
	At September 30,
	2019	2018
Stock options outstanding (1)	23,597,871	22,111,243
Restricted stock outstanding (2)	4,333,560	2,367,347
Common stock warrants (3)	115,824,142	115,824,142
Preferred stock (4)	15,019,838	2,782,075
Convertible Senior Notes (5)	59,133,896	43,630,020
Number of dilutive potential common stock	217,909,307	186,714,827

(1)

During the three months ended September 30, 2019 and 2018, the Company granted 2,572,500 and 2,170,000 stock options, respectively. During the nine months ended September 30, 2019 and 2018, the Company granted 2,936,892, and 2,654,667 stock options, respectively.

(2)

During the three months ended September 30, 2019 and 2018, the Company granted 2,562,500 and 2,160,000 shares of restricted stock, respectively. During the nine months ended September 30, 2019 and 2018, the Company granted 2,926,892 and 2,367,347 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 had been exercised as of September 30, 2019. All of these warrants were exercised on October 15, 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 had been exercised as of September 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 had been exercised as of September 30, 2019.

(4)

The preferred stock amount represents the dilutive potential on the shares of common stock as a result of the conversion of the Series C Redeemable Convertible Preferred Stock (Series C Preferred Stock) and Series E Preferred Stock, based on the conversion price of each preferred stock as of September 30, 2019 and 2018, respectively. Of the 10,431 Series C Preferred Stock issued on May 16, 2013, 7,811 had been converted to common stock at September 30, 2019 and 2018, with the remainder still outstanding. On November 1, 2018, the Company issued 35,000 shares of Series E Preferred Stock. As of September 30, 2019, 6,731 of the Series E Preferred Stock had been converted to common stock, with the remainder still outstanding.

(5)

In March 2018, the Company issued $100.0 million in aggregate principal amount of 5.5% Convertible Senior Notes. In September 2019, the Company issued a $40.0 million in aggregate principal amount of 7.5% Convertible Senior Note. See Note 8, Convertible Senior Notes.