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Stock-based Compensation | 12 Months Ended
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Dec. 31, 2023
Share-Based Payment Arrangement [Abstract] |
Stock-Based Compensation | 8. Stock-based Compensation The Company recognized stock-based compensation as follows (in thousands):
|  | Year Ended December 31, |
|  | 2023 |  |  | 2022 |  |  | 2021 |
Research and development |  | $ | 22,407 |  |  | $ | 24,895 |  |  | $ | 21,443 |
General and administrative |  |  | 20,391 |  |  |  | 21,251 |  |  |  | 19,342 |
Total stock-based compensation |  | $ | 42,798 |  |  | $ | 46,146 |  |  | $ | 40,785 |
Determination of Fair Value The estimated grant-date fair value of all the Company’s options to purchase common stock was calculated using the Black-Scholes option pricing model, based on the following assumptions:
|  | Year Ended December 31,
|  | 2023 |  | 2022 |  | 2021
Expected term (in years) |  | 5.3 – 6.0 |  | 5.3 – 6.1 |  | 5.2 – 6.1
Expected volatility |  | 79% – 80% |  | 79% – 80% |  | 78% – 81%
Risk free interest rate |  | 3.5% – 4.6% |  | 1.5% – 4.3% |  | 0.5% – 1.3%
Dividend yield |  | — |  | — |  | —
The fair value of each stock option was determined by the Company using the methods and assumptions discussed below. Each of these inputs is subjective and generally requires judgment and estimation by management. Expected Term—The expected term represents the period that stock-based awards are expected to be outstanding. The expected term was derived by using the simplified method which uses the midpoint between the average vesting term and the contractual expiration period of the stock-based award. Expected Volatility—The Company has limited information on the volatility of stock options as the shares were not actively traded on any public markets prior to February 7, 2019. The expected volatility was derived from the historical stock volatilities of comparable peer public companies within its industry. These companies are considered to be comparable to the Company’s business over a period equivalent to the expected term of the stock-based awards. In 2020, the Company began giving weight to in its own historical volatility in the determination of expected volatility. Risk-Free Interest Rate—The risk-free interest rate is based on the U.S. Treasury yield curve in effect at the date of grant for zero-coupon U.S. Treasury notes with maturities approximately equal to the expected term. Expected Dividend Rate—The expected dividend is zero as the Company has not paid nor does it anticipate paying any dividends on its common stock in the foreseeable future. 2019 Equity Incentive Plan and 2022 Inducement Plan On February 6, 2019, the Company adopted the 2019 Equity Incentive Plan (2019 Plan) under which the Board may issue incentive stock options, nonstatutory stock options, restricted stock, restricted stock units, stock appreciation rights, performance units, and performance shares to the Company’s employees, directors, and consultants. The Company’s 2017 Stock Option and Grant Plan (2017 Plan) was terminated; however, shares subject to awards granted under it will continue to be governed by the 2017 Plan. Shares reserved for issuance but not issued pursuant to, or not subject to, awards granted under the 2017 Plan were added to the available shares in the 2019 Plan. Shares subject to awards granted under the 2017 Plan that are repurchased by, or forfeited to, the Company will also be reserved for issuance under the 2019 Plan. The board of directors, or a committee appointed by the board of directors, has the authority to determine to whom options or shares will be granted, the number of shares, the term, and the exercise price. If an individual owns stock representing 10% or more of the outstanding shares, the exercise price of each share shall be at least 110% of the fair market value and the term of the award shall not exceed five years. All other options granted under the 2019 Plan must have an exercise price at least equal to the fair market value on the date of grant and have a term not to exceed ten years. The stock options generally vest over a four-year period with one forty-eighth of the shares vesting each month or over a four-year period with 25% vesting at the one-year cliff and monthly thereafter. The RSUs generally vest over a period of three years with one-twelfth of the shares vesting quarterly. On January 1, 2023, the Company added 4,144,785 shares to the shares reserved for issuance under the 2019 Equity Incentive Plan. As of December 31, 2023, the Company had reserved 23,018,705 shares of common stock under the 2019 Plan, of which 6,579,341 shares were available for issuance of future awards. On January 1, 2022, the Company adopted the 2022 Inducement Plan (Inducement Plan) and reserved 1,630,000 shares for issuance under the Inducement Plan for the grant of equity-based awards to individuals who were not previously employees or non-employee directors of the Company. On September 22, 2022, the Company increased the number of shares available for issuance under the 2022 Inducement Plan to a total of 3,300,000 shares. As of December 31, 2023, 1,407,389 shares were available for issuance of future awards under the Inducement Plan. Option activity is shown below:
|  | Number of Options |  |  | Weighted Average Exercise Price Per Share |  |  | Weighted Average Remaining Contractual Term |  |  | Aggregate Intrinsic Value |
|  |  |  |  |  |  |  | (In years) |  |  | (In thousands) |
Outstanding as of December 31, 2021 |  |  | 11,644,070 |  |  | $ | 16.41 |  |  |  |  |  |  |
Granted |  |  | 4,996,366 |  |  |  | 12.11 |  |  |  |  |  |  |
Exercised |  |  | (298,239 | ) |  |  | 10.26 |  |  |  |  |  |  |
Forfeited |  |  | (2,628,925 | ) |  |  | 18.28 |  |  |  |  |  |  |
Outstanding as of December 31, 2022 |  |  | 13,713,272 |  |  |  | 14.62 |  |  |  |  |  |  |
Granted |  |  | 572,538 |  |  |  | 6.98 |  |  |  |  |  |  |
Exercised |  |  | (132,192 | ) |  |  | 8.16 |  |  |  |  |  |  |
Forfeited |  |  | (2,057,381 | ) |  |  | 14.21 |  |  |  |  |  |  |
Outstanding as of December 31, 2023 |  |  | 12,096,237 |  |  | $ | 14.40 |  |  |  | 6.7 |  |  | $ | 676 |
Exercisable as of December 31, 2023 |  |  | 8,467,987 |  |  | $ | 14.80 |  |  |  | 6.0 |  |  | $ | 62 |
Vested and expected to vest as of December 31, 2023 |  |  | 12,096,237 |  |  | $ | 14.40 |  |  |  | 6.7 |  |  | $ | 676 |
The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying stock options and the fair value of the Company’s common stock for stock options that were in-the-money. The aggregate intrinsic value of options exercised was less than $0.1 million, $1.2 million, and $37.1 million for the years ended December 31, 2023, 2022, and 2021, respectively. The weighted-average grant-date fair value per share of options granted was $4.93, $8.42, and $14.70, for the years ended December 31, 2023, 2022, and 2021, respectively. As of December 31, 2023, total unrecognized stock-based compensation related to unvested stock options was $32.7 million, which the Company expects to recognize over a remaining weighted-average period of 2.1 years. Restricted Stock Unit Activity Activity for the RSUs is shown below. In May 2021 and January 2022, the Company issued RSUs with market conditions to certain executives, which are also included in the table below. The RSUs with market conditions are earned based on stock price performance and continued service by the employee. The RSUs with market conditions trigger vesting upon the Company’s stock price attaining a specified level over a specified period of time. The shares then vest quarterly over one year after attainment. The Company used a Monte Carlo simulation model to determine the fair value of the awards at the grant date. The Monte Carlo model uses the fair value inputs on the grant date to run simulations and take an average of possible outcomes. The total grant date fair value of the RSUs with market condition was $6.6 million to be amortized over an estimated weighted average service period of 2.1 years. Compensation expense related to awards with market-based conditions is recognized regardless of whether the market condition is ultimately satisfied if the related service has been provided.
|  | Number of Shares |  |  | Weighted Average Grant Date Fair Value per Share |
Unvested restricted stock units as of December 31, 2021 |  |  | 1,373,874 |  |  | $ | 18.35 |
Granted |  |  | 2,539,014 |  |  |  | 10.02 |
Vested |  |  | (414,577 | ) |  |  | 18.63 |
Forfeited |  |  | (453,693 | ) |  |  | 16.21 |
Unvested restricted stock units as of December 31, 2022 |  |  | 3,044,618 |  |  |  | 11.68 |
Granted |  |  | 5,142,444 |  |  |  | 6.62 |
Vested |  |  | (1,584,448 | ) |  |  | 10.41 |
Forfeited |  |  | (510,513 | ) |  |  | 10.03 |
Unvested restricted stock units as of December 31, 2023 |  |  | 6,092,101 |  |  | $ | 7.78 |
As of December 31, 2023, total unrecognized stock-based compensation related to unvested restricted common stock issued to employees was $41.5 million, which the Company expects to recognize over a remaining weighted-average period of 2.3 years. 2019 Employee Stock Purchase Plan The 2019 Employee Stock Purchase Plan (2019 ESPP) enables eligible employees of the Company to purchase shares of common stock at a discount. Each offering period is approximately six months long beginning on the first trading day on or after June 1 and December 1 each year. ESPP participants purchase shares of common stock at a price per share equal to 85% of the lesser of (1) the fair market value per share of the common stock on the first trading day of the offering period or (2) the fair market value of the common stock on the purchase date. On January 1, 2023, 591,397 shares were added to the shares reserved for issuance under the 2019 ESPP pursuant to the annual automatic increase. As of December 31, 2023, there was $0.4 million in unrecognized compensation expense related to the 2019 ESPP expected to be recognized over five months.

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