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Equity Incentive Plans | 12 Months Ended
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Dec. 31, 2023
Share-Based Payment Arrangement [Abstract] |
Equity Incentive Plans | EQUITY INCENTIVE PLANS 2008 Plan We granted options under our 2008 Stock Incentive Plan (2008 Plan) until June 2014 when it was terminated as to future awards, although it continues to govern the terms of options that remain outstanding under the 2008 Plan. The 2008 Plan provided for the granting of incentive and non-qualified stock options, and stock purchase rights to employees, directors and consultants at the discretion of the board of directors. Stock options granted generally vested over a period of four years from the date of grant. In connection with the board of directors and stockholders’ approval of the 2014 Plan, all remaining shares available for future award under the 2008 Plan were transferred to 2014 Plan, as discussed below, and the 2008 Plan was terminated. 2014 Plan The 2014 Equity Incentive Award Plan (2014 Plan) became effective on June 18, 2014. Under the 2014 Plan, 1.4 million shares of common stock were initially reserved for issuance pursuant to a variety of stock-based compensation awards, including stock options, stock appreciation rights (SARs), restricted stock awards, service-based restricted stock unit (RSU) awards, performance-based restricted stock unit (PRSU) awards, deferred stock awards, deferred stock unit awards, dividend equivalent awards, stock payment awards and performance awards. In addition, 35 thousand shares that had been available for future awards under the 2008 Plan as of June 18, 2014, were added to the initial reserve available under the 2014 Plan, bringing the total reserve upon the effective date of the 2014 Plan to 1.5 million shares. The number of shares initially reserved for issuance or transfer pursuant to awards under the 2014 Plan will be increased by (i) the number of shares represented by awards outstanding under 2008 Plan on June 18, 2014, that are either forfeited or lapse unexercised or that are repurchased for the original purchase price thereof, up to a maximum of 1.2 million shares, and (ii) if approved by the administrator of the 2014 Plan, an annual increase on the first day of each fiscal year ending in 2024 equal to the lesser of (A) four percent (4.0%) of the shares of stock outstanding (on an as converted basis) on the last day of the immediately preceding fiscal year and (B) such smaller number of shares of stock as determined by our board of directors; provided, however, that no more than 10.7 million shares of stock may be issued upon the exercise of incentive stock options. As of December 31, 2023, approximately 2.5 million shares of our common stock were available for future issuance under the 2014 Plan. 2016 Plan In November 2016, our board of directors approved the 2016 Employment Commencement Incentive Plan (Inducement Plan) under which 1.0 million shares were reserved. In January 2021, January 2022, December 2022 and January 2024, 0.5 million, 2.0 million, 3.0 million and 5.8 million shares, respectively, were added to the Inducement Plan. As of December 31, 2023, 6.0 million shares of our common stock were subject to inducement grants that were issued pursuant to the Inducement Plan. As of December 31, 2023, approximately 0.9 million shares of our common stock were available for future issuance under the 2016 Plan. Stock Options A summary of our stock option activity and related information during the year ended December 31, 2023 is as follows (in thousands, except per share dollar amounts and years):
|  |  | Options Issued and Outstanding |  | Weighted Average Remaining Contractual Term (in Years) |  | Aggregate Intrinsic Value
| Number of Shares |  | Weighted-Average Exercise Price per Share |  |
Balance at December 31, 2022 |  |  | 13,963 |  |  | $ | 4.83 |  |  |  |  |
Options granted |  |  | 8,914 |  |  | $ | 3.14 |  |  |  |  |
Options exercised |  |  | (226) |  |  | $ | 1.61 |  |  |  |  |
Options canceled |  |  | (483) |  |  | $ | 3.83 |  |  |  |  |
Balance at December 31, 2023 |  |  | 22,168 |  |  | $ | 4.20 |  |  | 7.3 |  | $ | 58,606 |
Vested and expected to vest at December 31, 2023 |  |  | 22,168 |  |  | $ | 4.20 |  |  | 7.3 |  | $ | 58,606 |
Exercisable at December 31, 2023 |  |  | 12,199 |  |  | $ | 5.30 |  |  | 6.1 |  | $ | 25,116 |
The aggregate intrinsic value represents the difference between the total pre-tax value (i.e., the difference between our stock price and the exercise price) of stock options outstanding as of December 31, 2023, based on our common stock closing price of $6.20 per share, which would have been received by the option holders if all their in-the-money options had been exercised as of that date. The intrinsic value of options exercised during the years ended December 31, 2023, 2022 and 2021, was $1.1 million, $30 thousand, and $1.7 million, respectively. The weighted-average grant-date estimated fair value of options granted during the years ended December 31, 2023, 2022 and 2021 was $2.36, $0.63 and $3.92 per share, respectively. The estimated grant date fair value of employee stock options was calculated using the Black-Scholes option-pricing model, based on the following weighted-average assumptions:
| Year Ended December 31,
| 2023 |  | 2022 |  | 2021
Expected term (years) | 5.1 |  | 4.9 |  | 5.0
Expected volatility | 97.6 | % |  | 92.1 | % |  | 77.0 | %
Risk-free interest rate | 3.8 | % |  | 2.2 | % |  | 4.7 | %
Dividend yield | — | % |  | — | % |  | — | %
Expected Term—We have limited historical information to develop reasonable expectations about future exercise patterns and post-vesting employment termination behavior for its stock-option grants. As such, the expected term was initially estimated using the simplified method whereby the expected term equals the arithmetic average of the vesting term and the original contractual term of the option. Beginning in 2021, we estimate the expected term of our options based upon historical exercises and post-vesting termination behavior, which has not resulted in a material difference as compared to using the simplified method. Expected Volatility—Since January 1, 2017, we use the historic volatility of our own stock over the retrospective period corresponding to the expected remaining term of the options, or the period since our shares were first quoted on The Nasdaq Global Market, if that is shorter, to compute our expected stock price volatility. Risk-Free Interest Rate—The risk-free interest rate assumption is based on the zero-coupon U.S. treasury instruments on the date of grant with a maturity date consistent with the expected term of our stock option grants. Dividend Yield—To date, we have not declared or paid any cash dividends and do not have any plans to do so in the future. Therefore, we use an expected dividend yield of zero. Restricted Stock Units A summary of our RSUs activity and related information for the year ended December 31, 2023 is as follows (in thousands, except per share dollar amounts):
| Number of RSUs |  | Weighted-Average Grant Date Fair Value Per Share |  |  |  |
Non-vested restricted stock units at December 31, 2022 | 1,406 |  |  | $ | 2.17 |  |  |  |  |
Granted | 3,269 |  |  | $ | 3.39 |  |  |  |  |
Vested | (942) |  |  | $ | 2.76 |  |  |  |  |
Forfeited | (87) |  |  | $ | 3.05 |  |  |  |  |
Non-vested restricted stock units at December 31, 2023 | 3,646 |  |  | $ | 3.09 |  |  |  |  |
The total estimated fair value of RSUs vested during the years ended December 31, 2023, 2022 and 2021 was $3.5 million, $2.6 million and $0.8 million, respectively. Issuance of Common Stock for Services During the years ended December 31, 2023, 2022 and 2021, we issued approximately 0.1 million, 0.7 million and 26 thousand shares, respectively, of common stock to members of the board of directors who elected to receive stock in lieu of their cash fees under our Non-Employee Director Compensation Program. The shares issued during the years ended December 31, 2023, 2022 and 2021 were valued at $0.3 million, $0.4 million and $0.2 million for each year, respectively, based on the fair value of the common stock on the date of grant. Employee Stock Purchase Plan We adopted the 2014 Employee Stock Purchase Plan (ESPP) and initially reserved approximately 0.2 million shares of common stock as of its effective date of June 18, 2014. If approved by the administrator of the ESPP, on the first day of each calendar year, ending in 2024, the number of shares in the reserve will increase by an amount equal to the lesser of (i) one percent (1.0%) of the shares of common stock outstanding on the last day of the immediately preceding fiscal year and (ii) such number of shares of common stock as determined by the board of directors; provided, however, no more than 2.2 million shares of our common stock may be issued under the ESPP. During the years ended December 31, 2023, 2022 and 2021, we issued approximately 436 thousand, 308 thousand and 387 thousand shares, respectively, at an average share price of $1.85, $0.63 and $2.12, respectively, pursuant to the ESPP. As of December 31, 2023, approximately 1.1 million shares of our common stock were available for future issuance under the ESPP. The following table illustrates the weighted-average assumptions for the Black-Scholes option-pricing model used in determining the fair value of ESPP purchase rights granted to our employees:
| Year Ended December 31,
| 2023 |  | 2022 |  | 2021
Expected term (years) | 0.5 |  | 0.5 |  | 0.5
Expected volatility | 86.0 | % |  | 97.2 | % |  | 123.0 | %
Risk-free interest rate | 5.3 | % |  | 1.9 | % |  | 0.7 | %
Dividend yield | — | % |  | — | % |  | — | %
Stock-based Compensation Expense Stock-based compensation expense recognized for stock options, RSUs, and our ESPP are recorded as operating expenses in our statements of operations and comprehensive loss, as follows (in thousands):
| Year Ended December 31,
| 2023 |  | 2022 |  | 2021
Selling, general and administrative | $ | 9,952 |  |  | $ | 7,525 |  |  | $ | 7,923 |
Research and development | 3,578 |  |  | 3,225 |  |  | 4,116 |
Total | $ | 13,530 |  |  | $ | 10,750 |  |  | $ | 12,039 |
A summary of our total unrecognized stock-based compensation expense, net of estimated forfeitures, as of December 31, 2023 is as follows (dollars in thousands):
| December 31, 2023
| Unrecognized Compensation Expense |  | Average Remaining Vesting Period (Years)
Stock option grants | $ | 19,960 |  |  | 2.75
RSU grants | $ | 10,579 |  |  | 3.03
ESPP | $ | 154 |  |  | 0.2

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