Convertible Preferred Stock and Stockholders’ Equity (Deficit) (Tables)

6 Months Ended

Jun. 30, 2019

Equity [Abstract]

Summary of Stock Option Activity Under 2018 Plan and 2006 Plan

A summary of the Company’s stock option activity under the 2018 Plan and 2006 Plan is as follows:

	Shares		Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term	Aggregate Intrinsic Value
Outstanding at January 1, 2019	1,282,847		$	6.90	9.2
Granted	545,374		4.15
Exercised	(50,047	)	1.30
Cancelled/Expired	(61,455	)	15.06
Outstanding at June 30, 2019	1,716,719		$	5.99	9.0	$	366,055
Vested and exercisable at June 30, 2019	566,997		$	6.86	8.4	$	248,379

Summary of Recognized Stock-Based Compensation Expense

The Company recognized stock-based compensation expense for the three and six months ended June 30, 2019 and 2018 as follows:

	Three Months Ended June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Research and development	$	57,687	$	9,253	$	110,590	$	18,867
General and administrative	277,979		43,728		514,471		88,559
Total stock-based compensation expense	$	335,666	$	52,981	$	625,061	$	107,426

Schedule of Weighted-Average Assumptions in Black -Scholes Option Pricing Model

The weighted-average assumptions used in the Black-Scholes option pricing model to determine the fair value of the employee stock option grants were as follows:

	Three Months Ended June 30,			Six Months Ended June 30,
	2019		2018	2019		2018
Risk-free interest rate	2.0	%	—	2.4	%	2.5	%
Expected volatility	65.2-66.1%		—	64.3-67.3%		63.0	%
Expected term (in years)	5.4		—	5.0		4.0
Expected dividend yield	0.0	%	—	0.0	%	0.0	%

Schedule of Common Stock Reserved for Future Issuance

Common stock reserved for future issuance consist of the following:

	June 30, 2019
Stock options issued and outstanding	1,716,719
Authorized for future stock awards, option grants, or employee stock purchase program	432,935
Common Warrants	4,224,494
Total	6,374,148