In April 2015, the Company’s shareholders approved the 2015 Stock Option and Grant Plan, or the 2015 Plan, and in July 2016, the Company’s shareholders approved the 2016 Stock Option and Incentive Plan, or the 2016 Plan. In May 2018, the Company’s shareholders approved the 2018 Stock Option and Incentive Plan, or the 2018 Plan (collectively, the “Plans”). Subsequent to the IPO, no further options were granted under the 2015 Plan. The Plans provide for the issuance of equity awards in the form of restricted shares, options to purchase common shares which may constitute incentive stock options, or ISOs, or non-statutory stock options, or NSOs, unrestricted stock unit grants, and qualified performance and market-based awards to eligible employees, officers, directors, non-employee consultants and other key personnel. Terms of the equity awards, including vesting requirements, are determined by our board of directors, subject to the provisions of the Plans. Options granted by the Company typically vest over four years and have a contractual life of ten years. Restricted stock unit grants typically vest over two to four years. At December 31, 2024, the Company had 29,405,365 common shares authorized for issuance under the 2018 Plan and 9,536,673 common shares available for future grant under the 2018 Plan.

The Company recognized stock-based compensation expense totaling $86.6 million, $81.0 million, and $97.9 million during the years ended December 31, 2024, 2023 and 2022, respectively. Stock‑based compensation expense by classification within the consolidated statements of operations and comprehensive loss is as follows (in thousands):


	Years Ended December 31,
	2024		2023		2022
Research and development	$	47,944	$	46,356	$	53,956
General and administrative		38,623		34,672		43,991
Total	$	86,567	$	81,028	$	97,947

As of December 31, 2024, there was $81.4 million and $95.5 million of unrecognized compensation expense related to unvested stock options and restricted stock units, respectively, that is expected to be recognized over a weighted-average period of 2.6 and 2.8 years, respectively.

The fair value of each option issued to employees was estimated at the date of grant using the Black-Scholes option pricing model with the following weighted-average assumptions:


	Years Ended December 31,
	2024			2023			2022
Options granted		1,566,536			1,860,485			1,492,589
Weighted-average exercise price	$	62.40		$	45.47		$	60.19
Weighted-average grant date fair value	$	38.63		$	28.39		$	38.54
Assumptions:
Expected volatility		63.9	%		65.1	%		70.2	%
Expected term (in years)		6.0			6.0			6.0
Risk-free interest rate		4.2	%		4.1	%		2.6	%
Expected dividend yield		0.0	%		0.0	%		0.0	%

The following table summarizes stock option activity under the Company’s equity award plans (intrinsic value in thousands):


	Shares			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (years)		Aggregate Intrinsic Value
Outstanding at December 31, 2023		7,204,372		$	55.05		6.7	$	115,681
Granted		1,566,536			62.40
Exercised		(900,136	)		33.80
Cancelled or forfeited		(581,889	)		69.92
Outstanding at December 31, 2024		7,288,883		$	58.07		6.6	$	14,050
Exercisable at December 31, 2024		4,851,838		$	58.64		5.5	$	14,050
Vested and expected to vest at December 31, 2024		7,288,883		$	58.07		6.6	$	14,050

The total intrinsic value (the amount by which the fair market value exceeded the exercise price) of stock options exercised during the year ended December 31, 2024, 2023 and 2022 was $32.2 million, $13.9 million, and $40.8 million, respectively.

As of December 31, 2024, options to purchase 1,085,467 common shares subject to performance-based vesting conditions were vested, as performance conditions were satisfied in prior years, and there were no options to purchase common shares subject to performance-based vesting conditions outstanding. Activity related to stock options subject to performance-based vesting conditions is included in the table above.

As of December 31, 2024, options to purchase 150,000 common shares subject to market-based vesting conditions were vested, as market conditions were satisfied in prior years. 100,000 options to purchase common shares subject to market-based vesting conditions were outstanding as of December 31, 2024.

The Company did not grant stock options subject to performance-based or market-based vesting conditions during 2024, 2023, and 2022.

The following table summarizes the restricted stock unit activity under the Company’s equity award plans:


	Shares			Weighted- Average Grant Date Fair Value
Unvested balance at December 31, 2023		1,781,415		$	61.00
Granted		1,284,894			60.26
Vested		(450,701	)		70.13
Cancelled or forfeited		(217,965	)		57.41
Unvested balance at December 31, 2024		2,397,643		$	59.21

During the years ended December 31, 2024, 2023 and 2022, the total fair value of restricted stock units vested was $30.0 million, $14.1 million, and $14.3 million, respectively.

During 2022, the Company granted 150,000 performance stock units with market-based vesting conditions in which the recipient is eligible to receive between zero and 150,000 common shares at the end of a three-year service period based upon achieving a specified average stock price. Expense for these awards is being recognized over the requisite service period. As of December 31, 2024, 150,000 of the performance stock units were unvested. Activity related to stock units subject to market-based vesting conditions is included in the table above.

The Company did not grant restricted stock units subject to performance-based or market-based vesting conditions during 2024 and 2023.

On July 19, 2016, our board of directors adopted its 2016 Employee Stock Purchase Plan, or the ESPP Plan, which was subsequently approved by its shareholders and became effective on October 19, 2016. The ESPP Plan authorizes the initial issuance of up to a total of 413,226 shares of the Company’s common stock to participating employees. The Company activated its ESPP Plan on January 1, 2020. The Company issued 37,245, 53,282, and 36,559 shares under the ESPP Plan during the years ended December 31, 2024, 2023 and 2022, respectively.