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Share-Based Compensation
 9 Months Ended
Sep. 30, 2020
Share-based Payment Arrangement [Abstract]  
Share-Based Compensation Share-Based Compensation
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 10,539,000 additional shares to the 2013 Plan, including for 2020, an increase of approximately 1,985,000 shares, or approximately 4% of the total number of shares of the Company's common stock outstanding as of January 1, 2020. As of September 30, 2020, the Company had approximately 2,666,000 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. As of September 30, 2020, the Company had approximately 610,000 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 September 30,Nine Months Ended September 30,
 2020201920202019
Stock options$1,089 $1,447 $3,556 $4,557 
Restricted stock units723 667 2,411 2,197 
Employee stock purchase plan35 12 77 49 
Total$1,847 $2,126 $6,044 $6,803 
The Company allocated stock-based compensation expense in the Company’s Consolidated Statements of Operations and Comprehensive Loss as follows:
 Three Months Ended September 30,Nine Months Ended September 30,
 2020201920202019
Research and development$882 $942 $2,986 $3,101 
General and administrative965 1,184 3,058 3,702 
Total$1,847 $2,126 $6,044 $6,803 
Stock Options
A summary of the stock option activity, weighted average exercise prices, options outstanding, exercisable and expected to vest as of September 30, 2020 is as follows (in thousands except weighted average exercise price):
 Number of Shares Underlying OptionsWeighted
Average
Exercise
Price
Outstanding, December 31, 20196,780 $10.89 
Granted990 $4.02 
Exercised(73)$2.55 
Forfeited(264)$16.21 
Expired(7)$1.59 
Outstanding, September 30, 20207,426 $9.88 
Vested and exercisable, September 30, 20203,968 $15.33 
Vested and expected to vest, September 30, 20207,114 $10.15 
As of September 30, 2020, there were approximately $7.2 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 2.7 years.
RSUs
The following table presents a summary of the Company's outstanding RSU awards granted as of September 30, 2020 (in thousands except weighted average grant-date fair value):
Restricted
Stock
Units		Weighted Average
Grant-Date
Fair Value
Outstanding, December 31, 20191,481 $7.79 
Awarded470 $3.77 
Vested(126)$19.78 
Forfeited(98)$6.74 
Outstanding, September 30, 20201,727 $5.88 
Outstanding, Expected to vest1,515 $3.90 
As of September 30, 2020, there were approximately $4.2 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 2.6 years.
ESPP
As of September 30, 2020, there were 805,361 shares available for future purchases under the ESPP. There were 32,821 and 38,944 shares issued under the ESPP during the three months ended September 30, 2020 and 2019. There were 76,402 and 70,466 shares of common stock issued under the ESPP during the nine months ended September 30, 2020 and 2019. Cash proceeds from ESPP purchases were $147 thousand and $81 thousand during the nine months ended September 30, 2020 and 2019, respectively.