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Stock Based Compensation (Tables) | 6 Months Ended
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Jun. 30, 2022
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, 2021 |  |  | 2,603,630 |  |  | $ | 7.70 |  |  |  | 7.46 |
Options granted |  |  | 1,314,200 |  |  | $ | 0.72 |  |  |  |  |
Options forfeited |  |  | (1,173,227 | ) |  | $ | 4.19 |  |  |  |  |
Options expired |  |  | (47,192 | ) |  | $ | 12.84 |  |  |  |  |
Outstanding — June 30, 2022 |  |  | 2,697,411 |  |  | $ | 5.64 |  |  |  | 7.85 |
Exercisable — June 30, 2022 |  |  | 1,375,625 |  |  | $ | 9.01 |  |  |  |  |
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, |
|  | 2022 |  |  | 2021 |  |  | 2022 |  |  | 2021 |
Employee Stock Options: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk-free rate |  |  | 2.93 | % |  |  | 0.96 | % |  |  | 2.22 | % |  |  | 0.74 | %
Expected term (in years) |  |  | 6.0 |  |  |  | 5.7 |  |  |  | 6.0 |  |  |  | 6.0 |
Dividend yield |  | — |  |  | — |  |  | — |  |  | — |
Volatility |  |  | 91.82 | % |  |  | 92.79 | % |  |  | 91.63 | % |  |  | 93.64 | %
Weighted-average fair value of stock options granted |  | $ | 0.87 |  |  | $ | 3.37 |  |  | $ | 0.55 |  |  | $ | 4.45 |
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, |
|  | 2022 |  |  | 2021 |  |  | 2022 |  |  | 2021 |
Research and development |  | $ | 83 |  |  | $ | 394 |  |  | $ | 211 |  |  | $ | 763 |
General and administrative(1) |  |  | 263 |  |  |  | 589 |  |  |  | 650 |  |  |  | 1,246 |
Total stock-based compensation expense |  | $ | 346 |  |  | $ | 983 |  |  | $ | 861 |  |  | $ | 2,009 |
| (1) | Included in general and administrative stock-based compensation for the six months ended June 30, 2022 is stock-based compensation expense related to 32,684 shares of common stock issued to certain board members in lieu of their cash compensation. No shares of common stock were issued to board members for the three months ended June 30, 2022.

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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 https://asc.fasb.org/extlink&oid=128089324&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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI https://asc.fasb.org/extlink&oid=128089324&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 https://asc.fasb.org/extlink&oid=128089324&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 https://asc.fasb.org/extlink&oid=128089324&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 https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901 + Details
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