Stock-based Compensation Expense (Tables)

9 Months Ended

Sep. 30, 2020

Summary of Stock Option Activity

Stock options granted by the Company typically vest over a four-year period and have a ten year contractual term. The following table summarizes the Company’s stock option activity under the 2019 Plan and 2016 Plan since December 31, 2019:

	Number of Shares			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value
Outstanding at December 31, 2019		2,023,828		$	9.31		8.94	$	2,730,209
Granted		1,032,506			15.73
Exercised		(118,281	)		7.59
Cancelled		(69,360	)		12.23
Outstanding at September 30, 2020		2,868,693		$	11.62		8.73	$	425,192
Exercisable at September 30, 2020		733,937		$	10.16		8.41	$	181,578

Weighted Average Assumptions Used to Calculate Fair Value of Stock Option

The fair value of stock options granted during the three and nine months ended September 30, 2020 and 2019 under the 2019 Plan and the 2016 Plan has been calculated on the date of grant using the following weighted average assumptions:

	Three Months Ended September 30, 2020			Three Months Ended September 30, 2019			Nine Months Ended September 30, 2020			Nine Months Ended September 30, 2019
Risk-free interest rate		0.3	%		1.7	%		1.4	%		2.4	%
Expected dividend yield		0.0	%		0.0	%		0.0	%		0.0	%
Expected term (years)		6.0			6.0			6.0			6.0
Expected stock price volatility		78.1	%		80.1	%		75.9	%		81.0	%

Summary of Stock-Based Compensation Expense Recognized

The total compensation cost recognized in the statements of operations associated with all stock-based compensation awards granted by the Company is as follows (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2020		2019		2020		2019
Research and development	$	779	$	533	$	2,723	$	1,398
General and administrative		997		646		2,950		1,552
Total stock-based compensation expense	$	1,776	$	1,179	$	5,673	$	2,950

2019 Plan and 2016 Plan

Summary of Restricted Stock Activity

The shares of common stock underlying restricted stock awards typically vest over a four-year period. The shares of common stock are recorded in stockholders’ equity as they vest.

The following table summarizes the Company’s restricted stock activity under the 2019 Plan and 2016 Plan since December 31, 2019:

	Number of Shares			Weighted Average Grant Date Fair Value
Unvested at December 31, 2019		346,423		$	3.05
Granted		—			—
Vested		(198,343	)		2.99
Repurchased		(24,010	)		3.02
Unvested at September 30, 2020		124,070		$	3.15

Grants Outside of 2016 Stock Incentive Plan and 2019 Stock Incentive Plan

Summary of Restricted Stock Activity

The following table summarizes the Company’s restricted stock activity outside of the 2019 Plan and 2016 Plan since December 31, 2019:

	Number of Shares			Weighted Average Grant Date Fair Value
Unvested at December 31, 2019		334,647		$	2.94
Granted		—			—
Vested		(277,504	)		2.94
Repurchased		—			—
Unvested at September 30, 2020		57,143		$	2.94