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Stock Based Compensation (Tables)

12 Months Ended

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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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.

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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.

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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.

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