Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2019

Summary of Stock-based Compensation Expense Recognized

During the three and six months ended June 30, 2019 and 2018, the Company recognized the following stock-based compensation expense (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Stock-based compensation expense by type of award:
Stock options	$	550	$	318	$	1,192	$	571
Restricted stock and restricted stock units		315		120		650		324
Employee stock purchase plan		57		192		114		346
Total stock-based compensation expense included    in expenses	$	922	$	630	$	1,956	$	1,241
Stock-based compensation expense by line item:
Research and development expenses	$	279	$	223	$	672	$	395
General and administrative expenses		643		407		1,284		846
Total stock-based compensation expense included    in expenses	$	922	$	630	$	1,956	$	1,241

Schedule of Unrecognized Stock-based Compensation Expense, Net of Estimated Forfeitures, by Type of Award and Weighted-average Recognition Period

The following table sets forth the Company’s unrecognized stock-based compensation expense, net of estimated forfeitures, by type of award and the weighted-average period over which that expense is expected to be recognized (in thousands, except for years):

	As of  June 30, 2019
	Unrecognized Expense, Net of Estimated Forfeitures		Weighted- average Recognition Period (in years)
Type of award:
Stock options	$	4,323		2.66
Restricted stock and restricted stock units	$	2,489		2.46

Summary of Stock Option Activity

The following table summarizes stock option activity during the six months ended June 30, 2019:

	Shares Subject to Stock Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value
Options outstanding at December 31, 2018		2,014,047		$	3.72
Granted		636,287		$	7.97
Exercised		(36,937	)	$	2.84				198,000
Cancelled		(13,988	)	$	6.07
Options outstanding at June 30, 2019		2,599,409		$	4.76		8.01		9,703,000
Options exercisable at June 30, 2019		1,221,405		$	3.75		7.21		5,813,000

Schedule of Weighted Average Assumptions using Black-Scholes Option Pricing Model

the fair value of each employee option grant during the six months ended June 30, 2019 was estimated on the date of grant using the Black-Scholes option pricing model with the following weighted average assumptions:

	Six Months Ended June 30, 2019
Expected volatility		78.34	%
Expected term (in years)		6.07
Risk-free interest rate		2.46	%
Expected dividend yield		0	%

Restricted stock [Member]

Summary of Stock Activity

The following table is a summary of restricted stock activity during the six months ended June 30, 2019:

	Shares of Restricted Stock		Weighted- Average Grant Date Fair Value
Unvested at December 31, 2018		183,095	$	0.17
Granted		—	$	—
Vested		—	$	—
Forfeited		—	$	—
Unvested at June 30, 2019		183,095	$	0.17

Restricted stock units [Member]

Summary of Stock Activity

The following table summarizes restricted stock unit activity during the six months ended June 30, 2019:

	Shares Subject to Restricted Stock Units			Weighted- Average Grant Date Value
Unvested at December 31, 2018		128,125		$	5.37
Granted		343,524		$	7.80
Vested		(42,622	)	$	5.90
Forfeited		(1,467	)	$	7.64
Unvested June 30, 2019		427,560		$	7.26

In January 2019, the Company issued 221,600 performance based restricted stock units (PRSU awards) to several of its employees, which are reflected in the above summary of restricted stock unit activity.  The shares subject to the PRSU awards shall vest upon the Company achieving certain milestones, with 100% of the PRSU awards vesting upon the achievement of three of the milestones over a four-year period, with any then-unvested portion of the PRSU awards to be cancelled on the four-year anniversary of the grant dates.  As of June 30, 2019, three of the milestones were deemed as probable of achievement, resulting in the Company recording stock-based compensation expense of $190,000 and $392,000 during the three and six months ended June 30, 2019, respectively.