrealized gains and losses on investments.

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 of potentially dilutive securities. Diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders since the effect of potentially dilutive securities is anti-dilutive given the net loss of the Company. For purposes of this calculation, redeemable convertible preferred stock, stock options, employee stock purchase rights, and unvested common stock subject to repurchase are considered to be common stock equivalents but are not included in the calculations of diluted net loss per share for the periods presented as their effect would be antidilutive.

Emerging Growth Company Status

The Company is an emerging growth company, as defined in the Jumpstart Our Business Startups Act of 2012 (“JOBS Act”). Under the JOBS Act, emerging growth companies can delay adopting new or revised accounting standards issued subsequent to the enactment of the JOBS Act until such time as those standards apply to private companies. The Company has elected to use this extended transition period for complying with certain new or revised accounting standards that have different effective dates for public and private companies until the earlier of the date that it is (1) no longer an emerging growth company or (2) affirmatively and irrevocably opt out of the extended transition period provided in the JOBS Act. As a result, these financial statements may not be comparable to companies that comply with the new or revised accounting pronouncements as of public company effective dates.

Recently Issued Accounting Pronouncements Not Yet Adopted

In June 2016, the FASB issued ASU 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. The objective of the standard is to provide information about expected credit losses on financial instruments at each reporting date and to change how other-than temporary impairments on investment securities are recorded. The guidance is effective for the Company beginning on January 1, 2023, with early adoption permitted. The Company is currently evaluating the impact the standard may have on its financial statements and related disclosures.

Fair Value Measurements

9 Months Ended

Sep. 30, 2021

Fair Value Disclosures [Abstract]

Fair Value Measurements

3. Fair Value Measurements

The Company follows authoritative accounting guidance, which among other things, defines fair value, establishes a consistent framework for measuring fair value, and expands disclosure for each major asset and liability category measured at fair value on either a recurring or nonrecurring basis. Fair value is defined as the exchange price that would be received to sell an asset or paid to transfer a liability (at exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. The three levels of inputs that may be used to measure fair value include:

Level 1: Quoted prices in active markets for identical assets or liabilities.

Level 2: Observable inputs other than Level 1 prices, such as quoted prices for similar assets or liabilities in active markets or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities.

Level 3: Unobservable inputs that are supported by little or no market activity.

The following table identifies the Company’s assets and liabilities that were measured at fair value on a recurring basis (in thousands):


September 30, 2021	Level	Amortized Cost		Gross unrealized gains		Gross unrealized losses		Fair Value
Money market funds	1	$	300,660	$	—	$	—	$	300,660
Long-term investments - U.S. Treasury securities	1		39,971		—	34			39,937
Total			340,631		—	34			340,597
December 31, 2020
Money market funds	1	$	386,369	$	—	$	—	$	386,369

There were no transfers between the Level 1 and Level 2 categories or into or out of the Level 3 category during the periods presented.

The Company’s long-term investments portfolio contains investments in U.S. Treasury securities that have an effective maturity date that is more than one year, but less than two years, from the respective balance sheet date. The Company evaluated its investments for other-than-temporary impairment and considers the decline in market value for the securities to be primarily attributable to current economic and market conditions. For the investments, it is not more-likely-than-not that the Company will be required to sell the investments, and the Company does not intend to do so prior to the recovery of the amortized cost basis.

Accrued Expenses

9 Months Ended

Sep. 30, 2021

Payables and Accruals [Abstract]

Accrued Expenses

4. Accrued Expenses

Accrued expenses consisted of the following (in thousands):


	September 30, 2021		December 31, 2020
Research and development expenses	$	8,638	$	2,063
Employee compensation and benefits		3,279		2,634
Professional services and other		620		581
Total accrued expenses	$	12,537	$	5,278

Leases

9 Months Ended

Sep. 30, 2021

Leases [Abstract]

Leases

5. Leases

The Company leases office and laboratory space in Seattle, Washington. The components of lease expense and related cash flows were as follows (in thousands):


	Three Months Ended September 30,				Nine Months Ended September 30,
	2021		2020		2021		2020
Lease expense
Operating lease expense	$	402	$	347	$	1,096	$	1,041
Variable lease expense	121		119			333		299
Total lease expense	523		466			1,429		1,340
Operating cash outflows from operating leases	$	499	$	396	$	1,333	$	1,183

On July 1, 2021, the Company entered into a sublease agreement for additional office space in Seattle, Washington. The commencement date of the sublease was August 1, 2021. The contractual term of the sublease is two years with an option to extend for one additional year and an option to terminate after one year subject to a termination fee. The annual minimum rent payable by the Company under the sublease is $0.3 million annually. The ROU asset obtained in exchange for the new operating lease liability is $0.6 million.

Other information related to the Company’s operating leases as of September 30, 2021 was as follows:


Weighted-average remaining lease term (in years)		4.7
Weighted-average discount rate		7.5%

During the three months ended September 30, 2021, there was a change in the expected lease term of the Company’s Seattle, Washington headquarters, whereby the Company became reasonably certain that it will not exercise its option to terminate the lease. As a result, the ROU asset and lease liability were remeasured using the present value of the remaining lease payments. The re-measurement resulted in an addition to the ROU asset and lease liability of $3.1 million. The change in the expected lease term has no effect on the classification of the lease.

Future minimum commitments due under the operating lease agreements as of September 30, 2021 are as follows (in thousands):


Years Ending December 31,	Amount
2021 (remaining 3 months)	$	268
2022		1,615
2023		1,513
2024		1,348
2025		1,389
Thereafter		1,186
Total undiscounted lease payments		7,319
Present value adjustment		(1,149	)
Total present value of lease payments	$	6,170

Term Loan Payable

9 Months Ended

Sep. 30, 2021

Debt Disclosure [Abstract]

Term Loan Payable

6. Term Loan Payable

In November 2016, the Company entered into a loan and security agreement with Silicon Valley Bank (“SVB”) and borrowed $3.5 million as a term loan. The outstanding principal amount of the term loan accrued interest at an annual rate of 1.75% per annum. At closing, the Company incurred de minimis debt issuance costs and owed a final payment fee of $0.3 million, both of which are amortized to interest expense over the remaining term of the debt under the effective interest method. The effective interest rate of the Company’s term loan was 5.14%.

The term loan’s original maturity date was November 1, 2020. However, in April 2020, the Company amended the loan and security agreement to defer principal payments for six months and extend the maturity date to May 1, 2021. There were no costs or additional warrant issuances in connection with this amendment. The Company accounted for the amendment as a debt modification and amortized the remaining debt discount over the remaining term.

On May 1, 2021, the Company made its final scheduled payment to SVB under the loan and security agreement including the final payment fee.

Stockholders' Equity (Deficit)

9 Months Ended

Sep. 30, 2021

Stockholders' Equity Note [Abstract]

Stockholders' Equity (Deficit)

7. Stockholders’ Equity (Deficit)

Redeemable Convertible Preferred Stock

Prior to its conversion to common stock in December 2020, the Company’s redeemable convertible preferred stock was classified as mezzanine equity on the Company’s balance sheets as the shares are contingently redeemable upon a deemed liquidation such as a change in control and in that event, there is no guarantee that all stockholders would be entitled to receive the same form of consideration. No accretion to redemption value was recorded during the nine months ended September 30, 2020 as a deemed liquidation event was not considered probable.

In March 2020, the Company issued 14,701,054 shares of its Series B redeemable convertible preferred stock, including 4,673,388 shares issued upon conversion of then outstanding convertible notes and accrued interest, and 10,027,666 shares issued for cash at a purchase price of $2.16 per share, resulting in gross proceeds of $21.5 million. The Series B purchase agreement provided that the Company would issue, and the Series B holders would purchase, an additional 21,732,862 shares of the Company’s Series B redeemable convertible preferred stock across two tranches for aggregate proceeds of $46.9 million in the event that certain agreed upon milestones were achieved or the preferred majority approved their closing. The Series B tranches did not meet the definition of freestanding instruments or the definition of derivatives, therefore, they were not accounted for separately or bifurcated.

On July 1, 2020, the Company issued 10,669,834 shares of its Series B redeemable convertible preferred stock for cash at a purchase price of $2.16 per share, resulting in gross proceeds of $23.0 million. Proceeds received in advance of the July 1, 2020 issuance of Series B redeemable convertible preferred stock totaled $12.0 million. Given the noncancelable nature of the Series B issuance on July 1, 2020 and in light of the call exercise notice mailed by the Company 20 days prior, the proceeds received in June 2020 were recorded as mezzanine equity at June 30, 2020 despite the related shares not becoming legally issued and outstanding until July 1, 2020.

In September 2020, the Company issued 11,063,028 additional shares of its Series B redeemable convertible preferred stock for cash at a purchase price of $2.16 per share, resulting in gross proceeds of $23.9 million, and 24,926,685 shares of its Series C redeemable convertible preferred stock for cash at a purchase price of $3.41 per share, resulting in gross proceeds of $84.9 million. The Series C purchase agreement did not contain any embedded features that meet the definition of freestanding instruments or the definition of derivatives, therefore, they were not accounted for separately or bifurcated.

Common Stock

The Company has reserved shares of common stock for the following potential future issuances:


	September 30, 2021		December 31, 2020
Shares underlying outstanding equity awards		6,451,285		6,316,569
Shares available for future equity award grants		2,877,457		1,506,806
Shares underlying early exercised equity awards		30,615		100,263
Shares underlying ESPP withholdings		22,668		4,393
Total		9,382,025		7,928,031

Stock-Based Compensation

9 Months Ended

Sep. 30, 2021

Share-based Payment Arrangement [Abstract]

Stock-Based Compensation

8. Stock-Based Compensation

Stock-based compensation expense recognized for all equity awards has been reported in the statements of operations and comprehensive loss as follows (in thousands):


	Three Months Ended September 30,				Nine Months Ended September 30,
	2021		2020		2021		2020
Research and development expense	$	2,144	$	87	$	6,147		191
General and administrative expense		2,875		104	$	7,887		175
Total stock-based compensation expense	$	5,019	$	191	$	14,034	$	366

As of September 30, 2021, the total unrecognized stock-based compensation expense was $56.7 million, which is expected to be recognized over a remaining weighted-average period of approximately 2.66 years.

Stock Option Awards

As of September 30, 2021, the Company’s equity incentive plans authorized a total of 9,564,798 shares, of which 2,877,457 shares are available for future grant, and 6,451,285 shares are outstanding. Not included in the outstanding option balance are 30,615 shares pursuant to stock options that were early exercised and subject to repurchase under the Company's 2016 Equity Incentive Plan that remain unvested as of September 30, 2021.

A summary of the Company’s stock option activity for the nine months ended September 30, 2021 is as follows (in thousands, except share and per share data and years):


	Stock Options Outstanding
	Shares Subject to Options Outstanding			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Life (Years)		Aggregate Intrinsic Value
Balance at December 31, 2020		6,316,569		$	10.93		9.5	$	223,647
Granted		436,773			36.64
Exercised		(234,710	)		2.65				1,721
Cancelled		(67,347	)		19.61
Balance at September 30, 2021		6,451,285			12.88		8.83		21,707
Vested at September 30, 2021		1,549,117			8.83		8.48		7,670

The total fair value of shares vested during the nine months ended September 30, 2021 and 2020 was $11.7 million and $0.2 million, respectively. The aggregate intrinsic value in the table above is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the Company’s common stock for all options that were in-the-money at September 30, 2021. The weighted-average grant date fair value per share of option grants for the nine months ended September 30, 2021 and 2020 was $24.92 and $1.47, respectively.

The grant date fair value of stock options was estimated using a Black-Scholes option pricing model with the following weighted-average assumptions:


	Nine Months Ended September 30,
	2021			2020
Expected term (in years)		5.9			6.0
Expected volatility		80	%		80	%
Risk-free interest rate		0.94	%		0.51	%
Expected dividend yield		—			—

The fair value of stock options was determined using the Black-Scholes option-pricing model and the assumptions below. Each of these inputs is subjective and generally requires significant judgement.

Fair Value of Common Stock. The grant date fair market value of the shares of common stock underlying stock options is determined by the Company’s board of directors. Following the closing of the Company’s IPO, the fair market value of the Company's common stock is based on its closing price as reported on the date of grant on the primary stock exchange on which the Company’s common stock is traded. Prior to the Company’s IPO, because there was no public market for the Company’s common stock, the board of directors exercised reasonable judgment and considered a number of objective and subjective factors to determine the best estimate of the fair market value, which included contemporaneous valuations performed by an independent third-party, the Company’s results of operations and financial position, including its levels of available capital resources, its stage of development and material risks related to the Company’s business, progress of the Company’s research and development activities, the Company’s business conditions and projections, the lack of marketability of the Company’s common stock and preferred stock as a private company, the prices at which the Company sold shares of its redeemable convertible preferred stock to outside investors in arms-length transactions, the rights, preferences and privileges of the Company’s redeemable convertible preferred stock relative to those of its common stock, the analysis of IPOs and the market performance of similar companies in the biopharmaceutical industry, the likelihood of achieving a liquidity event for the Company’s securityholders, such as an IPO or a sale of the company, given prevailing market conditions, the hiring of key personnel and the experience of management, trends and developments in the Company’s industry and external market conditions affecting the life sciences and biotechnology industry sectors.

Expected Term. The expected term represents the period that the options granted are expected to be outstanding. The expected term of stock options issued is determined using the simplified method (based on the mid-point between the vesting date and the end of the contractual term) as the Company has concluded that its stock option exercise history does not provide a reasonable basis upon which to estimate expected term.

Expected Volatility. Given the Company’s limited historical stock price volatility data, the Company derived the expected volatility from the average historical volatilities over a period approximately equal to the expected term of comparable publicly traded companies within its peer group that were deemed to be representative of future stock price trends as the Company has limited trading history for its common stock. 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.

Risk-free Interest Rate. The risk-free interest rate is based on the U.S. Treasury rate, with maturities similar to the expected term of the stock options.

Expected Dividend Yield. The Company has never paid dividends on its common stock and does not anticipate paying any dividends in the foreseeable future. Therefore, the Company uses an expected dividend yield of zero.

Licensing Agreement

9 Months Ended

Sep. 30, 2021

Licensing Agreement [Abstract]

Licensing Agreement

9. Licensing Agreement

Cell Line License Agreement with WuXi Biologics (Hong Kong) Limited

In October 2019, the Company entered into a cell line license agreement with WuXi Biologics (Hong Kong) Limited (“WuXi Bio”). Under the license agreement, WuXi Bio granted the Company a non-exclusive, worldwide, sublicensable, under certain of WuXi Bio’s intellectual property rights, know-how and biological materials (“WuXi Bio Licensed Technology”), to make, use, sell, offer for sale and import a product developed through the use of the WuXi Bio Licensed Technology (“WuXi Bio Licensed Product”). The WuXi Bio Licensed Technology is currently used to manufacture a component of the Company’s lead product, SBT6050. The Company has paid an aggregate of $150,000 in license fees that were recorded in research and development expense when incurred.

In the event the Company manufactures its commercial supplies of a product produced by the Licensed Cell Line using a manufacturer other than WuXi Bio or its affiliates, the Company will become obligated to pay WuXi Bio aggregate milestone payments, upon achievement of certain sales milestones, of up to $10.8 million.

The Company has the right to terminate the license by giving at least six months prior written notice to WuXi Bio and paying all amounts due to them through the termination date. In the event the Company fails to pay all amounts due to WuXi Bio under the license agreement, and fails to pay the amounts within 30 days after receiving written notice of such failure, WuXi Bio may terminate the license with 45 days written notice to the Company. In the event either party commits a material breach under the license and fails to cure the breach within 30 days after receiving written notice from the other party of such breach, either party may terminate the license immediately upon written notice to the other party.

Commitments and Contingencies

9 Months Ended

Sep. 30, 2021

Commitments and Contingencies Disclosure [Abstract]

Commitments and Contingencies

10. Commitments and Contingencies

Legal Proceedings

From time to time, the Company may have certain contingent liabilities that arise in the ordinary course of its business activities. The Company accrues a liability for such matters when it is probable that future expenditures will be made and can be reasonably estimated. Significant judgment is required to determine both probability and the estimated amount.

Indemnifications

In the ordinary course of business, the Company enters into agreements that may include indemnification provisions. Pursuant to such agreements, the Company may indemnify, hold harmless, and defend an indemnified party for losses suffered or incurred by the indemnified party. Some of the provisions will limit losses to those arising from third party actions. In some cases, the indemnification will continue after the termination of the agreement. The maximum potential amount of future payments the Company could be required to make under these provisions is not determinable. The Company has never incurred material costs to defend lawsuits or settle claims related to these indemnification provisions or related to any indemnification agreements. The Company has entered into indemnification agreements with its directors and officers that may require the Company to indemnify its directors and officers against liabilities that may arise by reason of their status or service as directors or officers to the fullest extent permitted by Delaware corporate law. The Company currently has directors’ and officers’ insurance coverage that reduces its exposure and enables the Company to recover a portion of any future amounts paid. The Company believes the estimated fair value of these indemnification agreements in excess of applicable insurance coverage is minimal.

COVID-19

The global COVID-19 pandemic continues to rapidly evolve, and management continue to monitor the situation closely. The extent of the impact of COVID-19 on the Company’s business, operations and clinical development and manufacturing timelines and plans remains uncertain, and will depend on certain developments, including the duration and spread of the outbreak and its impact on the Company’s clinical trial enrollment, trial sites, CROs, third-party manufacturers, supply chains necessary for research and development and manufacturing, and other third parties with whom the Company does business, as well as its impact on regulatory authorities and the Company’s key scientific and management personnel. The ultimate impact of the COVID-19 pandemic or a similar health epidemic is highly uncertain and subject to change. To the extent possible, management is conducting business as usual, with necessary or advisable modifications to employee travel and some of the Company’s non-lab based employees working remotely. Management will continue to actively monitor the rapidly evolving situation related to COVID-19 and may take further actions that alter Company operations, including those that may be required by federal, state or local authorities, or that management determines are in the best interests of the Company’s employees and other third parties with whom the Company does business. At this point, the extent to which the COVID-19 pandemic may affect the Company’s business, operations and clinical development timelines and plans, including the resulting impact on Company expenditures and capital needs, remains uncertain and is subject to change.

Employee Benefit Plans	9 Months Ended
Employee Benefit Plans	Sep. 30, 2021
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Abstract]
Employee Benefit Plans	11. Employee Benefit Plans The Company maintains a retirement plan, which is qualified under section 401(k) of the Internal Revenue Code of 1986, as amended, for the Company’s U.S. employees. The plan allows eligible employees to defer, at the employee’s discretion, pretax compensation up to the IRS annual limits. The Company does not match contributions made by employees.

Net Loss Per Share Attributable to Common Stockholders

9 Months Ended

Sep. 30, 2021

Earnings Per Share [Abstract]

Net Loss Per Share Attributable to Common Stockholders

12. Net Loss Per Share Attributable to Common Stockholders

The following outstanding shares of potentially dilutive securities were excluded from the computation of the diluted net loss per share attributable to common stockholders for the periods presented because their effect would have been anti-dilutive:


	As of September 30,
	2021		2020
Redeemable convertible preferred stock		—		20,758,098
Common stock options		6,451,285		2,468,163
Unvested common stock		30,615		124,233
Common stock warrants		—		9,154
ESPP withholdings		22,668		—
Total potentially dilutive shares		6,504,568		23,359,648

Subsequent Events

9 Months Ended

Sep. 30, 2021

Subsequent Events [Abstract]

Subsequent Events

13. Subsequent Events

On November 5, 2021, a securities class action complaint was filed against the Company and certain of the Company's officers and directors in the U.S. District for the Western District of Washington, captioned Dresner v. Silverback Therapeutics, Inc., et al., Case No. 2:21-cv-01499. The complaint alleges that between December 3, 2020 and September 10, 2021, the Company and certain of the Company's officers and directors violated (1) Sections 11 and 15 of the Securities Act; and (2) Sections 10(b) and 20(a) of the Exchange Act and SEC Rule 10b-5 promulgated thereunder, by making allegedly false and misleading statements in various SEC filings and press releases regarding the clinical and commercial prospects of the Company's lead product candidate, SBT6050. The complaint seeks unspecified damages and interest, as well as attorneys’ fees and other costs. The Company and the other defendants have not yet filed a response to the complaint, and do not anticipate doing so until after the court’s appointment of lead plaintiff and counsel, as well as the filing of an amended complaint.

The Company cannot predict the outcome of this suit, and failure by the Company to obtain a favorable resolution could have a material adverse effect on its business, results of operations and financial condition. The Company’s chances of success on the merits are still uncertain and any possible loss or range of loss cannot be reasonably estimated and as such the Company has not recorded a liability as of September 30, 2021.

Summary of Significant Accounting Policies (Polices)

9 Months Ended

Sep. 30, 2021

Accounting Policies [Abstract]

Basis of Presentation

Unaudited Interim Condensed Financial Statements

Use of Estimates

Fair Value of Financial Instruments

Cash and Cash Equivalents

Restricted Cash

Restricted cash consists of a deposit securing a collateral letter of credit issued in connection with the Company’s facility operating lease.


	September 30, 2021		December 31, 2020
Cash and cash equivalents	$	300,660	$	386,569
Restricted cash		350		350
Total cash and cash equivalents and restricted cash	$	301,010	$	386,919

Investments

Concentrations of Credit Risk

Leases

Research and Development Expenses

Stock-Based Compensation

Comprehensive Loss

Net Loss Per Share Attributable to Common Stockholders

Emerging Growth Company Status

Recently Issued Accounting Pronouncements Not Yet Adopted

Summary of Significant Accounting Policies (Tables)

9 Months Ended

Sep. 30, 2021

Accounting Policies [Abstract]

Summary of Reconciliation of Cash and Cash Equivalents and Restricted Cash


	September 30, 2021		December 31, 2020
Cash and cash equivalents	$	300,660	$	386,569
Restricted cash		350		350
Total cash and cash equivalents and restricted cash	$	301,010	$	386,919

Fair Value Measurements (Tables)

9 Months Ended

Sep. 30, 2021

Fair Value Disclosures [Abstract]

Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis

The following table identifies the Company’s assets and liabilities that were measured at fair value on a recurring basis (in thousands):


September 30, 2021	Level	Amortized Cost		Gross unrealized gains		Gross unrealized losses		Fair Value
Money market funds	1	$	300,660	$	—	$	—	$	300,660
Long-term investments - U.S. Treasury securities	1		39,971		—	34			39,937
Total			340,631		—	34			340,597
December 31, 2020
Money market funds	1	$	386,369	$	—	$	—	$	386,369

Accrued Expenses (Tables)

9 Months Ended

Sep. 30, 2021

Payables and Accruals [Abstract]

Summary of Accrued Expenses

Accrued expenses consisted of the following (in thousands):


	September 30, 2021		December 31, 2020
Research and development expenses	$	8,638	$	2,063
Employee compensation and benefits		3,279		2,634
Professional services and other		620		581
Total accrued expenses	$	12,537	$	5,278

Leases (Tables)

9 Months Ended

Sep. 30, 2021

Leases [Abstract]

Summary of Operating Lease Cost

The Company leases office and laboratory space in Seattle, Washington. The components of lease expense and related cash flows were as follows (in thousands):


	Three Months Ended September 30,				Nine Months Ended September 30,
	2021		2020		2021		2020
Lease expense
Operating lease expense	$	402	$	347	$	1,096	$	1,041
Variable lease expense	121		119			333		299
Total lease expense	523		466			1,429		1,340
Operating cash outflows from operating leases	$	499	$	396	$	1,333	$	1,183

Summary of other information related to Companys operating leases

Other information related to the Company’s operating leases as of September 30, 2021 was as follows:


Weighted-average remaining lease term (in years)		4.7
Weighted-average discount rate		7.5%

Summary of Future Minimum Commitments Due Under the Operating Lease Agreement

Future minimum commitments due under the operating lease agreements as of September 30, 2021 are as follows (in thousands):


Years Ending December 31,	Amount
2021 (remaining 3 months)	$	268
2022		1,615
2023		1,513
2024		1,348
2025		1,389
Thereafter		1,186
Total undiscounted lease payments		7,319
Present value adjustment		(1,149	)
Total present value of lease payments	$	6,170

Stockholders' Equity (Deficit) (Tables)

9 Months Ended

Sep. 30, 2021

Stockholders' Equity Note [Abstract]

Summary of Common Shares Reserved for Future Issuance

The Company has reserved shares of common stock for the following potential future issuances:


	September 30, 2021		December 31, 2020
Shares underlying outstanding equity awards		6,451,285		6,316,569
Shares available for future equity award grants		2,877,457		1,506,806
Shares underlying early exercised equity awards		30,615		100,263
Shares underlying ESPP withholdings		22,668		4,393
Total		9,382,025		7,928,031

Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2021

Share-based Payment Arrangement [Abstract]

Summary of Share-based Compensation Expense

Stock-based compensation expense recognized for all equity awards has been reported in the statements of operations and comprehensive loss as follows (in thousands):


	Three Months Ended September 30,				Nine Months Ended September 30,
	2021		2020		2021		2020
Research and development expense	$	2,144	$	87	$	6,147		191
General and administrative expense		2,875		104	$	7,887		175
Total stock-based compensation expense	$	5,019	$	191	$	14,034	$	366

Summary of Activity Related to Stock Option Grants

A summary of the Company’s stock option activity for the nine months ended September 30, 2021 is as follows (in thousands, except share and per share data and years):


	Stock Options Outstanding
	Shares Subject to Options Outstanding			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Life (Years)		Aggregate Intrinsic Value
Balance at December 31, 2020		6,316,569		$	10.93		9.5	$	223,647
Granted		436,773			36.64
Exercised		(234,710	)		2.65				1,721
Cancelled		(67,347	)		19.61
Balance at September 30, 2021		6,451,285			12.88		8.83		21,707
Vested at September 30, 2021		1,549,117			8.83		8.48		7,670

Summary of Fair Value of Each Option Grant Estimated Throughout Year Using Black-Scholes Option-pricing Model

The grant date fair value of stock options was estimated using a Black-Scholes option pricing model with the following weighted-average assumptions:


	Nine Months Ended September 30,
	2021			2020
Expected term (in years)		5.9			6.0
Expected volatility		80	%		80	%
Risk-free interest rate		0.94	%		0.51	%
Expected dividend yield		—			—

Net Loss Per Share Attributable to Common Stockholders (Tables)

9 Months Ended

Sep. 30, 2021

Earnings Per Share [Abstract]

Summary of Antidilutive Securities Excluded from Computation of Earnings Per Share Amount


	As of September 30,
	2021		2020
Redeemable convertible preferred stock		—		20,758,098
Common stock options		6,451,285		2,468,163
Unvested common stock		30,615		124,233
Common stock warrants		—		9,154
ESPP withholdings		22,668		—
Total potentially dilutive shares		6,504,568		23,359,648

Nature of Business - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Dec. 03, 2020	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Accumulated deficit		$ (162,752)	$ (96,734)
Cash, cash equivalents, and restricted cash		$ 300,660	$ 386,569
Common stock, shares, issued		35,067,751	34,801,537
Liquidity and Capital Resources [Member]
Cash, cash equivalents, and restricted cash		$ 340,900
Redeemable Convertible Preferred Stock [Member]
Redeemable convertible preferred stock outstanding				77,074,884	30,415,337	30,415,337	15,714,283
IPO [Member]
Offering price per share	$ 21.00
Net proceeds after deducting underwriting discounts and commissions and offering expenses	$ 255,300
Common stock, shares, issued	13,225,000
IPO [Member] \| Common Stock [Member]
Conversion of preferred stock, common shares issued	20,758,098
IPO [Member] \| Redeemable Convertible Preferred Stock [Member]
Redeemable convertible preferred stock outstanding	77,074,884

Summary of Significant Accounting Policies - Summary of Reconciliation of Cash and Cash Equivalents and Restricted Cash (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Cash and cash equivalents	$ 300,660	$ 386,569
Restricted cash	350	350
Total cash and cash equivalents and restricted cash	301,010	386,919	$ 142,867	$ 10,526
Restricted Cash [Member]
Cash and cash equivalents	300,660	386,569
Restricted cash	350	350
Total cash and cash equivalents and restricted cash	$ 301,010	$ 386,919

Fair Value Measurements - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Recurring [Member] - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	$ 340,631
Gross unrealized gains	0
Gross unrealized losses	34
Fair Value	340,597
Level 1 [Member] \| Long-Term Investments - U.S. Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	39,971
Gross unrealized gains	0
Gross unrealized losses	34
Fair Value	39,937
Level 1 [Member] \| Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	300,660	$ 386,369
Gross unrealized gains		0
Gross unrealized losses		0
Fair Value	$ 300,660	$ 386,369

Fair Value Measurements - Additional Information (Detail) - Fair Value, Recurring [Member] $ in Thousands	9 Months Ended
	Sep. 30, 2021 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value,assets transfers from Level 1 to Level 2	$ 0
Fair value,assets transfers from Level 2 to Level 1	0
Fair value, liabilities transfers from Level 1 to Level 2	0
Fair value, liabilities transfers from Level 2 to Level 1	0
Fair Value, assets transfers into (out of) Level 3	$ 0

Accrued Expenses - Summary of Accrued Expenses (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Payables and Accruals [Abstract]
Research and development expenses	$ 8,638	$ 2,063
Employee compensation and benefits	3,279	2,634
Professional services and other	620	581
Total accrued expenses	$ 12,537	$ 5,278

Leases - Summary of Operating Lease Cost (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Lease, Cost [Abstract]
Operating lease expense	$ 402	$ 347	$ 1,096	$ 1,041
Variable lease expense	121	119	333	299
Total lease expense	523	466	1,429	1,340
Operating cash outflows from operating leases	$ 499	$ 396	$ 1,333	$ 1,183

leases - Summary of other information related to companys operating leases (Details)	Sep. 30, 2021
Lessee, Lease, Description [Line Items]
Operating Lease Weighted Average Remaining Lease Term	4 years 8 months 12 days
Operating Lease Weighted Average Discount Rate	7.50%

Leases - Summary of Future Minimum Commitments Due Under Operating Lease Agreement (Detail) $ in Thousands	Sep. 30, 2021 USD ($)
Lessee, Operating Lease, Liability, Payment, Due [Abstract]
2021 (remaining 3 months)	$ 268
2022	1,615
2023	1,513
2024	1,348
2025	1,389
Thereafter	1,186
Total undiscounted lease payments	7,319
Present value adjustment	(1,149)
Total present value of lease payments	$ 6,170

Leases - Additional Information (Detail) - USD ($) $ in Thousands		3 Months Ended	9 Months Ended
	Jul. 01, 2021	Sep. 30, 2021	Sep. 30, 2021	Dec. 31, 2020
Lessee, Lease, Description [Line Items]
Operating Lease Weighted Average Remaining Lease Term		4 years 8 months 12 days	4 years 8 months 12 days
Operating Lease Weighted Average Discount Rate		7.50%	7.50%
Sublease agreement description			On July 1, 2021, the Company entered into a sublease agreement for additional office space in Seattle, Washington. The commencement date of the sublease was August 1, 2021. The contractual term of the sublease is two years with an option to extend for one additional year and an option to terminate after one year subject to a termination fee.
Contractual term of sublease	2 years
Option to Extend	option to extend for one additional year
Option to Terminate	option to terminate after one year subject to a termination fee
Annual minimum rent payable	$ 300
Right-of-use asset obtained in exchange for operating lease liability	$ 600	$ 3,100
ROU asset		$ 5,011	$ 5,011	$ 2,180

Term Loan Payable - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended		9 Months Ended
	Apr. 30, 2020	Nov. 30, 2016	Sep. 30, 2021	Sep. 30, 2020
Debt Instrument [Line Items]
Debt issuance costs			$ 2	$ 28
Term Loan [Member] \| Silicon Valley Bank [Member] \| Loan and Security Agreement [Member]
Debt Instrument [Line Items]
Term loan		$ 3,500
Annual Interest rate		1.75%
Debt issuance costs		$ 300
Effective interest rate			5.14%
Maturity date		Nov. 01, 2020
Term Loan [Member] \| Silicon Valley Bank [Member] \| Amended Loan and Security Agreement [Member]
Debt Instrument [Line Items]
Maturity date	May 01, 2021

Stockholders' Equity (Deficit) - Summary of Common Shares Reserved for Future Issuance (Detail) - shares	Sep. 30, 2021	Dec. 31, 2020
Class of Stock [Line Items]
Total	9,382,025	7,928,031
Shares Underlying Outstanding Equity Awards [Member]
Class of Stock [Line Items]
Total	6,451,285	6,316,569
Shares Available For Future Equity Award Grants [Member]
Class of Stock [Line Items]
Total	2,877,457	1,506,806
Shares Underlying Early Exercised Equity Awards [Member]
Class of Stock [Line Items]
Total	30,615	100,263
Shares Underlying ESPP Withholdings [Member]
Class of Stock [Line Items]
Total	22,668	4,393

Stockholders' Equity (Deficit) - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended		9 Months Ended
	Jul. 01, 2020	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2020
Redeemable Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Accretion to Redemption value				$ 0
Redeemable Stock Excersible On Agreed Upon Milestones [Member] \| Redeemable Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Issuance of Series B redeemable convertible preferred stock			21,732,862
Proceeds from Issuance of Convertible Preferred Stock			$ 46,900
Series B [Member] \| Redeemable Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Issuance of Series B redeemable convertible preferred stock			14,701,054
Issuance of Series B redeemable convertible preferred stock for cash	10,669,834	11,063,028	4,673,388
Issuance of Series B redeemable convertible preferred stock upon conversion of convertible notes			10,027,666
Preferred stock, Par or Stated Value Per Share	$ 2.16	$ 2.16	$ 2.16	$ 2.16
Proceeds from Issuance of Convertible Preferred Stock	$ 23,000	$ 23,900
Proceed received in advance for issuance of common stock	$ 12,000
Series B [Member] \| Redeemable Stock Excersible On Agreed Upon Milestones [Member]
Class of Stock [Line Items]
Proceeds from Issuance of Convertible Preferred Stock			$ 21,500
Series C [Member] \| Redeemable Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, Par or Stated Value Per Share		$ 3.41		$ 3.41
Proceeds from Issuance of Convertible Preferred Stock		$ 84,900
Issuance of series C redeemable convertible preferred stock for cash		24,926,685

Stock-Based Compensation - Summary of Share-based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	$ 5,019	$ 191	$ 14,034	$ 366
Research and development expense [Member]
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	2,144	87	6,147	191
General and administrative expense [Member]
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	$ 2,875	$ 104	$ 7,887	$ 175

Stock-Based Compensation - Summary of Activity Related to Stock Option Grants (Detail) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021 USD ($) $ / shares shares	Dec. 31, 2020 USD ($) $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares Subject to Options Outstanding, Beginning balance \| shares	6,316,569
Shares Subject to Options Outstanding, Granted \| shares	436,773
Shares Subject to Options Outstanding, Exercised \| shares	(234,710)
Shares Subject to Options Outstanding, Cancelled \| shares	(67,347)
Shares Subject to Options Outstanding, Ending balance \| shares	6,451,285	6,316,569
Shares Subject to Options Outstanding Vested, Ending balance \| shares	1,549,117
Weighted- Average Exercise Price, Beginning balance \| $ / shares	$ 10.93
Weighted- Average Exercise Price, Granted \| $ / shares	36.64
Weighted- Average Exercise Price, Exercised \| $ / shares	2.65
Weighted- Average Exercise Price, Cancelled \| $ / shares	19.61
Weighted- Average Exercise Price, Ending balance \| $ / shares	12.88	$ 10.93
Weighted- Average Exercise Price Vested, Ending balance \| $ / shares	$ 8.83
Weighted- Average Remaining Contractual Life (Years)	8 years 9 months 29 days	9 years 6 months
Weighted- Average Remaining Contractual Life (Years), Vested	8 years 5 months 23 days
Aggregate Intrinsic Value, Beginning balance \| $	$ 223,647
Aggregate Intrinsic Value, Exercised \| $	1,721
Aggregate Intrinsic Value, Ending balance \| $	21,707	$ 223,647
Aggregate Intrinsic Value, Vested Ending balance \| $	$ 7,670

Stock-Based Compensation - Summary of Fair Value of Each Option Grant Estimated Throughout Year Using Black-Scholes Option-pricing Model (Detail)	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Share-based Payment Arrangement [Abstract]
Expected term (in years)	5 years 10 months 24 days	6 years
Expected volatility	80.00%	80.00%
Risk-free interest rate	0.94%	0.51%
Expected dividend yield	0.00%	0.00%

Stock Based Compensation - Additional Information (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Share-based Payment Arrangement, Nonvested Award, Option, Cost Not yet Recognized, Amount	$ 56,700,000
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition	2 years 7 months 28 days
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	9,564,798
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number	6,451,285		6,316,569
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	6,504,568	23,359,648
Fair Value of Shares Vested	$ 11,700,000	$ 200,000
Share Based Compensation Number of Options Granted	436,773
Share Based Compensation Expected Dividend Yeild	$ 0
Shares Available for Future Equity Award Grants	9,382,025		7,928,031
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value	$ 24.92	$ 1.47
Stock Option Awards [Member]
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Shares Available for Future Equity Award Grants	2,877,457
Unvested Common Stock [Member]
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	30,615	124,233

Licensing Agreement - Additional Information (Detail) - WuXi Bio [Member] - USD ($)		9 Months Ended
	Oct. 31, 2019	Sep. 30, 2021
Aggregate milestone payments upon achievement of certain sales milestones		$ 10,800,000
Agreement payment and license Term desciption		In the event the Company fails to pay all amounts due to WuXi Bio under the license agreement, and fails to pay the amounts within 30 days after receiving written notice of such failure, WuXi Bio may terminate the license with 45 days written notice to the Company.
Research and Development Expense [Member]
License fees paid	$ 150,000

Net Loss Per Share Attributable to Common Stockholders - Summary of Antidilutive Securities Excluded from Computation of Earnings Per Share Amount (Detail) - shares	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive shares	6,504,568	23,359,648
Redeemable Convertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive shares	0	20,758,098
Common stock options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive shares	6,451,285	2,468,163
Unvested Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive shares	30,615	124,233
Common stock warrants [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive shares	0	9,154
ESPP withholdings [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive shares	22,668	0

Subsequent Events (Additional Information) (Details)	Nov. 05, 2021
Subsequent Event [Member]
Subsequent Event [Line Items]
Action Complaint Filed Date	November 5, 2021