Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2020

Equity [Abstract]

Summary of Stock Option Activity

The following table summarizes stock option activity as of December 31, 2020:

						Weighted
				Weighted		average		Aggregate
	Number			average		contractual		intrinsic
	of shares			exercise price		term		value
Outstanding at December 31, 2019		3,458,447			10.19
Granted		649,142			54.50
Exercised		(127,778	)		8.22
Forfeited		(207,628	)		35.85
Cancelled		(38,267	)		58.98
Outstanding at December 31, 2020		3,733,916			16.03		7.2		243,556,917
Vested and expected to vest at December 31, 2020		3,727,747			16.36		7.2		243,062,340
Exercisable at December 31, 2020		2,479,110			10.66		6.0		175,575,360

Summary of Black-Scholes Option Valuation Assumptions

The relevant data used to determine the value of the stock option grants for the years ended December 31, 2020, 2019, and 2018 is as follows:

Black-Scholes option valuation assumptions	2020		2019		2018
Risk-free interest rates	0.3 - 1.7%		1.4 - 2.7%		2.0 - 3.1%
Dividend yield		—		—		—
Volatility	67 - 99%		87 - 91 %		74 - 87%
Weighted average expected term	3.51 - 6.14 years		5.00 - 6.13 years		3.66 - 6.13 years

Schedule of RSU Activity

The following table sets forth the RSU activity for the year ended December 31, 2020:

				Weighted
				average
	Number			grant date
	of shares			fair value
Outstanding at December 31, 2019		—		$	—
Granted		161,268			37.42
Released		—			—
Forfeited		(13,466	)		86.88
Cancelled		(11,735	)		45.14
Outstanding at December 31, 2020		136,067		$	35.76

Schedule of Stock-Based Compensation Expense Recognized

Stock‑based compensation expense recognized for the years ended December 31, 2020, 2019, and 2018 was as follows:

	2020		2019		2018
Research and development	$	3,953,939	$	1,584,165	$	391,946
General and administrative		10,802,267		4,530,995		1,363,028
Total	$	14,756,206	$	6,115,160	$	1,754,974