[SCHEDULE 13D/A] MBX Biosciences, Inc. SEC Filing

Rhea-AI Filing Summary

OrbiMed affiliates report a change in beneficial ownership of MBX Biosciences common stock following an increase in the company's outstanding shares. The filing discloses that OrbiMed Advisors, OrbiMed Capital GP VII and OrbiMed Genesis collectively hold 4,002,887 shares, representing approximately 9.0% of 44,700,281 outstanding shares as stated in the issuer's prospectus. The reduction in reported percentage ownership is due to an increase in shares outstanding rather than a sale by the reporting persons. The filing also summarizes the relationships among the funds, their voting and dispositive authority and limited registration and piggyback rights under an Investors' Rights Agreement.

Positive

• Collective stake of 4,002,887 shares (approx. 9.0%) establishes OrbiMed as a significant minority holder.
• No sale or disposition reported; percentage change is attributed to increased shares outstanding rather than divestment.
• Clear contractual registration rights (demand, Form S-3 and piggyback) are documented, providing liquidity options for holders.

Negative

• Ownership diluted by an increase in outstanding shares, reducing reported percentage by more than 1%.
• No proposed strategic actions meaning investors receive no new corporate governance or operational catalysts from these reporting persons.

Insights

Financial Analyst

TL;DR A routine disclosure: OrbiMed group retains a material minority stake but was diluted by share issuance, not dispositions.

The filing is primarily informational and confirms OrbiMed entities collectively hold 4,002,887 shares, about 9.0% of the stated 44.7 million shares outstanding. The change reported arises from an increase in the issuer's outstanding shares, which lowered OrbiMed's percentage ownership by more than one percent. There is no indication of active disposal, governance changes or plans to effect corporate actions. For investors, this is a non-operational update clarifying ownership and voting/dispositive arrangements among affiliated investment vehicles.

Corporate Governance Expert

TL;DR Ownership structure and voting arrangements are clarified; no governance initiative or control attempt is disclosed.

The schedule details the chain of voting and dispositive power from limited partnerships to their general partners and to OrbiMed Advisors, and references an Investors' Rights Agreement that provides demand, Form S-3 and piggyback registration rights. Crucially, the reporting persons state they have not formulated plans to change board composition, capitalization or pursue extraordinary transactions. This maintains current governance status quo and documents contractual registration rights available to these holders.

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. 1)

MBX Biosciences, Inc.

(Name of Issuer)

COMMON STOCK

(Title of Class of Securities)

55287L101

(CUSIP Number)

OrbiMed Advisors LLC
601 Lexington Avenue, 54th Floor,
New York, NY, 10022
(212) 739-6400

OrbiMed Capital GP VII LLC
601 Lexington Avenue, 54th Floor,
New York, NY, 10022
(212) 739-6400

OrbiMed Genesis GP LLC
601 Lexington Avenue, 54th Floor,
New York, NY, 10022
(212) 739-6400

(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. 55287L101

1

Name of reporting person

ORBIMED ADVISORS LLC

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 4,002,887.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 4,002,887.00

11

Aggregate amount beneficially owned by each reporting person

4,002,887.00

12

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

13

Percent of class represented by amount in Row (11)

9.0 %

14

Type of Reporting Person (See Instructions)

IA

SCHEDULE 13D

CUSIP No. 55287L101

1

Name of reporting person

ORBIMED CAPITAL GP VII LLC

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 3,255,000.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 3,255,000.00

11

Aggregate amount beneficially owned by each reporting person

3,255,000.00

12

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

13

Percent of class represented by amount in Row (11)

7.3 %

14

Type of Reporting Person (See Instructions)

OO

SCHEDULE 13D

CUSIP No. 55287L101

1

Name of reporting person

OrbiMed Genesis GP LLC

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 747,887.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 747,887.00

11

Aggregate amount beneficially owned by each reporting person

747,887.00

12

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

13

Percent of class represented by amount in Row (11)

1.7 %

14

Type of Reporting Person (See Instructions)

OO

SCHEDULE 13D

Item 1.	Security and Issuer
(a)	Title of Class of Securities: COMMON STOCK
(b)	Name of Issuer: MBX Biosciences, Inc.
(c)	Address of Issuer's Principal Executive Offices: 11711 N. Meridian Street, Suite 300, Carmel, INDIANA , 46032.
Item 1 Comment: This Amendment No. 1 ("Amendment No. 1") to Schedule 13D supplements and amends the Statement on Schedule 13D of OrbiMed Advisors LLC ("OrbiMed Advisors"), OrbiMed Capital GP VII LLC, ("GP VII"), and OrbiMed Genesis GP LLC ("OrbiMed Genesis") (collectively, the "Reporting Persons") originally filed with the Securities and Exchange Commission (the "SEC") on September 23, 2024. This Amendment No. 1 is being filed to report that the beneficial ownership of the outstanding Shares held by the Reporting Persons decreased by more than 1% as a result of an increase in the number of Shares outstanding.
Item 2.	Identity and Background
(a)	OrbiMed Advisors is a limited liability company organized under the laws of Delaware and a registered investment adviser under the Investment Advisers Act of 1940, as amended. GP VII is a limited liability company organized under the laws of Delaware. OrbiMed Genesis is a limited liability company organized under the laws of Delaware. Carl L. Gordon is a United States citizen. Sven H. Borho is a German and Swedish citizen. W. Carter Neild is a United States citizen. Geoffrey C. Hsu is a United States citizen. C. Scotland Stevens is a United States citizen. David P. Bonita is a United States citizen. Peter A. Thompson is a United States citizen. Matthew S. Rizzo is a United States citizen. Mona Ashiya is a United States citizen. Trey Block is a United States citizen.
(b)	601 Lexington Avenue, 54th Floor, New York, New York 10022.
(c)	OrbiMed Advisors is the managing member or general partner of certain entities as more particularly described in Item 6 below. GP VII is the general partner of a limited partnership as more particularly described in Item 6 below. OrbiMed Genesis is the general partner of a limited partnership as more particularly described in Item 6 below. Carl L. Gordon is a member of OrbiMed Advisors. Sven H. Borho is a member of OrbiMed Advisors. W. Carter Neild is a member of OrbiMed Advisors. Geoffrey C. Hsu is a member of OrbiMed Advisors. C. Scotland Stevens is a member of OrbiMed Advisors. David P. Bonita is a member of OrbiMed Advisors. Peter A. Thompson is a member of OrbiMed Advisors. Matthew S. Rizzo is a member of OrbiMed Advisors. Mona Ashiya is a member of OrbiMed Advisors. Trey Block is the Chief Financial Officer of OrbiMed Advisors.
(d)	Not applicable.
(e)	Not applicable.
(f)	Item 2(a) is incorporated herein by reference.
Item 3.	Source and Amount of Funds or Other Consideration
	Not applicable.
Item 4.	Purpose of Transaction
	The Reporting Persons from time to time intend to review their investment in the Issuer on the basis of various factors, including the Issuer's business, financial condition, results of operations and prospects, general economic and industry conditions, the securities markets in general and those for the Issuer's Shares in particular, as well as other developments and other investment opportunities. Based upon such review, the Reporting Persons will take such actions in the future as the Reporting Persons may deem appropriate in light of the circumstances existing from time to time. If the Reporting Persons believe that further investment in the Issuer is attractive, whether because of the market price of Shares or otherwise, they may acquire Shares or other securities of the Issuer either in the open market or in privately negotiated transactions. Similarly, depending on market and other factors, the Reporting Persons may determine to dispose of some or all of the Shares currently owned by the Reporting Persons or otherwise acquired by the Reporting Persons either in the open market or in privately negotiated transactions. Except as set forth in this Schedule 13D, the Reporting Persons have not formulated any plans or proposals which relate to or would result in: (a) the acquisition by any person of additional securities of the Issuer or the disposition of securities of the Issuer, (b) an extraordinary corporate transaction, such as a merger, reorganization or liquidation, involving the Issuer or any of its subsidiaries, (c) a sale or transfer of a material amount of the assets of the Issuer or any of its subsidiaries, (d) any change in the present Board of Directors or management of the Issuer, including any plans or proposals to change the number or term of directors or to fill any existing vacancies on the board, (e) any material change in the Issuer's capitalization or dividend policy of the Issuer, (f) any other material change in the Issuer's business or corporate structure, (g) any change in the Issuer's charter or bylaws or other instrument corresponding thereto or other action which may impede the acquisition of control of the Issuer by any person, (h) causing a class of the Issuer's securities to be deregistered or delisted from a national securities exchange or to cease to be authorized to be quoted in an inter-dealer quotation system of a registered national securities association, (i) a class of equity securities of the Issuer becoming eligible for termination of registration pursuant to Section 12(g)(4) of the Act or (j) any action similar to any of those enumerated above.
Item 5.	Interest in Securities of the Issuer
(a)	The following disclosure is based upon 44,700,281 outstanding Shares, as set forth in the Issuer's Rule 424(b)(5) Prospectus filed with the SEC on September 25, 2025. As of the date of this filing, OrbiMed Private Investments VII, LP ("OPI VII"), a limited partnership organized under the laws of Delaware, holds 3,255,000 Shares, constituting approximately 7.3% of the issued and outstanding Shares. GP VII is the general partner of OPI VII, pursuant to the terms of the limited partnership agreement of OPI VII, and OrbiMed Advisors is the managing member of GP VII, pursuant to the terms of the limited liability company agreement of GP VII. As a result, OrbiMed Advisors and GP VII share power to direct the vote and disposition of the Shares held by OPI VII and may be deemed directly or indirectly, including by reason of their mutual affiliation, to be the beneficial owners of the Shares held by OPI VII. OrbiMed Advisors exercises this investment and voting power through a management committee comprised of Carl L. Gordon, Sven H. Borho, and W. Carter Neild, each of whom disclaims beneficial ownership of the Shares held by OPI VII. In addition, OrbiMed Advisors and GP VII, pursuant to their authority under the limited partnership agreement of OPI VII, caused OPI VII to enter into the agreements referred to in Item 6 below. As of the date of this filing, OrbiMed Genesis Master Fund, L.P. ("Genesis"), a limited partnership organized under the laws of the Cayman Islands, holds 747,887 Shares, constituting approximately 1.7% of the issued and outstanding Shares. OrbiMed Genesis is the general partner of Genesis, pursuant to the terms of the limited partnership agreement of Genesis, and OrbiMed Advisors is the managing member of OrbiMed Genesis, pursuant to the terms of the limited liability company agreement of OrbiMed Genesis. As a result, OrbiMed Advisors and OrbiMed Genesis share power to direct the vote and disposition of the Shares held by Genesis and may be deemed, directly or indirectly, including by reason of their mutual affiliation, to be the beneficial owners of the Shares held by Genesis. OrbiMed Advisors exercises this investment and voting power through a management committee comprised of Carl L. Gordon, Sven H. Borho and W. Carter Neild, each of whom disclaims beneficial ownership of the Shares held by Genesis. In addition, OrbiMed Advisors and OrbiMed Genesis, pursuant to their authority under the limited partnership agreement of Genesis, caused Genesis to enter into the agreements referred to in Item 6 below.
(b)	Item 5(a) is incorporated by reference herein.
(c)	Not applicable.
(d)	Not applicable.
(e)	Not applicable.
Item 6.	Contracts, Arrangements, Understandings or Relationships With Respect to Securities of the Issuer
	In addition to the relationships between the Reporting Persons described in Items 2 and 5 above, GP VII is the general partner of OPI VII, pursuant to the terms of the limited partnership agreement of OPI VII. Pursuant to this agreement and relationship, GP VII has discretionary investment management authority with respect to the assets of OPI VII. Such authority includes the power to vote and otherwise dispose of securities held by OPI VII. The number of outstanding Shares of the Issuer attributable to OPI VII is 3,255,000 Shares. GP VII, pursuant to its authority under the limited partnership agreement of OPI VII, may be considered to hold indirectly 3,255,000 Shares. OrbiMed Genesis is the general partner of Genesis, pursuant to the terms of the limited partnership agreement of Genesis. Pursuant to this agreement and relationship, OrbiMed Genesis has discretionary investment management authority with respect to the assets of Genesis. Such authority includes the power to vote and otherwise dispose of securities held by Genesis. The number of outstanding Shares of the Issuer attributable to Genesis is 747,887 Shares. OrbiMed Genesis, pursuant to its authority under the limited partnership agreement of Genesis, may be considered to hold indirectly 747,887 Shares. In addition to the relationships between the Reporting Persons described in Items 2 and 5 above, OrbiMed Advisors is the managing member of GP VII and OrbiMed Genesis, pursuant to the terms of the limited liability company agreements of GP VII and OrbiMed Genesis. Pursuant to these agreements and relationships, OrbiMed Advisors and GP VII have discretionary investment management authority with respect to the assets of OPI VII, and OrbiMed Advisors and OrbiMed Genesis have discretionary investment management authority with respect to the assets of Genesis. Such authority includes the power of GP VII to vote and otherwise dispose of securities held by OPI VII and OrbiMed Genesis to vote and otherwise dispose of securities held by Genesis. The number of outstanding Shares attributable to OPI VII is 3,255,000 Shares, and the number of outstanding Shares attributable to Genesis is 747,887 Shares. OrbiMed Advisors, pursuant to its authority under the terms of the limited liability company agreements of GP VII and Genesis may also be considered to hold indirectly 4,002,887 Shares. Investors' Rights Agreement In addition, OPI VII, Genesis, and certain other stockholders of the Issuer entered into a Second Amended and Restated Investors' Rights Agreement with the Issuer, dated as of August 2, 2204 (the "Investors' Rights Agreement"). Pursuant to the Investors' Rights Agreement and subject to the terms and conditions therein, the parties agreed that: Demand Registration Rights At any time beginning 180 days following the effective date of the registration statement of the IPO, the holders of at least 40% of the Issuer's registrable securities may request that the Issuer file a registration statement under the Securities Act of 1933, as amended (the "Securities Act") on a Form S-1 at the Issuer's expense, subject to certain exceptions, with an anticipated aggregate offering price, net of offering expenses, of not less than $10 million, in which case the Issuer will be required to effect the registration as soon as practicable, and in any event within 60 days after the date of such registration request. The Issuer is not obligated to effect more than one of these registrations. Form S-3 Registration Rights At any time after the Issuer is qualified to file a registration statement on Form S-3 under the Securities Act, and subject to limitations and conditions specified in the Investors' Rights Agreement, holders of at least 20% of the registrable securities under the Investors' Rights Agreement may request that the Issuer prepare and file a registration statement on Form S-3 at the Issuer's expense covering the Shares of such holders having an anticipated aggregate offering price, net of offering expenses, of at least $5 million, in which case the Issuer will be required to effect the registration as soon as practicable, and in any event within 45 days after receiving such request. The Issuer is not obligated to effect more than two of these registrations in any 12-month period. Piggyback Registration Rights Whenever the Issuer proposes to file a registration statement under the Securities Act, other than with respect to certain excluded registrations, OPI VII and Genesis will each be entitled to notice of the registration and have the right, subject to limitations that the underwriters may impose on the number of Shares included in the registration, to include the Shares held by them in the registration. The foregoing description of the Investors' Rights Agreement does not purport to be complete and is qualified in its entirety by reference to the full text of the Investors' Rights Agreement, which is filed as Exhibits 2 and incorporated herein by reference.
Item 7.	Material to be Filed as Exhibits.
	Exhibit Description 1. Joint Filing Agreement among OrbiMed Advisors LLC, OrbiMed Capital GP VII LLC, and OrbiMed Genesis GP LLC. 2. Second Amended and Restated Investors' Rights Agreement among the Issuer and certain of its stockholders, dated August 2, 2024 (incorporated by reference to Exhibit 4.1 to the Issuer's Registration Statement on Form S-1/A (File No. 333-281764) filed with the SEC on September 9, 2024). https://www.sec.gov/Archives/edgar/data/1776111/000119312524206227/d790774dex41.htm

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.

ORBIMED ADVISORS LLC

Signature:	/s/ Carl L. Gordon
Name/Title:	Carl L. Gordon/Member
Date:	09/30/2025

ORBIMED CAPITAL GP VII LLC

Signature:	/s/ Carl L. Gordon
Name/Title:	Carl L. Gordon/Member of OrbiMed Advisors LLC
Date:	09/30/2025

OrbiMed Genesis GP LLC

Signature:	/s/ Carl L. Gordon
Name/Title:	Carl L. Gordon/Member of OrbiMed Advisors LLC
Date:	09/30/2025

FAQ

How many MBX shares do OrbiMed entities report owning?

The reporting persons disclose beneficial ownership of 4,002,887 shares, representing approximately 9.0% of the stated 44,700,281 shares outstanding.

Why did OrbiMed's reported percentage ownership decrease for MBX?

The filing states the reported percentage decreased by more than 1% due to an increase in shares outstanding, not because the reporting persons sold shares.

Do OrbiMed entities have voting or dispositive power over these MBX shares?

Yes. Through their ownership and management relationships, OrbiMed Advisors, GP VII and OrbiMed Genesis share voting and dispositive authority over the shares held by affiliated limited partnerships.

Are there registration rights associated with the OrbiMed-held MBX shares?

Yes. The filing references a Second Amended and Restated Investors' Rights Agreement granting demand registration, Form S-3 rights and piggyback rights subject to the agreement's terms.

Does the filing indicate any plans to change MBX management or pursue a transaction?

No. The reporting persons state they have not formulated any plans relating to changes in the board, management, corporate structure or extraordinary transactions.