Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Stock Option Activity

The following table summarizes stock option activity for the 2002 Plan for the nine months ended September 30, 2018:

				Weighted-		Weighted-
				Average		Average
				Exercise		Remaining		Aggregate
	Total			Price Per		Contractual		Intrinsic
	Options			Share		Term (in years)		Value
Outstanding at December 31, 2017		1,075,209		$	1.60		7.3	$	711,328
Granted		260,301		$	3.40
Exercised		(19,885	)	$	2.68
Outstanding at September 30, 2018		1,315,625		$	1.94		7.2	$	8,105,669
Vested and expected to vest at September 30, 2018		1,217,885		$	1.90		7.1	$	7,553,919
Vested and exercisable at September 30, 2018		807,177		$	1.60		6.2	$	5,243,664

Estimated Fair Value of Option on Grant Date using Weighted Average Assumptions

The fair value of each option was estimated on the date of grant using the weighted average assumptions in the table below:

	Nine Months Ended
	September 30,
	2018		2017
Risk-free interest rate	2.87%		2.28%
Expected term (in years)	7.0		7.1
Expected volatility	69%		68%
Expected dividend yield		—		—
Estimated fair value of common stock	$	2.32	$	1.00

Stock-based Compensation Expense

Stock-based compensation expense has been reported in the Company’s statements of operations for the three months ended September 30, 2018 and 2017 and for the nine months ended September 30, 2018 and 2017 as follows (in thousands):

	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2018		2017		2018		2017
General and administrative	$	66	$	10	$	119	$	30
Research and development		31		9		143		28
Total stock-based compensation	$	97	$	19	$	262	$	58