As of December 31, 2022, 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 18, 2022 and approved by the Company’s stockholders on June 7, 2022. The Inducement Plan was most recently amended and restated on August 26, 2022. The ESPP was most recently amended and restated on April 1, 2021 and approved by the Company’s stockholders on May 25, 2021.

Stock-based compensation expense of $44,701 ($36,716 of expense related to the Incentive Plan, $6,550 of expense related to the Inducement Plan, and $1,435 of expense related to the ESPP) was recognized during 2022, while $34,640 ($27,062 of expense related to the Incentive Plan, $6,055 of expense related to the Inducement Plan, and $1,523 of expense related to the ESPP) was recognized during 2021 and $14,794 ($12,938 of expense related to the Incentive Plan, $1,494 of expense related to the Inducement Plan, and $362 of expense related to the ESPP) was recognized during 2020.

There was approximately $148,700 of total unrecognized compensation expense related to non-vested stock option and restricted stock unit awards granted by the Company as of December 31, 2022. As of December 31, 2022, the Company expected to recognize that expense as follows: $50,713 in 2023, $46,329 in 2024, $34,537 in 2025 and $17,121 in 2026. In addition, the Company has outstanding performance-based stock options and restricted stock unit awards for which no compensation expense is recognized until “performance” has occurred and the award vests.


	Years Ended December 31,
	2022		2021		2020
Research and development	$	24,936	$	20,179	$	10,222
Selling, general and administrative	19,765		14,461		4,572
Total stock-based compensation expense	$	44,701	$	34,640	$	14,794

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 August 2013, December 2014 and December 2019, the Company issued 1,032, 1,250 and 315 performance-based stock options, respectively. These awards vest upon successful completion of specific development milestones. As of December 31, 2022, 100%, 85% and 100% of these August 2013, December 2014 and December 2019 grants, respectively, have vested. During 2020, the Company recognized $1,768 and $684 of stock compensation expense related to milestones within the August 2013 and December 2019 grants for which achievement became probable.

In January 2022, the Company issued 221 performance-based restricted stock unit awards. Contingent upon successful achievement of specific commercial or operational objectives in 2022, the awards become 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. During 2022, the Company recognized $158 of stock compensation expense related to certain milestones within the January 2022 grants for which achievement became probable.

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. 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.

All stock option awards have contractual terms of 10 years. 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.


	Awards Available	Options Outstanding	Weighted Average Exercise Price
Balance at December 31, 2019	968	21,050	$	5.96
Plan amendment	8,000	—	—
Restricted stock unit awards granted	(31)	—	—
Stock option awards granted	(7,469)	7,469	8.06
Stock option awards exercised	—	(510)	3.56
Stock option awards cancelled	3,124	(3,124)	6.93
Balance at December 31, 2020	4,592	24,885	$	6.52
Plan amendment	7,500	—	—
Restricted stock unit awards granted	(1,936)	—	—
Stock option awards granted	(6,753)	6,753	11.57
Stock option awards exercised	—	(2,705)	4.36
Stock option awards cancelled	248	(248)	7.62
Balance at December 31, 2021	3,651	28,685	$	7.90
Plan amendment	8,000	—	—
Restricted stock unit awards granted	(2,948)	—	—
Restricted stock unit awards cancelled	254	—	—
Stock option awards granted	(5,784)	5,784	10.70
Stock option awards exercised	—	(2,257)	5.00
Stock option awards cancelled	1,033	(1,033)	10.02
Balance at December 31, 2022	4,206	31,179	$	8.56

For stock option awards granted under the Incentive Plan during 2022, 2021, and 2020, the fair value was estimated on the date of grant using a Black-Scholes option pricing model and the assumptions noted in the table following the next subsection. The weighted average grant date fair value of these awards granted during 2022, 2021, and 2020 was $7.57, $7.93, and $5.48, respectively. The fair value of the stock option awards is amortized to expense over the vesting periods using a straight-line expense attribution method. For restricted stock unit awards granted under the Incentive Plan, the fair value of the awards was 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 2022 and 2021 was $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.

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.


	Awards Available	Options Outstanding	Weighted Average Exercise Price
Balance at December 31, 2019	171	1,329	$	3.60
Plan amendment	2,900	—	—
Stock option awards granted	(3,002)	3,002	4.02
Stock option awards cancelled	160	(160)	4.15
Balance at December 31, 2020	229	4,171	$	3.88
Plan amendment	1,500	—	—
Stock option awards granted	(1,003)	1,003	13.91
Stock option awards exercised	—	(592)	3.63
Stock option awards cancelled	174	(174)	3.67
Balance at December 31, 2021	900	4,408	$	6.20
Plan amendment	1,926	—	—
Restricted stock unit awards granted	(603)	—	—
Restricted stock unit awards cancelled	15	—	—
Stock option awards granted	(1,819)	1,819	13.54
Stock option awards exercised	—	(358)	3.45
Stock option awards cancelled	528	(528)	7.44
Balance at December 31, 2022	947	5,341	$	8.80

For stock option awards granted under the Inducement Plan during 2022, 2021, and 2020, the fair value was estimated on the date of grant using a Black-Scholes option pricing model and the assumptions noted in the table below. The weighted average grant date fair value of these awards granted during 2022, 2021, and 2020 was $9.54, $9.65, and $2.73, respectively. The fair value of the stock option awards is amortized to expense over the vesting periods using a straight-line expense attribution method. For restricted stock unit awards granted under the Inducement Plan, the fair value of the awards was 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 2022 was $13.21. 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 or 2020.

The following table summarizes the key assumptions used by the Company to value the stock option awards granted under all plans during 2022, 2021, and 2020, respectively. The expected life is 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. 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.

The total intrinsic value of stock option awards exercised under the Incentive Plan was $21,150 during 2022, $25,484 during 2021, and $1,562 during 2020. The intrinsic value represents the total proceeds (fair market value at the date of exercise, less the exercise price, times the number of stock option awards exercised) received by all individuals who exercised stock option awards during the period. The total intrinsic value of stock option awards exercised under the Inducement Plan was $3,710 and $6,700 during 2022 and 2021, respectively. No stock option awards were exercised under the Inducement Plan during 2020.

The following table summarizes, at December 31, 2022, by price range: (1) for stock option awards outstanding under the Incentive and Inducement Plans, the number of stock option awards outstanding, their weighted average remaining life and their weighted average exercise price; and (2) for stock option awards exercisable under the Incentive and Inducement Plans, the number of stock option awards exercisable and their weighted average exercise price:


				Outstanding				Exercisable
Range				Number	Weighted Average Remaining Life	Weighted Average Exercise Price		Number	Weighted Average Exercise Price

$	0	to	3	1,145	6.3	$	2.65	777	$	2.66
3		to	6	8,653	5.7	4.23		7,183	4.32
6		to	9	9,704	7.2	7.90		6,332	7.70
9		to	12	13,390	8.3	10.89		3,287	11.01
12		to	15	2,189	7.5	13.06		927	12.80
15		to	18	1,439	8.4	16.02		322	15.89
$	0	to	18	36,520	7.3	$	8.59	18,828	$	7.17

The aggregate intrinsic value of stock option awards outstanding and exercisable under the Incentive and Inducement Plans at December 31, 2022 was $83,783. The aggregate intrinsic value represents the value (the period’s closing market price, less the exercise price, times the number of in-the-money stock option awards) that would have been received by all stock option award holders under the Incentive and Inducement Plans had they exercised their stock option awards at the end of the year.

As of December 31, 2022, the number of stock option awards vested and expected to vest under the Incentive and Inducement Plans is 32,837. The weighted average exercise price of these stock option awards is $8.52 and their weighted average remaining contractual life is 7.2 years.


	Non-Vested Stock Option Awards	Weighted Average Grant-Date Fair Value
Balance December 31, 2021	17,532	$	6.06
Stock option awards granted	7,603	8.03
Stock option awards vested	(5,908)	5.68
Stock option awards forfeited	(1,535)	6.24
Balance December 31, 2022	17,692	$	7.02

The Company has reserved a total of 7,975 shares of common stock to be purchased under the ESPP, of which 5,792 shares remain available for purchase at December 31, 2022. 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 3 shares may be purchased by any one employee at the six-month purchase dates and no employee may purchase stock having a fair market value at the commencement date of $25 or more in any one calendar year.

There were 260, 321, and 246 shares of common stock purchased under the ESPP in 2022, 2021, and 2020, respectively, at a weighted average price per share of $11.03, $6.20, and $2.56, respectively. Expense of $1,435, $1,523, and $362 related to the ESPP was recognized during 2022, 2021, and 2020, 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 2022, 2021, and 2020, were $6.02, $2.80, and $1.47, respectively.