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Stock Plans | 6 Months Ended
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Jun. 30, 2018
Share-based Compensation [Abstract] |
Stock Plans [Text Block] |
| 2. | 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.

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- Definition The entire disclosure for compensation-related costs for equity-based compensation, which may include disclosure of policies, compensation plan details, allocation of equity compensation, incentive distributions, equity-based arrangements to obtain goods and services, deferred compensation arrangements, employee stock ownership plan details and employee stock purchase plan details. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -URI http://asc.fasb.org/topic&trid=2228938 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 50 -URI http://asc.fasb.org/subtopic&trid=2208855 + Details
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