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Stock-Based Compensation
 9 Months Ended
Sep. 30, 2023
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 (IRC), 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-month 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 nine months ended September 30, 2023, 48,280 shares of common stock were purchased under the 2016 ESPP.
Employee Stock Incentive Plans
Effective February 2003, the Company implemented the 2003 Equity Incentive Plan (2003 Plan), and it was amended and approved by the Company's stockholders in 2005. Stock options granted under the 2003 Plan may be either incentive stock options as defined by the IRC, or non-qualified stock options. In 2013, the 2003 Plan was terminated, and no further awards
may be issued under the plan. Any shares of common stock subject to awards under the 2003 Plan that expire, terminate, or are otherwise surrendered, canceled, forfeited or repurchased without having been fully exercised, or resulting in any common stock being issued, will become available for issuance under the 2013 Equity Incentive Plan (2013 Plan), up to a specified number of shares.
In October 2013, the Company implemented the 2013 Plan. The 2013 Plan provides for the grant of stock options and other stock-based awards, as well as cash-based performance awards. 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).
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. Initially, the maximum number of shares of the Company’s common stock that may be issued under the 2023 Plan will not exceed 4,850,000 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.
The following stock-based compensation expense was recognized for the periods indicated (in thousands):
Three Months Ended September 30,Nine Months Ended September 30,
2023202220232022
Research and development$2,335 $2,571 $6,996 $7,621 
Selling, general and administrative2,455 2,552 7,018 8,076 
Total stock-based compensation expense$4,790 $5,123 $14,014 $15,697 
Employee stock options
The fair value of each option award is estimated on the date of grant using the Black-Scholes option-pricing model using the assumptions in the following table for options issued during the period indicated:
Nine Months Ended September 30,
20232022
Expected dividend yield0%0%
Expected volatility
76.1% -94.9%
87.8% - 90.8%
Risk-free interest rate
3.5% - 4.4%
1.4% - 3.6%
Expected term5.88 years5.95 years
The following table summarizes stock option activity during the nine months ended September 30, 2023:
SharesWeighted-
Average
Exercise Price		Weighted-Average
Remaining
Contractual Term
(Years)		Aggregate
Intrinsic
Value
 (in thousands)
Outstanding, December 31, 202210,098,929 $18.58 6.5
Granted3,267,727 4.91 
Exercised(26,146)3.34 
Forfeited(293,161)9.78 
Expired(367,054)19.77 
Outstanding, September 30, 202312,680,295 15.26 6.7$301 
As of September 30, 2023:
Exercisable7,836,655 19.50 5.4186 
Vested and expected to vest11,733,021 15.75 6.5278 
The weighted-average grant-date fair value of options granted during the nine months ended September 30, 2023 and 2022 was $3.78 and $6.89, respectively. The total intrinsic value of options exercised during the nine months ended September 30, 2023 and 2022 was $0.1 million and $0.5 million, respectively. The total cash received for options exercised during the nine months ended September 30, 2023 and 2022 was $0.1 million and $0.3 million, respectively. The total fair value of shares vested in the nine months ended September 30, 2023 and 2022 was approximately $13.6 million and $15.1 million, respectively. As of September 30, 2023, the total unrecognized compensation expense related to unvested stock options, net of related forfeiture estimates, was approximately $22.6 million, which the Company expects to recognize over a weighted-average period of approximately 1.3 years.
Restricted Stock Units
The Company awards restricted stock units (RSUs) to employees. 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 during the nine months ended September 30, 2023:
Shares
Weighted-Average
Grant Date Fair Value
Outstanding, December 31, 2022415,084 $8.83 
Granted696,635 4.84 
Vested(135,396)8.32 
Forfeited(68,046)7.53 
Outstanding, September 30, 2023908,277 5.95 

At September 30, 2023, there was $2.9 million of total unrecognized compensation cost related to unvested RSUs, which the Company expects to recognize over a remaining weighted-average period of approximately 1.3 years.