Stock Incentive Plans (Tables)

9 Months Ended

Sep. 30, 2020

The assumptions used in the Black-Scholes option pricing model are as follows:

	Three months ended September 30,		Nine months ended September 30,
	2020	2019	2020	2019
Expected volatility	64.6% — 65.2%	62.4% — 63.2%	63.2% — 65.4%	62.4% — 64.7%
Weighted-average risk-free interest rate	0.31% — 0.40%	1.36% — 1.94%	0.31% — 1.73%	1.36% — 2.60%
Expected dividend yield	— %	— %	— %	— %
Expected term (in years)	6.0-6.25	6.25	5.5-6.25	6.25
Underlying common stock fair value	$10.10 — $16.48	$15.92 — $21.45	$10.10 — $21.75	$15.92 — $30.34

The Company recorded stock-based compensation expense related to stock options and shares purchased under the 2018 ESPP as follows:

	Three months ended September 30,				Nine months ended September 30,
	2020		2019		2020		2019
	(in thousands)
Research and development	$	1,572	$	1,124	$	4,640	$	2,786
General and administrative		1,774		863		5,042		2,413
	$	3,346	$	1,987	$	9,682	$	5,199

A summary of option activity under the Plans during the nine months ended September 30, 2020 is as follows:

	Number of Options			Weighted- Average Exercise Price per Share		Weighted- Average Remaining Contractual Term (in Years)		Aggregate Intrinsic Value
								(in thousands)
Outstanding at January 1, 2020		4,843,018		$	14.78		8.5	$	33,809
Granted		1,200,650		$	18.43
Exercised		(41,593	)	$	3.84
Cancelled/Forfeited		(189,243	)	$	23.31
Outstanding at September 30, 2020		5,812,832		$	15.33		8.1	$	11,299
Vested and expected to vest at September 30, 2020		5,812,832		$	15.33		8.1	$	11,299
Exercisable at September 30, 2020		2,574,662		$	11.70		7.3	$	9,618

A summary of the Company’s unvested common stock from early exercises that is subject to repurchase by the Company is as follows: