Wellington Management reports ownership of 10.79% of MBX Biosciences, Inc. common stock as of 09/30/2025. The holding is recorded across four related Wellington entities with an aggregate beneficial ownership of 4,823,336 shares, of which shared voting power is reported at 4,558,297 and shared dispositive power at 4,823,336. The filing states these securities are owned of record by clients of Wellington investment advisers and were acquired and are held in the ordinary course of business, not for the purpose of changing or influencing control. The Schedule 13G/A identifies the organizational relationships among the reporting entities and lists Wellington Management Company LLP and several regional Wellington advisers as relevant investment-adviser entities.
Positive
Reported beneficial ownership of 10.79% signals a material passive stake
Full disclosure of shared voting (4,558,297) and dispositive power (4,823,336) across Wellington entities
Negative
None.
Insights
Wellington holds a meaningful passive stake in MBX at 10.79%.
The stake of 4,823,336 shares represents a sizeable position for a single investment manager and exceeds the 5% Schedule 13G reporting threshold, which can affect liquidity and shareholder composition.
The filing classifies the position as held in the ordinary course and not intended to influence control; investors should monitor subsequent filings for changes in percent ownership or voting power, especially around quarterly reporting windows.
Filing complies with Schedule 13G disclosure and identifies affiliated reporting entities.
The document names four Wellington entities, states citizenship/organization jurisdictions, and discloses shared voting and dispositive powers, which meets Item disclosure requirements.
Watch for any future amendments that would switch to Schedule 13D if intent or control objectives change; a conversion-triggering event would typically appear in an amended filing within required SEC timeframes.
FAQ
AI-generated questions and answers. How Rhea-AI works. Not financial advice.
What stake does Wellington Management report in MBX (MBX)?
Wellington reports aggregate beneficial ownership of 4,823,336 shares, equal to 10.79% of MBX common stock.
Is Wellington seeking to influence control of MBX according to the filing?
No. The filing certifies the securities are held in the ordinary course of business and were not acquired to change or influence control.
Which Wellington entities are named as reporting persons on the Schedule 13G/A?
The filing names Wellington Management Group LLP, Wellington Group Holdings LLP, Wellington Investment Advisors Holdings LLP, and Wellington Management Company LLP.
What voting and dispositive powers are disclosed?
Shared voting power is reported as 4,558,297 shares and shared dispositive power as 4,823,336 shares.
As of what date does the Schedule 13G/A report MBX ownership?
The ownership is reported as of 09/30/2025 and the filing is signed on 10/07/2025.
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549
SCHEDULE 13G
UNDER THE SECURITIES EXCHANGE ACT OF 1934
(Amendment No. 3)
MBX Biosciences, Inc.
(Name of Issuer)
Common Stock
(Title of Class of Securities)
55287L101
(CUSIP Number)
09/30/2025
(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
4,558,297.00
7
Sole Dispositive Power
0.00
8
Shared Dispositive Power
4,823,336.00
9
Aggregate Amount Beneficially Owned by Each Reporting Person
4,823,336.00
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)
10.8 %
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
4,558,297.00
7
Sole Dispositive Power
0.00
8
Shared Dispositive Power
4,823,336.00
9
Aggregate Amount Beneficially Owned by Each Reporting Person
4,823,336.00
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)
10.8 %
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
4,558,297.00
7
Sole Dispositive Power
0.00
8
Shared Dispositive Power
4,823,336.00
9
Aggregate Amount Beneficially Owned by Each Reporting Person
4,823,336.00
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)
10.8 %
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
4,556,077.00
7
Sole Dispositive Power
0.00
8
Shared Dispositive Power
4,722,906.00
9
Aggregate Amount Beneficially Owned by Each Reporting Person
4,823,336.00
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)
10.6 %
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:
10.79 %
(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:
Not Applicable.
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.
