The Company’s 2017 Stock Incentive Plan (the “2017 Plan”) was approved by the Company’s stockholders on June 16, 2017 and became effective on June 28, 2017. Under the 2017 Plan, the Company may grant incentive stock options, nonstatutory stock options, stock appreciation rights, restricted stock awards, awards of restricted stock units and other stock-based awards. The Company’s employees, officers, directors, consultants and advisors are eligible to receive awards under the 2017 Plan; however, incentive stock options may only be granted to employees. The 2017 Plan is administered by the board of directors or, at the discretion of the board of directors, by a committee of the board. The number of shares of common stock covered by options and the date those options become exercisable, type of options to be granted, exercise prices, vesting and other restrictions are determined at the discretion of the board of directors, or its committee if so delegated.

Stock options granted under the 2017 Plan with service-based vesting conditions generally vest over four years and may not have a duration in excess of ten years, although options have been granted with vesting terms of less than four years.

The total number of shares of common stock that may be issued under the 2017 Plan was 5,319,537 as of June 30, 2020, of which 358,416 shares remained available for grant. The Company initially reserved 1,244,816 shares of common stock plus the number of shares equal to the sum of the number of shares of common stock then available for issuance under the Company’s 2016 Stock Incentive Plan (the “2016 Plan”), which was 424,601 shares, and the number of shares of common stock subject to outstanding awards under the Company’s 2006 Stock Incentive Plan, as amended (the “2006 Plan”) and the 2016 Plan that expire, terminate or are otherwise surrendered, canceled, forfeited or repurchased by the Company at their original issuance price pursuant to a contractual repurchase right. Pursuant to the terms of the 2017 Plan, the number of shares of common stock that may be issued under the 2017 Plan will automatically increase on each January 1, beginning with the fiscal year ending December 31, 2018 and continuing for each fiscal year until, and including, the fiscal year ending December 31, 2027, equal to the least of (i) 1,244,816 shares, (ii) 4% of the outstanding shares of common stock on such date and (iii) an amount determined by the Company’s board of directors. On January 1, 2019 and January 1, 2020, the number of shares issuable under the 2017 Plan automatically increased by 589,939 shares and 1,112,414 shares, respectively.

During the six months ended June 30, 2020, pursuant to the terms of the 2017 Plan, the Company granted options to employees and directors to purchase 1,771,000 shares of common stock at a weighted average exercise price of $0.72 per share.

Shares that are expired, terminated, surrendered or canceled without having been fully exercised will be available for future awards. In addition, shares of common stock that are tendered to the Company by a participant to exercise an award are added to the number of shares of common stock available for the grant of awards.

The exercise price for stock options granted may not be less than the fair market value of the common stock as of the date of grant.

On June 16, 2017, the Company’s stockholders approved the 2017 Employee Stock Purchase Plan (the “2017 ESPP”), which became effective on June 28, 2017. A total of 150,000 shares of common stock were initially reserved for issuance under this plan. Under the 2017 ESPP, the number of shares of common stock that may be issued under the 2017 ESPP will automatically increase on each January 1, beginning with the fiscal year ending December 31, 2018 and continuing for each fiscal year until, and including, the fiscal year ending December 31, 2027, equal to the least of (i) 622,408 shares, (ii) 1% of the outstanding shares of common stock on such date and (iii) an amount determined by the Company’s board of directors. The compensation committee of the board of directors has determined that the number of shares of common stock that may be issued under the 2017 ESPP would not be increased on January 1, 2019 or January 1, 2020. The Company has not issued any shares under the 2017 ESPP.

The 2016 Plan provided for the Company to grant incentive stock options or nonqualified stock options, restricted stock, restricted stock units and other equity awards to employees, directors and consultants of the Company. The 2016 Plan was administered by the board of directors or, at the discretion of the board of directors, by a committee of the board. The exercise prices, vesting and other restrictions were determined at the discretion of the board of directors, or its committee if so delegated.

Stock options granted under the 2016 Plan with service-based vesting conditions vest over four years and expire after ten years.

As of the effective date of the 2017 Plan, the board of directors determined to grant no further awards under the 2016 Plan. No stock options or other awards have been made under the 2016 Plan since the adoption of the 2017 Plan.

Shares that are expired, terminated, surrendered or canceled without having been fully exercised will be available for future awards under the 2017 Plan. In addition, shares of common stock that are tendered to the Company by a participant to exercise an award are added to the number of shares of common stock available for the grant of awards under the 2017 Plan.

The 2006 Plan provided for the Company to grant incentive stock options or nonqualified stock options, restricted stock, restricted stock units and other equity awards to employees, directors and consultants of the Company. The 2006 Plan was administered by the board of directors or, at the discretion of the board of directors, by a committee of the board. The exercise prices, vesting and other restrictions were determined at the discretion of the board of directors, or its committee if so delegated.

Stock options granted under the 2006 Plan with service-based vesting conditions generally vest over four years and expire after ten years, although options have been granted with vesting terms of less than four years.

The 2006 Plan expired in 2016. Since its expiration no further awards have been made under the 2006 Plan.

The assumptions that the Company used to determine the grant-date fair value of the stock options granted to employees and directors during the six months ended June 30, 2020 and 2019 were as follows, presented on a weighted average basis:

	Six Months Ended June 30, 2020			Six Months Ended June 30, 2019
Risk-free interest rate		1.03	%		2.53	%
Expected term (in years)		6.3			6.3
Expected volatility		76.0	%		76.0	%
Expected dividend yield		0	%		0	%

The following table summarizes the Company’s stock option activity since January 1, 2020:

	Number of Shares			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(in years)
Outstanding at December 31, 2019		3,320,706		$	3.87		7.4	$	—
Granted		1,771,000			0.72
Exercised		—			0.00
Canceled		(401,754	)		6.54
Forfeited		—			0.00
Outstanding at June 30, 2020		4,689,952		$	2.45		8.5	$	—
Options exercisable at June 30, 2020		1,879,181		$	3.68		7.6	$	—
Options vested and expected to vest at June 30, 2020		4,587,708		$	2.48		8.5	$	—
Options exercisable at December 31, 2019		1,415,900		$	5.56		5.2	$	—
Options vested and expected to vest at December 31, 2019		3,264,851		$	3.89		7.4	$	—

The weighted average grant-date fair value of stock options granted during the six months ended June 30, 2020 and 2019 was $0.48 and $1.11, respectively.

The aggregate fair value of stock options that vested during the six months ended June 30, 2020 and 2019 was $1,173 and $1,142, respectively.

The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those stock options that had exercise prices lower than the fair value of the Company’s common stock. The aggregate intrinsic value of stock options exercised during the six months ended June 30, 2020 and 2019 was $0 and $109, respectively.

On April 15, 2019, the Company granted restricted stock units under the 2017 Plan. The following table summarizes the Company’s restricted stock unit activity during the six months ended June 30, 2020:

				Weighted-Average
				Grant Date
	Units			per Unit
Outstanding, non-vested at December 31, 2019		50,000		$	1.75
Issued		—			—
Vested		(40,000	)	$	1.75
Canceled/forfeited		—			—
Outstanding, non-vested at June 30, 2020		10,000		$	1.75

The Company recorded stock-based compensation expense related to stock options and restricted stock units in the following expense categories of its statements of operations and comprehensive loss:

	Three Months Ended June 30,				Six Months Ended June 30,
	2020		2019		2020		2019
Research and development expenses	$	200	$	112	$	349	$	193
General and administrative expenses		360		348		716		830
	$	560	$	460	$	1,065	$	1,023

During the three months ended June 30, 2020 and 2019 the Company recognized stock-based compensation expense of $111 and $23, respectively, included in the table above, related to performance-based awards for which achievement of such performance-based conditions were deemed probable. During the six months ended June 30, 2020 and 2019 the Company recognized stock-based compensation expense of $160 and $23, respectively, included in the table above, related to performance-based awards for which achievement of such performance-based conditions were deemed probable.

As of June 30, 2020, the Company had an aggregate of $2,925 of unrecognized stock-based compensation expense, which it expects to recognize over a weighted average period of 2.4 years.