Summary of Significant Accounting Policies (Tables)
 12 Months Ended
Dec. 31, 2021
Accounting Policies [Abstract]  
Reconciliation of Cash and Restricted Cash
The following table provides a reconciliation of cash, cash equivalents and restricted cash, reported within the consolidated statements of cash flows (in thousands): 
Years Ended December 31,
20212020
Cash and cash equivalents$7,732 $48,892 
Restricted cash5,056 22,559 
Restricted cash included in other noncurrent assets800 800 
Total cash, cash equivalents and restricted cash presented in the consolidated statements of cash flows$13,588 $72,251 
Summary of Stock Option Expensing Policies
The following table summarizes the Company’s stock-based awards expensing policies for employees and nonemployees: 
   Employees and
Nonemployee Consultants
Service only condition  Straight-line based on the grant date fair value
Performance criterion is probable of being met:
Service criterion is complete  Recognize the grant date fair value of the award(s) once the performance criterion is considered probable of occurrence
Service criterion is not complete  
Expense using an accelerated multiple-option approach(1) over the remaining requisite service period
Performance criterion is not probable of being met and:No expense is recognized until the performance criterion is considered probable at which point expense is recognized using an accelerated multiple-option approach
________________
(1)The accelerated multiple-option approach results in compensation expense being recognized for each separately vesting tranche of the award as though the award was in substance multiple awards and, therefore, results in accelerated expense recognition during the earlier vesting periods.
Summary of Potentially Dilutive Securities Excluded from Calculation of Diluted Net Loss Per Share Potentially dilutive securities excluded from the calculation of diluted net loss per share are summarized in the table below. Common shares were calculated for the Series B-2 Convertible Preferred Stock and the convertible debt using the if-converted method.
Years Ended December 31,
20212020
Unvested restricted common stock subject to repurchase— 80,000 
Common stock to be purchased under the 2019 ESPP508,656 204,664 
Options to purchase common stock10,625,062 8,935,801 
Warrants to purchase common stock67,767,107 10,426,107 
Series B-2 convertible preferred stock 8,333,334 — 
Convertible debt17,882,508 — 
Total105,116,667 19,646,572