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Equity Incentive Plans
 12 Months Ended
Dec. 31, 2023
Share-Based Payment Arrangement [Abstract]  
Equity Incentive Plans Equity Incentive Plans
2017 Equity Incentive Plan
In January 2017, our Board of Directors and stockholders approved and adopted the 2017 Equity Incentive Plan (the “2017 Plan”). Under the 2017 Plan, we may grant stock options, stock appreciation rights, restricted stock, restricted stock units and other awards to individuals who are then our employees, officers, directors or consultants. In addition, the number of shares of stock available for issuance under the 2017 Plan will be automatically increased each January 1, beginning on January 1, 2018, by 4% of the aggregate number of outstanding shares of our common stock as of the immediately preceding December 31 or such lesser number as determined by our Board of Directors. The 2017 Plan automatically increased by 1,140,527 shares as of January 1, 2023. As of December 31, 2023, 2,272,850 shares were available for future issuance.
Employee Stock Purchase Plan
In January 2017, our Board of Directors and stockholders approved and adopted the 2017 Employee Stock Purchase Plan or the ESPP. In addition, the number shares of stock available for issuance under the ESPP will be automatically increased each January 1, beginning on January 1, 2018, by 1% of the aggregate number of outstanding shares of our common stock as of the immediately preceding December 31 or such lesser number as determined by our Board of Directors. The ESPP automatically increased by 285,131 shares as of January 1, 2023. As of December 31, 2023, 98,000 shares have been issued under the ESPP. As of December 31, 2023, 1,734,840 shares were available for future issuance under the ESPP.
Stock Options
Stock options granted to employees and non-employees generally vest over a four-year period while stock options granted to directors generally vest over a one-year period. Each stock option award has a maximum term of 10 years from the date of grant, subject to earlier cancellation prior to vesting upon cessation of service to us. A summary of the activity related to stock option awards during the year ended December 31, 2023 is as follows:
Shares
Subject to
Options
Weighted-Average
Exercise
Price per
Share
Weighted-Average
Remaining
Contractual
Term (in years)
Aggregate
Intrinsic
Value (in
thousands)
Outstanding at January 1, 20233,673,208 $32.04 6.28$18,686 
Granted1,579,331 $22.02 
Exercises(90,883)$18.95 
Forfeitures and cancellations(936,041)$37.52 
Outstanding at December 31, 2023
4,225,615 $27.36 7.42$5,827 
Exercisable at December 31, 20232,154,408 $30.60 6.10$4,741 
Total cash received from the exercise of stock options was approximately $1.8 million during the year ended December 31, 2023.
Time-based Restricted Stock Units
Each Restricted Stock Unit (“RSU”) represents one equivalent share of our common stock to be issued after satisfying the applicable continued service-based vesting criteria over a specified period. The fair value of these RSUs is based on the closing price of our common stock on the date of the grant. We measure compensation expense over the expected vesting period on a straight-line basis. The RSUs do not entitle the participants to the rights of holders of common stock, such as voting rights, until the shares are issued.
Number of Restricted Stock UnitsWeighted-Average Grant Date Fair Value
Weighted-Average
Remaining
Contractual
Term
(in years)
Aggregate
Intrinsic
Value (in
thousands)
Outstanding at January 1, 20231,028,843 $26.14 1.17$31,884 
Granted568,574 $22.02 
Released(68,900)$23.89 
Forfeitures and cancellations(46,945)$21.87 
Outstanding at December 31, 20231,481,572 $24.80 0.73$31,735 
RSU expected to vest at December 31, 20231,481,572 $24.80 0.73$31,735 
Stock-Based Compensation Expense
We recognize stock-based compensation expense for awards issued to employees and non-employees over the requisite service period based on the estimated grant-date fair value of such awards. We record the expense for stock-based compensation awards subject to performance-based milestone vesting over the requisite service period when management determines that achievement of the milestone is probable. Management evaluates when the achievement of a performance-based milestone is probable based on the expected satisfaction of the performance conditions at each reporting date. The
estimated fair values of stock option awards granted were determined on the date of grant using the Black-Scholes option valuation model with the following weighted-average assumptions:
Year Ended
December 31,
202320222021
Risk-free interest rate3.8 %2.5 %0.7 %
Expected volatility85.7 %87.4 %92.9 %
Expected dividend yield— %— %— %
Expected term (in years)5.826.096.18
Weighted average grant date fair value per share$16.06$21.58$22.34
We determine the appropriate risk-free interest rate, expected term for employee stock-based awards, contractual term for non-employee stock-based awards, and volatility assumptions. The weighted-average expected option term for employee and non-employee stock-based awards reflects the historical option term for 2023 and the simplified life method for 2022 and 2021, which defines the life as the average of the contractual term of the options and the weighted-average vesting period for all option tranches. Expected volatility incorporates the historical volatility of our stock price. The risk-free interest rate is based upon U.S. Treasury securities with remaining terms similar to the expected or contractual term of the stock-based payment awards. The assumed dividend yield is based on our expectation of not paying dividends in the foreseeable future.
Total non-cash stock-based compensation expense for all stock awards that was recognized in the consolidated statements of operations and comprehensive loss is as follows:
Year Ended
December 31,
(in thousands)202320222021
Research and development$10,159 $6,775 $5,856 
General and administrative23,046 20,581 9,491 
Total
$33,205 $27,356 $15,347 
On March 20, 2022, our then Chief Executive Officer (“former CEO”), resigned by mutual agreement with the Board of Directors. In connection with his separation agreement, we modified certain equity awards and recognized approximately $3.2 million in non-cash stock-based compensation expense. Given the former CEO had substantially rendered the required services per his separation agreement, we recorded the full expense related to the modification in the period ending March 31, 2022. Additionally, on March 21, 2022, we awarded our newly appointed Interim President and Chief Executive Officer RSUs for 887,043 shares of the Company’s common stock. The fair value of the award will be recognized as part of compensation cost, occurring ratably over the stated 24-month requisite service period. During the year ended December 31, 2023, we recognized $11.7 million and $9.2 million during the year ended December 31, 2022 of non-cash stock-based compensation cost related to the award.
At December 31, 2023, there was $31.6 million of unrecognized compensation cost related to unvested stock option awards, which is expected to be recognized over a remaining weighted average vesting period of 2.67 years, $12.2 million of unrecognized cost related to unvested RSU awards, which is expected to be recognized over a period of 1.23 years, and $0.2 million of unrecognized compensation cost related to the ESPP, which is expected to be recognized over a remaining weighted-average vesting period of 0.37 years.