Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2018

Stock-Based Compensation Expense Related to Common Stock Options, Restricted Common Stock and Common Stock Warrants

Following is the stock-based compensation expense related to common stock options, common stock, restricted common stock and common stock warrants:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2018		2017		2018		2017
Research and development	$	252	$	109	$	1,413	$	407
General and administrative		411		121		1,880		422
Total stock-based compensation expense	$	663	$	230	$	3,293	$	829

Summary of Stock Option Activity

The following table summarizes the stock option activity in the Company’s equity incentive plans, including non-plan grants to Company executives, from December 31, 2017 through September 30, 2018:

	Shares			Weighted Average Exercise Price
Outstanding, December 31, 2017		5,155,263		$	4.11
Granted		1,011,875			5.02
Exercised		(2,193,366	)		1.99
Options forfeited/cancelled		(209,124	)		4.74
Outstanding, September 30, 2018		3,764,648		$	5.55

Weighted Average Assumptions Used to Determine Fair Value of Options Granted

The fair value of all other options granted is determined using the Black-Scholes option-pricing model. The following weighted average assumptions were used:

	Nine Months Ended September 30, 2018			Nine Months Ended September 30, 2017
Risk-free interest rate		2.47	%		—
Expected life of the options		5.7 years			—
Expected volatility of the underlying stock		104	%		—
Expected dividend rate		0	%		—