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Capital structure
12 Months Ended
Dec. 31, 2018
Stockholders' Equity Note [Abstract]  
Capital structure
Capital structure
Common stock
In April 2018, the Company closed an underwritten public offering of its common stock pursuant to a registration statement on Form S-3. The Company issued and sold an aggregate of 4,600,000 shares of common stock under the registration statement at a public offering price of $27.04 per share, including 600,000 shares issued upon exercise by the underwriters of their option to purchase additional shares.  The Company received net proceeds of approximately $117.9 million after deducting underwriting discounts and commissions and other offering expenses payable by the Company.
As of December 31, 2018, the Company’s number of authorized shares of common stock was 125,000,000.
Warrants
All of the Company’s outstanding warrants are classified as liabilities as of December 31, 2018 and 2017 because they contain non-standard antidilution provisions.
The following is a summary of the Company’s outstanding warrants as of December 31, 2018:
 
 
Warrant shares
 
Exercise price
 
Expiration
Common stock
 
7,030

 
$
128.00

 
2019
Common stock
 
130

 
$
2,520.00

 
2019
The following is a summary of the Company’s outstanding warrants as of December 31, 2017:
 
 
Warrant shares
 
Exercise price
 
Expiration
Common stock
 
7,030

 
$
128.00

 
2019
Common stock
 
130

 
$
2,520.00

 
2019