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Stock Based Compensation (Tables) | 12 Months Ended
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Dec. 31, 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) |  |  | Aggregate Intrinsic Value (thousands) |
Outstanding — December 31, 2019 |  |  | 1,577,541 |  |  | $ | 10.85 |  |  |  | 8.15 |  |  | $ | 1,350 |
Options granted |  |  | 1,091,250 |  |  |  | 6.26 |  |  |  |  |  |  |  |  |
Options exercised |  |  | (44,605 | ) |  |  | 5.04 |  |  |  |  |  |  |  |  |
Options forfeited |  |  | (250,641 | ) |  |  | 10.32 |  |  |  |  |  |  |  |  |
Options expired |  |  | (17,930 | ) |  |  | 51.12 |  |  |  |  |  |  |  |  |
Outstanding — December 31, 2020 |  |  | 2,355,615 |  |  | $ | 8.59 |  |  |  | 7.96 |  |  | $ | 1,337 |
Options granted |  |  | 1,493,238 |  |  |  | 5.31 |  |  |  |  |  |  |  |  |
Options exercised |  |  | (5,000 | ) |  |  | 4.63 |  |  |  |  |  |  |  |  |
Options forfeited |  |  | (1,239,630 | ) |  |  | 6.26 |  |  |  |  |  |  |  |  |
Options expired |  |  | (593 | ) |  |  | 566.33 |  |  |  |  |  |  |  |  |
Outstanding — December 31, 2021 |  |  | 2,603,630 |  |  | $ | 7.70 |  |  |  | 7.46 |  |  | $ | — |
Exercisable — December 31, 2021 |  |  | 1,513,039 |  |  | $ | 9.37 |  |  |  | 6.35 |  |  | $ | — |
Shares available to be granted — December 31, 2021 |  |  | 3,333,791 |  |  |  |  |  |  |  |  |  |  |  |  |
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:
|  | Year Ended December 31, |
|  | 2021 |  |  | 2020 |
Employee Stock Options: |  |  |  |  |  |  |  |
Expected term (in years) |  |  | 6.00 |  |  |  | 5.81 |
Risk-free interest rate |  |  | 0.84 | % |  |  | 0.91 | %
Dividend yield |  |  | — |  |  |  | — |
Volatility |  |  | 93.25 | % |  |  | 113.36 | %
Summary of Stock-Based Compensation Recognized | Total stock-based compensation recognized was as follows (in thousands):
|  | Year Ended December 31, |
|  | 2021 |  |  | 2020 |
Research and development |  | $ | 1,202 |  |  | $ | 1,487 |
General and administrative (1) |  |  | 2,203 |  |  |  | 2,140 |
Total stock-based compensation |  | $ | 3,405 |  |  | $ | 3,627 |
(1) | Included in general and administrative stock-based compensation for the years ended December 31, 2021 and 2020 is $0.3 million and $0.3 million in expense related to 56,912 shares and 51,056 shares of common stock, respectively, issued to certain board members in lieu of their cash compensation.

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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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