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Equity Incentive Plans, Employee Stock Purchase Plan and Stock-Based Compensation (Tables) | 9 Months Ended
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Sep. 30, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract] |
Summary of Option Activity | The following table summarizes option activity under the Company’s 2004 Plan and 2018 Plan:
|  | Shares |  |  | Weighted- Average Exercise Price |  |  | Weighted- Average Remaining Contract Term (Years) |  |  | Aggregate Intrinsic Value (in thousands) |
Stock options outstanding at December 31, 2019 |  |  | 3,872,664 |  |  | $ | 12.89 |  |  |  | 7.88 |  |  | $ | 2,119 |
Granted |  |  | 1,655,078 |  |  | $ | 8.96 |  |  |  |  |  |  |  |  |
Exercised |  |  | (26,502 | ) |  | $ | 6.49 |  |  |  |  |  |  |  |  |
Canceled and forfeited |  |  | (112,989 | ) |  | $ | 10.45 |  |  |  |  |  |  |  |  |
Stock options outstanding at September 30, 2020 |  |  | 5,388,251 |  |  | $ | 11.77 |  |  |  | 7.82 |  |  | $ | 3,425 |
Stock options exercisable at September 30, 2020 |  |  | 2,482,870 |  |  | $ | 12.48 |  |  |  | 6.69 |  |  | $ | 1,389 |
Schedule of Employee Stock Options Valuation | For determining stock-based compensation expense, the fair-value-based measurement of each employee stock option was estimated as of the date of grant using the Black-Scholes option-pricing model with assumptions as follows:
|  | Nine Months Ended |
|  | September 30, |
|  | 2020 |  |  | 2019 |
Expected term (in years) |  | 5.06-7.01 |  |  | 5.07-7.01 |
Expected volatility |  | 73.15%-84.24% |  |  | 72.89%-74.89% |
Risk-free interest rate |  | 0.16%-1.62% |  |  | 1.42%-2.55% |
Expected dividend |  |  | – |  |  |  | – |
Summary of Status and Activity of Non-vested RSUs | A summary of the status and activity of non-vested RSUs at September 30, 2020 is as follows:
|  | Number of shares |  |  | Weighted Average Grant-Date Fair Value |
Non-vested December 31, 2019 |  |  | 335,799 |  |  | $ | 13.49 |
Granted |  |  | 505,250 |  |  | $ | 8.21 |
Vested and released |  |  | (151,976 | ) |  | $ | 14.03 |
Canceled and forfeited |  |  | (36,898 | ) |  | $ | 9.10 |
Non-vested September 30, 2020 |  |  | 652,175 |  |  | $ | 9.52 |
Schedule of Fair Value of ESPP Shares Using Option Pricing Model | For the nine months ended September 3 0, 20 20, the fair value of ESPP shares was estimated using the following assumptions:
| Nine Months Ended |
| September 30, |
| 2020 |  |  | 2019 |
Expected term (in years) | 0.5 |  |  | 0.5 |
Expected volatility |  | 94.10 | % |  |  | 63.04 | %
Risk-free interest rate |  | 0.38 | % |  |  | 1.92 | %
Expected dividend |  | – |  |  |  | – |
Schedule of Stock-Based Compensation Expense Recognized | Total stock-based compensation expense recognized was as follows:
|  | Three Months Ended |  |  | Nine Months Ended |
|  | September 30, |  |  | September 30, |
|  | 2020 |  |  | 2019 |  |  | 2020 |  |  | 2019 |
|  | (in thousands) |  |  | (in thousands) |
Research and development expense: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock options |  | $ | 403 |  |  | $ | 259 |  |  | $ | 987 |  |  | $ | 732 |
Restricted stock units |  |  | 210 |  |  |  | 190 |  |  |  | 617 |  |  |  | 493 |
ESPP |  |  | 148 |  |  |  | 218 |  |  |  | 368 |  |  |  | 295 |
Subtotal |  |  | 761 |  |  |  | 667 |  |  |  | 1,972 |  |  |  | 1,520 |
General and administrative expense: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stock options |  |  | 1,799 |  |  |  | 1,766 |  |  |  | 5,257 |  |  |  | 4,920 |
Restricted stock units |  |  | 532 |  |  |  | 381 |  |  |  | 1,521 |  |  |  | 1,095 |
ESPP |  |  | 20 |  |  |  | 66 |  |  |  | 80 |  |  |  | 94 |
Subtotal |  |  | 2,351 |  |  |  | 2,213 |  |  |  | 6,858 |  |  |  | 6,109 |
Total |  | $ | 3,112 |  |  | $ | 2,880 |  |  | $ | 8,830 |  |  | $ | 7,629 |
Summary of the Status of Call Options | A summary of the status of the call options at September 30, 2020 and at December 31, 2019 is as follows:
|  | September 30, |  |  | December 31, |
|  | 2020 |  |  | 2019 |
|  | Shares |  |  | Shares |
Options vested and exercised |  |  | 191,153 |  |  |  | 191,153 |
Options vested and outstanding |  |  | 66,681 |  |  |  | - |
Options unvested and outstanding |  |  | 8,890 |  |  |  | 75,571 |
Total options granted |  |  | 266,724 |  |  |  | 266,724 |

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