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Stock-Based Compensation
 12 Months Ended
Dec. 31, 2022
Share-Based Payment Arrangement [Abstract]  
Stock-Based Compensation STOCK-BASED COMPENSATION
2017 Stock Incentive Plan
The Board adopted the 2017 Stock Incentive Plan (the "2017 Plan") in February 2017, and the stockholders approved the 2017 Plan in March 2017. The 2017 Plan was most recently amended in March 2020.
As of December 31, 2022, there were an aggregate of 584,646 shares of common stock issuable upon the exercise of outstanding options under the 2017 Plan. Any options or awards outstanding under the 2017 Plan remain outstanding and effective.
2020 Equity Incentive Plan
In April 2020, the 2020 Equity Incentive Plan (the "2020 Plan") became effective, and, as a result, no further awards will be made under the 2017 Plan. The 2020 Plan provides for the grant of stock options qualifying as incentive stock options ("ISOs"), to employees and for the grant of nonstatutory stock options ("NSOs"), restricted stock awards, restricted stock unit awards, stock appreciation rights, performance stock awards and other forms of stock compensation to employees, consultants and directors. The 2020 Plan also provides for the grant of performance cash awards to employees, consultants and directors. Any previously granted awards under the 2017 Plan will remain outstanding in accordance with their respective terms.
Under the 2020 Plan, there is an annual increase on January 1 of each year from January 1, 2021 continuing through January 1, 2030, by 4.0% of the total number of shares of common stock outstanding on December 31 of the preceding calendar year, or a lesser number of shares as may be determined by the Board. On January 1, 2022, the Company increased the number of shares available for future grant under the 2020 Plan by 958,993 shares. On January 1, 2023, the Company increased the number of shares available for future grant under the 2020 Plan by 1,101,738 shares. The awards granted under the 2020 Plan typically vest over a four-year period and have a 10-year contractual term.
As of December 31, 2022, there were an aggregate of 2,948,523 shares of common stock issuable upon the exercise of outstanding options under the 2020 Plan. Additionally, there were an aggregate of 972,377 shares reserved for future issuance under the 2020 Plan, including shares forfeited from the 2017 Plan.
Stock Option Valuation
The assumptions that the Company used in the Black-Scholes option-pricing model to determine the grant-date fair value of stock options granted for the years ended December 31, 2022 and 2021 were as follows:
YEAR ENDED DECEMBER 31,
20222021
Weighted-average risk-free interest rate2.24 %0.88 %
Expected term (in years)6.066.05
Expected volatility83.03 %83.00 %
Expected dividend yield0.00 %0.00 %
Fair value of underlying common stock$31.18 $42.29 

A summary of option activity during the years ended December 31, 2022 and 2021 is as follows (in thousands except share and per share data):
NUMBER OF
OPTIONS		WEIGHTED-
AVERAGE
EXERCISE PRICE		WEIGHTED-AVERAGE
REMAINING
CONTRACTUAL
TERM (IN YEARS)		AGGREGATE
INTRINSIC VALUE
Outstanding as of January 1, 20222,810,684 $24.62 8.11$100,009 
Granted1,086,458 43.66 
Exercised(158,235)5.17 $5,637 
Cancelled or forfeited(195,428)42.44 
Expired(10,310)(59.46)
Outstanding as of December 31, 20223,533,169 $30.26 7.77$72,434 
Options vested as of December 31, 20221,749,888 $19.59 6.93$53,895 
Options unvested as of December 31, 20221,783,281 $40.72 
The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those stock options that had exercise prices lower than the fair value of the Company’s common stock.
The weighted-average grant date fair value price per share of options granted during each of the years ended December 31, 2022 and 2021 was $31.18 and $42.29, respectively. As of December 31, 2022, there was $44.4 million of unrecognized stock-based compensation expense related to unvested stock options. The unrecognized stock-based compensation expense is estimated to be recognized over a period of 2.51 years.
The total fair value of options vested during the years ended December 31, 2022 and 2021 was $16.5 million and $10.1 million, respectively.
Stock-Based Compensation Expense
Total stock-based compensation expense recorded as research and development and general and administrative expenses, respectively, for employees, directors and non-employees during the years ended December 31, 2022 and 2021 is as follows (in thousands):
YEAR ENDED DECEMBER 31,
20222021
Research and development$8,205 $4,258 
General and administrative10,477 7,464 
Total stock-based compensation expense$18,682 $11,722