UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, DC 20549

SCHEDULE 13G

Under the Securities Exchange Act of 1934

(Amendment No. )*

Compass Therapeutics, Inc.

(Name of Issuer)

Common Stock, $0.0001 par value per share

(Title of Class of Securities)

20454B104

(CUSIP Number)

July 23, 2024

(Date of Event which Requires Filing of this Statement)

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

☐ Rule 13d-1(b)

☒ Rule 13d-1(c)

☐ Rule 13d-1(d)

The information required on the remainder of this cover page shall not be deemed to be “filed” for the 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).

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Compass Therapeutics, Inc. (the “Issuer”)

80 Guest Street, Suite 601

Boston, MA 02135

This Schedule 13G is being filed by Blackstone Annex Master Fund L.P. (“Annex Fund”), Spruce Street Aggregator L.P. (“Spruce”), Blackstone Alternative Asset Management Associates LLC (“BAAMA”), Blackstone Holdings II L.P. (“Holdings II”), Blackstone Holdings I/II GP L.L.C. (“Holdings GP”), Blackstone Inc. (“Blackstone”), Blackstone Group Management L.L.C. (“Blackstone Management”), and Stephen A. Schwarzman (together with Annex Fund, Spruce, BAAMA, Holdings II, Holdings GP, Blackstone, and Blackstone Management, the “Reporting Persons”).

The principal business address of each of the Reporting Persons is:

345 Park Avenue, 28th Floor

New York, NY 10154

Annex Fund is a Cayman Islands limited partnership. Spruce is a Cayman Islands exempted limited partnership. BAAMA is a limited liability company organized under the laws of the State of Delaware. Holdings II is a limited partnership organized under the laws of the State of Delaware. Holdings GP is a limited liability company organized under the laws of the State of Delaware. Blackstone is a corporation organized under the laws of the State of Delaware. Blackstone Management is a limited liability company organized under the laws of the State of Delaware. Mr. Schwarzman is a citizen of the United States of America.

Common Stock, $0.0001 par value per share per share (“Common Stock”)

20454B104

Not Applicable.

Items 5 through 9 and 11 of each of the cover pages to this Schedule 13G are incorporated herein by reference. Annex Fund holds 3,607,287 shares of Common Stock (the “Annex Fund Shares”) and Spruce holds 4,382,126 shares of Common Stock (the “Spruce Shares”). The Annex Fund Shares and the Spruce Shares represent beneficial ownership of approximately 2.6% and 3.2% of the outstanding shares of Common Stock, respectively. Funds or accounts that may be deemed to be indirectly controlled by a subsidiary of Blackstone Inc. hold an aggregate of 1,505,738 shares of Common Stock representing beneficial ownership of approximately 1.1% of the outstanding shares of Common Stock. Calculations are based on 137,589,171 shares of Common Stock of the Issuer outstanding as of May 10, 2024, as disclosed in the Issuer’s Quarterly Report on Form 10-Q filed with the Securities and Exchange Commission on May 13, 2024.

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BAAMA is the general partner of the Annex Fund and Spruce. Holdings II is the sole member of BAAMA. Holdings GP is the general partner of Holdings II. Blackstone is the sole member of Holdings GP. Blackstone Management is the sole holder of the Series II preferred stock of Blackstone. Blackstone Management is wholly owned by its senior managing directors and controlled by its founder, Stephen A. Schwarzman.

Each Reporting Person may be deemed to beneficially own the Common Stock of the Issuer directly or indirectly controlled by it or him, but neither the filing of this Schedule 13G nor any of its contents shall be deemed to constitute an admission that any such Reporting Person (other than any Reporting Person to the extent they directly hold Issuer securities) is the beneficial owner of Common Stock of the Issuer referred to herein for purposes of Section 13(d) of the Securities Exchange Act of 1934, as amended (the “Act”), or for any other purpose and each of the Reporting Persons expressly disclaims beneficial ownership of such shares of Common Stock. The filing of this statement should not be construed to be an admission that any member of the Reporting Persons are members of a “group” for the purposes of Section 13(d) and 13(g) of the Act.

If this statement is being filed to report the fact that as of the date hereof the reporting person has ceased to be the beneficial owner of more than five percent of the class of securities, check the following: ☐

Not applicable.

Not applicable.

Not applicable.

Not applicable.

By signing below I certify that, to the best of my 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.

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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.

August 2, 2024

EXHIBIT LIST

Exhibit 99.1

JOINT FILING AGREEMENT

Pursuant to and in accordance with the Securities Exchange Act of 1934, as amended (the “Exchange Act”), and the rules and regulations thereunder, the undersigned hereby agree to the joint filing on behalf of each of them of any filing required by such party under Section 13 of the Exchange Act or any rule or regulation thereunder (including any amendment, restatement, supplement, and/or exhibit thereto) with respect to securities of Compass Therapeutics, Inc., a Delaware corporation, and further agree to the filing, furnishing, and/or incorporation by reference of this Joint Filing Agreement (this “Agreement”) as an exhibit thereto. Each of them is responsible for the timely filing of such filings and any amendments thereto, and for the completeness and accuracy of the information concerning such person contained therein; but none of them is responsible for the completeness or accuracy of the information concerning the other persons making the filing, unless such person knows or has reason to believe that such information is inaccurate. This Agreement shall remain in full force and effect until revoked by any party hereto in a signed writing provided to each other party hereto, and then only with respect to such revoking party. This Agreement may be executed in any number of counterparts all of which taken together shall constitute one and the same instrument.

Dated: August 2, 2024