Equity Incentive Plans, Equity Inducement Plans, Employee Stock Purchase Plan and Stock-Based Compensation (Tables)
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12 Months Ended |
Dec. 31, 2022 |
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| Share-Based Payment Arrangement [Abstract] |
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| Summary of Option Activity |
The following table summarizes option activities under the Company’s 2004 Plan, 2018 Plan and 2021 Plan:
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Outstanding
Options |
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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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Balances at December 31, 2021 |
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6,512,086 |
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$ |
13.86 |
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7.39 |
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$ |
14,955 |
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Granted |
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1,376,500 |
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7.16 |
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Exercised |
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(49,654 |
) |
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5.40 |
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Canceled/Forfeited |
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(528,321 |
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13.54 |
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Balances 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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Exercisable at December 31, 2022 |
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5,201,489 |
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$ |
13.05 |
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5.88 |
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$ |
991 |
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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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2022 |
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2021 |
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2020 |
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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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3.1-7.0 |
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Expected volatility |
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81.8%-83.5% |
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80.9%-84.9% |
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73.2%-87.4% |
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Risk-free interest rate |
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1.7%-4.2% |
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0.6%-1.3% |
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0.2%-1.6% |
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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, 2022 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, 2021 |
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2,403,826 |
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$ |
18.43 |
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Granted |
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2,688,000 |
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7.62 |
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Released |
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(620,647 |
) |
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17.80 |
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Canceled |
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(512,701 |
) |
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14.44 |
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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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| Schedule of Fair Value of ESPP Shares Using Option Pricing Model |
For the years ended December 31, 2022, 2021 and 2020, the fair value of ESPP shares was estimated using the following assumptions:
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Year Ended December 31, |
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2022 |
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2021 |
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2020 |
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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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65.9-88.1% |
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65.9-111.4% |
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63.0%-111.4% |
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Risk-free interest rate |
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0.1%-3.8% |
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0.1 |
% |
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0.1%-1.9% |
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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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2022 |
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2021 |
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2020 |
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(in thousands) |
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Research and development expense: |
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Stock options |
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$ |
2,287 |
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$ |
2,208 |
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$ |
1,405 |
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Restricted stock units |
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7,227 |
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4,280 |
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|
770 |
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ESPP |
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592 |
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|
638 |
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|
512 |
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Subtotal |
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10,106 |
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7,126 |
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2,687 |
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General and administrative expense: |
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Stock options |
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10,261 |
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11,045 |
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7,098 |
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Restricted stock units |
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5,781 |
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4,920 |
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2,021 |
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ESPP |
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156 |
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150 |
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|
111 |
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Subtotal |
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16,198 |
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16,115 |
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9,230 |
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Total |
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$ |
26,304 |
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$ |
23,241 |
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$ |
11,917 |
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