Stock and Employee Benefit Plans (Tables)

12 Months Ended

Dec. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Schedule of Fair Value Assumptions

The fair value of shares expected to be purchased under the 2018 ESPP using the Black-Scholes model with the following assumptions:

	Years Ended December 31,
	2018
Risk free interest rate	2.48	%
Expected term (in years)	0.5
Dividend yield	—
Expected volatility	57.57	%

The Company estimates the fair value of each stock award on the grant date using the Black-Scholes option-pricing model based on the following assumptions:

	Years Ended December 31,
	2018	2017
Risk free interest rate	2.40% - 3.06%	1.77% - 2.22%
Expected term (in years)	4.75 - 5.73	5.00 - 5.73
Dividend yield	—	—
Expected volatility	77.1% - 80.5%	75.1% - 78.9%

Schedule of Stock Option Activity

The following table summarizes stock option activity for these inducement options (in thousands):

	Number of Options		Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Life	Aggregate Intrinsic Value (in thousands)
Outstanding, December 31, 2017	1,531,250		$	3.62		$	6,015
Exercised	(406,250	)	$	1.45		$	2,530
Outstanding, December 31, 2018	1,125,000		$	4.41	7.75 years	$	—
Vested or expected to vest	950,000		$	4.14	7.57 years	$	—
Exercisable, December 31, 2018	590,625		$	3.94	7.26 years	$	—

The following table summarizes stock option activity for employees and nonemployees:

	Number of Options		Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Life	Aggregate Intrinsic Value (in thousands)
Outstanding, December 31, 2017	5,443,485		$	2.33		$	27,822
Granted	1,881,660		7.59			—
Exercised	(190,019	)	2.09			861
Canceled	(285,079	)	3.28			519
Outstanding, December 31, 2018	6,850,047		$	3.76	7.68 years	$	3,257
Vested or expected to vest, December 31, 2018	6,850,047		$	3.76	7.68 years	$	3,257
Exercisable, December 31, 2018	3,796,264		$	2.31	6.82 years	$	2,522

Schedule of Stock-Based Compensation Expense

Total stock-based compensation expense is recorded in operating expenses based upon the functional responsibilities of the individuals holding the respective options as follows (in thousands):

	Years Ended December 31,
	2018		2017
Research and development	$	1,984	$	844
General and administrative	2,959		2,202
Total stock-based compensation	$	4,943	$	3,046