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Stock Based Compensation (Tables) | 3 Months Ended
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Mar. 31, 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 |  |  | 879,200 |  |  | $ | 0.52 |  |  |  |  |
Options forfeited |  |  | (658,947 | ) |  | $ | 5.20 |  |  |  |  |
Options expired |  |  | (46,173 | ) |  | $ | 9.81 |  |  |  |  |
Outstanding — March 31, 2022 |  |  | 2,777,710 |  |  | $ | 5.92 |  |  |  | 8.00 |
Exercisable — March 31, 2022 |  |  | 1,305,020 |  |  | $ | 9.42 |  |  |  |  |
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 March 31, |
|  | 2022 |  |  | 2021 |
Employee Stock Options: |  |  |  |  |  |  |  |
Risk-free interest rate |  |  | 1.87 | % |  |  | 0.69 | %
Expected term (in years) |  |  | 6.1 |  |  |  | 6.1 |
Dividend yield |  |  | — |  |  |  | — |
Volatility |  |  | 91.54 | % |  |  | 93.83 | %
Weighted-average fair value of stock options granted |  | $ | 0.39 |  |  | $ | 4.70 |
Summary of Stock-Based Compensation Recognized | Total stock-based compensation expense recognized was as follows (in thousands):
|  | Three Months Ended March 31, |
|  | 2022 |  |  | 2021 |
Research and development |  | $ | 128 |  |  | $ | 369 |
General and administrative(1) |  |  | 387 |  |  |  | 657 |
Total stock-based compensation expense |  | $ | 515 |  |  | $ | 1,026 |
| (1) | Included in general and administrative stock-based compensation for the three months ended March 31, 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.

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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=122041274&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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