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Equity-Based Compensation (Tables) | 3 Months Ended
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Mar. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract] |
Equity-Based Compensation Expense Classification in Statement of Operations | Equity-based compensation expense is classified in the statements of operations as follows:
| Three Months Ended March 31,
| 2018 |  | 2017
Research and development expense | $ | 96 |  |  | $ | 44 |
General and administrative expense | 269 |  |  | 336 |
| $ | 365 |  |  | $ | 380 |
Assumptions Used to Determine Fair Value of Common Stock Option Grants | The fair value of the underlying common stock and the exercise price for the common stock options granted during the three months ended March 31, 2018 and 2017 are summarized in the table below:
Common Stock Options Granted During Period Ended: | Fair Value of Underlying Common Stock |  | Exercise Price of Common Stock Option
Three months ended March 31, 2018 | $3.00 |  | $3.00
Three months ended March 31, 2017 | $4.21 |  | $4.21
In addition to an assumption on the expected term of the option grants as discussed below, application of the Black-Scholes model requires additional inputs for which we have assumed the values described in the table below:
| Three Months Ended March 31,
| 2018 |  | 2017
Expected term | 5.9 to 6.0 years |  |  | 5.3 to 6.5 years |
Risk-free interest rate | 2.72% |  |  | 1.97% to 2.17%; weighted avg. 2.07% |
Expected volatility | 82.4% |  |  | 80.8% to 83.1%; weighted avg. 81.0% |
Forfeiture rate | 5 | % |  | 5 | %
Expected dividend yield | — | % |  | — | %
Common Stock Option Activity | A summary of common stock option activity as of the periods indicated is as follows:
| Options |  | Weighted-Average Exercise Price |  | Weighted-Average Remaining Contractual Term (years) |  | Aggregate Intrinsic Value (thousands)
Outstanding - December 31, 2017 | 3,672,620 |  |  | $ | 1.79 |  |  | 7.5 |  | $ | 5,221 |
Granted | 674,279 |  |  | 3.00 |  |  |  |  |
Exercised | (8,532 | ) |  | 0.86 |  |  |  |  |
Forfeited | (2,638 | ) |  | 1.03 |  |  |  |  |
Outstanding - March 31, 2018 | 4,335,729 |  |  | $ | 1.98 |  |  | 7.5 |  | $ | 5,221 |
Exercisable - March 31, 2018 | 2,597,078 |  |  | $ | 1.51 |  |  | 7.0 |  | $ | 4,245 |
Vested and Expected to Vest - March 31, 2018 | 3,916,640 |  |  | $ | 2.11 |  |  | 7.5 |  | $ | 5,190 |

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- Definition Tabular disclosure of the amount of total share-based compensation cost, including the amounts attributable to each share-based compensation plan and any related tax benefits. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure for stock option plans. Includes, but is not limited to, outstanding awards at beginning and end of year, grants, exercises, forfeitures, and weighted-average grant date fair value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure of the significant assumptions used during the year to estimate the fair value of stock options, including, but not limited to: (a) expected term of share options and similar instruments, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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