Stock Based Compensation (Tables)

6 Months Ended

Jun. 30, 2021

Stock Based Compensation [Abstract]

Weighted-Average Assumptions for Stock Options

The weighted average assumptions that the Company used in the Black-Scholes pricing model to determine the fair value of the stock options granted to employees, non-employees and directors were as follows:

	Six Months Ended June 30,
	2021			2020
Risk-free interest rate		0.74	%		1.27	%
Expected term (in years)		5.84			5.84
Expected volatility		69.19	%		77.39	%
Expected dividend yield		0.00	%		0.00	%
Exercise price	$	57.18		$	21.56
Fair value of common stock	$	57.18		$	21.56

Stock Option Activity

The following table summarizes stock option activity for the six months ended June 30, 2021 under the Second Amended and Restated 2014 Stock Option and Incentive Plan:

	Number of Shares			Weighted Average Exercise Price		Weighted Average Contractual Term (Years)		Aggregate Intrinsic Value
Outstanding as of December 31, 2020		11,050,931		$	9.10		6.55	$	504,079
Granted		1,243,509			57.18		9.32
Exercised		(1,125,270	)		8.76				53,345
Cancelled		(128,473	)		34.15
Outstanding as of June 30, 2021		11,040,697		$	14.27		6.40	$	346,476
Options vested and exercisable as of June 30, 2021		8,127,520		$	6.27		5.43	$	308,590
Options unvested as of June 30, 2021		2,913,177		$	36.57		9.11

Stock-Based Compensation Expense by Award Type

Stock-based compensation expense recognized by award type was as follows:

	Three Months Ended June 30,				Six Months Ended June 30,
	2021		2020		2021		2020
Stock options	$	7,215	$	4,489	$	15,041	$	8,450
Restricted stock units		96		-		170		-
Total share based compensation expense	$	7,311	$	4,489	$	15,211	$	8,450

Stock-Based Compensation Expense

Stock-based compensation expense by classification included within the consolidated statements of operations and comprehensive loss was as follows:

	Three Months Ended June 30,				Six Months Ended June 30,
	2021		2020		2021		2020
Research and development	$	3,148	$	1,679	$	6,064	$	3,414
General and administrative		4,163		2,810		9,147		5,036
Total share based compensation expense	$	7,311	$	4,489	$	15,211	$	8,450

RSU Activity

The following table summarizes the Company's RSU activity for the six months ended June 30, 2021:

	Number of Shares		Weighted Average Grant Date Fair Value
Unvested as of December 31, 2020		20,000	$	25.06
Granted		3,500		62.32
Unvested as of June 30, 2021		23,500		30.61