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Share-Based Payments
 3 Months Ended
Mar. 31, 2024
Share-Based Payment Arrangement [Abstract]  
Share-Based Payments Share-Based Payments
2023 Stock Incentive Plan and Inducement Grants
In June 2023, our stockholders approved the 2023 Stock Incentive Plan, or the 2023 Plan. The 2023 Plan provides for the grant of incentive stock options, nonstatutory stock options, stock appreciation rights, restricted stock awards, restricted stock units, or RSUs, performance-based share units, or PSUs, and other stock-based awards to employees, advisors, consultants and non-employee directors.
Following the adoption of the 2023 Plan, we ceased granting equity awards under the 2013 Stock Incentive Plan, or the 2013 Plan. Any outstanding equity awards that were previously granted under the 2013 Plan continue to be governed by their terms. Following adoption of the 2013 Plan, we ceased granting equity awards under the 2007 Stock Incentive Plan, or the 2007 Plan. There are no outstanding equity awards under the 2007 Plan.
In connection with the start of employment of our Chief Executive Officer and Chief Financial Officer in 2022, and our Chief Commercial Officer in 2023, our board of directors granted each of them equity awards in the form of stock options, RSUs and
PSUs, which awards were made outside our equity incentive plans as inducements material to their respective entry into employment with us in accordance with Nasdaq Listing Rule 5635(c)(4).
As of March 31, 2024, the maximum number of shares reserved under the 2013 Plan, the 2023 Plan and the inducement grants described above was 11,250,432, and we had 3,076,274 shares available for future issuance under the 2023 Plan.
Stock options
The following table presents stock option activity for the three months ended March 31, 2024:
Number of
Stock Options
Weighted-Average
Exercise Price
Outstanding at December 31, 20235,263,681 $44.94 
Granted808,415 32.20 
Exercised(148,626)31.04 
Cancelled/Forfeited(9,357)41.86 
Expired(5,250)26.15 
Outstanding at March 31, 20245,908,863 $43.56 
Exercisable at March 31, 20243,475,385 $52.67 
Vested and expected to vest at March 31, 20245,908,863 $43.56 
At March 31, 2024, there was approximately $38.0 million of total unrecognized compensation expense related to unvested stock option awards, which we expect to recognize over a weighted-average period of approximately 2.86 years.
Restricted stock units
The following table presents RSU activity for the three months ended March 31, 2024:
Number of
Stock Units
Weighted-Average
Grant Date Fair 
Value
Unvested shares at December 31, 20231,346,701 $29.67 
Granted989,378 30.91 
Vested(460,470)33.54 
Forfeited(10,424)29.75 
Unvested shares at March 31, 20241,865,185 $29.37 
As of March 31, 2024, there was approximately $47.9 million of total unrecognized compensation expense related to RSUs, which we expect to recognize over a weighted-average period of approximately 2.33 years.
Performance-based stock units
The following table presents PSU activity for the three months ended March 31, 2024:
Number of
Stock Units
Weighted-Average
Grant Date Fair 
Value
Unvested shares at December 31, 2023362,133 $30.66 
Granted183,000 32.27 
Vested(145,023)36.04 
Unvested shares at March 31, 2024400,110 $29.45 
Stock-based compensation expense associated with these PSUs is recognized if the underlying performance condition is considered probable of achievement using our management’s best estimates.
As of March 31, 2024, there was no unrecognized compensation expense related to PSUs with performance-based vesting criteria that are considered probable of achievement, and $11.8 million of total unrecognized compensation expense related to PSUs with performance-based vesting criteria that are considered not probable of achievement.
Market-based stock units
The following table presents market-based stock unit, or MSU, activity for the three months ended March 31, 2024:
Number of
Stock Units		Weighted-Average
Grant Date Fair
Value
Unvested shares at December 31, 202342,695 $41.50 
Expired(42,695)41.50 
Unvested shares at March 31, 2024— $— 
The fair value of MSUs are estimated using a Monte Carlo simulation model. Assumptions and estimates utilized in the model include the risk-free interest rate, dividend yield, expected stock volatility and the estimated period to achievement of the market condition. As of March 31, 2024, there was no remaining unrecognized compensation expense related to MSUs.
2013 Employee Stock Purchase Plan
In June 2013, our Board of Directors adopted, and in July 2013 our stockholders approved, the 2013 Employee Stock Purchase Plan, or the 2013 ESPP. We issued and sold 52,514 and 54,867 shares of common stock during the three months ended March 31, 2024 and 2023, respectively, under the 2013 ESPP. The 2013 ESPP provides participating employees with the opportunity to purchase up to an aggregate of 2,363,636 shares of our common stock. As of March 31, 2024, we had 1,633,525 shares of common stock available for future issuance under the 2013 ESPP.
Stock-based compensation expense
Stock-based compensation expense by award type included within the condensed consolidated statements of operations is as follows:
Three Months Ended
March 31,
(In thousands)20242023
Stock options$4,071 $4,957 
Restricted stock units4,966 4,947 
Employee stock purchase plan197 235 
Total stock-based compensation expense$9,234 $10,139 

Expenses related to stock options and stock-based awards were allocated as follows in the condensed consolidated statements of operations:
Three Months Ended
March 31,
(In thousands)20242023
Research and development expense$3,775 $4,355 
Selling, general and administrative expense5,459 5,784 
Total stock-based compensation expense$9,234 $10,139