Equity-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2020

Share-based Payment Arrangement [Abstract]

Equity-Based Compensation Expense Classification in Statement of Operations

Equity-based compensation expense is classified in the statements of operations as follows:

	Year Ended December 31,
	2020		2019
Research and development expense	$	878	$	535
General and administrative expense	1,306		1,305
	$	2,184	$	1,840

Assumptions Used to Determine Fair Value of Common Stock Option Grants

In addition to an assumption on the expected term of the option grants as discussed below, application of the Black-Scholes model requires additional inputs for which we have assumed the values described in the table below:

	Year Ended December 31,
	2020		2019
Expected term	5.2 to 6.1 years		5.3 to 6.1 years
Risk-free interest rate	0.31% to 1.68%; weighted avg. 0.61%		1.55% to 2.56%; weighted avg. 1.94%
Expected volatility	81.0% to 85.8%; weighted avg. 83.8%		80.2% to 86.7%; weighted avg. 82.6%
Forfeiture rate	5	%	5	%
Expected dividend yield	—	%	—	%

The fair value of the underlying common stock and the exercise price for the common stock options granted during the years ended December 31, 2020 and 2019 are summarized in the table below:

Common Stock Options Granted During Period Ended:	Fair Value of Underlying Common Stock	Exercise Price of Common Stock Option
Year ended December 31, 2020	$1.19 to $2.80; weighted avg. $1.89	$1.19 to $2.80; weighted avg. $1.93
Year ended December 31, 2019	$2.32 to $3.05; weighted avg. $2.86	$2.32 to $3.05; weighted avg. $2.86

Common Stock Option Activity

A summary of common stock option activity as of the periods indicated is as follows:

	Options	Weighted-Average Exercise Price		Weighted-Average Remaining Contractual Term (years)	Aggregate Intrinsic Value (thousands)
Outstanding - December 31, 2019	5,697,714	$	2.34	6.7	$	4,625
Granted	2,755,248	1.93
Exercised	(500,905)	0.73
Forfeited	(724,761)	2.90
Outstanding - December 31, 2020	7,227,296	$	2.24	6.8	$	1,719
Exercisable - December 31, 2020	4,216,647	$	2.23	5.1	$	1,275
Vested and Expected to Vest - December 31, 2020	7,013,117	$	2.24	6.7	$	1,684