Equity Incentive Plans (Tables)

9 Months Ended

Sep. 30, 2020

Share-based Payment Arrangement [Abstract]

Summary of Activity Related to Stock Option Awards

A summary of the activity related to stock option awards during the nine months ended September 30, 2020 is as follows:

	Shares Subject to Options	Weighted-Average Exercise Price per Share		Weighted-Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value (in thousands)
Outstanding at January 1, 2020	3,039,880	$	29.40
Granted	461,425	$	18.30
Exercises	(91,146)	$	4.06
Forfeitures and cancellations	(534,096)	$	37.54
Outstanding at September 30, 2020	2,876,063	$	26.91	7.47	$	6,747
Exercisable at September 30, 2020	1,355,530	$	27.48	5.64	$	6,365

Summary of Weighted Average Assumptions in Stock Option Valuations

The estimated fair values of stock option awards granted to employees were determined on the date of grant using the Black-Scholes option valuation model with the following weighted-average assumptions:

	Nine Months Ended September 30,
	2020			2019
Risk-free interest rate		0.5	%		2.5	%
Expected volatility		90.5	%		68.4	%
Expected dividend yield		—	%		—	%
Expected term (in years)		6.25			6.25
Weighted-average grant date fair value per share	$	13.88		$	43.08

Summary of Non-cash Stock-based Compensation Expense

Total non-cash stock-based compensation expense for all stock awards that was recognized in the consolidated statements of operations and comprehensive loss is as follows:

	Three Months Ended September 30,				Nine Months Ended September 30,
(in thousands)	2020		2019		2020		2019
Research and development	$	926	$	1,580	$	3,030	$	4,469
General and administrative	1,879		1,564		5,491		5,185
Total	$	2,805	$	3,144	$	8,521	$	9,654