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Stock-based Compensation
 12 Months Ended
Dec. 31, 2024
Share-Based Payment Arrangement [Abstract]  
Stock-based Compensation Stock-based Compensation
Employee Stock Purchase Plan
In May 2017, the Company’s stockholders approved the 2016 Employee Stock Purchase Plan (the 2016 ESPP). The 2016 ESPP is structured as a qualified employee stock purchase plan under Section 423 of the Internal Revenue Code of 1986, as amended, and is not subject to the provisions of the Employee Retirement Income Security Act of 1974. The Company reserved 800,000 shares of common stock for issuance under the 2016 ESPP. The 2016 ESPP allows eligible employees to purchase shares of the Company’s common stock at a discount through payroll deductions of up to 10% of their eligible compensation, subject to any plan limitations. The 2016 ESPP provides for six months offering periods ending on May 31 and November 30 of each year. At the end of each offering period, employees are able to purchase shares at 85% of the fair market value of the Company’s common stock on the last day of the offering period. During the year ended December 31, 2024, employees purchased 94,619 shares of common stock under the 2016 ESPP for net proceeds to the Company of approximately $0.3 million.
Employee Stock Incentive Plans
In October 2013, the Company implemented the 2013 Equity Incentive Plan (2013 Plan). In May 2023, the 2013 Plan was terminated, and no further awards may be issued under the plan. If an option granted under the 2013 Plan expires or terminates for any reason without having been fully exercised, if any shares of restricted stock are forfeited, or if any award terminates, expires or is settled without all or a portion of the shares of common stock covered by the award being issued, such shares will become available for issuance under the 2023 Equity Incentive Plan (2023 Plan). As of December 31, 2024, under the 2013 Plan, there were options to purchase an aggregate of 10,613,229 shares of common stock outstanding at a weighted average exercise price of $14.96 per share. As of December 31, 2024, there were 404,449 unvested RSUs outstanding under the 2013 Plan.
The 2023 Plan was effective as of stockholder approval in May 2023. The 2023 Plan provides for grants of stock options and other stock-based awards, as well as cash-based performance awards. The 2023 Plan authorized the issuance of up to an aggregate of 4,850,000 shares of common stock. In May 2024, the board and stockholders of the Company approved an amendment to the 2023 Plan to increase the number of shares of common stock available for issuance thereunder by 2,000,000 shares. Accordingly, the maximum number of shares of common stock authorized for issuance under the 2023 Plan is 6,850,000 shares. If an option expires or terminates for any reason without having been fully exercised, if any shares of restricted stock are forfeited, or if any award terminates, expires or is settled without all or a portion of the shares of common stock covered by the award being issued, such shares are available for the grant of additional awards. However, any shares that are withheld (or delivered) to pay withholding taxes or to pay the exercise price of an option are not available for the grant of additional awards. As of December 31, 2024, under the 2023 Plan, there were options to purchase an aggregate of 2,326,288 shares of common stock outstanding at a weighted average exercise price of $14.06 per share. As of December 31, 2024, there were 664,545 unvested RSUs outstanding under the 2023 Plan.
The following stock-based compensation amounts were recognized for the periods indicated (in thousands):
Year Ended December 31,
202420232022
Research and development$11,759 $9,190 $10,094 
Selling, general and administrative17,680 9,183 10,344 
Total stock-based compensation expense$29,439 $18,373 $20,438 
On October 25, 2024, Dr. Scott Koenig, the President and Chief Executive Officer of the Company and the Company entered into a separation and consulting agreement (the Separation Agreement), which provides for the terms of Dr. Koenig’s separation from employment, effective February 28, 2025 (the Separation Date). Under the Separation Agreement and in accordance with the terms of his employment agreement, the Company accelerated vesting of 50% of the unvested stock option awards and RSU awards granted to Dr. Koenig during his employment outstanding as of the Separation Date. Additionally, pursuant to the Separation Agreement, following Dr. Koenig’s Separation Date, Dr. Koenig will serve as an advisor to the Company. As compensation for the advisory services, all remaining unvested option awards and RSU awards as of the Separation Date will continue to vest during the advisory services period. Lastly, all outstanding, accelerated and continued vesting options during the advisory services were amended to extend the exercisability period. The Company evaluated the impacts of the Separation Agreement and the related modifications to Dr. Koenig’s option awards and RSU awards. As a result, the Company recognized all additional stock-based compensation expense in 2024 in accordance with ASC Topic 718, Compensation – Stock Compensation (ASC 718) in the amount of $6.1 million. On February 25, 2025, Dr. Koenig and the Company amended the Separation Agreement to extend Dr. Koenig’s Separation Date to a date to be determined by the Board
of Directors of the Company as the special executive search committee of the Board of Directors continues its process to identify the next Chief Executive Officer of the Company.

The assumptions used to estimate the fair value of Dr. Koenig’s modified awards in 2024 were as follows:

Expected dividend yield
0%
Expected volatility
108% -174%
Risk-free interest rate
4.2% - 4.3%
Expected term
1.00 year - 5.50 years
Employee Stock Options
The Company accounts for stock-based compensation to employees and non-employee directors in accordance with ASC 718. The Company estimates the fair value of stock option awards using the Black-Scholes option pricing model on the date of grant using the assumptions in the table below. Stock options granted to employees generally vest over four years and have a term of ten years. Stock-based compensation expense for stock options is recognized as expense over the requisite service period, which is the vesting period. As the Company has not paid dividends since inception, nor does it expect to pay any dividends for the foreseeable future, the expected dividend yield assumption is zero. The expected volatility is based on the historical stock volatility of the Company’s own common stock over a period equal to the expected term of the options. The risk-free rate of the stock options is based on the U.S. Treasury rate in effect at the time of grant for the expected term of the stock options. The Company calculates expected term based on the historical experience with similar awards, giving consideration to the contractual terms of the share-based awards, vesting schedules and expectations of future employee behavior. In addition, the Company estimates the expected forfeiture rate and only recognizes expense for those shares expected to vest. The Company estimates the forfeiture rate based on historical experience and its expectations regarding future pre-vesting termination behavior of employees. The Company reviews its estimate of the expected forfeiture rate annually, and stock-based compensation expense is adjusted accordingly.
Year Ended December 31,
202420232022
Expected dividend yield0%0%0%
Expected volatility
95% -116%
76% - 96%
88% - 92%
Risk-free interest rate
3.5% - 4.7%
3.5% - 4.8%
1.4% - 4.0%
Expected term6.06 years5.88 years5.95 years

The following table summarizes stock option activity for 2024:
Shares
Weighted-Average
Exercise Price
Weighted-Average
Remaining
Contractual Term
(Years)
Aggregate Intrinsic Value
(in thousands)
Outstanding, December 31, 2023
12,223,637 $15.11 6.7
Granted1,986,182 15.95 
Exercised(333,522)9.38 
Forfeited(165,111)11.44 
Expired(771,669)25.67 
Outstanding, December 31, 2024
12,939,517 $14.80 6.4$15 
As of December 31, 2024:
Exercisable9,004,928 $16.68 5.5$9 
Vested and expected to vest12,456,248 $14.96 6.3$14 
As of December 31, 2024, the total unrecognized compensation expense related to unvested stock options, net of related forfeiture estimates, was approximately $20.1 million, which the Company expects to recognize over a weighted-
average period of approximately 1.4 years. The following table summarizes additional information on stock options (in thousands, except per share amounts):
Year Ended December 31,
202420232022
Shares of common stock issued with stock options exercises
333,522 34,608 120,900 
Weighted-average fair value per share of stock options granted
$12.61 $3.83 $6.84 
Total intrinsic value of stock options exercised
$2,617 $93 $634 
Total cash received for stock options exercised
$3,128 $129 $172 
Total grant date fair value of stock options vested
$17,992 $16,435 $19,573 
Restricted Stock Units
RSUs are valued based on the closing price of the Company’s common stock on the date of the grant. The fair value of RSUs is recognized and amortized on a straight-line basis over the requisite service period of the award.
The following table summarizes RSU activity for 2024:
Shares
Weighted-Average
Grant Date Fair Value
Outstanding, December 31, 2023905,614 $5.97 
Granted679,715 16.43 
Vested(483,181)6.91 
Forfeited or expired(33,154)13.27 
Outstanding, December 31, 20241,068,994 $11.97 
At December 31, 2024, there was $6.4 million of total unrecognized compensation cost related to unvested RSUs, which the Company expects to recognize over a remaining weighted-average period of 1.4 years.