Share-Based Compensation

6 Months Ended

Jun. 30, 2021

Share-based Payment Arrangement [Abstract]

Pursuant to the evergreen provisions of the Company's 2013 stock incentive plan (the “2013 Plan”), annual increases have resulted in the addition of an aggregate of approximately 13,081,000 additional shares to the 2013 Plan, including for 2021, an increase of approximately 2,542,000 shares. As of June 30, 2021, the Company had approximately 2,847,000 shares available for grant under the 2013 Plan.

In October 2019, the Company's board of directors adopted its 2019 Inducement Stock Incentive Plan (the “2019 Inducement Plan”) to reserve initially 1,000,000 shares of its common stock to be used exclusively for grants of awards to individuals who were not previously employees or directors of the Company as a material inducement to such individuals’ entry into employment with the Company within Rule 5635(c)(4) of the Nasdaq Listing Rules. The terms and conditions of the 2019 Inducement Plan are substantially similar to those of the 2013 Plan. In March 2020, the Company's board of directors amended the 2019 Inducement Plan to reserve an additional 1,000,000 shares of its common stock for issuance under the plan and in February 2021, the Company's board of directors further amended the 2019 Inducement Plan to reserve an additional 600,000 shares of its common stock for issuance under the plan. As of June 30, 2021, the Company had approximately 731,000 shares available for grant under the 2019 Inducement Plan.

Share-based compensation expense, net of estimated forfeitures, includes expenses related to stock options and RSUs granted to employees, non-employee directors and consultants, as well as options granted to employees to purchase shares

under the ESPP. Stock-based compensation by award type was as follows:


	Three Months Ended June 30,				Six Months Ended June 30,
	2021		2020		2021		2020
Stock options	$	1,042	$	1,093	$	2,511	$	2,467
Restricted stock units	1,003		752		1,795		1,688
Employee stock purchase plan	34		28		65		42
Total	$	2,079	$	1,873	$	4,371	$	4,197

The Company allocated stock-based compensation expense in the Company’s Consolidated Statements of Operations and Comprehensive Loss as follows:


	Three Months Ended June 30,				Six Months Ended June 30,
	2021		2020		2021		2020
Research and development	$	1,319	$	951	$	2,662	$	2,104
General and administrative	760		922		1,709		2,093
Total	$	2,079	$	1,873	$	4,371	$	4,197

Stock Options

A summary of the stock option activity, weighted average exercise prices, options outstanding, exercisable and expected to vest as of June 30, 2021 is as follows (in thousands except weighted average exercise price):


	Number of Shares Underlying Options	Weighted Average Exercise Price
Outstanding, December 31, 2020	8,928	$	9.22
Granted	549	$	6.66
Exercised	(144)	$	3.18
Forfeited	(100)	$	4.91


Outstanding, June 30, 2021	9,233	$	9.47
Vested and exercisable, June 30, 2021	5,148	$	12.67
Vested and expected to vest, June 30, 2021	8,903	$	9.62

As of June 30, 2021, there were approximately $14.5 million of unrecognized compensation costs, net of estimated forfeitures, related to stock option awards grants, which are expected to be recognized over a remaining weighted average period of 3.0 years.

RSUs

The following table presents a summary of the Company's outstanding RSU awards granted as of June 30, 2021 (in thousands except weighted average grant-date fair value):


	Restricted Stock Units	Weighted Average Grant-Date Fair Value
Outstanding, December 31, 2020	1,958	$	6.95
Awarded	201	$	6.57
Vested	(98)	$	3.31
Forfeited	(108)	$	36.14
Outstanding, June 30, 2021	1,953	$	5.48
Outstanding, Expected to vest	1,795	$	5.48

As of June 30, 2021, there were approximately $8.0 million of unrecognized compensation costs, net of estimated forfeitures, related to RSUs grants, which are expected to be recognized over a remaining weighted average period of 2.8 years.

ESPP

As of June 30, 2021, there were 780,939 shares available for future purchases under the ESPP. There were no shares issued under the ESPP during the three months ended June 30, 2021 and 2020, respectively. There were 24,422 shares of common stock issued under the ESPP during the six months ended June 30, 2021. Cash proceeds from ESPP purchases were $121 thousand during the six months ended June 30, 2021. There were 43,581 shares of common stock issued under the ESPP during the six months ended June 30, 2020. Cash proceeds from ESPP purchases were $42 thousand during the six months ended June 30, 2020.