Stock-Based Compensation
Total compensation cost recognized for all stock-based compensation awards in the consolidated statements of operations was as follows (in thousands):
Year Ended
December 31,
20242023
Research and development$8,642 $9,842 
General and administrative12,775 9,956 
Total stock-based compensation expense$21,417 $19,798 
Restricted Stock and Restricted Stock Unit Awards
The following table summarizes restricted stock and restricted stock unit awards activity for the instruments discussed above as of December 31, 2024 and 2023 is as follows:
SharesWeighted
Average
Grant Date
Fair Value
Per Share
Unvested restricted stock and restricted stock unit awards as of December 31, 20232,107,147$11.96 
Issued1,504,321$9.17 
Vested(730,157)$12.58 
Forfeited(554,788)$12.71 
Unvested restricted stock and restricted stock unit awards as of December 31, 20242,326,523$9.78 
The expense related to restricted stock and restricted stock unit awards granted for the years ended December 31, 2024, 2023 and 2022 was $5.3 million, $5.1 million, and $7.9 million, respectively.
The restricted stock and restricted stock units granted in the year ended December 31, 2024 include 422,000 units granted to certain employees that contain performance-based vesting provisions. The expense related to the performance-based vesting of restricted stock units was $3.5 million, $0.5 million and $4.4 million for the years ended December 31, 2024, 2023 and 2022, respectively.
As of December 31, 2024, total unrecognized compensation expense related to unvested restricted stock and restricted stock unit awards was $12.2 million, which the Company expects to recognize over a remaining weighted-average period of 2.3 years.
Stock Options
The following is a summary of stock option activity for the year ended December 31, 2024:
SharesWeighted Average
Exercise Price
Remaining
Contractual Life (years)
Aggregate Intrinsic
Value (in thousands)
Outstanding at December 31, 20236,149,645$16.47 6.98$3,195 
Granted2,966,836$8.07 
Exercised(23,809)$8.10 
Cancelled(1,436,662)$30.43 
Outstanding at December 31, 20247,656,010$14.70 7.19$— 
Exercisable at December 31, 20243,964,202$19.87 5.76$— 
The total intrinsic value of options exercised for the years ended December 31, 2024, 2023 and 2022 was $0.04 million, $0.03 million, and $0.09 million, respectively.
Using the Black-Scholes option pricing model, the weighted average fair value of options containing service-based vesting granted during the years ended December 31, 2024, 2023, and 2022 was $5.62, $6.04, and $15.87, respectively. The expense related to options containing service-based vesting was $12.2 million, $13.9 million, and $14.1 million for the years ended December 31, 2024, 2023, and 2022, respectively.
The fair value of each service-based vesting option issued was estimated at the date of grant using the Black-Scholes option pricing model with the following weighted-average assumptions:
Year Ended
December 31,
202420232022
Expected volatility75.6 %75.2 %64.2 %
Expected option term (in years)6.206.196.25
Risk free interest rate4.2 %4.2 %1.7 %
Expected dividend yield— — — 
As of December 31, 2024, total unrecognized compensation expense related to stock options was $22.1 million, which the Company expects to recognize over a remaining weighted-average period of 2.60 years.

About Stock Compensation Disclosures

Stock-based compensation disclosures detail the equity awards granted to employees and executives — including stock options, restricted stock units (RSUs), and performance shares — along with the valuation methods and assumptions used to expense them. This section reveals the true cost of talent retention and the alignment between management incentives and shareholder interests.

Key signals: total unrecognized compensation expense and its expected recognition period signal future earnings headwinds from already-granted awards. For stock options, examine Black-Scholes assumptions — expected volatility, risk-free rate, and expected term — as understating any of these reduces reported compensation expense. Compare stock compensation expense as a percentage of revenue against peers to assess dilution cost. Watch vesting schedules for acceleration clauses tied to change-of-control events. Performance-based awards with undemanding targets may indicate weak governance. Add back stock compensation to operating cash flow to calculate a more conservative free cash flow figure.