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Equity-based Compensation and Equity Incentive Plans (Tables)

12 Months Ended

Dec. 31, 2021

Schedule of Restricted Common Stock Activity

The following table shows restricted common stock activity:

	Restricted stock awards
				Weighted average grant date
	Number of			fair value
	shares			(per share)
Unvested at December 31, 2020		478,207		$	2.19
Granted		242,454			3.50
Vested		(343,619	)		2.39
Forfeited		(77,794	)		2.96
Unvested at December 31, 2021		299,248		$	2.83

Schedule of Equity-based Compensation Expenses

The following table summarizes equity‑based compensation expense within the Company’s consolidated statements of operations and comprehensive loss for the years ended December 31, 2021 and 2020 (in thousands):

	Years ended December 31,
	2021		2020
Research and development	$	1,373	$	1,724
General and administrative		1,988		2,183
	$	3,361	$	3,907

Schedule of Equity-based Compensation Expenses by Award Type

The following table summarizes equity‑based compensation expense by type of award for the years ended December 31, 2021 and 2020 (in thousands):

	Years ended December 31,
	2021		2020
Stock options	$	2,969	$	3,200
Restricted stock awards		334		680
ESPP		58		27
	$	3,361	$	3,907

2015 and 2017 Plan

Schedule of Weighted Average Assumption Used Black-Scholes Option-pricing Model for Stock Options Issued to Employees and Non-employees

The weighted average assumptions used in the Black-Scholes option-pricing model for stock options issued to employees and non-employees under the 2015 Plan and the 2017 Plan, during the years ended December 31, 2021 and 2020 were:

	Year ended December 31,
Employees:	2021		2020
Expected term	6.2 years		6.2 years
Weighted-average, risk-free interest rate	1.0%		0.6%
Expected volatility	84.9%		77.2%
Dividend yield		—		—

Schedule of Stock Option Activity

The following table summarizes stock option activity under the 2015 and 2017 Plans.

	Stock options outstanding
						Weighted
						average
				Weighted		remaining		Aggregate
				average		contractual		intrinsic
	Number of			exercise		term		value^(a)
	options			price		(in years)		(in thousands)
Outstanding at December 31, 2020		3,343,836		$	6.00		8.4	$	399
Granted		2,215,796		$	3.37				—
Exercised		(139,772	)	$	2.24				160
Cancelled/Forfeited		(918,947	)	$	4.30				—
Outstanding at December 31, 2021		4,500,913		$	5.17		8.2	$	588

Vested or expected to vest at December 31, 2021		4,500,913		$	5.17		8.2	$	588
Exercisable at December 31, 2021		2,032,515		$	7.33		7.2	$	258

^(a)

^{The aggregate intrinsic value is calculated as the difference between the exercise price of the options and the fair market value of the underlying common stock for the options that were in the money at December 31, 2021 and 2020. 1,210,026 and 970,826 options were in the money at December 31, 2021 and 2020, respectively.}