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Stock Incentive Plans (Tables) | 6 Months Ended
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Jun. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
Schedule of Weighted Average Assumptions Used | The estimated grant date fair values of employee stock option awards granted under the 2016 Plan and the 2018 Inducement Incentive Award Plan were calculated using the Black-Scholes option pricing model, based on the following weighted-average assumptions:
| Three Months Ended June 30, |  | Six Months Ended June 30,
| 2020 |  | 2019 |  | 2020 |  | 2019
Risk-free interest rate | 0.39 | % |  | 2.18 | % |  | 1.61 | % |  | 2.41 | %
Dividend yield | — |  |  | — |  |  | — |  |  | — |
Expected term | 5.56 |  |  | 5.87 |  |  | 6.03 |  |  | 6.03 |
Expected volatility | 89.70 | % |  | 88.71 | % |  | 88.12 | % |  | 87.56 | %
Weighted-average fair value of common stock | $ | 2.92 |  |  | $ | 2.48 |  |  | $ | 2.36 |  |  | $ | 2.42 |
Summary of Activity Under the 2008 Plan and the 2016 Plan | The following table summarizes the activity under the 2008 Plan, 2016 Plan, and 2018 Inducement Incentive Award Plan:
|  |  |  |  | Weighted‑average |  |
|  |  |  |  | remaining |  | Aggregate
| Number of |  | Weighted-average |  | contractual term |  | intrinsic value
| options |  | exercise price ($) |  | (in years) |  | (in thousands)
Employee awards |  |  |  |  |  |  |
Outstanding at December 31, 2019 | 6,323,596 |  |  | $ | 4.91 |  |  | 8.71 |  | $ | 1,716 |
Granted | 1,878,309 |  |  | $ | 2.36 |  |  |  |  |
Exercised | (42,628 | ) |  | $ | 2.42 |  |  |  |  |
Forfeited | (1,158,737 | ) |  | $ | 5.84 |  |  |  |  |
Outstanding at June 30, 2020 | 7,000,540 |  |  | $ | 4.09 |  |  | 8.69 |  | $ | 3,736 |
Vested at June 30, 2020 | 1,632,355 |  |  | $ | 7.62 |  |  | 7.37 |  | $ | 337 |
Vested and expected to vest at June 30, 2020 | 6,472,944 |  |  | $ | 4.22 |  |  | 8.64 |  | $ | 3,381 |
Non‑employee awards |  |  |  |  |  |  |
Outstanding at December 31, 2019 | 473,073 |  |  | $ | 5.89 |  |  | 6.23 |  | $ | 38 |
Granted | — |  |  | $ | — |  |  |  |  |
Exercised | — |  |  | $ | — |  |  |  |  |
Forfeited | — |  |  | $ | — |  |  |  |  |
Outstanding at June 30, 2020 | 473,073 |  |  | $ | 5.89 |  |  | 5.73 |  | $ | 72 |
Vested at June 30, 2020 | 413,191 |  |  | $ | 5.45 |  |  | 5.37 |  | $ | 72 |
Vested and expected to vest at June 30, 2020 | 473,073 |  |  | $ | 5.89 |  |  | 5.73 |  | $ | 72 |
Disclosure of Share-based Compensation Arrangements by Share-based Payment Award | The following table summarizes the status of the Company’s restricted stock units:
| Number of shares |  | Weighted average grant date fair value ($)
Unvested at December 31, 2019 | 181,250 |  |  | $ | 5.00 |
Granted | — |  |  | — |
Vested | 21,875 |  |  | 6.03 |
Forfeited | — |  |  | — |
Unvested at June 30, 2020 | 159,375 |  |  | $ | 5.05 |
Schedule of Stock-Based Compensation Expense Related to Stock Options and Restricted Common Stock | The Company recorded stock-based compensation expense related to stock option awards, restricted stock units and the ESPP in the following expense categories of its consolidated statements of operations and comprehensive loss (in thousands):
| Three Months Ended June 30, |  | Six Months Ended June 30,
| 2020 |  | 2019 |  | 2020 |  | 2019
Research and development | $ | 618 |  |  | $ | 445 |  |  | $ | 1,241 |  |  | $ | 964 |
General and administrative | 863 |  |  | 806 |  |  | 1,649 |  |  | 1,467 |
Total stock-based compensation expense | $ | 1,481 |  |  | $ | 1,251 |  |  | $ | 2,890 |  |  | $ | 2,431 |
Non-employee stock options |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
Schedule of Weighted Average Assumptions Used | The estimated grant date fair values of non-employee stock option awards granted under the 2016 Plan were calculated using the Black-Scholes option pricing model, based on the following weighted-average assumptions:
| Three Months Ended June 30, |  | Six Months Ended June 30,
| 2020 |  | 2019 |  | 2020 |  | 2019
Risk-free interest rate | — | % |  | 2.07 | % |  | — | % |  | 2.10 | %
Dividend yield | — |  |  | — |  |  | — |  |  | — |
Expected life (in years) | 0 |  |  | 5.49 |  |  | 0 |  |  | 5.50 |
Expected volatility | — | % |  | 88.46 | % |  | — | % |  | 88.44 | %
Weighted-average fair value of common stock | $ | — |  |  | $ | 2.04 |  |  | $ | — |  |  | $ | 2.04 |

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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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=120381028&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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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