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Stock Based Compensation (Tables)

6 Months Ended

Jun. 30, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Option Activity Under Company's Equity Incentive Plans

The following table summarizes stock option activity under the Company’s equity incentive plans and related information:

		Number of Shares Underlying Outstanding Options				Weighted- Average Exercise Price				Weighted- Average Remaining Contractual Term (Years)
Outstanding — December 31, 2018			1,361,977			$	12.04				8.71
Options granted			395,300			$	8.10
Options exercised			(41,219	)		$	4.57
Options forfeited			(60,290	)		$	8.14
Options expired			(7,474	)		$	74.90
Outstanding — June 30, 2019			1,648,294			$	11.14				8.60
Exercisable — June 30, 2019			547,822			$	15.74				7.95
Vested and expected to vest — June 30, 2019			1,648,294			$	11.14				8.60

Summary of Weighted Average Valuation Assumptions Used to Estimate Fair Value of Employee Stock Options

The fair value of employee stock options was estimated using the following weighted-average assumptions for the six months ended June 30, 2019 and 2018:

		Six Months Ended June 30,
		2019				2018
Employee Stock Options:
Risk-free interest rate			2.44	%			2.48	%
Expected term (in years)			5.98				5.93
Dividend yield			—				—
Volatility			88.21	%			104.37	%
Weighted-average fair value of stock options granted		$	5.96			$	13.38

Summary of Stock-Based Compensation Recognized

Total stock-based compensation recognized was as follows (in thousands):

		Three Months Ended June 30,								Six Months Ended June 30,
		2019				2018				2019				2018
Research and development		$	284			$	137			$	522			$	252
General and administrative⁽¹⁾			619				414				1,210				904
Total stock-based compensation		$	903			$	551			$	1,732			$	1,156

(1)	Included $0.03 and $0.1 million in modifications related to certain Board member stock options in the three and six months ended June 30, 2019 and for the six months ended June 30, 2018, respectively.