Common Stock, par value $0.001 per share

89bio, Inc.

655 MONTGOMERY STREET, SAN FRANCISCO, CA, 94111

This statement is filed by: i. Adage Capital Management, L.P., a Delaware limited partnership ("ACM"), as the investment manager of Adage Capital Partners, L.P., a Delaware limited partnership ("ACP"), with respect to the shares of Common Stock directly held by ACP; ii. Robert Atchinson ("Mr. Atchinson"), as (i) managing member of Adage Capital Advisors, L.L.C., a Delaware limited liability company ("ACA"), managing member of Adage Capital Partners GP, L.L.C., a Delaware limited liability company ("ACPGP"), general partner of ACP and (ii) managing member of Adage Capital Partners LLC, a Delaware limited liability company ("ACPLLC"), general partner of ACM, with respect to the shares of Common Stock directly held by ACP; and iii. Phillip Gross ("Mr. Gross"), as (i) managing member of ACA, managing member of ACPGP and (ii) managing member of ACPLLC, general partner of ACM, with respect to the shares of Common Stock directly held by ACP. The foregoing persons are hereinafter sometimes collectively referred to as the "Reporting Persons." Any disclosures herein with respect to persons other than the Reporting Persons are made on information and belief after making inquiry to the appropriate party. The filing of this statement should not be construed in and of itself as an admission by any Reporting Person as to beneficial ownership of the securities reported herein.

The address of the business office of each of the Reporting Persons is 200 Clarendon Street, 52nd Floor, Boston, Massachusetts 02116.

The principal business of ACM is to serve as investment manager to ACP. The principal business of each of Messrs. Atchinson and Gross is the management of investments in securities.

ACM consented to an Order Instituting Cease-And-Desist Proceedings ("Order") by the Securities and Exchange Commission ("SEC"). The Order was issued on September 25, 2024. In the Order, the SEC found violations (i.e., late filings) of Sections 13(d) and 16(a) of the Act and Rules 13d-1 and 16a-3 promulgated thereunder in 2020 and 2021 with respect to two separate issuers of securities held by ACP. Without admitting or denying any wrongdoing, ACM agreed to cease and desist from committing or causing any violations or future violations of Sections 13(d) and 16(a) of the Exchange Act and Rules 13d-1 and 16a-3 thereunder, and paid a civil money penalty in the amount of $200,000. The Order notes that there is no state of mind requirement for violations of Section 16(a) and 13(d) of the Act and the rules thereunder and that the failure to timely file a required report, even if inadvertent, constitutes a violation. The Order further notes that, in determining to accept the offer, the SEC considered certain remedial acts undertaken by ACM and cooperation afforded to SEC staff. ACM has since implemented additional policies and procedures to protect against future inadvertent Section 13 and Section 16 violations. Except as described above, during the last five years no Reporting Person was a party to a civil proceeding of a judicial or administrative body of competent jurisdiction and as a result of such proceeding was or is subject to a judgment, decree or final order enjoining future violations of, or prohibiting or mandating activities subject to, federal or state securities laws or finding any violation with respect to such laws. Schedule A attached hereto sets forth the information required by Instruction C of the instructions to Schedule 13D.

ACM is a Delaware limited partnership. Messrs. Atchinson and Gross are citizens of the United States.

See rows (11) and (13) of the cover pages to this Schedule 13D for the aggregate number of shares of Common Stock and percentages of the shares of Common Stock beneficially owned by each of the Reporting Persons. The percentages used in this Schedule 13D are calculated based upon 148,492,428 shares of Common Stock outstanding as of September 16, 2025, as reported in the Merger Agreement attached as Exhibit 2.1 to the Issuer's Current Report on Form 8-K, filed with the SEC on September 18, 2025.

See rows (7) through (10) of the cover pages to this Schedule 13D for the number of shares of Common Stock as to which each Reporting Person has the sole or shared power to vote or direct the vote and sole or shared power to dispose or to direct the disposition.

The transactions in the shares of Common Stock within the past sixty (60) days by the Reporting Persons, which were all in the open market, are set forth in Schedule B, and are incorporated herein by reference.

No person other than the Reporting Persons is known to have the right to receive, or the power to direct the receipt of dividends from, or proceeds from the sale of, such shares of Common Stock.

Not applicable.