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Stock-based Compensation
 3 Months Ended
Mar. 31, 2024
Share-Based Payment Arrangement [Abstract]  
Stock-based Compensation Stock-based Compensation
2015 Stock Plan
The Company’s 2015 Stock Plan, as amended (the “2015 Stock Plan”), is one of the Company’s equity incentive compensation plans through which equity based grants are awarded. The 2015 Stock Plan provides for the grant of incentive stock options, non-statutory stock options, restricted stock, restricted stock units and other stock-based compensation awards to employees, officers, directors, and consultants of the Company. The administration of the 2015 Stock Plan is under the general supervision of the Compensation Committee of the Board. The terms of stock options awarded under the 2015 Stock Plan, in general, are determined by the Compensation Committee, provided the exercise price per share generally shall not be set at less than the fair market value of a share of the common stock on the date of grant and the term shall not be greater than ten years from the date the option is granted. As of March 31, 2024, 456,241 shares were available for future issuance under the 2015 Stock Plan.
2023 Inducement Plan
In September 2023, the Company adopted the 2023 Inducement Plan (the “Inducement Plan”), pursuant to which the Company may from time to time make equity grants to new employees as a material inducement to their employment. The Inducement Plan was adopted without stockholder approval, pursuant to Nasdaq Listing Rule 5635(c)(4), and is administered by the Compensation Committee of the Board. The Inducement Plan provides for the granting of non-statutory stock options, restricted stock, restricted stock units, performance stock units and other stock-based compensation awards to new employees, but does not allow for the granting of incentive stock options. The terms of the stock options under the Inducement Plan, in general, are determined by the Compensation Committee, provided the exercise price per share generally shall not be set at less than the fair market value of a share of the common stock on the date of grant and the
term shall not be greater than ten years from the date the option or award is granted. A total of 500,000 shares of the Company’s common stock were reserved for issuance under the Inducement Plan. As of March 31, 2024, 23,366 shares were available for future issuance under the 2023 Inducement Plan.
Stock Options
The following table summarizes stock option activity during the three months ended March 31, 2024:
 
Shares
Weighted
average exercise
price
Outstanding at December 31, 20232,355,779$79.94 
Options granted135,518229.77 
Options exercised(18,319)74.90 
Options cancelled(14,751)88.80 
Outstanding at March 31, 20242,458,227$88.18 
Exercisable at March 31, 20241,850,988$75.49 
The total cash received by the Company as a result of stock option exercises was $1.4 million and $17.9 million, respectively, for the three months ended March 31, 2024 and 2023. The total intrinsic value of options exercised was $3.8 million and $32.5 million, respectively, for the three months ended March 31, 2024 and 2023. The weighted-average grant date fair values, based on the Black-Scholes option model, of options granted during the three months ended March 31, 2024 and 2023 were $152.05 and $219.02, respectively.
Restricted Stock Units
The Company’s 2015 Stock Plan provides for awards of restricted stock units (“RSUs”) to employees, officers, directors and consultants to the Company. The Company’s Inducement Plan provides for awards of RSUs to new employees. RSUs vest over a period of months or years, or upon the occurrence of certain performance criteria or the attainment of stated goals or events, and are subject to forfeiture if employment or service terminates before vesting. As of March 31, 2024, the Company had 530,671 restricted stock units outstanding, with a weighted average grant date fair value of $231.44 per unit.

The following table summarizes RSU activity, excluding performance-based RSUs, during the three months ended March 31, 2024:

Shares
Weighted average grant date fair value
Unvested at December 31, 2023
376,117$241.45 
RSUs granted
209,347229.11
RSUs vested
(47,114)297.22
RSUs forfeited
(7,679)254.45
Unvested at March 31, 2024
530,671$231.44 

Performance-Based Restricted Stock Units
The Company has granted various performance-based restricted stock units (“PSUs”) to certain members of senior leadership. Depending on the terms of the PSUs and the outcome of the pre-established performance criteria, which may include a market and/or performance condition, a recipient may ultimately earn the target number of PSUs granted or a specified multiple thereof at the end of the vesting period.
As of March 31, 2024, the Company granted 101,202 PSUs. The maximum number of outstanding PSUs eligible to be earned as of March 31, 2024 are 252,404 PSUs, with a weighted average grant date fair value of $268.63 per unit.
Outstanding Awards
As of March 31, 2024, the Company had restricted stock units, performance stock units, and options outstanding pursuant to which an aggregate of 3,241,302 shares of its common stock may be issued pursuant to the terms of all awards granted under the 2015 Stock Plan and Inducement Plan.
Stock-Based Compensation Expense
Stock-based compensation expense during the three months ended March 31, 2024 and 2023 was as follows (in thousands):
Three Months Ended March 31,
20242023
Stock-based compensation expense by type of award:
Stock options$6,886 $8,598 
Restricted stock units8,625 2,652 
Performance-based restricted stock units4,391 — 
Total stock-based compensation expense$19,902 $11,250 
Effect of stock-based compensation expense by line item:
Research and development$5,907 $5,385 
Selling, general and administrative13,995 5,865 
Total stock-based compensation expense included in net loss$19,902 $11,250 
Unrecognized stock-based compensation expense as of March 31, 2024 was $177.3 million with a weighted average remaining period of 3.00 years.