In 2017, the Company adopted the 2017 Stock Incentive Plan (the “2017 Plan”). Under the 2017 Plan, a total of 537,914 shares of the Company’s common stock were reserved for the issuance of stock options to employees, directors, and consultants under terms and provisions established by the Board of Directors (the “Board”). Under the terms of the 2017 Plan, options were granted at an exercise price not less than fair market value. The terms of options granted under the 2017 Plan may not exceed ten years. The Board determined the terms and conditions of a Restricted Stock Award, including the conditions for vesting and repurchase (or forfeiture) and the issue price, if any. On October 11, 2017, the Company increased the number of shares of common stock available for issuance under the 2017 Plan from 537,914 shares to 630,662 shares. On November 29, 2017, the Company increased the number of shares of common stock available for issuance under the 2017 Plan from 630,662 shares to 1,866,009 shares.

In connection with the Company’s initial public offering completed in January 2018, the Board adopted and the Company’s stockholders approved the 2018 Stock Incentive Plan (“2018 Plan”), which became effective on the date immediately preceding the date on which the Company’s registration statement became effective. The 2018 Plan provides for the grant of incentive stock options, nonstatutory stock options, restricted stock awards, restricted stock units, stock appreciation rights, and other stock-based awards. The Company’s employees, officers, directors, consultants and advisors are eligible to receive awards under the 2018 Plan. The number of shares of common stock that were initially reserved for issuance under the 2018 Plan were 2,200,260 shares. The 2018 Plan provides that the number of shares reserved and available for issuance under the plan will automatically increase each January 1, beginning on January 1, 2019, by 4% of the outstanding number of shares of the Company’s common stock on the immediately preceding December 31 or such lesser number of shares as determined by the Board. On January 1, 2019, as a result of the foregoing evergreen provision, the number of shares of common stock available for issuance under the 2018 Plan automatically increased from 2,200,260 to 3,322,473 shares.

Since the date of effectiveness of the 2018 Plan, the Company has not and will not grant any further awards under the 2017 Plan. However, any shares of common stock subject to awards under the 2017 Plan that expire, terminate, or otherwise are surrendered, canceled, forfeited or repurchased without having been fully exercised or resulting in any common stock being issued will become available for issuance under the 2018 Plan.

Total stock-based compensation expense is recognized for stock options granted to employees and non-employees and has been reported in the Company’s condensed consolidated statements of operations as follows:

	Three Months Ended June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Research and development	$	499	$	101	$	776	$	726
General and administrative		444		301		830		857
Total stock-based compensation expense	$	943	$	402	$	1,606	$	1,583

	Shares Available for Grant			Number of Options Outstanding			Weighted- Average Exercise Price per Option		Weighted- Average Remaining Contract Term		Aggregate Intrinsic Value
											(In thousands)
Outstanding, December 31, 2018		1,350,582			1,122,677		$	11.63		9.22
Shares reserved for issuance		1,122,213
Options granted		(1,049,524	)		1,049,524			8.30
Options exercised		—			(7,029	)		0.79
Options cancelled		77,900			(77,900	)		10.52
Outstanding, June 30, 2019		1,501,171			2,087,272			10.04		9.23	$	3,167
Exercisable, June 30, 2019					274,954			11.19		8.51		457
Vested and expected to vest, June 30, 2019					2,087,272			10.04		9.23		3,167

The aggregate intrinsic values of options outstanding, exercisable, vested and expected to vest were calculated as the difference between the exercise price of the options and the fair value of the Company’s common stock as of June 30, 2019. A total of 7,029 options were exercised for cash proceeds of approximately $9,000 during the six months ended June 30, 2019.

During the six months ended June 30, 2019, the Company granted options to employees and directors to purchase an aggregate of 1,039,684 common shares with a weighted-average grant date fair value of $6.18 per share. During the six months ended June 30, 2019, the Company granted options to non-employees to purchase an aggregate of 9,840 common shares with a weighted-average grant date fair value of $5.79 per share. The expense related to options granted to employees and directors for the three and six months ended June 30, 2019 was $0.9 million and $1.5 million, respectively. The expense related to options granted to non-employees for the three and six months ended June 30, 2019 was $40,000 and $48,000. The expense related to options granted to employees and directors was $0.4 million and $0.7 million for the three and six months ended June 30, 2018, respectively. The expense related to options granted to non-employees was $24,000 and $27,000, for the three and six months ended June 30, 2018, respectively.

As of June 30, 2019, the total unrecognized compensation expense related to unvested options granted to employees and directors was $12.0 million, which the Company expects to recognize over an estimated weighted-average period of 3.11 years. As of June 30, 2019, the total unrecognized compensation expense related to unvested non-employee options was $0.2 million, which the Company expects to recognize over an estimated weighted-average period of 2.75 years.

The fair value of stock options for employees and non-employees was estimated using a Black-Scholes option pricing model with the following assumptions:

	Three Months Ended June 30,			Six Months Ended June 30,
	2019	2018		2019	2018
Employees and directors:
Fair value of common stock	$6.97 - $8.08	$8.57 - $9.91		$6.97 - $8.90	$8.57 - $15.45
Expected term (in years)	5.5 - 6.1		6.6	5.5 - 6.1	5.8 - 6.6
Expected volatility	94.5% - 104.8%	77.4% - 77.7%		93.7% - 104.8%	75.9% - 90.6%
Risk-free interest rate	1.9% - 2.4%	2.6% - 2.8%		1.9% - 2.6%	2.4% - 2.8%
Expected dividend yield	0.0%	0.0%		0.0%	0.0%
Non-employees:
Fair value of common stock	$8.90 - $10.20		—	$6.82 - $10.20	$15.45
Expected term (in years)	8.0 - 9.7		—	8.0 - 10.0		10.0
Expected volatility	93.4% - 94.2%		—	90.0% - 94.9%	77.9%
Risk-free interest rate	1.9% - 2.1%		—	1.9% - 2.6%	2.7%
Expected dividend yield	0.0%		—	0.0%	0.0%

On April 17, 2018, the Company granted 2,000 shares of restricted stock to a consultant. The restrictions lapsed in four equal quarterly installments and were fully vested on the first anniversary of such grant. Compensation expenses of such unvested shares were remeasured at fair value until vested at each reporting date.

The Company recognized $2,000 and $4,000 of stock-based compensation expense related to restricted shares during the three and six months ended June 30, 2019, respectively. The Company recognized $10,000 and $0.9 million of stock-based compensation expense related to restricted shares during the three and six months ended June 30, 2018, respectively. As of June 30, 2019, there was no unrecognized stock-based compensation expense related to unvested restricted stock. There were no restricted stock awards during the six months ended June 30, 2019.

In May 2018, the Company granted 24,960 restricted stock units to an employee with a grant date fair value of $9.03 per share.

The Company recognized $14,000 and $ 28,000 of stock-based compensation expense related to restricted stock units during the three and six months ended June 30, 2019, respectively. As of June 30, 2019, there was $0.2 million of unrecognized stock-based compensation expense related to unvested restricted stock units. This amount is expected to be recognized over a remaining weighted-average period of 2.89 years. There were no restricted stock units granted to employees or non-employees during the three and six months ended June 30, 2019. The Company recognized $6,000 of stock-based compensation expense related to restricted stock units during the three and six months ended June 30, 2018.

The Board adopted and the Company’s stockholders approved the 2018 Employee Stock Purchase Plan (“2018 ESPP”), which became effective on the date immediately preceding the date on which the Company’s registration statement became effective. The 2018 ESPP enables eligible employees to purchase shares of the Company’s Common Stock at a discount. The number of shares of common stock that are reserved for issuance under the 2018 ESPP are 275,030 shares. The 2018 ESPP provides that the number of shares reserved and available for issuance will automatically increase each January 1, beginning on January 1, 2019 and increasing each January 1 thereafter through January 1, 2028, by the least of (i) 1% of the outstanding number of shares of the Company’s common stock on the immediately preceding December 31; (ii) 543,926 shares or (iii) such number of shares as determined by the ESPP administrator. On January 1, 2019, as a result of the foregoing evergreen provision, the number of shares of common stock available for issuance under the 2018 ESPP automatically increased from 275,030 to 555,583 shares.