Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Stock-Based Compensation Expense

Total stock-based compensation expense is recognized for stock options granted to employees and non-employees and has been reported in the Company’s consolidated statements of operations as follows:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2018		2017		2018		2017
Research and development	$	229	$	47	$	955	$	152
General and administrative		400		44		1,257		149
Total stock-based compensation expense	$	629	$	91	$	2,212	$	301

Summary of Stock Option Activity

The following table summarizes stock option activity under the Plans:

	Shares Available for Grant			Number of Options Outstanding		Weighted- Average Exercise Price per Option		Weighted- Average Remaining Contract Term		Aggregate Intrinsic Value
										(In thousands)
Outstanding, December 31, 2017		1,670,341			195,668	$	4.49		9.67
Shares reserved for issuance		641,239
Options granted(1)		(871,010	)		871,010		13.09
Restricted stock granted		(2,000	)
Restricted stock units granted		(24,960	)
Outstanding, September 30, 2018		1,413,610			1,066,678		11.52		9.43	$	3,901
Exercisable, September 30, 2018					25,487		0.79		8.70		366
Vested and expected to vest, September 30, 2018					1,066,678		11.52		9.43		3,901

(1) The Company granted 7,200 stock options to non-employees during the nine months ended September 30, 2018.

Schedule of Assumptions to Estimate Fair Value of Stock Options for Employees and Non-Employees Using Black-Scholes Option Pricing Model

The fair value of stock options for employees and non-employees was estimated using a Black-Scholes option pricing model with the following assumptions:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2018		2017		2018		2017
Employees:
Fair value of common stock	$9.03 - $13.94		$0.79		$8.57 - $15.45		$0.79
Expected term (in years)	5.9 - 6.1			6.1	5.8 - 6.6			6.1
Expected volatility	89.8% - 90.6%		74.4%		75.9% - 90.6%		74.4%
Risk-free interest rate	2.8% - 2.9%		1.9%		2.4% - 2.9%		1.9%
Expected dividend yield	0.0%		0.0%		0.0%		0.0%
Non-employees:
Fair value of common stock		—	$1.00		$15.45		$0.79 - $1.00
Expected term (in years)		—		10.0		10.0		10.0
Expected volatility		—	76.9%		77.9%		74.6% - 76.9%
Risk-free interest rate		—	2.3%		2.7%		2.3%
Expected dividend yield		—	0.0%		0.0%		0.0%

Summary of Restricted Stock Activity

The summary of restricted stock activity and related information follows:

	Number of Restricted Shares Outstanding
Unvested shares — December 31, 2017		1,096,449
Granted		2,000
Vested		(1,096,949	)
Unvested shares — September 30, 2018		1,500

Summary of Restricted Stock Units Activity

The summary of restricted stock unit activity and related information follows:

	Number of Restricted Stock Units Outstanding
Unvested shares — December 31, 2017		—
Granted		24,960
Unvested shares — September 30, 2018		24,960