Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2021

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Stock-Based Compensation Expense on All Employee and Non-Employee Awards

The Company recognized stock-based compensation expense on all employee and non-employee awards as follows:

	For the Three Months Ended September 30,				For the Nine Months Ended September 30,
(in thousands)	2021		2020		2021		2020
Research and development		535		522		2,115		1,587
General and administrative		1,836		1,270		7,742		3,806
Stock-based compensation expense	$	2,371	$	1,792	$	9,857	$	5,393

Fair Value of Stock Options Assumptions Using Black-Scholes Option Valuation Model

For the three and nine months ended September 30, 2021 and 2020, the fair value of stock options was estimated on the date of grant using a Black-Scholes option valuation model with the following assumptions:

	For the Three Months Ended September 30,						For the Nine Months Ended September 30,
	2021			2020			2021			2020
Risk-free interest rate	0.89 - 0.96%			0.36 - 0.39%			0.50 - 1.15%			0.36 - 1.68%
Expected life in years	6.00 - 6.25			5.75 - 6.25			5.50 - 6.25			5.75 - 6.25
Expected volatility	72.53 - 72.84%			73.59 - 74.18%			72.53 - 74.80%			71.11 - 74.18%
Expected dividend yield		—	%		—	%		—	%		—	%

Stock Option Activity under Stock Option Plan

Stock option activity under the Company’s stock option plans for the nine months ended September 30, 2021 is as follows:

(in thousands, except share and per share data)	Number of Shares			Weighted- Average Exercise Price		Weighted- Average Contractual Term (Years)		Aggregate Intrinsic Value
Outstanding, December 31, 2020		6,840,719		$	3.81
Granted		7,150,438			2.97
Exercised		(363,109	)		2.86
Cancelled		(2,555,154	)		3.54
Outstanding, September 30, 2021		11,072,894		$	3.36		8.32	$	473
Options exercisable, September 30, 2021		5,743,820		$	4.08		7.24	$	—
Options exercisable, December 31, 2020		3,596,315		$	4.17		6.90	$	598
Options available for future grant		2,224,159

Summary of Unvested Restricted Stock

A summary of the status of unvested restricted stock for the nine months ended September 30, 2021 is as follows:

	Number of Shares			Weighted-Average Grant Date Fair Value
Unvested, December 31, 2020		786,280		$	3.08
Granted		1,601,224			2.60
Vested		(466,436	)		3.68
Cancelled		(410,413	)		3.46
Unvested, September 30, 2021		1,510,655		$	2.28