Stock-based compensation expense was as follows:


	Three Months Ended September 30,				Nine Months Ended September 30,
(in thousands)	2024		2023		2024		2023
Research and development	$	312	$	3,254	$	232	$	10,191
General and administrative		4,635		7,214		15,657		20,594
Total stock-based compensation expense	$	4,947	$	10,468	$	15,889	$	30,785

In connection with the reduction in the Company’s workforce (“RIF”) (see Note 13), the Company accelerated certain unvested stock options and restricted common stock scheduled to vest in the three month period following the employees’ separation date. The Company determined that the acceleration of the unvested units constituted a Type III modification in accordance with ASC 718, resulting in a new measurement of compensation cost. As of September 30, 2024, 664,248 units were accelerated. For the nine months ended September 30, 2024, the acceleration resulted in the recognition of $0.1 million of stock-based compensation expense using the reassessed fair value on the modification date and a reversal of $4.0 million in stock-based compensation expense for previously recognized expense using the original grant date fair value, of which $2.4 million was related to research and development expense and $1.6 million was related to general and administrative expense. There were no units accelerated during the three months ended September 30, 2024.

At September 30, 2024, there is an aggregate of 15,091,301 shares reserved for future issuance under the 2020 Plan.

The following table summarizes restricted common stock activity for the nine months ended September 30, 2024:


	Shares			Weighted Average Grant Date Fair Value
Unvested at January 1, 2024		3,254,863		$	8.73
Granted		245,000			0.77
Forfeited		(1,126,973	)		8.34
Vested		(1,252,791	)		8.65
Unvested at September 30, 2024		1,120,099		$	7.45

At September 30, 2024, there was $7.0 million of unrecognized stock-based compensation expense related to restricted stock, which is expected to be recognized over a weighted average period of 1.60 years. The total fair value of restricted stock vested was $0.9 million and $3.0 million for the nine months ended September 30, 2024 and 2023, respectively.

The following table summarizes stock option activity (in thousands, except share and per share data):


	Shares			Weighted Average Exercise Price		Weighted Average Contractual Life (in years)		Aggregate Intrinsic Value
Options outstanding at January 1, 2024		10,439,751		$	13.81		7.95	$	—
Granted		—			—		—		—
Exercised		—			—		—		—
Forfeited		(4,367,732	)		11.32		—		—
Options outstanding at September 30, 2024		6,072,019		$	15.60		6.88	$	—
Options vested and exercisable at September 30, 2024		4,579,700		$	17.51		6.56	$	—

The aggregate intrinsic value of options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those stock options that had exercise prices lower than the fair value of the common stock as of the end of the period.

There were no options granted during the nine months ended September 30, 2024. The weighted-average grant date fair value of stock options granted during the nine months ended September 30, 2023 was $4.92 per share. At September 30, 2024, there was $8.9 million of unrecognized stock-based compensation expense related to unvested stock options, which is being recognized over a period of 1.52 years.

The Company issued 36,337 common shares under the Employee Stock Purchase Plan (the “ESPP”) during the nine months ended September 30, 2024 at an average price per share of $0.58. Cash received from purchases under the ESPP for the nine months ended September 30, 2024 and 2023 were $0.0 million and $0.3 million, respectively. The Company recognized $0.0 and $0.3 million of compensation expense for the ESPP during the nine months ended September 30, 2024 and 2023, respectively.

At September 30, 2024, there was an aggregate of 1,585,257 shares reserved for future issuance under the ESPP.