Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2021

Summary of Stock Option Activity

Stock option activity is summarized as follows:

	Number of Outstanding Stock Options			Weighted Average Exercise Price		Weighted Remaining Contractual Term		Aggregate Intrinsic Value
Outstanding as of December 31, 2020		576,534		$	23.33
Granted		892,239		$	5.08
Exercises		(10,751	)	$	5.78
Canceled/forfeited/expired		(45,472	)	$	22.38
Outstanding as of December 31, 2021		1,412,550		$	12.01		8.46	$	3,134,377
Options vested and expected to vest as of December 31, 2021		1,412,550		$	12.01		8.46	$	3,134,377
Options exercisable as of December 31, 2021		472,553		$	25.55		7.10	$	763,461

Summary of Assumptions Used to Determine Fair Value of Employee Stock Purchase Plan

The assumptions used in the Black-Scholes option pricing model to determine the fair value of the ESPP offering were as follows:

	Years Ended December 31,
	2021		2020		2019
Expected term (in years)		0.50		0.50		0.50
Risk-free interest rate	0.04% – 0.12%		0.1% – 1.6%		1.6% – 2.5%
Expected volatility	89.7% – 108.12%		89.7% – 143.2%		99.7% – 141.7%
Expected dividend yield		0.0%		0.0%		0.0%

Schedule of Restricted Stock Units Activity

Occasionally, we grant restricted stock units to employees. The fair value of restricted stock is determined by the closing price of the Company's common stock reported on the Nasdaq Global Select Market on the date of grant. Restricted stock unit activity is summarized as follows:

	Number of Outstanding Restricted Stock Units			Weighted Average Grant Date Fair Value
Balance as of December 31, 2020		7,677		$	5.32
Granted		4,000		$	3.89
Released		(4,177	)	$	6.18
Balance as of December 31, 2021		7,500		$	4.08

Schedule of Allocation of Stock-Based Compensation for All Options Including Performance Options with Market Condition and Restricted Stock Units

The allocation of stock-based compensation for all options, including performance options with market condition and restricted stock units is as follows (in thousands):

	Years Ended December 31,
	2021		2020		2019
Research and development	$	295	$	254	$	354
General and administrative		1,319		1,211		1,429
Total stock-based compensation expense	$	1,614	$	1,465	$	1,783

Summary of Warrants Outstanding

Warrants outstanding for the purchase of common stock as of December 31, 2021 were as follows:

Number		Exercise Price		Expiration
Outstanding		Per Share		Date
	1,066	$	281.50	July 2023
	6,830	$	43.93	November 2023
	2,978	$	50.37	June 2024
	2,886	$	51.98	December 2024
	13,760

Summary of Common Stock Reserved for Future Issuance

Common stock reserved for future issuance was as follows:

	December 31, 2021
Common stock warrants		13,760
Common stock options and restricted stock units		1,420,050
Shares available under the 2015 equity incentive plan		420,896
Shares available under the employee stock purchase plan		71,933
		1,926,639

Employee Stock Option [Member]

Summary of Assumptions Used to Determine Fair Value of Employee Stock Option Grants and Performance Options with Market Condition

The assumptions used in the Black-Scholes option pricing model to determine the fair value of the employee stock option grants were as follows:

	Years Ended December 31,
	2021		2020		2019
Expected term (in years)	5.50 – 6.08		5.50 – 6.08		5.51 – 6.07
Risk-free interest rate	0.6% – 1.3%		0.3% – 1.5%		1.4% – 2.6%
Expected volatility	86.3% – 104.8%		102.2% – 109.7%		97.2% – 105.4%
Expected dividend yield		0.0%		0.0%		0.0%