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Stock Based Compensation (Tables) | 6 Months Ended
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Jun. 30, 2021
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract] |
Summary of Option Activity Under Company's Equity Incentive Plans | The following table summarizes stock option activity under the Company’s 2018 Plan and related information:
|  | Number of Shares Underlying Outstanding Options |  |  | Weighted- Average Exercise Price |  |  | Weighted- Average Remaining Contractual Term (Years) |
Outstanding — December 31, 2020 |  |  | 2,355,615 |  |  | $ | 8.59 |  |  |  | 8.0 |
Options granted |  |  | 1,157,488 |  |  | $ | 5.90 |  |  |  |  |
Options exercised |  |  | (5,000 | ) |  | $ | 4.63 |  |  |  |  |
Options forfeited |  |  | (270,942 | ) |  | $ | 7.12 |  |  |  |  |
Options expired |  |  | (569 | ) |  | $ | 582.27 |  |  |  |  |
Outstanding — June 30, 2021 |  |  | 3,236,592 |  |  | $ | 7.66 |  |  |  | 8.2 |
Exercisable — June 30, 2021 |  |  | 1,338,239 |  |  | $ | 9.65 |  |  |  |  |
Summary of Weighted Average Valuation Assumptions Used to Estimate Fair Value of Employee Stock Options | The fair value of employee stock options was estimated using the following weighted-average assumptions:
|  | Three Months Ended June 30, |  |  | Six Months Ended June 30, |
|  | 2021 |  |  | 2020 |  |  | 2021 |  |  | 2020 |
Employee Stock Options: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk-free rate |  | 0.96% |  |  | 0.40% |  |  | 0.74% |  |  | 1.29% |
Expected term (in years) |  |  | 5.7 |  |  |  | 5.9 |  |  |  | 6.0 |  |  |  | 5.6 |
Dividend yield |  | 0.00% |  |  | 0.00% |  |  | 0.00% |  |  | 0.00% |
Volatility |  | 92.79% |  |  | 115.20% |  |  | 93.64% |  |  | 111.27% |
Weighted-average fair value of stock options granted |  | $ | 3.37 |  |  | $ | 5.25 |  |  | $ | 4.45 |  |  | $ | 5.50 |
Summary of Stock-Based Compensation Expense Recognized | Total stock-based compensation expense recognized was as follows (in thousands):
|  | Three Months Ended June 30, |  |  | Six Months Ended June 30, |
|  | 2021 |  |  | 2020 |  |  | 2021 |  |  | 2020 |
Research and development |  | $ | 394 |  |  | $ | 314 |  |  | $ | 763 |  |  | $ | 658 |
General and administrative(1) |  |  | 589 |  |  |  | 520 |  |  |  | 1,246 |  |  |  | 981 |
Total stock-based compensation expense |  | $ | 983 |  |  | $ | 834 |  |  | $ | 2,009 |  |  | $ | 1,639 |
(1) | Included in general and administrative for the three and six months ended June 30, 2021 is stock-based compensation expense related to 13,713 shares and 23,862 shares, respectively, of common stock issued to certain board members in lieu of their cash compensation.

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- Definition Tabular disclosure of allocation of amount expensed and capitalized for award under share-based payment arrangement to statement of income or comprehensive income and statement of financial position. Includes, but is not limited to, corresponding line item in financial statement. + 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 (h)(1) -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 S99 -Paragraph 1 -Subparagraph (SAB Topic 14.F) -URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5047-113901 + 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://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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