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Equity Incentive Plans | 6 Months Ended
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Jun. 30, 2021
Share-based Payment Arrangement [Abstract] |
Equity Incentive Plans | Equity Incentive Plans 2017 Equity Incentive Plan On January 12, 2017, our board of directors and stockholders approved and adopted the 2017 Equity Incentive Plan (the “2017 Plan”). The 2017 Plan became effective upon the execution and delivery of the underwriting agreement for our initial public offering on January 26, 2017 and replaced our existing 2006 Equity Incentive Plan. Under the 2017 Plan, we may grant stock options, stock appreciation rights, restricted stock, restricted stock units and other awards to individuals who are then our employees, officers, directors or consultants. In addition, the number of shares of stock available for issuance under the 2017 Plan will be automatically increased each January 1, beginning on January 1, 2018, by 4% of the aggregate number of outstanding shares of our common stock as of the immediately preceding December 31 or such lesser number as determined by our board of directors. The 2017 Plan automatically increased by 1,094,223 shares as of January 1, 2021. Employee Stock Purchase Plan On January 12, 2017, our board of directors and stockholders approved and adopted the 2017 Employee Stock Purchase Plan or the ESPP. The ESPP became effective upon the execution and delivery of the underwriting agreement for our initial public offering on January 26, 2017. In addition, the number shares of stock available for issuance under the ESPP will be automatically increased each January 1, beginning on January 1, 2018, by 1% of the aggregate number of outstanding shares of our common stock as of the immediately preceding December 31 or such lesser number as determined by our board of directors. The ESPP automatically increased by 273,555 shares as of January 1, 2021. The initial offering period for the ESPP began in May 2021. Stock Options Stock options granted to employees and non-employees generally vest over a four-year period while stock options granted to directors vest over a one year period. Each stock option award has a maximum term of 10 years from the date of grant, subject to earlier cancellation prior to vesting upon cessation of service to us. A summary of the activity related to stock option awards during the six months ended June 30, 2021 is as follows:
| Shares Subject to Options |  | Weighted-Average Exercise Price per Share |  | Weighted-Average Remaining Contractual Term (in years) |  | Aggregate Intrinsic Value (in thousands)
Outstanding at January 1, 2021 | 2,920,700 |  |  | $ | 26.67 |  |  |  |  |
Granted | 1,037,374 |  |  | $ | 29.52 |  |  |  |  |
Exercises | (77,034) |  |  | $ | 9.86 |  |  |  |  |
Forfeitures and cancellations | (149,232) |  |  | $ | 29.47 |  |  |  |  |
Outstanding at June 30, 2021 | 3,731,808 |  |  | $ | 27.70 |  |  | 7.51 |  | $ | 24,823 |
Exercisable at June 30, 2021 | 1,744,975 |  |  | $ | 27.82 |  |  | 5.76 |  | $ | 17,109 |
Stock-Based Compensation Expense We recognize stock-based compensation expense for awards issued to employees and non-employees over the requisite service period based on the estimated grant-date fair value of such awards. We record the expense for stock-based compensation awards subject to performance-based milestone vesting over the requisite service period when management determines that achievement of the milestone is probable. Management evaluates when the achievement of a performance-based milestone is probable based on the expected satisfaction of the performance conditions at each reporting date. The estimated fair values of stock option awards granted to employees were determined on the date of grant using the Black-Scholes option valuation model with the following weighted-average assumptions:
| Six Months Ended June 30, |
| 2021 |  | 2020 |  |  |
Risk-free interest rate |  | 0.7 | % |  |  | 1.2 | % |  |  |  |
Expected volatility |  | 93.1 | % |  |  | 91.8 | % |  |  |  |
Expected dividend yield |  | — |  | % |  |  | — |  | % |  |  |  |
Expected term (in years) |  | 6.18 |  |  |  | 6.25 |  |  |  |  |
Weighted-average grant date fair value per share | $ | 22.39 |  |  | $ | 11.97 |  |  |  |  |
We determine the appropriate risk-free interest rate, expected term for employee stock-based awards, contractual term for non-employee stock-based awards, and volatility assumptions. The weighted-average expected option term for employee and non-employee stock-based awards reflects the application of the simplified method, which defines the life as the average of the contractual term of the options and the weighted-average vesting period for all option tranches. Estimated volatility for 2021 incorporates the historical volatility of our stock price, and volatility for 2020 incorporates our stock price as well as similar entities whose share prices are publicly available. The risk-free interest rate is based upon U.S. Treasury securities with remaining terms similar to the expected or contractual term of the stock-based payment awards. The assumed dividend yield is based on our expectation of not paying dividends in the foreseeable future. Total non-cash stock-based compensation expense for all stock awards that was recognized in the consolidated statements of operations and comprehensive loss is as follows:
| Three Months Ended June 30, |  | Six Months Ended June 30,
(in thousands) | 2021 |  | 2020 |  | 2021 |  | 2020
Research and development | $ | 1,435 |  |  | $ | 958 |  |  | $ | 2,629 |  |  | $ | 2,104 |
General and administrative | 2,255 |  |  | 1,783 |  |  | 4,376 |  |  | 3,612 |
Total | $ | 3,690 |  |  | $ | 2,741 |  |  | $ | 7,005 |  |  | $ | 5,716 |
At June 30, 2021, there was $36.8 million of unrecognized compensation cost related to unvested stock option awards, which is expected to be recognized over a remaining weighted-average vesting period of 2.71 years. At June 30, 2021, there was $0.2 million of unrecognized compensation cost related to the ESPP, which is expected to be recognized over a remaining weighted-average vesting period of 0.38 years.

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