Quarterly report pursuant to Section 13 or 15(d)

Stock Plans

Stock Plans

6 Months Ended

Jun. 30, 2018

Stock Plans

The following table summarizes the stock-based compensation expense recorded for awards under the stock option plans for the three and six-month periods ended June 30, 2018 and 2017:

	Three Months Ended ‎ June 30,				Six Months Ended ‎ June 30,
(in thousands, except per share amounts)	2018		2017		2018		2017
Research and development	$	156	$	128	$	318	$	245
General and administrative		246		254		516		558
Total stock-based compensation expenses	$	402	$	382	$	834	$	803

No tax benefit was recognized related to stock-based compensation expense since we have accumulated operating losses and we have established a full valuation allowance to offset all the potential tax benefits associated with our deferred tax assets.

We use the Black-Scholes-Merton option-pricing model with the following assumptions to estimate the stock-based compensation expense for the three and six-month periods ended June 30, 2018 and 2017:

	Three Months Ended ‎ June 30,						Six Months Ended ‎ June 30,
	2018			2017			2018			2017
Weighted-average risk-free interest rate		2.9	%		2.0	%		2.7	%		2.1	%
Expected dividend payments		—			—			—			—
Expected holding period (years) 1		6.4			6.5			6.4			6.5
Weighted-average volatility factor 2		0.91			0.88			0.89			0.88
Estimated forfeiture rates 3		25	%		27	%		26	%		28	%

(1)

Expected holding period is based on historical experience of similar awards, giving consideration to the contractual terms of the stock-based awards, vesting schedules and the expectations of future employee behavior.

(2)

Weighted average volatility is based on the historical volatility of our common stock.

(3)

Estimated forfeiture rates are based on historical data.

Options to purchase approximately 5,000 and 60,000 shares of common stock were granted during the three-month periods ended June 30, 2018 and 2017, respectively. Options to purchase approximately 950,000 and 496,000 shares of common stock were granted during the six-month periods ended June 30, 2018 and 2017, respectively.

The following table summarizes option activity for the six-month period ended June 30, 2018:

(in thousands, except per share amounts)	Options			Weighted Average Exercise Price		Weighted Average Remaining Option Term		Aggregate Intrinsic Value
Outstanding at January 1, 2018		2,728		$	4.32		5.75	$	30
Granted		950			0.97
Exercised		—			—
Expired		(18	)		8.36
Cancelled or forfeited		(12	)		5.46

Outstanding at June 30, 2018		3,648		$	3.42		6.44	$	115

Exercisable at June 30, 2018		2,796		$	4.00		5.88	$	57

No shares of restricted stock were awarded to employees, directors and consultants during the three and six-month periods ended June 30, 2018 and 2017.

As of June 30, 2018, there was approximately $

0.6

million of total unrecognized compensation expense related to non-vested stock options. This expense is expected to be recognized over a weighted-average period of 0.7 years.