Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

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

Summary of Stock Option Activity

The following table summarizes stock option activity for the year ended December 31, 2018:

	Number of Options		Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value
Outstanding — January 1, 2018	369,619		$	56.92			$	—
Granted	674,104		$	4.68
Exercised	—			—
Forfeited or expired	(71,150	)	$	13.92
Outstanding — December 31, 2018	972,573		$	23.85		8.46	$	—
Exercisable — December 31, 2018	516.087		$	36.31		7.84	$	—
Vested and expected to vest — December 31, 2018	959,929		$	24.04		8.45	$	—

Calculation of Fair Value Assumptions Using Black Scholes Option Model

The estimated fair values of employee stock options granted during the year ended December 31, 2018 and 2017, were determined on the date of grant using the Black-Scholes option-pricing model with the following assumptions:

	For the year ended December 31,
	2018		2017
Expected option life (years)		5.00 – 6.50		5.50–9.41
Risk-free interest rate		2.74% – 2.84%		1.89% – 2.25%
Expected volatility		78.5% – 82.9%		74.0% – 86.3%
Expected dividend yield		0%		0%

Stock-Based Compensation Expense

The following table presents total stock-based compensation expense for the years ended December 31, 2018 and 2017, respectively:

	For the years ended December 31,
	2018		2017
Research and development	$	970	$	710
General and administrative		2,003		2,095
Total stock-based compensation expense	$	2,973	$	2,805