[SCHEDULE 13D] 89bio, Inc. SEC Filing

Rhea-AI Filing Summary

Adage Capital Management, L.P. and affiliated individuals reported beneficial ownership of 9,097,587 shares of 89bio, Inc. common stock, representing 6.13% of the outstanding class based on 148,492,428 shares. The shares were acquired for investment purposes using working capital of Adage Capital Partners, L.P., with a disclosed purchase cost of approximately $125,505,893 inclusive of commissions. The filing notes a Merger Agreement dated September 17, 2025 between the Issuer and Roche Holdings, Inc., disclosed by the Issuer on September 18, 2025. Adage states it has no present plan to change control but may review its position and engage with management or other parties regarding strategic matters. The filing also discloses a prior SEC Order against ACM dated September 25, 2024 involving late filings and a $200,000 civil penalty.

Positive

• 9,097,587 shares reported, representing 6.13% ownership
• Total disclosed purchase cost approximately $125,505,893, confirming material economic commitment
• Filing states acquisition was for investment purposes and not intended to change control

Negative

• Adage Capital Management disclosed an SEC Order dated September 25, 2024 and a $200,000 civil penalty for late filings
• Reporting Persons previously required to implement additional policies and procedures for Section 13/16 compliance

Insights

Securities Ownership Analyst

Adage holds a meaningful 6.13% stake in 89bio, acquired for $125.5M.

Holding 9,097,587 shares and reporting shared voting and dispositive power indicates a sizeable passive investment position disclosed under Schedule 13D rules.

This filing is material because Schedule 13D signals a significant shareholder that may engage with management, and the filing explicitly allows future purchases or sales depending on the Merger Agreement and other developments.

Regulatory & Compliance Specialist

ACM disclosed a prior SEC Order (Sept 25, 2024) and a $200,000 penalty for late filings.

The disclosure is factual: the Order relates to violations of Sections 13(d) and 16(a) and notes remedial steps taken by ACM. This history is relevant to investors assessing the reporting track record and compliance risk.

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

89bio, Inc.

(Name of Issuer)

Common Stock, par value $0.001 per share

(Title of Class of Securities)

282559103

(CUSIP Number)

Robert Atchinson
Adage Capital Partners GP, L.L.C., 200 Clarendon Street, 52nd Floor
Boston, MA, 02116
617-867-2800

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

09/26/2025

(Date of Event Which Requires Filing of This Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§ 240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box.

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

SCHEDULE 13D

CUSIP No. 282559103

1

Name of reporting person

Adage Capital Management, L.P.

2

Check the appropriate box if a member of a Group (See Instructions)

(a)
(b)

3

SEC use only

4

Source of funds (See Instructions)

AF

5

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

6

Citizenship or place of organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 9,097,587.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 9,097,587.00

11

Aggregate amount beneficially owned by each reporting person

9,097,587.00

12

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

13

Percent of class represented by amount in Row (11)

6.13 %

14

Type of Reporting Person (See Instructions)

IA, PN

SCHEDULE 13D

CUSIP No. 282559103

1

Name of reporting person

Robert Atchinson

2

Check the appropriate box if a member of a Group (See Instructions)

(a)
(b)

3

SEC use only

4

Source of funds (See Instructions)

AF

5

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

6

Citizenship or place of organization

UNITED STATES

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 9,097,587.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 9,097,587.00

11

Aggregate amount beneficially owned by each reporting person

9,097,587.00

12

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

13

Percent of class represented by amount in Row (11)

6.13 %

14

Type of Reporting Person (See Instructions)

IN, HC

SCHEDULE 13D

CUSIP No. 282559103

1

Name of reporting person

Phillip Gross

2

Check the appropriate box if a member of a Group (See Instructions)

(a)
(b)

3

SEC use only

4

Source of funds (See Instructions)

AF

5

Check if disclosure of legal proceedings is required pursuant to Items 2(d) or 2(e)

6

Citizenship or place of organization

UNITED STATES

Number of Shares Beneficially Owned by Each Reporting Person With:

7	Sole Voting Power 0.00
8	Shared Voting Power 9,097,587.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 9,097,587.00

11

Aggregate amount beneficially owned by each reporting person

9,097,587.00

12

Check if the aggregate amount in Row (11) excludes certain shares (See Instructions)

13

Percent of class represented by amount in Row (11)

6.13 %

14

Type of Reporting Person (See Instructions)

IN, HC

SCHEDULE 13D

Item 1.	Security and Issuer
(a)	Title of Class of Securities: Common Stock, par value $0.001 per share
(b)	Name of Issuer: 89bio, Inc.
(c)	Address of Issuer's Principal Executive Offices: 655 MONTGOMERY STREET, SUITE 1500, SAN FRANCISCO, CALIFORNIA , 94111.
Item 1 Comment: This statement on Schedule 13D (the "Schedule 13D") relates to the shares of common stock, par value $0.001 per share (the "Common Stock"), of 89bio, Inc., a Delaware corporation (the "Issuer").
Item 2.	Identity and Background
(a)	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.
(b)	The address of the business office of each of the Reporting Persons is 200 Clarendon Street, 52nd Floor, Boston, Massachusetts 02116.
(c)	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.
(d)	None of the Reporting Persons has, during the last five years, been convicted in a criminal proceeding (excluding traffic violations or similar misdemeanors).
(e)	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.
(f)	ACM is a Delaware limited partnership. Messrs. Atchinson and Gross are citizens of the United States.
Item 3.	Source and Amount of Funds or Other Consideration
	Funds for the purchase of the 9,097,587 shares of Common Stock to which this Schedule 13D relates were derived from working capital of ACP made in the ordinary course of business. A total of approximately $125,505,893, inclusive of commissions, was paid to acquire the 9,097,587 shares of Common Stock reported herein.
Item 4.	Purpose of Transaction
	The Reporting Persons acquired the shares of Common Stock to which this Schedule 13D relates for investment purposes in the ordinary course of business, and not with the purpose nor with the effect of changing or influencing the control or management of the Issuer. The Reporting Persons acquired the shares of Common Stock pursuant to investment strategies, including merger arbitrage and event driven strategies, because they believed that the shares of Common Stock reported herein, when purchased, represented an attractive investment opportunity. Accordingly, the Reporting Persons may not be eligible to report this position on a Schedule 13G. See Colish, Faith (No-Act., Available March 24, 1980). On September 18, 2025, the Issuer filed a Current Report on Form 8-K disclosing it had entered into an Agreement and Plan of Merger and Reorganization dated as of September 17, 2025 (the "Merger Agreement") with Roche Holdings, Inc., a Delaware corporation ("Parent"), and Bluefin Merger Subsidiary, Inc., a Delaware corporation and wholly owned subsidiary of Parent. The transactions contemplated by the Merger Agreement are hereinafter referred to as the "Proposed Transaction". Except as set forth herein, the Reporting Persons have no present plan or proposal that would relate to or result in any of the matters set forth in subparagraphs (a)-(j) of Item 4 of Schedule 13D. The Reporting Persons have not entered into any agreement with any third party to act together for the purpose of acquiring, holding, voting or disposing of the shares of Common Stock reported herein. The Reporting Persons intend to review their investment in the Issuer on a continuing basis and may from time to time engage in discussions with management, the Board of Directors, other shareholders of the Issuer and other relevant parties, including representatives of any of the foregoing, concerning matters with respect to the Reporting Persons' investment in the shares of Common Stock, including, without limitation, the business, operations, governance, management, strategy and future plans of the Issuer. Depending on various factors, including, without limitation, the terms of the Proposed Transaction and any other offers or developments related thereto, the outcome of any discussions referenced above, the Issuer's financial position and strategic direction, actions taken by the Board of Directors, price levels of the shares of Common Stock, other investment opportunities available to the Reporting Persons, conditions in the securities market and general economic and industry conditions, the Reporting Persons may in the future take such actions with respect to their investment in the Issuer as they deem appropriate, including, without limitation, purchasing additional shares of Common Stock or selling some or all of their shares of Common Stock, engaging in short selling of or any hedging or similar transactions with respect to the shares of Common Stock and/or otherwise changing their intention with respect to any and all matters referred to in Item 4 of the Schedule 13D. The Reporting Persons may, at any time and from time to time, review or reconsider their position and/or change their purpose and/or formulate plans or proposals with respect to their investment in the shares of Common Stock.
Item 5.	Interest in Securities of the Issuer
(a)	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.
(b)	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.
(c)	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.
(d)	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.
(e)	Not applicable.
Item 6.	Contracts, Arrangements, Understandings or Relationships With Respect to Securities of the Issuer
	Other than the Joint Filing Agreement attached as Exhibit 99.1 hereto, there are no contracts, arrangements, understandings or relationships (legal or otherwise) among the persons named in Item 2 hereof and between such persons and any person with respect to any securities of the Issuer, including any class of the Issuer's securities used as a reference security, in connection with any of the following: call options, put options, security-based swaps or any other derivative securities, transfer or voting of any of the securities, finder's fees, joint ventures, loan or option arrangements, guarantees of profits, division of profits or loss, or the giving or withholding of proxies.
Item 7.	Material to be Filed as Exhibits.
	Exhibit 99.1 Joint Filing Agreement Statement as required by Rule 13d-1(k)(1) under the Act.

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.

Adage Capital Management, L.P.

Signature:	/s/ Robert Atchinson
Name/Title:	By: Adage Capital Partners LLC, its General Partner, By: Robert Atchinson, its Managing Member
Date:	10/03/2025

Robert Atchinson

Signature:	/s/ Robert Atchinson
Name/Title:	Robert Atchinson, Individually
Date:	10/03/2025

Phillip Gross

Signature:	/s/ Phillip Gross
Name/Title:	Phillip Gross, Individually
Date:	10/03/2025

FAQ

What stake does Adage report in 89bio (ETNB)?

Adage reports beneficial ownership of 9,097,587 shares, equal to 6.13% of 89bio's outstanding common stock.

How much did Adage pay for the reported 89bio shares?

The filing states approximately $125,505,893 was paid to acquire the 9,097,587 shares, inclusive of commissions.

Does Adage intend to seek control of 89bio?

The Reporting Persons state the shares were acquired for investment purposes and that they have no present plan to change or influence control or management.

Is there any disclosure of a potential transaction involving 89bio?

Yes, the filing references a Merger Agreement dated September 17, 2025 between 89bio and Roche Holdings, Inc., disclosed by the Issuer on September 18, 2025.

Were there recent compliance issues disclosed by Adage?

The filing discloses an SEC Order from September 25, 2024 finding late filings and a $200,000 civil money penalty; ACM implemented additional procedures thereafter.