Stock based compensation

3 Months Ended

Mar. 31, 2020

Stock based compensation

Stock based compensation

9. Stock-based compensation

Stock-Based Compensation Expense

Total compensation cost recognized for all stock-based compensation awards in the condensed consolidated statements of operations and comprehensive loss is as follows:

	Three Months Ended
	March 31,
	2020		2019
	(in thousands)
Research and development	$	1,685	$	1,416
General and administrative		2,355		2,102
Total stock-based compensation expense	$	4,040	$	3,518

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

	Three Months Ended
	March 31,
	2020		2019
	(in thousands)
Stock options	$	3,208	$	2,776
Restricted stock awards and units		741		683
Employee stock purchase plan awards		91		59
Total stock-based compensation expense	$	4,040	$	3,518

Restricted Stock Units

A summary of the status of and changes in unvested restricted stock unit activity under the Company’s equity award plan for the three months ended March 31, 2020 was as follows:

		Weighted
		Average
		Grant Date
		Fair Value
	Units	Per Unit
Unvested restricted stock units as of December 31, 2019	455,404	$	12.16
Granted	429,103	$	13.05
Vested	(108,600)	$	9.10
Forfeited	(28,480)	$	10.50
Unvested restricted stock units as of March 31, 2020	747,427	$	13.18

Stock-based compensation of restricted stock units is based on the fair value of the Company’s common stock on the date of grant and recognized over the vesting period. The weighted average fair value of restricted stock units granted to employees during the three months ended March 31, 2020 was $13.05 per share. The weighted average fair value of restricted stock units granted to employees during the three months ended March 31, 2019 was $9.10 per share. The restricted stock units granted in the three months ended March 31, 2020 and March 31, 2019 vest in equal amounts, annually over three years. The expense related to restricted stock units granted to employees was $0.7 million for each of the three months ended March 31, 2020 and 2019. As of March 31, 2020, the Company had unrecognized stock-based compensation expense related to its unvested restricted stock units of $8.5 million, which is expected to be recognized over the remaining average vesting period of 2.8 years.

Stock Options

The following is a summary of stock option activity for the three months ended March 31, 2020:

		Weighted		Remaining	Aggregate
		Average		Contractual	Intrinsic
		Exercise		Life	Value
	Shares	Price		(in years)	(in thousands)
Outstanding at December 31, 2019	5,317,326	$	15.98
Granted	899,085	$	12.88
Exercised	(3,035)	$	11.30
Cancelled or forfeited	(148,221)	$	15.66
Outstanding at March 31, 2020	6,065,155	$	15.53	7.9	$	391
Exercisable at March 31, 2020	2,723,252	$	14.77	6.6	$	350
Vested and expected to vest at March 31, 2020	6,065,155	$	15.53	7.9	$	391

Using the Black‑Scholes option pricing model, the weighted-average fair value of options granted to employees and directors during the three months ended March 31, 2020 was $8.25 per share. The expense related to options granted to employees and directors was $3.2 million for the three months ended March 31, 2020. The weighted-average fair value of options granted to employees and directors during the three months ended March 31, 2019 was $6.23 per share. The expense related to options granted to employees and directors was $2.8 million for the three months ended March 31, 2019.

The fair value of each option issued to employees and directors was estimated at the date of grant using the Black-Scholes option pricing model with the following weighted-average assumptions:

	Three Months Ended
	March 31,
	2020		2019
Risk-free interest rate	1.4	%	2.5	%
Expected dividend yield	—	%	—	%
Expected term (in years)	6.0		6.0
Expected volatility	72.3	%	75.0	%

There were no new options granted to non‑employees during the three months ended March 31, 2020 and 2019. The expense related to awards previously granted to non‑employees was $0.1 million for each of the three months ended March 31, 2020 and 2019, respectively.

As of March 31, 2020, the Company had unrecognized stock-based compensation expense related to its unvested stock options of $30.8 million which is expected to be recognized over the remaining weighted-average vesting period of 2.8 years.