Stock Based Compensation

3 Months Ended

Mar. 31, 2021

Stock Based Compensation [Abstract]

Stock Based Compensation

8. Stock Based Compensation

Stock Option Valuation

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:

	Three Months Ended March 31,
	2021			2020
Risk-free interest rate		0.62	%		1.48	%
Expected term (in years)		5.87			5.87
Expected volatility		69.41	%		77.75	%
Expected dividend yield		0.00	%		0.00	%
Exercise price	$	36.99		$	22.47
Fair value of common stock	$	36.99		$	22.47

The following table summarizes stock option activity for the three months ended March 31, 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		916,575			60.95		10.64
Exercised		(991,432	)		8.31				51,892
Cancelled		(83,244	)		31.82
Outstanding as of March 31, 2021		10,892,830		$	13.42		6.57	$	349,406
Options vested and exercisable as of March 31, 2021		7,856,137		$	5.70		5.55	$	300,452
Options unvested as of March 31, 2021		3,036,693		$	33.17		9.20

The weighted average grant-date fair value per share of stock options granted during the three months ended March 31, 2021 and 2020 was $36.99 and $15.03, respectively.

The total fair value of options vested during the three months ended March 31, 2021 and 2020 was $8.3 million and $17.3 million, respectively.

Stock-Based Compensation

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

	Three Months Ended March 31,
	2021		2020
Stock options	$	7,826	$	3,961
Restricted stock units		74		-
Total share based compensation expense	$	7,900	$	3,961

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

	Three Months Ended March 31,
	2021		2020
Research and development	$	2,916	$	1,735
General and administrative		4,984		2,226
Total share based compensation expense	$	7,900	$	3,961

As of March 31, 2021, the Company had an aggregate of $55.7 million of unrecognized stock-based compensation expense, which is expected to be recognized over the weighted average period of 3.01 years.

Restricted Stock Units (“RSU”)

The following table summarizes our RSU activity for the three months ended March 31, 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
Vested		-		-
Forfeited		-		-
Unvested as of March 31, 2021		23,500		30.61