
	At June 30,
	2018	2017
Stock options outstanding (1)	19,977,576	15,016,609
Restricted stock outstanding	207,347	248,077
Common stock warrants (2)	115,824,142	60,537,546
Preferred stock (3)	2,782,075	5,554,594
Convertible Senior Notes	43,630,020	—
Number of dilutive potential common shares	182,421,160	81,356,826

(1)

During the three months ended June 30, 2018 and 2017, the Company granted 484,667 and 314,511 stock options, respectively. During the six months ended June 30, 2018 and 2017, the Company granted 484,667 and 450,863 stock options, respectively.

(2)

In February 2013, the Company issued 23,637,500 warrants as part of an underwritten public offering with an exercise price of $0.15 per warrant. Of these warrants issued in February 2013, zero and 100 were unexercised as of June 30, 2018 and 2017.

In January 2014, the Company issued 4,000,000 warrants as part of an underwritten public offering with an exercise price of $4.00 per warrant. In December 2016, as a result of additional public offerings, and pursuant to the effect of the anti-dilution provisions of these warrants, the exercise price of the $4.00 warrants was reduced to $0.65. Of these warrants issued in January 2014, all 4,000,000 warrants were exercised during 2017, as described in Note 9, Stockholders’ Equity.

In December 2016, the Company issued 10,501,500 warrants as part of two concurrent underwritten public offerings with an exercise price of $1.50 per warrant. Of these warrants issued in December 2016, all 10,501,500 warrants were exercised during 2017, as described in Note 9, Stockholders’ Equity.

In April 2017, the Company issued 5,250,750 warrants with an exercise price of $2.69 per warrant, as described in Note 9, Stockholders’ Equity. Of these warrants issued in April 2017, none have been exercised as of June 30, 2018.

In April 2017, the Company issued warrants to acquire up to 55,286,696 of the Company’s common stock as part of a transaction agreement, subject to certain vesting events, as described in Note 11, Warrant Transaction Agreements. Of these warrants issued, none have been exercised as of June 30, 2018.

In July 2017, the Company issued warrants to acquire up to 55,286,696 of the Company’s common stock as part of a transaction agreement, subject to certain vesting events, as described in Note 11, Warrant Transaction Agreements. Of these warrants issued, none have been exercised as of June 30, 2018.

(3)

The preferred stock amount represents the dilutive potential common shares of the Series C redeemable convertible preferred stock, based on the conversion price of the preferred stock as of June 30, 2018 and 2017, respectively. Of the 10,431 Series C redeemable preferred stock issued on May 16, 2013, 7,811 and 5,200 had been converted to common stock through June 30, 2018 and 2017, respectively, with the remainder still outstanding. Of the 18,500 Series D redeemable convertible preferred stock issued on December 22, 2016, 3,700 shares were redeemed during the three months ended March 31, 2017 and the remaining 14,800 were converted to common stock during the second quarter of 2017.