Stock-based compensation

9 Months Ended

Sep. 30, 2021

Stock-based compensation

Stock-based compensation

11. Stock-based compensation

Stock options

A summary of the Company’s stock option activity and related information for the nine months ended September 30, 2021 is as follows:

	Shares	Weighted-average exercise price per share		Weighted-average remaining contractual term (years)	Aggregate intrinsic value (in thousands)
Outstanding at December 31, 2020	12,690,745	$	3.90	6.5	$	3,390
Granted	1,652,863		2.78
Exercised	(565,891)		1.59
Forfeited/cancelled	(830,708)		3.71
Outstanding at September 30, 2021	12,947,009	$	3.87	6.1	$	10,140
Vested at September 30, 2021	8,454,091	$	4.54	4.8	$	6,720

The fair value of each stock option granted during the nine months ended September 30, 2021 and 2020 was estimated on the grant date using the Black-Scholes option-pricing model using the following weighted-average assumptions:

	September 30,
	2021		2020
Risk-free interest rate	0.76	%	0.75	%
Volatility	95	%	94	%
Dividend yield	—		—
Expected term (years)	5.9		5.8

Restricted stock units (RSUs)

A summary of the Company’s RSU activity and related information for the nine months ended September 30, 2021 is as follows:

	Shares	Weighted-average grant date fair value per share
Outstanding at December 31, 2020	2,649,317	$	1.73
Granted	795,962	$	2.76
Vested	(1,426,160)	$	1.40
Forfeited/cancelled	(269,140)	$	2.11
Outstanding at September 30, 2021	1,749,979	$	2.41

Employee stock purchase plan

At the Special Meeting of Stockholders, held on December 18, 2018, the stockholders approved the 2018 Employee Stock Purchase Plan (2018 ESPP). On June 21, 2019, the board of directors of the Company amended and restated the 2018 ESPP, to account for certain non-material changes to the plan’s administration (the Amended and Restated 2018 ESPP). The Amended and Restated 2018 ESPP provides eligible employees with the opportunity, through regular payroll deductions, to purchase shares of the Company’s common stock at 85% of the lesser of the fair market value of the common stock (a) on the date the option is granted, which is the first day of the purchase period, and (b) on the exercise date, which is the last business day of the purchase period. The Amended and Restated 2018 ESPP generally allows for two six-month purchase periods per year beginning in January and July, or such other periods as determined by the compensation committee of the Company’s board of directors. The Company has reserved 2,000,000 shares of common stock for the administration of the Amended and Restated 2018 ESPP. The fair value of shares expected to be purchased under the Amended and Restated 2018 ESPP was calculated using the following weighted-average assumptions:

	Nine months ended September 30,
	2021		2020
Risk-free interest rate	0.07	%	1.04	%
Volatility	68	%	109	%
Dividend yield	—		—
Expected term (years)	0.5		0.5

For the nine months ended September 30, 2021 and 2020, the Company has recognized $0.1 million of stock-based compensation expense under the Amended and Restated 2018 ESPP. During the nine months ended September 30, 2021 the Company issued 110,060 shares of common stock for proceeds of $0.2 million under the Amended and Restated 2018 ESPP.