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

Rhea-AI Filing Summary

Wellington Management affiliates report beneficial ownership of 2,266,628 shares of MBX Biosciences common stock, shown on the cover pages as 6.8% of the class and noted elsewhere as 6.78%. Four Wellington entities are listed as reporting persons and the schedules show no sole voting or dispositive power, with voting and disposition attributed on a shared basis.

The filing includes an Item 10 certification that the securities are held in the ordinary course of business and were not acquired to change or influence control. The statement indicates these positions are held of record by clients of Wellington investment advisers and names two funds with specified interests.

Positive

• Aggregate ownership of 2,266,628 shares reported, representing a material institutional stake of ~6.8% of MBX common stock.
• Item 10 certification specifies holdings are in the ordinary course of business and not held to influence control, clarifying intent.
• Coordinated disclosure by multiple Wellington entities provides transparent reporting of the group's aggregate position.

Negative

• No sole voting or dispositive power is reported (sole power = 0), indicating Wellington does not control vote or disposition unilaterally.
• Investor-level exposures are largely aggregated; only two specific funds are named, leaving most client-level holders unidentified.

Insights

Financial Analyst

TL;DR: Wellington holds a material passive stake — 2.27M shares (~6.8%) — without sole control, signaling institutional ownership not seeking control.

Wellington Management affiliates report an aggregate of 2,266,628 shares, referenced as 6.8% on cover pages and 6.78% in Item 4(b). The filing discloses 0 shares with sole voting or dispositive power and attributes voting/dispositive authority on a shared basis. For investors, this is a notable institutional position that increases ownership concentration but, per the Item 10 certification, is presented as passive and not intended to influence corporate control.

Corporate Governance Expert

TL;DR: Document is a passive Schedule 13G disclosure; joint reporting and Item 10 language indicate no control intent.

The filing names four Wellington entities as reporting persons and classifies most as holding companies (HC) with one reporting entity as an investment adviser (IA). The certification explicitly states the securities are held in the ordinary course and not for the purpose of changing or influencing control. The combination of shared voting/dispositive power and the 13G filing form supports a governance view that Wellington is a substantial but passive shareholder.

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13G

UNDER THE SECURITIES EXCHANGE ACT OF 1934

(Amendment No. 2)

MBX Biosciences, Inc.

(Name of Issuer)

Common Stock

(Title of Class of Securities)

55287L101

(CUSIP Number)

06/30/2025

(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)

SCHEDULE 13G

CUSIP No. 55287L101

1

Names of Reporting Persons

Wellington Management Group LLP

2

Check the appropriate box if a member of a Group (see instructions)

(a)
(b)

3

Sec Use Only

4

Citizenship or Place of Organization

MASSACHUSETTS

Number of Shares Beneficially Owned by Each Reporting Person With:

5	Sole Voting Power 0.00
6	Shared Voting Power 2,226,590.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 2,266,628.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

2,266,628.00

10

Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)

11

Percent of class represented by amount in row (9)

6.8 %

12

Type of Reporting Person (See Instructions)

HC

SCHEDULE 13G

CUSIP No. 55287L101

1

Names of Reporting Persons

Wellington Group Holdings LLP

2

Check the appropriate box if a member of a Group (see instructions)

(a)
(b)

3

Sec Use Only

4

Citizenship or Place of Organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

5	Sole Voting Power 0.00
6	Shared Voting Power 2,226,590.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 2,266,628.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

2,266,628.00

10

Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)

11

Percent of class represented by amount in row (9)

6.8 %

12

Type of Reporting Person (See Instructions)

HC

SCHEDULE 13G

CUSIP No. 55287L101

1

Names of Reporting Persons

Wellington Investment Advisors Holdings LLP

2

Check the appropriate box if a member of a Group (see instructions)

(a)
(b)

3

Sec Use Only

4

Citizenship or Place of Organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

5	Sole Voting Power 0.00
6	Shared Voting Power 2,226,590.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 2,266,628.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

2,266,628.00

10

Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)

11

Percent of class represented by amount in row (9)

6.8 %

12

Type of Reporting Person (See Instructions)

HC

SCHEDULE 13G

CUSIP No. 55287L101

1

Names of Reporting Persons

Wellington Management Company LLP

2

Check the appropriate box if a member of a Group (see instructions)

(a)
(b)

3

Sec Use Only

4

Citizenship or Place of Organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

5	Sole Voting Power 0.00
6	Shared Voting Power 2,224,487.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 2,264,525.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

2,266,628.00

10

Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)

11

Percent of class represented by amount in row (9)

6.8 %

12

Type of Reporting Person (See Instructions)

IA

SCHEDULE 13G

Item 1.
(a)	Name of issuer: MBX Biosciences, Inc.
(b)	Address of issuer's principal executive offices: 11711 North Meridian Street, Suite 300, Carmel IN 46032
Item 2.
(a)	Name of person filing: Wellington Management Group LLP Wellington Group Holdings LLP Wellington Investment Advisors Holdings LLP Wellington Management Company LLP
(b)	Address or principal business office or, if none, residence: c/o Wellington Management Company LLP, 280 Congress Street, Boston MA 02210
(c)	Citizenship: Wellington Management Group LLP - Massachusetts Wellington Group Holdings LLP - Delaware Wellington Investment Advisors Holdings LLP - Delaware Wellington Management Company LLP - Delaware
(d)	Title of class of securities: Common Stock
(e)	CUSIP No.: 55287L101
Item 3.	If this statement is filed pursuant to §§ 240.13d-1(b) or 240.13d-2(b) or (c), check whether the person filing is a:
(a)	Broker or dealer registered under section 15 of the Act (15 U.S.C. 78o);
(b)	Bank as defined in section 3(a)(6) of the Act (15 U.S.C. 78c);
(c)	Insurance company as defined in section 3(a)(19) of the Act (15 U.S.C. 78c);
(d)	Investment company registered under section 8 of the Investment Company Act of 1940 (15 U.S.C. 80a-8);
(e)	An investment adviser in accordance with § 240.13d-1(b)(1)(ii)(E);
(f)	An employee benefit plan or endowment fund in accordance with § 240.13d-1(b)(1)(ii)(F);
(g)	A parent holding company or control person in accordance with § 240.13d-1(b)(1)(ii)(G);
(h)	A savings associations as defined in Section 3(b) of the Federal Deposit Insurance Act (12 U.S.C. 1813);
(i)	A church plan that is excluded from the definition of an investment company under section 3(c)(14) of the Investment Company Act of 1940 (15 U.S.C. 80a-3);
(j)	A non-U.S. institution in accordance with § 240.13d-1(b)(1)(ii)(J). If filing as a non-U.S. institution in accordance with § 240.13d-1(b)(1)(ii)(J), please specify the type of institution:
(k)	Group, in accordance with Rule 240.13d-1(b)(1)(ii)(K).
Item 4.	Ownership
(a)	Amount beneficially owned: See the responses to Item 9 on the attached cover pages.
(b)	Percent of class: 6.78 %
(c)	Number of shares as to which the person has:
	(i) Sole power to vote or to direct the vote: 0
	(ii) Shared power to vote or to direct the vote: See the responses to Item 6 on the attached cover pages.
	(iii) Sole power to dispose or to direct the disposition of: 0
	(iv) Shared power to dispose or to direct the disposition of: See the responses to Item 8 on the attached cover pages.
Item 5.	Ownership of 5 Percent or Less of a Class.
Item 6.	Ownership of more than 5 Percent on Behalf of Another Person.
	If any other person is known to have the right to receive or the power to direct the receipt of dividends from, or the proceeds from the sale of, such securities, a statement to that effect should be included in response to this item and, if such interest relates to more than 5 percent of the class, such person should be identified. A listing of the shareholders of an investment company registered under the Investment Company Act of 1940 or the beneficiaries of employee benefit plan, pension fund or endowment fund is not required. The securities as to which this Schedule is filed are owned of record by clients of one or more investment advisers identified in Item 7 directly or indirectly owned by Wellington Management Group LLP. Those clients have the right to receive, or the power to direct the receipt of, dividends from, or the proceeds from the sale of, such securities. No such client is known to have such right or power with respect to more than five percent of this class of securities, except as follows: Wellington Biomedical Innovation Master Investors (Cayman) II L.P. Wellington Biomedical Innovation Master Investors (Cayman) II, L.P.
Item 7.	Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on by the Parent Holding Company or Control Person.
	If a parent holding company has filed this schedule, pursuant to Rule 13d-1(b)(ii)(G), so indicate under Item 3(g) and attach an exhibit stating the identity and the Item 3 classification of the relevant subsidiary. If a parent holding company has filed this schedule pursuant to Rule 13d-1(c) or Rule 13d-1(d), attach an exhibit stating the identification of the relevant subsidiary. Pursuant to the instructions in Item 7 of Schedule 13G, the following lists the identity and Item 3 classification of each relevant entity that beneficially owns shares of the security class being reported on this Schedule 13G. Wellington Group Holdings LLP - HC Wellington Investment Advisors LLP - HC Wellington Management Global Holdings, Ltd. - HC One or more of the following investment advisers (the "Wellington Investment Advisers"): Wellington Management Company LLP - IA Wellington Management Canada LLC - IA Wellington Management Singapore Pte Ltd - IA Wellington Management Hong Kong Ltd - IA Wellington Management International Ltd - IA Wellington Management Japan Pte Ltd - IA Wellington Management Australia Pty Ltd - IA The securities as to which this Schedule is filed by Wellington Management Group LLP, as parent holding company of certain holding companies and the Wellington Investment Advisers, are owned of record by clients of the Wellington Investment Advisers. Wellington Investment Advisors Holdings LLP controls directly, or indirectly through Wellington Management Global Holdings, Ltd., the Wellington Investment Advisers. Wellington Investment Advisors Holdings LLP is owned by Wellington Group Holdings LLP. Wellington Group Holdings LLP is owned by Wellington Management Group LLP.
Item 8.	Identification and Classification of Members of the Group.
	Not Applicable
Item 9.	Notice of Dissolution of Group.
	Not Applicable

Item 10.

Certifications:

By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were acquired and are held in the ordinary course of business and 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, other than activities solely in connection with a nomination under ?? 240.14a-11.

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.

Wellington Management Group LLP

Signature:	Taisia Lowe
Name/Title:	Regulatory Analyst
Date:	08/12/2025

Wellington Group Holdings LLP

Signature:	Taisia Lowe
Name/Title:	Regulatory Analyst
Date:	08/12/2025

Wellington Investment Advisors Holdings LLP

Signature:	Taisia Lowe
Name/Title:	Regulatory Analyst
Date:	08/12/2025

Wellington Management Company LLP

Signature:	Taisia Lowe
Name/Title:	Regulatory Analyst
Date:	08/12/2025

FAQ

How many MBX (MBX) shares does Wellington Management report owning?

The aggregate beneficial ownership reported is 2,266,628 shares.

What percentage of MBX does Wellington's stake represent?

The filing shows the stake as 6.8% on the cover pages and as 6.78% in Item 4(b).

Does Wellington have sole voting or dispositive power over these MBX shares?

No. The filing reports 0 shares with sole voting power and 0 with sole dispositive power; voting and dispositive powers are reported as shared.

Which Wellington entities filed the Schedule 13G for MBX?

The reporting persons are Wellington Management Group LLP, Wellington Group Holdings LLP, Wellington Investment Advisors Holdings LLP, and Wellington Management Company LLP.

Is Wellington indicating intent to influence MBX's control?

No. The Item 10 certification states the securities are held in the ordinary course of business and were not acquired to change or influence control.

Are the shares held on behalf of clients or specific funds?

Yes. The securities are owned of record by clients of Wellington investment advisers; two named funds are Wellington Biomedical Innovation Master Investors (Cayman) II L.P. and the similarly named limited partner.