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Stock-based compensation (Tables)

6 Months Ended

Jun. 30, 2020

Stock-based compensation expense is classified in the condensed consolidated statement of operations as follows:

(in thousands)	THREE MONTHS ENDED JUNE 30,				SIX MONTHS ENDED JUNE 30,
	2020		2019		2020		2019
Research and development expenses	$	346	$	-	$	629	$	80
General and administrative expenses		676		788		1,076		1,786
Total stock-based compensation expense	$	1,022	$	788	$	1,705	$	1,866

The fair values of stock options granted are estimated using the Black-Scholes option pricing model with the following weighted average assumptions:

A summary of the Company’s stock option activity and related information for the six months ended June 30, 2020 is as follows:

	NUMBER OF SHARES			WEIGHTED AVERAGE EXERCISE PRICE		WEIGHTED AVERAGE REMAINING CONTRACTUAL LIFE (IN YEARS)		AGGREGATE INTRINSIC VALUE (IN THOUSANDS)
Outstanding at December 31, 2019		5,840,538		$	5.11		7.72	$	45,037
Options granted		711,700			10.58
Options exercised		(475,193	)		7.79
Options forfeited		(463,708	)		15.83
Outstanding at June 30, 2020		5,613,337		$	4.69		8.65	$	66,953
Exercisable as of June 30, 2020		983,334		$	7.66		5.76	$	8,861

A summary of the Company’s restricted stock unit activity and related information for the six months ended June 30, 2020 is as follows:

	NUMBER OF SHARES			WEIGHTED AVERAGE GRANT DATE FAIR VALUE
Non-vested at December 31, 2019		72,000		$	3.47
Restricted stock units granted		75,000			6.44
Restricted stock units vested		(14,500	)		3.47
Restricted stock units forfeited		(1,500	)		3.47
Non-vested at June 30, 2020		131,000		$	5.17