Stock-Based Compensation (Tables)

9 Months Ended

12 Months Ended

Sep. 30, 2019

Dec. 31, 2018

Stock-Based Compensation

Summary of stock option activity

	Options Outstanding and Exercisable
				Weighted
		Weighted		Average	Aggregate
		Average		Remaining	Intrinsic
	Number of	Exercise		Contractual	Value(1)
	Shares	Price		Life (Years)	(in thousands)
Outstanding as of December 31, 2018	2,056,891	$	4.74	9.4	$	5,946
Granted	247,275	$	18.18	10.0
Exercised	(53,963)	$	1.20	8.1
Forfeitures	(1,250)	$	7.63	9.5
Outstanding as of September 30, 2019	2,248,953	$	6.30	9.1	$	55,305
Vested and expected to vest as of September 30, 2019	2,248,953	$	6.30		$	55,305
Exercisable as of September 30, 2019	450,866	$	1.15		$	12,335

The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock for the options that were in the money at September 30, 2019 and December 31, 2018.

A summary of stock option activity for awards under the 2015 Plan is presented below:

	Options Outstanding and Exercisable
				Weighted
		Weighted		Average	Aggregate
		Average		Remaining	Intrinsic
	Number of	Exercise		Contractual	Value(1)
	Shares	Price		Life (Years)	(in thousands)
Outstanding as of January 1, 2017	189,189	$	0.48	8.6	$	137
Granted	317,397		1.21
Outstanding as of December 31, 2017	506,586		0.94	9.0		137
Granted	1,570,136		5.92	9.9		2,684
Exercised	(5,599)		0.84
Forfeitures	(14,232)		1.11
Outstanding as of December 31, 2018	2,056,891		4.74	9.4		5,946
Vested and expected to vest as of December 31, 2018	2,056,891		4.74	9.4		5,946
Exercisable as of December 31, 2018	242,079		0.88	7.9		1,634

(1) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock for the options that were in the money at December 31, 2018 and 2017.

Summary of stock based compensation expense recorded

Stock‑based compensation expense recorded as research and development and general and administrative expenses is as follows (in thousands):

	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2019		2018		2019		2018
Research and development	$	182	$	41	$	482	$	62
General and administrative		342		49		837		76
Total stock-based compensation expense	$	524	$	90	$	1,319	$	138

Stock‑based compensation expense recorded as research and development and general and administrative expenses is as follows (in thousands):

	December 31,
	2018		2017
Research and development	$	85	$	35
General and administrative		178		40
Total stock-based compensation expense	$	263	$	75