On May 19, 2022, our stockholders approved the 2022 Equity Incentive Plan (the “2022 Plan”), succeeding our 2013 Stock Incentive Plan (the “2013 Plan”), which has expired and under which no further grants may be made. The 2022 Plan provides for the

grant of incentive stock options, non-statutory stock options, stock appreciation rights, restricted stock awards, RSU awards and other stock-based awards. On May 24, 2023, our stockholders approved an amendment to the 2022 Plan to increase the number of shares of our common stock available for issuance under the 2022 Plan by 5,000,000 shares. As of December 31, 2023, there were 8,591,362 shares available for future grants under the 2022 Plan.

Under the terms of the 2022 Plan, 2013 Plan and the 2010 Stock Incentive Plan, which was replaced by the 2013 Plan, we granted stock options, RSUs, and PSUs to our employees, officers, directors, consultants and advisors. Stock options have a ten-year term and an exercise price equal to the fair market value of a share of our common stock on the grant date. Stock options vest over four years with 25% vesting on the one-year anniversary of the grant date and the remainder vesting in equal monthly installments thereafter. RSUs generally vest over a period of one to four years in annual installments beginning on the first anniversary of the grant date. PSUs will vest if certain performance goals are achieved over a certain performance period. Certain portions of certain PSU awards vest based on continuous service to the Company throughout the performance period even if the performance goal is not achieved.

During 2023, 2022, and 2021, we also granted stock options and RSUs through inducement grants outside of our stockholder approved equity compensation plans as permitted under the Nasdaq Stock Market listing rules to certain employees to induce them to accept employment with us (collectively, “Inducement Awards”). In February 2022, our Board approved the 2022 Inducement Stock Incentive Plan (the “2022 Inducement Plan”) under which 850,000 shares of common stock were initially reserved for issuance for inducement awards to be granted to newly hired full-time employees. In May 2022 and February 2023, the Board increased the number of shares reserved for issuance under the 2022 Inducement Plan by 850,000 and 1,200,000, respectively. We assessed the terms of these Inducement Awards and determined there was no possibility that we would have to settle these awards in cash and therefore, equity accounting was applied. As of December 31, 2023, there were 874,498 shares available for future grants under the 2022 Inducement Plan.

As of December 31, 2023, we had 25,753,096 shares reserved for issuance, which includes shares available for future grants and outstanding stock options, RSUs and PSUs under the 2010 Plan, 2013 Plan, 2022 Plan, and Inducement Awards (including the Inducement Awards granted under the 2022 Inducement Plan).

During the year ended December 31, 2022, we accelerated the vesting and extended the exercise date of certain stock-based awards granted to our former Chief Executive Officer and former Chief Scientific Officer in connection with their departure from the Company in May 2022. These modifications resulted in the recognition of incremental stock-based compensation expense of $7.4 million.

In connection with all stock-based payment awards, total stock-based compensation expense recognized was as follows (in thousands):


	For the Years Ended December 31,
	2023		2022		2021
Cost of sales	$	370	$	226	$	154
Research and development		6,529		14,351		11,842
Selling, general and administrative		14,810		20,822		17,787
Total	$	21,709	$	35,399	$	29,783

The total stock-based compensation expense recognized by award type was as follows (in thousands):


	For the Years Ended December 31,
	2023		2022		2021
Options	$	8,862	$	21,513	$	19,288
RSUs		10,662		12,587		9,348
PSUs		1,222		—		—
ESPP		963		1,299		1,147
Total	$	21,709	$	35,399	$	29,783

The following table summarizes stock option activity related to the 2010 Plan, 2013 Plan, 2022 Plan, and Inducement Awards (including the stock option Inducement Awards granted under the 2022 Inducement Plan):


	Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (years)		Aggregate Intrinsic Value (in thousands)
Options outstanding at December 31, 2022		13,026,359		$	11.46		5.3	$	6
Granted		306,000		$	2.23
Exercised		(13,193	)	$	3.46
Forfeited		(1,120,412	)	$	10.31
Expired		(3,577,944	)	$	12.65
Options outstanding at December 31, 2023		8,620,810		$	10.79		5.3	$	—
Options exercisable at December 31, 2023		6,739,120		$	11.59		4.4	$	—

The total intrinsic value of stock options exercised for the year ended December 31, 2023 was less than $0.1 million. The total intrinsic value of stock options exercised for the years ended December 31, 2022 and 2021 was $0.3 million and $1.0 million, respectively.

The fair value of each stock option granted is estimated on the date of grant using the Black-Scholes option-pricing model. The following table summarizes the assumptions used in calculating the fair value of the stock option awards:


	For the Years Ended December 31,
	2023	2022	2021
Volatility	80%	79%-81%	76%-78%
Expected term (in years)	5.5-5.9	5.5-6.1	5.9-6.1
Risk-free interest rate	3.75%-4.21%	1.69%-4.23%	0.62%-1.35%
Dividend	—%	—%	—%

We use the simplified method to calculate the expected term as our historical exercise data does not provide a reasonable basis upon which to estimate the expected term. The expected term is applied to the stock option grant group as a whole, as we do not expect substantially different exercise or post-vesting termination behavior among our employee population. Our expected stock price volatility assumption is based on the historical volatility of our publicly traded stock. The risk-free interest rate is based on a treasury instrument whose term is consistent with the expected term of the stock options. We account for forfeitures as they occur.

Using the Black-Scholes option-pricing model, the weighted-average grant date fair values of options granted during the years ended December 31, 2023, 2022 and 2021 was $1.54, $5.83 and $7.57 per share, respectively.

At December 31, 2023, there was $9.7 million of unrecognized compensation related to unvested stock option awards, which are expected to be recognized over a weighted-average period of 1.7 years.

The following is a summary of RSU and PSU activity for the 2010 Plan, 2013 Plan, 2022 Plan, and Inducement Awards (including the RSU Inducement Awards granted under the 2022 Inducement Plan):


	Number of Shares Underlying RSUs and PSUs			Weighted -Average Grant Date Fair Value
Unvested at December 31, 2022		3,402,825		$	10.35
Granted		7,075,522		$	2.84
Forfeited		(1,761,720	)	$	6.01
Vested		(1,050,201	)	$	10.55
Unvested at December 31, 2023		7,666,426		$	4.40

As of December 31, 2023, there was $24.0 million of unrecognized compensation costs related to unvested RSUs and PSUs, which are expected to be recognized over a weighted-average period of 1.9 years.

We have an ESPP that permits eligible employees to enroll in six-month offering periods. Participants may purchase shares of our common stock, through payroll deductions, at a price equal to 85% of the fair market value of the common stock on the first or last day of the applicable six-month offering period, whichever is lower. Purchase dates under the ESPP occur on or about May 1 and November 1 each year. In 2013, our stockholders approved an increase in the number of shares of common stock authorized for issuance pursuant to the ESPP to 242,424 shares of common stock, plus an annual increase to be added on the first day of each fiscal year, commencing on January 1, 2015 and ending on December 31, 2023, equal to the lesser of 484,848 shares of our common stock, 1% of the number of outstanding shares on such date, or an amount determined by the Board (the "Evergreen Provision"). On May 24, 2023, our stockholders approved an amendment and restatement (the “Amended and Restated ESPP”) of our ESPP. The Amended and Restated ESPP (i) eliminated the Evergreen Provision and (ii) increased the number of shares of common stock authorized for issuance under the ESPP from 2,508,923 shares to 4,008,923 shares.

As of December 31, 2023, 1,632,818 shares of our common stock remained available for issuance under the Amended and Restated ESPP.

During the years ended December 31, 2023, 2022 and 2021, $1.1 million, $2.3 million and $2.0 million, respectively, was withheld from employees, on an after-tax basis, in order to purchase 638,182, 508,391 and 330,257 shares of our common stock, respectively. As of December 31, 2023, there was $0.2 million of total unrecognized stock-based compensation expense related to the Amended and Restated ESPP. The expense is expected to be recognized over a period of four months.

The fair value of the option component of the shares purchased under the Amended and Restated ESPP was estimated using the Black-Scholes option-pricing model with the following weighted-average assumptions:


	For the Years Ended December 31,
	2023	2022	2021
Volatility	77% - 92%	68% - 131%	58% - 68%
Expected term (in years)	0.5	0.5	0.5
Risk-free interest rate	4.58% - 5.51%	0.06% - 4.58%	0.03% - 0.06%
Dividend	—%	—%	—%