Equity Incentive Plans, Equity Inducement Plans, Employee Stock Purchase Plan and Stock-Based Compensation (Tables)
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12 Months Ended |
Dec. 31, 2023 |
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| Share-Based Payment Arrangement [Abstract] |
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| Summary of Option Activity |
The following table summarizes option activity under the Company’s 2004 Plan, 2018 Plan and 2021 Plan:
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Shares |
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Weighted-
Average
Exercise Price |
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Weighted-
Average
Remaining
Contract Term
(Years) |
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Aggregate
Intrinsic Value
(in thousands) |
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Stock options outstanding
at December 31, 2022 |
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7,310,611 |
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$ |
12.68 |
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|
6.66 |
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$ |
2,187 |
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Granted |
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1,611,500 |
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5.35 |
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Exercised |
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(53,060 |
) |
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5.92 |
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Canceled and forfeited |
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(964,019 |
) |
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12.63 |
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Stock options outstanding
at December 31, 2023 |
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7,905,032 |
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$ |
11.24 |
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|
6.02 |
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$ |
180 |
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Stock options exercisable
at December 31, 2023 |
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5,717,050 |
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$ |
12.42 |
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5.14 |
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$ |
14 |
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| 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:
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Year Ended December 31, |
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2023 |
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2022 |
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2021 |
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Expected term (in years) |
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5.3-6.1 |
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5.3-6.1 |
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|
5.3-6.1 |
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Expected volatility |
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80.7%-84.1% |
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81.8%-83.5% |
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80.9%-84.9% |
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Risk-free interest rate |
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3.6%-4.7% |
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1.7%-4.2% |
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0.6%-1.3% |
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Expected dividend |
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- |
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- |
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- |
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| Summary of Status and Activity of Non-vested RSUs |
A summary of the status and activity of non-vested RSUs for the year ended December 31, 2023 is as follows:
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Number of
Shares |
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Weighted
Average
Grant-Date
Fair Value |
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Non-vested December 31, 2022 |
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3,958,478 |
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$ |
11.70 |
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Granted |
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2,823,725 |
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|
5.41 |
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Released |
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(1,155,644 |
) |
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12.31 |
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Canceled |
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(381,686 |
) |
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|
9.92 |
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Non-vested December 31, 2023 |
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5,244,873 |
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$ |
8.31 |
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| Schedule of Fair Value of ESPP Shares Using Option Pricing Model |
For the years ended December 31, 2023, 2022 and 2021, the fair value of ESPP shares was estimated using the following assumptions:
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Year Ended December 31, |
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2023 |
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2022 |
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2021 |
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Expected term (in years) |
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0.5 |
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0.5 |
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0.5 |
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Expected volatility |
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79.6-89.9% |
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65.9-88.1% |
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65.9-111.4% |
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Risk-free interest rate |
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3.8%-5.5% |
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0.1%-3.8% |
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0.1 |
% |
Expected dividend |
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- |
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- |
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- |
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| Schedule of Stock-Based Compensation Expense Recognized |
Total stock-based compensation expense recognized was as follows:
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Year Ended December 31, |
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2023 |
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2022 |
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2021 |
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(in thousands) |
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Research and development expense: |
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Stock options |
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$ |
2,183 |
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$ |
2,287 |
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$ |
2,208 |
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Restricted stock units |
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8,885 |
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7,227 |
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|
4,280 |
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ESPP |
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|
764 |
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|
592 |
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|
638 |
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Subtotal |
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11,832 |
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|
10,106 |
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7,126 |
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General and administrative expense: |
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Stock options |
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6,066 |
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10,261 |
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11,045 |
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Restricted stock units |
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6,796 |
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|
5,781 |
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|
4,920 |
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ESPP |
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|
214 |
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|
156 |
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|
150 |
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Subtotal |
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13,076 |
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16,198 |
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|
16,115 |
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Total |
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$ |
24,908 |
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$ |
26,304 |
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$ |
23,241 |
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