Stockholders' Equity and Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2020

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Summary of Reserved Shares of Common Stock for Issuance

The Company has reserved shares of common stock for issuance as follows:

	As of June 30,
	2020		2019
Options issued and outstanding		1,832,415		1,454,461
Options available for future grants		1,375,223		537,345
Employee Stock Purchase Plan shares available for future issuance		89,398		104,540
Total		3,297,036		2,096,346

Summary of Stock Option Activity

The following table summarizes the Company’s stock option activity for the six months ended June 30, 2020:

							Weighted- Average		Weighted- Average		Aggregate
	Options						Exercise		Remaining		Intrinsic
	Available for			Options			Price Per		Contractual		Value
	Grant			Outstanding			Share		Term (years)		(in thousands)
Outstanding—December 31, 2019		1,935,054			1,335,755		$	16.91		8.77	$	54,071
Options granted		(567,123	)		567,123		$	46.77
Options exercised					(63,171	)	$	6.28
Options cancelled		7,292			(7,292	)	$	16.88
Outstanding — June 30, 2020		1,375,223			1,832,415		$	26.52		8.76	$	38,756
Options exercisable – June 30, 2020					483,345		$	16.70		8.25	$	14,969
Options vested and expected to    vest – June 30, 2020					1,832,415		$	26.52		8.76	$	38,756

Schedule of Total Stock-based Compensation Expense Related to Stock-based Awards

Total stock-based compensation expense related to all our stock-based awards was recorded in the statements of operations as follows (in thousands):

	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2020		2019		2020		2019
Research and development	$	1,448	$	552	$	2,363	$	1,004
General and administrative		1,270		614		2,282		1,126
Total stock-based compensation expense	$	2,718	$	1,166	$	4,645	$	2,130

Employees

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Summary of Fair Value of Employee and Non-employee Stock Options Granted

The fair value of employee and non-employee director stock option awards was estimated at the date of grant using a Black-Scholes option-pricing model with the following assumptions:

	Three Months Ended						Six Months Ended
	June 30,						June 30,
	2020			2019			2020			2019
Expected term in years		6.06			6.07			6.06			6.07
Expected volatility		72.48	%		72.27	%		72.29	%		72.31	%
Risk-free interest rate		0.45	%		2.07	%		0.57	%		2.10	%
Dividend yield		—			—			—			—
Weighted average fair value of share-based awards granted	$	29.03		$	18.62		$	29.65		$	18.22

Non Employees

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Summary of Fair Value of Employee and Non-employee Stock Options Granted

The fair value of the stock options granted to non-employees was calculated at each reporting date using the Black-Scholes option-pricing model with the following assumptions:

	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2020			2019	2020			2019
Expected term in years		6.06		N/A		6.06			6.08
Expected volatility		72.48	%	N/A		72.45	%		73.54	%
Risk-free interest rate		0.41	%	N/A		0.44	%		2.74	%
Dividend yield		—		N/A		—			—