The Company’s equity incentive plans, the 2018 Long Term Incentive Plan, as amended (the “2018 LTIP”), 2020 Employment Inducement Incentive Plan, as amended (the “2020 EIIP”), and previously, the Amended and Restated 2012 Long Term Incentive Plan (the “2012 LTIP”), reserve ordinary shares for the issuance of stock options, stock appreciation rights, restricted shares, RSUs, performance bonus awards, performance share units awards, dividend equivalents and other share or cash-based awards to eligible individuals. Options granted under each of the 2018 LTIP, 2020 EIIP, and 2012 LTIP expire no later than ten years from the date of grant.

In May 2023, the Company’s shareholders approved an amendment to the 2018 LTIP to increase the number of ordinary shares available for issuance under the 2018 LTIP by 2,000,000 ordinary shares. As of March 31, 2024, the number of ordinary shares authorized under the 2018 LTIP was 14,620,433. Upon adoption of the 2018 LTIP, no new awards are permitted under the 2012 LTIP.

As of March 31, 2024, the number of ordinary shares authorized under the 2020 EIIP was 1,485,000 and 66,250 ordinary shares remained available for future awards under the 2020 EIIP. The Company’s Board of Directors has adopted a series of amendments to increase the ordinary shares available for issuance under the 2020 EIIP and it reserves the right to both amend the 2020 EIIP to increase the number of ordinary shares available and make additional awards to key new hires.

The Company’s option awards generally vest over four years, while RSUs vest over two years. As of March 31, 2024, 1,662,669 ordinary shares remained available for grant under its equity plans.

Share-based compensation expense recorded in these Condensed Consolidated Financial Statements was based on awards granted under the 2012 LTIP, the 2018 LTIP, and the 2020 EIIP. The estimated forfeiture rate as of March 31, 2024 was 7%. Changes in our estimates and assumptions relating to forfeitures may cause us to realize material changes in stock-based compensation expense in the future.

The amount of unearned share-based compensation related to unvested stock options at March 31, 2024, is $115.2 million. The weighted-average period over which this unearned share-based compensation is expected to be recognized is 2.94 years.


	Three Months Ended March 31,
	2024		2023
Research and development	$	5,455	$	4,362
General and administrative	6,928		4,428
Total share-based compensation expense	$	12,383	$	8,790

The Company recognized tax benefits from share-based awards of $2.1 million and $1.6 million for the three months ended March 31, 2024 and 2023, respectively.

The fair value of the options granted to employees and non-employee directors during the three months ended March 31, 2024 and 2023 was estimated as of the grant date using the Black-Scholes option-pricing model using the key assumptions listed in the following table:


	Three Months Ended March 31,
	2024				2023
Expected term (in years)	4.60		-	5.51	4.53		-	5.40
Expected volatility	74.8	%	-	78.6%	76.4	%	-	89.7%
Risk-free interest rate	3.8	%	-	4.4%	3.5	%	-	4.4%
Expected dividend yield	—%				—%
Weighted average grant date fair value	$19.57				$35.37

The fair value of employee stock options is amortized on a straight-line basis over the requisite service period for each award. Each of the inputs discussed above is subjective and generally requires significant management judgment to determine.


	Options	Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (years)	Aggregate Intrinsic Value (in thousands)
Outstanding at December 31, 2023	9,866,337	$	29.06	6.60	$	118,447
Granted	1,998,950	30.02
Exercised	(38,338)	23.24
Forfeited	(65,792)	36.36
Expired	(10,375)	28.50
Outstanding at March 31, 2024	11,750,782	$	29.20	6.92	$	42,936
Vested and expected to vest at March 31, 2024	11,258,566	$	28.84	6.82	$	42,907
Vested at March 31, 2024	6,909,977	$	22.39	5.48	$	41,926

The total intrinsic value of options exercised was $0.4 million, and $6.8 million during the three months ended March 31, 2024 and 2023, determined as of the date of exercise.


	Number of Units	Weighted Average Grant-Date Fair Value	Weighted Average Remaining Contractual Term (years)	Aggregate Intrinsic Value (in thousands)
Unvested at December 31, 2023	25,250	$58.01	1.09	$918
Units Granted	—	—
Units Vested	—	—
Units Forfeited	—	—
Unvested at March 31, 2024	25,250	$58.01	0.84	$625
Unvested and expected to vest at March 31, 2024	23,760	$58.16	0.83	$589

The fair value of RSUs was determined on the date of grant based on the market price of the Company’s ordinary shares as of that date. The fair value of the RSUs is recognized as an expense on a straight-line basis over the vesting period of each RSU. Upon the vesting of the RSUs, a portion of the shares vested are sold by the employee to satisfy employee withholding tax requirements (sell-to-cover). As of March 31, 2024, total compensation cost not yet recognized related to unvested RSUs was $0.8 million, which is expected to be recognized over a weighted-average period of 0.92 years. RSUs settle into ordinary shares upon vesting.