Stock-based compensation

3 Months Ended

Mar. 31, 2021

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,
	2021		2020
	(in thousands)
Research and development	$	1,371	$	1,685
General and administrative		2,230		2,355
Total stock-based compensation expense	$	3,601	$	4,040

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,
	2021		2020
	(in thousands)
Stock options	$	2,523	$	3,208
Restricted stock awards and units		975		741
Employee stock purchase plan awards		103		91
Total stock-based compensation expense	$	3,601	$	4,040

Restricted Stock Units

A summary of the status of and changes in unvested restricted stock unit activity under the Company’s equity award plans for the three months ended March 31, 2021 was as follows:

		Weighted
		Average
		Grant Date
		Fair Value
	Units	Per Unit
Unvested restricted stock units as of December 31, 2020	638,471	$	12.74
Granted	1,203,684	$	6.28
Vested	(184,217)	$	11.55
Forfeited	(50,607)	$	9.40
Unvested restricted stock units as of March 31, 2021	1,607,331	$	8.15

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. Restricted stock units granted by the Company typically vest in equal amounts, annually over three years. In the three months ended March 31, 2021, the Company granted 503,684 restricted stock units vesting in equal amounts, annually over three years, and 700,000 restricted stock units vesting in equal amounts, annually over two years. All of the restricted stock units granted in the three months ended March 31, 2020 vest in equal amounts, annually over three years. The stock-based compensation expense related to restricted stock units and awards was $0.9 million and $0.7 million for the three months ended March 31, 2021 and 2020, respectively.

As of March 31, 2021, the Company had unrecognized stock-based compensation expense related to its unvested restricted stock units of $11.8 million, which is expected to be recognized over the remaining weighted-average vesting period of 2.2 years.

Stock Options

The following is a summary of stock option activity for the three months ended March 31, 2021:

		Weighted		Remaining	Aggregate
		Average		Contractual	Intrinsic
		Exercise		Life	Value
	Shares	Price		(in years)	(in thousands)
Outstanding at December 31, 2020	5,485,078	$	14.77
Granted	1,031,704	$	7.42
Exercised	(3,811)	$	7.27
Cancelled or forfeited	(261,109)	$	14.01
Outstanding at March 31, 2021	6,251,862	$	13.59	7.7	$	—
Exercisable at March 31, 2021	3,165,450	$	15.18	6.5	$	—

Using the Black‑Scholes option pricing model, the weighted-average fair value of options granted to employees during the three months ended March 31, 2021 was $4.81 per share. The expense related to stock options granted to employees and directors was $2.5 million for the three months ended March 31, 2021. The expense related to stock options granted to employees and directors was $3.2 million for the three months ended March 31, 2020.

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,
	2021		2020
Risk-free interest rate	0.7	%	1.4	%
Expected dividend yield	—	%	—	%
Expected term (in years)	6.0		6.0
Expected volatility	74.8	%	72.3	%

There were no new stock options granted to non‑employees during the three months ended March 31, 2021 and 2020. The expense related to stock options previously granted to non‑employees was de minimis for the three months ended March 31, 2021 and 2020, respectively.

As of March 31, 2021, the Company had unrecognized stock-based compensation expense related to its unvested stock options of $22.0 million which is expected to be recognized over the remaining weighted-average vesting period of 2.8 years.