As of December 31, 2023, the Company had three stock-based employee compensation plans: the Amended and Restated Stock Incentive Plan (“Incentive Plan”), the Amended and Restated Inducement Equity Incentive Plan (“Inducement Plan”) and the Amended and Restated Employee Stock Purchase Plan (“ESPP”). The Incentive Plan was most recently amended and restated on April 24, 2023 and approved by the Company’s stockholders on June 13, 2023. The Inducement Plan was most recently amended and restated by the Company’s Board of Directors on October 26, 2023. The ESPP was most recently amended and restated by the Company’s Board of Directors on July 7, 2023.


	Years Ended December 31,
	2023		2022		2021
Incentive Plan	$	44,581	$	36,716	$	27,062
Inducement Plan	9,958		6,550		6,055
ESPP	1,076		1,435		1,523
Stock-based compensation expense	$	55,615	$	44,701	$	34,640


	Years Ended December 31,
	2023		2022		2021
Research and development	$	29,377	$	24,936	$	20,179
Selling, general and administrative	26,238		19,765		14,461
Total stock-based compensation expense	$	55,615	$	44,701	$	34,640

The Company grants stock option awards, restricted stock and restricted stock units to its employees, directors, and consultants under the Incentive Plan. Under the Incentive Plan, stock option awards are granted with an exercise price equal to the market price of the Company’s common stock at the date of grant. Stock option awards and restricted stock units granted to employees generally vest 25% each year until fully vested after four years.

In December 2014, the Company issued 1,250 performance-based stock options. These awards vest upon successful completion of specific development milestones. As of December 31, 2023, 85% of these grants have vested.

In January 2022, the Company issued 221 performance-based restricted stock unit awards. 21 of the awards met the performance objectives in 2022 and became eligible for vesting at 50% on the first anniversary of the grant date and 25% on each of the second and third anniversaries of the grant date, until fully vested after three years. The Company recognized $34 and $158 of stock compensation expense related to these awards during the years ended December 31, 2023 and December 31, 2022, respectively.

Stock option awards and restricted stock unit awards granted to non-employee directors of the Company generally vest over one year. Stock option awards granted to new non-employee directors when they first join the Company’s Board of Directors generally vest, subject to the terms of the Incentive Plan, in 36 equal monthly installments over a three-year period measured from the grant date. All stock option awards have contractual terms of 10 years. Restricted stock unit awards granted to new non-employee directors when they first join the Company’s Board of Directors generally vest, subject to the terms of the Incentive Plan, in three equal annual installments beginning on the first anniversary of the grant date. The vesting and exercise provisions of all awards granted under the Incentive Plan are subject to acceleration in the event of certain stockholder-approved transactions, or upon the occurrence of a change in control as defined in the Incentive Plan.


	Shares (in thousands)	Weighted Average Exercise Price per Share		Weighted Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value (in thousands)
Outstanding at December 31, 2022	31,179	$	8.56
Granted	6,625	6.67
Exercised	(935)	5.21			$	3,601
Cancelled or Forfeited	(1,364)	10.00
Outstanding at December 31, 2023	35,505	$	8.24	6.85	$	12,280
Exercisable at December 31, 2023	21,008	$	7.87	5.44	$	12,280
Vested and expected to vest at December 31, 2023	33,557	$	8.19	6.77	$	12,280

The total intrinsic value of stock option awards exercised under the Incentive Plan was $21,150 and $25,484 during the years ended December 31, 2022 and 2021, respectively. The aggregate intrinsic value represents the total proceeds (calculated as the difference between the exercise price of the underlying stock options and the estimated fair value of the Company’s common stock on the date of exercise for those stock options that had exercise prices lower than the fair value of the Company’s common stock on the exercise date) received by all individuals who exercised stock option awards during the period.


	Shares (in thousands)	Weighted Average Grant Date Fair Value
Unvested at December 31, 2022	4,159	$	10.69
Granted	3,106	6.71
Vested	(1,137)	11.04
Forfeited	(536)	7.91
Unvested at December 31, 2023	5,592	$	8.67

For restricted stock unit awards granted under the Incentive Plan, the fair value of the awards is determined based on the market value of the Company’s shares on the grant date. The weighted average grant date fair value of these awards granted during 2023, 2022, and 2021 was $6.71, $11.20, and 11.36, respectively. The fair value of the restricted stock unit awards is amortized to expense over the vesting periods using a straight-line expense attribution method.

As of December 31, 2023, total unrecognized compensation cost related to unvested restricted stock unit awards granted under the Incentive Plan was $40,981, which is expected to be recognized over a weighted average period of 2.0 years.

The Company has the ability to grant stock option and restricted stock unit awards to newly-hired employees as inducements material to each employee entering employment with the Company. Awards granted to newly hired employees generally vest 25% each year until fully vested after four years and are subject to the terms and conditions of the Inducement Plan. Each stock option has a term of 10 years. The vesting and exercise provisions of all awards granted under the Inducement Plan are subject to acceleration in the event of certain stockholder-approved transactions, or upon the occurrence of a change in control as defined in the Inducement Plan.


	Shares (in thousands)	Weighted Average Exercise Price per Share		Weighted Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value (in thousands)
Outstanding at December 31, 2022	5,341	$	8.80
Granted	1,196	8.15
Exercised	(344)	3.70			$	1,803
Cancelled or Forfeited	(666)	10.47
Outstanding at December 31, 2023	5,527	$	8.77	7.76	$	4,463
Exercisable at December 31, 2023	2,448	$	7.17	6.89	$	3,434
Vested and expected to vest at December 31, 2023	5,122	$	8.68	7.72	$	4,291

The total intrinsic value of stock option awards exercised under the Inducement Plan was $3,710 and $6,700 during the years ended December 31, 2022 and 2021, respectively. The aggregate intrinsic value represents the total proceeds (calculated as the difference between the exercise price of the underlying stock options and the estimated fair value of the Company’s common stock on the date of exercise for those stock options that had exercise prices lower than the fair value of the Company’s common stock on the exercise date) received by all individuals who exercised stock option awards during the period.


	Shares (in thousands)	Weighted Average Grant Date Fair Value
Unvested at December 31, 2022	588	$	13.14
Granted	581	7.81
Vested	(139)	13.13
Forfeited	(115)	11.98
Unvested at December 31, 2023	915	$	9.90

For restricted stock unit awards granted under the Inducement Plan, the fair value of the awards is determined based on the market value of the Company’s shares on the grant date. The weighted average grant date fair value of these awards granted during 2023 and 2022 was $7.81 and 13.21, respectively. The fair value of the restricted stock unit awards is amortized to expense over the vesting periods using a straight-line expense attribution method. No restricted stock unit awards were granted under the Inducement Plan during 2021.

As of December 31, 2023, total unrecognized compensation cost related to unvested restricted stock unit awards granted under the Inducement Plan was $7,044, which is expected to be recognized over a weighted average period of 1.8 years.

For stock option awards granted under the Incentive Plan and the Inducement Plan, the fair value is estimated on the date of grant using a Black-Scholes option pricing model and the assumptions noted below. The fair value of the stock option awards is amortized to expense over the vesting periods using a straight-line expense attribution method.

Historically, the expected life was based on the average of the assumption that all outstanding stock option awards will be exercised at full vesting and the assumption that all outstanding stock option awards will be exercised at the midpoint of the current date (if already vested) or at full vesting (if not yet vested) and the full contractual term. Effective July 1, 2023, the expected life is based on the historical settlement of options by taking into account exercises and post-

vesting terminations and weighing them based on the number of options settled. This change in approach did not have a significant impact on the value of the stock option awards granted. The expected volatility represents the historical volatility on the Company’s publicly-traded common stock. The Company has assumed no expected dividend yield, as dividends have never been paid to stock or option holders and will not be paid for the foreseeable future. The weighted average risk-free interest rate is the implied yield currently available on zero-coupon government issues with a remaining term equal to the expected term.


	Years Ended December 31,
	2023			2022			2021
Expected Life in Years	5.7			5.5			5.5
Expected Volatility	82.5		%	84.2		%	84.2		%
Expected Dividend Yield	0.0		%	0.0		%	0.0		%
Risk-Free Interest Rate	3.9		%	3.6		%	1.1		%
Weighted average grant date fair value per share	$	4.75		$	7.57		$	7.93

The total fair value of the stock option awards vested under the Incentive Plan was $33,731, $28,916, and $20,510 during the years ended December 31, 2023, 2022, and 2021, respectively. As of December 31, 2023, total unrecognized compensation cost related to unvested stock option awards granted under the Incentive Plan was $75,164, which is expected to be recognized over a weighted average period of 1.9 years.

The following table summarizes the key assumptions used by the Company to value the stock option awards granted under the Inducement Plan during the years ended December 31, 2023, 2022, and 2021:


	Years Ended December 31,
	2023			2022			2021
Expected Life in Years	5.6			5.5			5.5
Expected Volatility	83.5		%	84.2		%	84.2		%
Expected Dividend Yield	0.0		%	0.0		%	0.0		%
Risk-Free Interest Rate	4.0		%	3.2		%	0.9		%
Weighted average grant date fair value per share	$	5.79		$	9.54		$	9.65

The total fair value of the stock option awards vested under the Inducement Plan was $7,698, $4,659, and $2,885 during the years ended December 31, 2023, 2022, and 2021, respectively. As of December 31, 2023, total unrecognized compensation cost related to unvested stock option awards granted under the Inducement Plan was $16,216, which is expected to be recognized over a weighted average period of 1.5 years.

The Company has reserved a total of 7,975 shares of common stock to be purchased under the ESPP, of which 5,454 shares remain available for purchase at December 31, 2023. Eligible employees may authorize up to 15% of their salary to purchase common stock at the lower of 85% of the beginning or 85% of the ending price during six-month purchase intervals. No more than three thousand shares may be purchased by any one employee at each purchase date, and no employee may purchase stock having a fair market value at the commencement date of $25 or more in any one calendar year.

During the years ended December 31, 2023, 2022, and 2021, the Company issued 338, 260, and 321 shares of common stock under the ESPP, respectively, at a weighted average price per share of $7.68, $11.03, and $6.20,

respectively. Compensation expense for shares purchased under the ESPP related to the purchase discount and the “look-back” option were determined using a Black-Scholes option pricing model. The weighted average grant date fair values of shares granted under the ESPP during the years ended December 31, 2023, 2022, and 2021, were $3.82, $6.02, and $2.80, respectively.