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Equity Incentive Plans | 9 Months Ended
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Sep. 30, 2020
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 1,090,203 shares as of January 1, 2020. 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 272,550 shares as of January 1, 2020. 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 nine months ended September 30, 2020 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, 2020 | 3,039,880 |  |  | $ | 29.40 |  |  |  |  |
Granted | 461,425 |  |  | $ | 18.30 |  |  |  |  |
Exercises | (91,146) |  |  | $ | 4.06 |  |  |  |  |
Forfeitures and cancellations | (534,096) |  |  | $ | 37.54 |  |  |  |  |
Outstanding at September 30, 2020 | 2,876,063 |  |  | $ | 26.91 |  |  | 7.47 |  | $ | 6,747 |
Exercisable at September 30, 2020 | 1,355,530 |  |  | $ | 27.48 |  |  | 5.64 |  | $ | 6,365 |
Stock-Based Compensation Expense 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:
| Nine Months Ended September 30, |
| 2020 |  | 2019 |  |  |
Risk-free interest rate |  | 0.5 |  | % |  |  | 2.5 |  | % |  |  |  |
Expected volatility |  | 90.5 |  | % |  |  | 68.4 |  | % |  |  |  |
Expected dividend yield |  | — |  | % |  |  | — |  | % |  |  |  |
Expected term (in years) |  | 6.25 |  |  |  | 6.25 |  |  |  |  |
Weighted-average grant date fair value per share | $ | 13.88 |  |  |  | $ | 43.08 |  |  |  |  |  |
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 incorporates historical volatility of 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 September 30, |  | Nine Months Ended September 30,
(in thousands) | 2020 |  | 2019 |  | 2020 |  | 2019
Research and development | $ | 926 |  |  | $ | 1,580 |  |  | $ | 3,030 |  |  | $ | 4,469 |
General and administrative | 1,879 |  |  | 1,564 |  |  | 5,491 |  |  | 5,185 |
Total | $ | 2,805 |  |  | $ | 3,144 |  |  | $ | 8,521 |  |  | $ | 9,654 |
At September 30, 2020, there was $24.8 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 3.03 years.

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- Definition The entire disclosure for share-based payment arrangement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -URI http://asc.fasb.org/topic&trid=2228938 + Details
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