SECURITIES AND EXCHANGE COMMISSION

Washington, DC 20549

SCHEDULE 13G

(Rule 13d-102)

INFORMATION TO BE INCLUDED IN STATEMENTS FILED PURSUANT TO

RULES 13d-1(b), (c) AND (d) AND AMENDMENTS THERETO FILED

PURSUANT TO RULE 13d-2(b)

Opexa Therapeutics, Inc.

(Name of Issuer)

Common Stock, $.01 par value

(Title of Class of Securities)

68372T202

(CUSIP Number)

April 9, 2015

(Date of Event Which Requires Filing of this Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule is filed:

x Rule 13d-1(b)

x Rule 13d-1(c)

¨ Rule 13d-1(d)

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SCHEDULE 13G

This Schedule 13G (this “Schedule 13G”) is being filed on behalf of Frigate Ventures LP (d/b/a Anson Group), a Texas limited partnership (“Frigate”), Admiralty Advisors LLC, a Texas limited liability company (“Admiralty”), Mr. Bruce R. Winson, the principal of Frigate and Admiralty, M5V Advisors Inc. (d/b/a Anson Group Canada), an Ontario, Canada corporation (“M5V”), Mr. Adam Spears, a director of M5V, and Mr. Moez Kassam, a director of M5V, relating to Common Stock, $.01 par value (the “Common Stock”), of Opexa Therapeutics, Inc., a Texas corporation (the “Issuer”).

This Schedule 13G relates to Common Stock of the Issuer purchased by a private fund to which Frigate and M5V serve as co-investment advisors (the “Fund”). Frigate and M5V serve as co-investment advisors to the Fund and may direct the vote and disposition of the 5,536,999 shares of Common Stock held by the Fund. As the general partner of Frigate, Admiralty may direct the vote and disposition of the 5,536,999 shares of Common Stock held by the Fund. As the principal of Frigate and Admiralty, Mr. Winson may direct the vote and disposition of the 5,536,999 shares of Common Stock held by the Fund. As directors of M5V, Mr. Spears and Mr. Kassam may each direct the vote and disposition of the 5,536,999 shares of Common Stock held by the Fund.

Opexa Therapeutics, Inc.

2635 TECHNOLOGY FOREST BLVD.

THE WOODLANDS TX 77381

Frigate Ventures LP (“Frigate”), Admiralty Advisors LLC (“Admiralty”), Mr. Bruce R. Winson, M5V Advisors Inc. (“M5V”), Mr. Adam Spears and Mr. Moez Kassam

For Frigate, Admiralty and Mr. Winson:

5950 Berkshire Lane, Suite 210

Dallas, Texas 75225

For M5V, Mr. Spears and Mr. Kassam:

111 Peter Street, Suite 904

Toronto, ON

M5V 2H1

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Frigate is a limited partnership organized under the laws of the State of Texas. Admiralty is a limited liability company organized under the laws of the State of Texas. Mr. Winson is a United States citizen. M5V is a corporation organized under the laws of Ontario, Canada. Mr. Spears and Mr. Kassam are each Canadian citizens.

Common Stock, $.01 par value (the “Common Stock”).

68372T202

If this statement is filed pursuant to Rule 13d-1(b), or 13d-2(b), check whether the person filing is a:

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Inapplicable.

Inapplicable.

Inapplicable.

Inapplicable.

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Inapplicable.

For Frigate, Admiralty and Mr. Winson:

By signing below each of the Reporting Persons certifies that, to the best of such person’s knowledge and belief, the securities referred to above were acquired and held in the ordinary course of business and were not acquired and were not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect.

For the M5V, Mr. Spears and Mr. Kassam:

By signing below each of the Reporting Persons certifies that, to the best of such person’s knowledge and belief, the securities referred to above were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect.

Joint Filing Agreement dated April 16, 2015, by and among Frigate, Admiralty, Mr. Winson, M5V, Mr. Spears and Mr. Kassam.

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SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Date: April 16, 2015

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EXHIBIT 99.1

JOINT FILING AGREEMENT

In accordance with Rule 13d-1(k) under the Securities Exchange Act of 1934, as amended, the undersigned agree to the joint filing on behalf of each of them of a Statement on Schedule 13G (including any and all amendments thereto) with respect to the Common Stock, $.01 par value, of Opexa Therapeutics, Inc., a Texas corporation, and further agree that this Joint Filing Agreement shall be included as an Exhibit to such joint filings.

The undersigned further agree that each party hereto is responsible for the timely filing of such Statement on Schedule 13G and any amendments thereto, and for the accuracy and completeness of the information concerning such party contained therein; provided, however, that no party is responsible for the accuracy or completeness of the information concerning any other party, unless such party knows or has reason to believe that such information is inaccurate.

This Joint Filing Agreement may be signed in counterparts with the same effect as if the signature on each counterpart were upon the same instrument.

IN WITNESS WHEREOF, the undersigned have executed this Agreement as of April 16, 2015.

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