(Amendment No. 15)*

If the filing person has previously filed a statement on Schedule 13G to report the acquisition which is the subject of this Schedule 13D, and is filing this schedule because of Rule 13d-1(e), Rule 13d-1(f) or Rule 13d-1(g), check the following box  ¨.

The information required on the remainder of this cover page shall not be deemed to be “filed” for purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

*Includes (i) 3,068,951 shares of Common Stock held individually by Dr. Frost; (ii) 395,000 shares of Common Stock held by Frost Nevada, which is controlled by Dr. Frost as sole trustee; (iii) options to acquire 825,000 shares of Common Stock, which are exercisable within 60 days; and (iv) 164,234,443 shares of Common Stock held by Gamma Trust, which is controlled by Dr. Frost as sole trustee.

**Includes (i) 2,851,830 shares of Common Stock held by the Phillip and Patricia Frost Philanthropic Foundation, Inc., which is controlled by Dr. Frost and his wife, and (ii) 20,091,062 shares of Common Stock held by Frost Group.

***Includes (i) 3,068,951 shares of Common Stock held individually by Dr. Frost; (ii) 395,000 shares of Common Stock held by Frost Nevada, which is controlled by Dr. Frost as sole trustee; (iii) options to acquire 825,000 shares of Common Stock, which are exercisable within 60 days; (iv) 164,234,443 shares of Common Stock held by

Gamma Trust, which is controlled by Dr. Frost as sole trustee; (v) 2,851,830 shares of Common Stock held by the Phillip and Patricia Frost Philanthropic Foundation, Inc., which is controlled by Dr. Frost and his wife; and (vi) 20,091,062 shares of Common Stock held by Frost Group.

This Amendment No. 15 (the “Amendment”) amends and supplements the statement on Schedule 13D filed on February 15, 2007, as amended by Amendment No. 1 to the Schedule 13D filed on April 6, 2007, as amended by Amendment No. 2 to the Schedule 13D filed on August 2, 2007, as amended by Amendment No. 3 to the Schedule 13D filed on February 14, 2008, as amended by Amendment No. 4 to Schedule 13D filed on August 21, 2008, as amended by Amendment No. 5 to the Schedule 13D filed on February 26, 2009, as amended by Amendment No. 6 to the Schedule 13D filed on March 19, 2009, as amended by Amendment No. 7 to the Schedule 13D filed on October 1, 2009, as amended by Amendment No. 8 to the Schedule 13D filed on March 28, 2011, as amended by Amendment No. 9 to the Schedule 13D filed on October 24, 2011, as amended by Amendment No. 10 to the Schedule 13D filed on October 11, 2012, as amended by Amendment No. 11 to the Schedule 13D filed on March 21, 2013, as amended by Amendment No. 12 to the Schedule 13D filed on September 16, 2013, as amended by Amendment No. 13 to the Schedule 13D filed on February 17, 2015, as amended by Amendment No. 14 to the Schedule 13D filed on December 18, 2015 (collectively, the “Original Schedule 13D”), by The Frost Group, LLC (“Frost Group”), Frost Gamma Investments Trust (“Gamma Trust”), Frost Nevada Investments Trust (“Frost Nevada”), and Phillip Frost, M.D. (“Dr. Frost”). This Amendment is filed pursuant to the Joint Filing Agreement as executed by the reporting persons listed on the cover pages to this Amendment.

Gamma Trust acquired 5,899,900 shares of Common Stock, for investment purposes, in a series of transactions from December 18, 2015 to August 25, 2017 for an aggregate purchase price of $45,498,037.76. The source of funds used to acquire the Shares was working capital of Gamma Trust.

Frost Nevada acquired 395,000 shares of Common Stock, for investment purposes, in a series of transactions from September 14, 2017 to September 19, 2017 for an aggregate purchase price of $2,366,133.95. The source of funds used to acquire the Shares was working capital of Frost Nevada.

Gamma Trust acquired 5,899,900 shares of Common Stock, for investment purposes, in a series of transactions from December 18, 2015 to August 25, 2017.

Frost Nevada acquired 395,000 shares of Common Stock, for investment purposes, in a series of transactions from September 14, 2017 to September 19, 2017.

(a)-(b) Frost Group beneficially owns 20,091,062 shares of Common Stock. The 20,091,062 shares of Common Stock beneficially owned by Frost Group constitute approximately 3.6% of the Company’s outstanding shares of Common Stock, based upon 559,404,941 shares of Common Stock outstanding as of August 1, 2017. Frost Group shares the power to vote and the power to dispose such shares with Gamma Trust and Dr. Frost.

Gamma Trust directly beneficially owns 164,234,443 shares of Common Stock. Also, Gamma Trust, as the controlling member of Frost Group, may be deemed to beneficially own the 20,091,062 shares of Common Stock beneficially owned by Frost Group. The 184,325,505 total shares of Common Stock beneficially owned by Gamma Trust constitute 33.0% of the Company’s outstanding shares of Common Stock, based upon 559,404,941 shares of Common Stock outstanding as of August 1, 2017. Dr. Frost is the sole trustee of Gamma Trust and holds sole voting and dispositive power with respect to 168,523,394 shares of Common Stock. Gamma Trust has shared voting and dispositive power with respect to the 20,091,062 shares of Common Stock owned by Frost Group.

Frost Nevada directly beneficially owns 395,000 shares of Common Stock. The 395,000 total shares of Common Stock beneficially owned by Frost Nevada constitute 0.1% of the Company’s outstanding shares of Common Stock, based upon 559,404,941 shares of Common Stock outstanding as of August 1, 2017. Dr. Frost is the sole trustee of Frost Nevada and holds sole voting and dispositive power with respect to 395,000 shares of Common Stock.

In addition, the Phillip and Patricia Frost Philanthropic Foundation, Inc., which is controlled by Dr. Frost and his wife, owns 2,851,830 shares of Common Stock. Dr. Frost, as the sole trustee of Gamma Trust, which is the controlling member of Frost Group, may be deemed to beneficially own the 20,091,062 shares of Common Stock beneficially owned by Frost Group and the 164,234,443 shares of Common Stock beneficially owned by Gamma Trust. Dr. Frost, as the sole trustee of Frost Nevada may be deemed to beneficially own the 395,000 shares of Common Stock beneficially owned by Frost Nevada. The 191,466,286 shares of Common Stock beneficially owned by Dr. Frost constitute 34.2% of the Company’s outstanding shares of Common Stock, based upon 559,404,941 shares of Common Stock outstanding as of August 1, 2017. Dr. Frost, as sole trustee of Gamma Trust, has sole voting and dispositive power over 164,234,443 shares of Common Stock. Dr. Frost, as sole trustee of Frost Nevada, has sole voting and dispositive power over 395,000 shares of Common Stock. Dr. Frost has shared voting and dispositive power with respect to 20,091,062 shares of Common Stock owned by Frost Group and 2,851,830 shares of Common Stock owned by the Phillip and Patricia Frost Philanthropic Foundation, Inc. Dr. Frost individually owns 3,068,951 shares of Common Stock and options to acquire 825,000 shares of Common Stock, which are exercisable within 60 days, and has sole voting and dispositive power over each.

(c) In the last 60 days prior to the filing of this Amendment No. 15, neither the Frost Group nor Dr. Frost has effected any transactions in the shares of Common Stock.

In the last 60 days prior to the filing of this Amendment No. 15, Gamma Trust has acquired a total of 522,100 shares of Common Stock purchased on the open market at prices ranging from $5.875 to $6.48 per share. The following table sets forth the information with regard to Common Stock purchased by Gamma Trust on the open market in the last 60 days.

In the last 60 days prior to the filing of this Amendment No. 15, Frost Nevada has acquired a total of 395,000 shares of Common Stock purchased on the open market at prices ranging from $5.925 to $6.03 per share. The following table sets forth the information with regard to Common Stock purchased by Frost Nevada on the open market in the last 60 days.

Joint Filing Agreement, dated as of September 20, 2017, by and among the Reporting Persons.