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Stockholders' Equity
 12 Months Ended
Dec. 31, 2023
Equity [Abstract]  
Stockholders' Equity Stockholders’ Equity
Amended and Restated Certificate of Incorporation
On June 16, 2020, the Company’s certificate of incorporation was amended and restated to authorize 400,000,000 shares of common stock and 40,000,000 shares of undesignated preferred stock, each with a par value of $0.0001 per share. There was no preferred stock outstanding as of December 31, 2023, 2022, or 2021.
Common Stock
On June 16, 2020, the Company completed its IPO in which it sold 16,560,000 shares of common stock at an offering price of $18.00 per share. Proceeds from the IPO, net of underwriting discounts, commissions and offering costs of $24.0 million, were $274.1 million.
On July 2, 2021, the Company entered into a sales agreement (the 2021 Sales Agreement) with Cowen and Company, LLC (the Sales Agent), under which the Company may, from time to time, sell shares of its common stock having an aggregate offering price of up to $150.0 million through the Sales Agent. Sales of the Company’s common stock made pursuant to the 2021 Sales Agreement are made under the Company’s shelf registration statement on Form S-3, which became automatically effective upon filing on July 2, 2021 (the Shelf Registration Statement). On November 8, 2022, the Company entered into a sales agreement (the 2022 Sales Agreement) with the Sales Agent, with substantially similar terms as the 2021 Sales Agreement (collectively the Sales Agreements). Under the 2022 Sales Agreement, the Company may, from time to time, sell shares of its common stock having an aggregate offering price of up to $200.0 million through the Sales Agent. Sales of the shares of common stock, if any, will be made at prevailing market prices at the time of sale, or as otherwise agreed with the Sales Agent. The Company is not obligated to sell, and the Sales Agent is not obligated to buy or sell, any shares of common stock under the 2022 Sales Agreement. Upon entry into the 2022 Sales Agreement, the 2021 Sales Agreement was terminated. During the years ended December 31, 2023 and 2022, the Company sold 4,107,810 and 7,771,812 shares of its common stock, respectively, pursuant to the Sales Agreements and received net proceeds of $60.5 million and $121.1 million, respectively, after deducting offering-related transaction costs and commissions of $1.4 million and $3.7 million, respectively.
On August 6, 2021, the Company completed a public offering of 9,200,000 shares of its common stock at a public offering price of $18.00 per share. The net proceeds from the offering were $155.1 million, after deducting underwriting discounts, commissions and offering costs of $10.5 million. The shares sold in the offering were registered pursuant to the Company’s Shelf Registration Statement.
On December 15, 2022, the Company completed a public offering of 13,800,000 shares of its common stock at a public offering price of $17.25 per share. The net proceeds from the offering were $223.8 million, after deducting underwriting discounts, commissions and offering costs of $14.3 million. The shares sold in the offering were registered pursuant to the Company’s Shelf Registration Statement.
On November 27, 2023, the Company sold 5,075,304 unregistered shares of its common stock to BMS in a private placement under the terms of the BMS Purchase Agreement. The approximate proceeds related to the sale were $40.0 million of which $31.2 million was recorded as common stock and additional paid in capital, net of issuance costs of $0.1 million, and $8.7 million was recorded as deferred revenue (Note 5). The Company is required to file a registration statement covering these shares with the SEC on or before April 25, 2024.
Equity Incentive Plans
The Company's board of directors adopted, and the company's shareholders approved, the 2013 Equity Incentive Plan (the 2013 Plan) and the 2020 Incentive Award Plan (the 2020 Plan). Under the plans, the Company may grant stock options, restricted stock, dividend equivalents, restricted stock units, stock appreciation rights, and other stock or cash-based awards to individuals who are then employees, officers, non-employee directors or consultants of the Company. The Company ceased granting awards under the 2013 Plan in June 2020 upon adopting the 2020 Plan.
A total of 3,900,000 shares of common stock were initially reserved for issuance under the 2020 Plan. The number of shares of common stock available for issuance under the 2020 Plan will be increased annually on the first day of each fiscal year during the term of the 2020 Plan, beginning with the 2021 fiscal year, by an amount equal to the lesser of (a) 5% of the shares of common stock outstanding on the final day of the immediately preceding calendar year or (b) such smaller number of shares as determined by the Company’s board of directors. At December 31, 2023, 499,402 shares were available for grant under the 2020 Plan, inclusive of 3,500,000 additional shares which were reserved for issuance during the year ended December 31, 2023.
In December 2022, the Company’s board of directors adopted the 2022 Employment Inducement Incentive Award Plan (the Inducement Plan). Under the Inducement Plan, the Company may grant non-qualified stock options, restricted stock, restricted stock units, stock appreciation rights, and other stock or cash-based awards to an employee in connection with his or her commencement of employment with the Company or an affiliate. A total of 1,500,000 shares of common stock were reserved for issuance under the Inducement Plan. At December 31, 2023, 737,475 shares were available for grant under the Inducement Plan.
Stock Options
Options granted from the 2013 Plan, 2020 Plan, and the Inducement Plan are exercisable at various dates and will expire no more than ten years from their date of grant. Options generally vest over a four-year period. Prior to the IPO, the exercise price of options was determined by the Company’s board of directors. Following the IPO, the Company grants options with an exercise price equal to the fair market value of the Company’s stock on the date of the option grant.
Stock option activity in 2023 for employee and non-employee awards and related information is as follows (in thousands, except per share and contractual term data):
Number of Outstanding OptionsWeighted-
Average
Exercise
Price Per
Share		Weighted-
Average
Remaining
Contractual
Term
(in years)		Aggregate
Intrinsic
Value
Balance at December 31, 20229,352$15.28 
Granted4,01514.47 
Exercised(149)3.81 
Forfeited/expired(723)19.52 
Balance at December 31, 202312,495$14.91 7.91$15,833 
Vested and expected to vest at December 31, 202312,495$14.91 7.91$15,833 
Exercisable at December 31, 20236,246$14.39 7.11$13,061 
The aggregate intrinsic values presented in the table above were calculated as the difference between the closing price of the Company’s common stock at December 31, 2023 and the exercise price of stock options that had strike prices below the closing price.
The following summarizes additional information regarding stock options (in thousands, except per share data):
Year Ended December 31,
202320222021
Cash received from options exercised$573 $470 $930 
Intrinsic value of options exercised$2,421 $6,724 $3,436 
Weighted-average grant date fair value per share$10.33 $11.11 $16.78 
The total intrinsic values of options exercised were calculated as the difference between the fair value of the Company’s common stock at the time of the option exercise and the exercise price of that stock option.
Restricted Stock Units and Performance Stock Units
During the year ended December 31, 2023, under the 2020 Incentive Award Plan and the 2022 Employment Inducement Incentive Award Plan, the Company granted restricted stock units (RSUs) and performance stock units (PSUs) to employees of the Company. PSUs were only granted to the Company's officers.
RSUs are valued at the market price of a share of the Company’s stock on the date of grant. RSUs vest ratably on an annual basis over a four-year service period and are payable in shares of common stock on the vesting date. Compensation expense for RSUs is recognized on a straight-line basis over the four-year service period.
The following table summarizes the RSU activity for the year ended December 31, 2023 (in thousands, except per share data):
Number of SharesWeighted-Average Grant Date Fair Value
Unvested at December 31, 2022— $— 
Granted822 18.89 
Vested— — 
Forfeited(64)20.78 
Unvested at December 31, 2023
758 $18.73 
During the year ended December 31, 2023, the Company granted 750,000 PSUs at a weighted-average grant date fair value of $6.57 per share. The PSUs vest upon achievement of certain clinical milestones and continued employment thereafter. The PSUs have a two-year term and any unvested awards at the end of the term will be forfeited. Compensation expense for PSUs is recognized on a straight-line basis over the requisite service periods when the achievement of the performance condition is determined to be probable. If a performance condition is not determined to be probable or is not met, no stock-based compensation expense is recognized, and any previously recognized expense is reversed.
As of December 31, 2023, no PSUs were vested and no stock-based compensation expense was recognized as the performance conditions were not deemed probable.
Employee Stock Purchase Plan
In June 2020, the Company adopted the ESPP, which permits participants to contribute up to 15% of their eligible compensation during defined rolling six-month offering periods to purchase the Company’s common stock. The purchase price of the shares will be 85% of the lower of the fair market value of the Company’s common stock on the first day of trading of the offering period or on the applicable purchase date. The Company issued 175,511, 90,535, and 33,147 shares of common stock under the ESPP during the years ended December 31, 2023, 2022, and 2021, respectively. The Company had an outstanding liability of $0.1 million at December 31, 2023, which is included in accounts payable and accrued liabilities on the balance sheet, for employee contributions to the ESPP for shares pending issuance at the end of the current offering period. As of December 31, 2023, 372,517 shares of common stock were available for issuance under the ESPP.
Stock-Based Compensation Expense
The assumptions used in the Black-Scholes model to determine the fair value of stock option grants and shares purchasable under the ESPP were as follows:
OptionsESPP
Year Ended December 31,Year Ended December 31,
202320222021202320222021
Risk-free interest rate
3.5% - 4.9%
1.5% - 4.2%
0.5% - 1.3%
5.4%
2.2% - 4.7%
0.1%
Expected volatility
78% - 82%
85%
86% - 88%
68% - 76%
78% - 79%
69% - 75%
Expected term (in years)
5.5 - 6.1
5.5 - 6.1
5.5 - 6.1
0.50.50.5
Expected dividend yield—%—%—%—%—%—%
Risk-Free Interest Rate. The Company bases the risk-free interest rate assumption for equity awards on the rates for U.S. Treasury zero-coupon bonds with maturities similar to those of the expected term of the award being valued.
Expected Volatility. The expected volatility of stock options is estimated based on the average historical volatilities of common stock of comparable publicly traded companies and the Company's own volatility. The comparable companies are chosen based on their size and stage in the life cycle. 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. Prior to 2023, the Company exclusively used peer group companies to determine expected volatility. The expected volatility for employee stock purchases under the ESPP is based on the Company's own historical volatility for the prior six months to conform with the six-month ESPP offering period.
Expected Term. The Company's limited option exercise history does not provide a reasonable basis for estimating expected term, therefore the Company has estimated the expected life of its stock options using the simplified method, whereby the expected life equals the average of the vesting term and the original contractual term of the option. The expected life assumption for employee stock purchases under the ESPP is six months to conform with the six-month ESPP offering period.
Expected Dividend Yield. The Company’s expected dividend yield assumption is zero as it has never paid dividends and has no present intention to do so in the future.
The allocation of stock-based compensation expense was as follows (in thousands):
Year Ended December 31,
202320222021
Research and development expense$22,007 $15,222 $9,228 
General and administrative expense16,215 11,917 7,828 
Total stock-based compensation expense$38,222 $27,139 $17,056 
As of December 31, 2023, the unrecognized compensation cost related to outstanding time-based options and RSUs was $66.0 million and $11.1 million, respectively, which is expected to be recognized over a weighted-average period of 2.5 years and 3.2 years, respectively. Unrecognized compensation cost related to PSUs, which the Company has concluded are not probable of vesting, was $4.9 million. As of December 31, 2023, the unrecognized compensation cost related to stock purchase rights under the ESPP was $0.4 million, which is expected to be recognized over a weighted-average period of 0.5 years.