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Stockholders' Equity (Tables)
 12 Months Ended
Dec. 31, 2019
Stockholders' Equity Note [Abstract]  
Summary of Warrant Activity and Changes in Warrants Outstanding
A summary of warrant activity and changes in warrants outstanding, including both liability and equity classifications, is presented below:
 
 
Number of
Warrants (1)
 
Weighted-Average
Exercise Price
Per Share (1)
 
Weighted-Average Remaining Contractual Term (1)
Balance outstanding, December 31, 2017
322,774

 
$
68.07

 
4.4 years
Granted
3,450,000

 
$
6.60

 
 
Exercised
(74,518
)
 
$
21.60

 
 
Expired
(48,915
)
 
$
217.17

 
 
Balance outstanding, December 31, 2018
3,649,341

 
$
8.91

 
4.4 years
Granted
7,437,454

 
$
1.87

 
 
Exercised
(497,313
)
 
$
6.60

 
 
Balance outstanding, December 31, 2019
10,589,482

 
$
4.08

 
3.7 years

 
(1)Balance outstanding, December 31, 2019 excludes 605,072 pre-funded warrants to purchase shares of common stock at a nominal exercise price of $0.01 per share. The pre-funded warrants expire when exercised in full. As of February 20, 2020 all of the pre-funded warrants have been exercised.