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Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2023

Stock-Based Compensation

Summary of stock option activity

A summary of stock option activity for awards under the Plans is presented below:

​

​	​	​	​	​	​	​	​	​	​	​
​	​	Options Outstanding and Exercisable
​	​	​	​	​	​	​	Weighted	​	​	​
​	​	​	​	Weighted		​	Average	​	Aggregate
​	​	​	​	Average		​	Remaining	​	Intrinsic
​	​	Number of	​	Exercise		​	Contractual	​	Value(1)
​		Shares		Price			Life (Years)		(in thousands)
Outstanding as of January 1, 2022	​	4,545,794	​	$	14.15	​	8.1	​	$	2,860
Granted		1,739,350	​	$	5.34		—	​		—
Exercised	​	(50,420)	​	$	1.32	​	—	​	​	—
Forfeitures	​	(972,545)	​	$	13.77	​	—	​	​	—
Outstanding as of December 31, 2022		5,262,179	​	$	11.44		7.6	​	$	115
Granted	​	2,074,750	​	$	1.55	​	—	​	​	—
Exercised	​	(5,057)	​	$	0.99	​	—	​	​	—
Forfeitures	​	(514,770)	​	$	6.25	​	—	​	​	—
Outstanding as of December 31, 2023	​	6,817,102	​	$	8.83	​	7.3	​	​	​
Exercisable as of December 31, 2023		3,851,843	​	$	12.50		​	​	$	52

(1) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the fair value of the common stock for the options that were in the money at December 31, 2023 and 2022.

Summary of stock based compensation expense recorded

Stock-based compensation expense recorded as research and development and general and administrative expenses is as follows:

​	​	​	​	​	​	​
​	​	December 31,
(in thousands)	​	2023			2022
Research and development	​	$	2,924	​	$	3,056
General and administrative	​		5,264	​		6,452
Total stock-based compensation expense	​	$	8,188	​	$	9,508

Summary of assumptions used in the Black Scholes option pricing model for stock options granted

​

The assumptions used in the Black-Scholes option-pricing model for stock options granted were as follows:

​

​	​	​	​	​	​	​	​
​	​	Year Ended					​
​	​	December 31,					​
​		2023			2022		​
Expected term		6.1	years	​	5.5 - 6.1	years	​
Expected volatility		81.4	%	​	79.7 - 81.4	%	​
Risk free interest rate		3.5 - 4.09	%	​	1.8 - 4.2	%	​
Expected dividend yield		—	%	​	—	%	​

​

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Tabular disclosure for stock option plans. Includes, but is not limited to, outstanding awards at beginning and end of year, grants, exercises, forfeitures, and weighted-average grant date fair value.

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Tabular disclosure of the significant assumptions used during the year to estimate the fair value of stock options, including, but not limited to: (a) expected term of share options and similar instruments, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions.

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