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Stock-Based Compensation (Tables) | 9 Months Ended
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Sep. 30, 2025
Stock-Based Compensation |
Schedule of the Company's option activity | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | Weighted- | ​ | ​ | ​
​ | ​ | ​ | ​ | Weighted- | ​ | Average | ​ | ​ | ​
​ | ​ | ​ | ​ | Average | ​ | Remaining | ​ | Aggregate
​ |  | ​ |  | Exercise |  | Contractual |  | Intrinsic
​ | ​ | ​ | ​ | Price Per | ​ | Term | ​ | Value
​ | ​ | Options | ​ | Share | ​ | (in years) | ​ | (in thousands)
Outstanding as of December 31, 2024 |  | 7,670,908 | ​ | $ | 9.59 |  | 8.2 | ​ | $ | 1,777
Stock options assumed in connection with the ACELYRIN Merger | ​ | 4,712,186 | ​ | $ | 12.99 | ​ | ​ | ​ | ​ | ​
Granted |  | 6,983,120 | ​ | $ | 4.60 | ​ | ​ | ​ | ​ | ​
Exercised |  | — | ​ | $ | — | ​ | ​ | ​ | $ | —
Forfeited or expired |  | (956,679) | ​ | $ | 9.07 | ​ | ​ | ​ | ​ | ​
Outstanding, vested and expected to vest as of September 30, 2025 |  | 18,409,535 | ​ | $ | 8.60 | ​ | 7.0 | ​ | $ | 388
Exercisable as of September 30, 2025 | ​ | 10,661,758 | ​ | $ | 10.66 | ​ | 5.2 | ​ | $ | 116
​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | ​ |  | ​ | ​ |  | Weighted- |  | ​ | ​
​ | ​ | ​ | ​ | Weighted- | ​ | Average | ​ | ​ | ​
​ | ​ | ​ | ​ | Average | ​ | Remaining | ​ | Aggregate
​ | ​ | ​ | ​ | Exercise | ​ | Contractual | ​ | Intrinsic
​ | ​ | ​ | ​ | Price Per | ​ | Term | ​ | Value
​ | ​ | Options | ​ | Share | ​ | (in years) | ​ | (in thousands)
Outstanding as of December 31, 2024 |  | 1,787,880 | ​ | $ | 10.19 |  | ​ | 9.3 | ​ | $ | —
Granted |  | — | ​ | $ | — |  | ​ | ​ | ​ |  |
Forfeited or expired |  | (107,056) | ​ | $ | 10.19 | ​ |  | ​ | ​ |  | ​
Outstanding, vested and expected to vest as of September 30, 2025 |  | 1,680,824 | ​ | $ | 10.19 | ​ |  | 8.6 | ​ | $ | —
Exercisable as of September 30, 2025 |  | — | ​ | $ | — | ​ |  | — | ​ | $ | —
Schedule of various significant assumptions used to estimate the fair value of stock options | ​
​ | ​ | ​ | ​ | ​
​ | ​ | Nine Months Ended September 30,
​ |  | 2025 |  | 2024
Expected term (in years) | ​ | 5.23 - 6.08 |  | 5.75 - 6.08
Volatility | ​ | 98.84% - 104.32% |  | 103.65% - 109.54%
Risk-free interest rate | ​ | 3.69% - 4.65% |  | 3.47% - 4.48%
Dividend yield | ​ | 0.00% | ​ | 0.00%
Schedule of activity for the Company's early exercisable shares | ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | Weighted-
​ | ​ | ​ | ​ | Average
​ | ​ | Number of | ​ | Exercise Price
​ |  | Shares |  | Per Share
Unvested as of December 31, 2024 |  | 146,683 | ​ | $ | 5.53
Vested |  | (125,805) | ​ | $ | 4.96
Unvested as of September 30, 2025 |  | 20,878 | ​ | $ | 8.96
schedule of estimating the fair value of the stock purchase rights under the ESPP | ​
​ | ​ | ​
​ | ​ | Nine Months Ended
​ |  | September 30, 2025
Expected term (in years) | ​ | 0.32 - 0.50
Volatility | ​ | 79.42% - 133.00%
Risk-free interest rate | ​ | 4.25% - 4.33%
Dividend yield | ​ | 0.00%
Schedule of unvested RSU activity | ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | Weighted-
​ | ​ | ​ | ​ | Average
​ | ​ | ​ | ​ | Grant Date
​ | ​ | Number of | ​ | Fair Value
​ |  | Shares |  | Per Share
RSUs assumed in connection with the ACELYRIN Merger |  | 1,470,868 | ​ | $ | 13.46
RSUs granted |  | 2,449,945 | ​ | $ | 3.95
RSUs released | ​ | (1,115,413) | ​ | $ | 13.76
RSUs forfeited |  | (134,495) | ​ | $ | 5.48
Unvested as of September 30, 2025 |  | 2,670,905 | ​ | $ | 5.01
Schedule of the stock-based compensation expense | The following table summarizes the stock-based compensation expense recognized in the Company’s unaudited condensed consolidated statements of operations and comprehensive loss for the three and nine months ended September 30, 2025 and 2024 (in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended September 30, | ​ | Nine Months Ended September 30,
​ |  | 2025 |  | 2024 |  | 2025 |  | 2024
Research and development | ​ | $ | 5,773 | ​ | $ | 3,720 | ​ | $ | 16,717 | ​ | $ | 6,805
General and administrative | ​ |  | 4,014 | ​ | ​ | 3,589 | ​ | ​ | 18,700 | ​ | ​ | 6,294
Total stock-based compensation expense | ​ | $ | 9,787 | ​ | $ | 7,309 | ​ | $ | 35,417 | ​ | $ | 13,099
The following table summarizes the stock-based compensation expense recognized in the Company’s unaudited condensed consolidated statements of operations and comprehensive loss by award type for the three and nine months ended September 30, 2025 and 2024 (in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended September 30, | ​ | Nine Months Ended September 30,
​ |  | 2025 |  | 2024 |  | 2025 |  | 2024
Stock options | ​ | $ | 7,979 | ​ | $ | 7,291 | ​ | $ | 28,771 | ​ | $ | 13,045
RSUs | ​ | ​ | 1,542 | ​ | ​ | — | ​ | ​ | 5,939 | ​ | ​ | —
ESPP | ​ | ​ | 266 | ​ | ​ | — | ​ | ​ | 701 | ​ | ​ | —
Restricted stock awards | ​ |  | — | ​ | ​ | 18 | ​ | ​ | 6 | ​ | ​ | 54
Total stock-based compensation expense | ​ | $ | 9,787 | ​ | $ | 7,309 | ​ | $ | 35,417 | ​ | $ | 13,099

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