Summary of Assumptions Used in Black-Scholes Option Pricing Model


	Three Months Ended
	March 31,
	2020	2019
Risk-free interest rate	0.54% - 1.57%	2.54% - 2.62%
Expected volatility	73.72% - 74.71%	74.29% - 75.19%
Expected dividend yield	0%	0%
Expected life of employee options (in years)	6.25	6.25
Expected life of non-employee options (in years)	—	—

A summary of stock option award activity related to employees, non-employee members of the Company’s Board of Directors and non-employee consultants as of and for the three months ended March 31, 2020 is presented below:


		Weighted		Aggregate
	Number of	Average Exercise		Intrinsic
	Shares	Price		Value
Outstanding, December 31, 2019	4,450,517	$	14.73
Granted	673,350		16.11
Exercised	(7,500)		9.94
Expired	(161,089)		13.64
Forfeited	(24,688)		16.51
Outstanding, March 31, 2020	4,930,590	$	14.95	$	5,099
Weighted average remaining contractual life as of March 31, 2020 (in years)	7.68
Options exercisable, March 31, 2020	2,484,710	$	13.10	$	4,763
Weighted average remaining contractual life as of March 31, 2020 (in years)	6.47
Options vested and expected to vest as of March 31, 2020	4,930,590	$	14.95	$	5,099
Weighted average remaining contractual life as of March 31, 2020 (in years)	7.68

Summary of Assumptions Used in Black-Scholes Option Pricing Model

On January 1, 2019, the Company used the Black-Scholes option valuation model to remeasure the fair value of all outstanding unvested options that had been granted to non-employee consultants in accordance with ASU 2018-07, Compensation – Stock Compensation (Topic 718), Improvements to Non-employee Share-Based Payment Accounting. The range of assumptions used by the Company on January 1, 2019 were as follows:



		January 1, 2019
Risk-free interest rate		2.59% - 2.62%
Expected volatility		58.9% - 84.6%
Expected dividend yield		0%
Expected life of non-employee options (in years)		0.81 - 8.19