
	At June 30,
	2019	2018
Stock options outstanding (1)	21,258,304	19,977,576
Restricted stock outstanding (2)	2,504,392	207,347
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	201,150,449	182,421,160

(1)

During the three months ended June 30, 2019 and 2018, the Company granted 339,392 and 484,667 stock options, respectively. During the six months ended June 30, 2019 and 2018, the Company granted 364,392, and 484,667 stock options, respectively.

(2)

During the three months ended June 30, 2019 and 2018, the Company granted 339,392 and zero shares of restricted stock, respectively. During the six months ended June 30, 2019 and 2018, the Company granted 364,392 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 June 30, 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 June 30, 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 June 30, 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 June 30, 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 June 30, 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 June 30, 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.