
	At March 31,
	2019	2018
Stock options outstanding (1)	21,109,998	19,733,986
Restricted stock outstanding (2)	2,372,347	234,744
Common stock warrants (3)	115,824,142	115,824,142
Preferred stock (4)	17,933,591	2,782,075
Convertible Senior Notes (5)	43,630,020	43,630,020
Number of dilutive potential common shares	200,870,098	182,204,967

(1)

During the three months ended March 31, 2019 and 2018, the Company granted 25,000, and zero stock options, respectively.

(2)

During the three months ended March 31, 2019 and 2018, the Company granted 25,000 and zero shares of restricted stock, respectively.

(3)

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 March 31, 2019.

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 March 31, 2019.

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 March 31, 2019.

(4)

The preferred stock amount represents the dilutive potential common shares of the Series C and E redeemable convertible preferred stock, based on the conversion price of the preferred stock as of March 31, 2019 and 2018, respectively. Of the 10,431 Series C redeemable preferred stock issued on May 16, 2013, 7,811 had been converted to common stock through March 31, 2019 and 2018, with the remainder still outstanding. On November 1, 2018, the Company issued 35,000 shares of Series E redeemable convertible preferred stock. As of March 31, 2019, all Series E redeemable convertible preferred stock are outstanding.

(5)

In March 2018, the Company issued $100.0 million in aggregate principal amount of 5.5% Convertible Senior Notes. See Note 8, Convertible Senior Notes.