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

Total unrecognized compensation cost for all stock-based compensation plans was $12.6 million as of June 30, 2020. This cost is expected to be recognized over a weighted average remaining vesting period of 3.06 years.

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

	THREE MONTHS ENDED JUNE 30,				SIX MONTHS ENDED JUNE 30,
	2020			2019	2020			2019
Expected volatility		93.61	%	NA		92.09	%	NA
Expected dividends		0	%	NA		0	%	NA
Expected terms (years)		5.82		NA		5.91		NA
Risk free rate		0.41	%	NA		0.56	%	NA

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

During the six months ended June 30, 2020, the Company granted stock options to purchase 536,700 shares of common stock to employees and 175,000 shares to non-employee directors. The stock options granted to employees have an exercise price per share ranging from $6.44 to $13.58. The stock options granted to non-employee directors during the six months ended June 30, 2020 have an exercise price per share ranging from $12.84 and $13.58.

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

During the six months ended June 30, 2020, the Company granted 75,000 restricted stock units to employees with a grant date fair value per share of $6.44.

The Company’s 2020 Employee Stock Purchase Plan (“2020 ESPP”) was adopted by the Company’s Board of Directors in March 2020 and approved by the Company’s stockholders in May 2020. A total of 759,936 shares of common stock have been reserved for issuance under the 2020 ESPP.

Subject to share and dollar limits as described in the plan, the 2020 ESPP allows eligible employees to contribute, through payroll deductions, up to 15% of their earnings for the purchase of the Company’s shares of common stock at the lower of 85% of the closing price of the Company’s common stock on the first trading day of the offering period or 85% of the closing price of the Company’s common stock on the last trading day of the offering period. There are two six-month offering periods during each fiscal year, ending on May 15 and November 15. The first offering period commenced on May 16, 2020.