Stock Incentive Plans (Tables)

3 Months Ended

Mar. 31, 2020

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Schedule of Weighted Average Assumptions Used

The estimated grant date fair values of employee stock option awards granted under the 2016 Plan and the 2018 Inducement Incentive Award Plan were calculated using the Black-Scholes option pricing model, based on the following weighted-average assumptions:

	Three Months Ended March 31,
	2020			2019
Risk-free interest rate	1.73		%	2.45		%
Dividend yield	—			—
Expected term	6.08			6.06
Expected volatility	87.96		%	87.35		%
Weighted-average fair value of common stock	$	2.30		$	2.41

Summary of Activity Under the 2008 Plan and the 2016 Plan

The following table summarizes the activity under the 2008 Plan, 2016 Plan, and 2018 Inducement Incentive Award Plan:

					Weighted‑average
					remaining	Aggregate
	Number of		Weighted-average		contractual term	intrinsic value
	options		exercise price ($)		(in years)	(in thousands)
Employee awards
Outstanding at December 31, 2019	6,323,596		$	4.91	8.71	$	1,716
Granted	1,708,309		$	2.30
Exercised	(5,128	)	$	0.47
Forfeited	(753,914	)	$	3.62
Outstanding at March 31, 2020	7,272,863		$	4.44	8.71	$	1,728
Vested at March 31, 2020	1,822,514		$	8.39	7.01	$	115
Vested and expected to vest at March 31, 2020	6,661,467		$	4.61	8.64	$	1,541
Non‑employee awards
Outstanding at December 31, 2019	473,073		$	5.89	6.23	$	38
Granted	—		$	—
Exercised	—		$	—
Forfeited	—		$	—
Outstanding at March 31, 2020	473,073		$	5.89	5.98	$	40
Vested at March 31, 2020	346,860		$	5.77	4.98	$	22
Vested and expected to vest at March 31, 2020	473,073		$	5.89	5.98	$	40

Disclosure of Share-based Compensation Arrangements by Share-based Payment Award

The following table summarizes the status of the Company’s restricted stock units:

	Number of shares	Weighted average fair value ($)
Unvested at December 31, 2019	181,250	$	5.00
Granted	—	—
Vested	10,937	6.03
Forfeited	—	—
Unvested at March 31, 2020	170,313	$	4.93

Schedule of Stock-Based Compensation Expense Related to Stock Options and Restricted Common Stock

The Company recorded stock-based compensation expense related to stock option awards, restricted stock units and the ESPP in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands):

	Three Months Ended March 31,
	2020		2019
Research and development	$	623	$	519
General and administrative	786		661
Total stock-based compensation expense	$	1,409	$	1,180