SC 13G 1 SEC13G_Filing.htm SEC SCHEDULE 13G

UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13G

Under the Securities Exchange Act of 1934

(Amendment No. _ )^*

MBX Biosciences, Inc.

(Name of Issuer)

Common Stock

(Title of Class of Securities)

55287L101

(CUSIP Number)

September 13, 2024

(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)
[X]	Rule 13d-1(c)
[ ]	Rule 13d-1(d)

^* The remainder of this cover page shall be filled out for a reporting person's initial filing on this form with respect to the subject class of securities, and for any subsequent amendment containing information which would alter the disclosures provided in a prior cover page.

The information required in 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 (the "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.)

CUSIP No.

55287L101

1.	NAMES OF REPORTING PERSONS Wellington Biomedical Innovation Master Investors (Cayman) II, L.P.
2.	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (a) [ ] (b) [ ]
3.	SEC USE ONLY
4.	CITIZENSHIP OR PLACE OF ORGANIZATION
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH		5. SOLE VOTING POWER	0
		6. SHARED VOTING POWER	1,790,122
		7. SOLE DISPOSITIVE POWER	0
		8. SHARED DISPOSITIVE POWER	1,790,122
9.	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 1,790,122
10.	CHECK IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES [ ]
11.	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 5.62%
12.	TYPE OF REPORTING PERSON PN

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
Item 2.
	(a)	Name of Person Filing Wellington Biomedical Innovation Master Investors (Cayman) II, L.P.
	(b)	Address of Principal Business Office or, if None, Residence c/o Wellington Management Company LLP 280 Congress Street Boston, MA 02210
	(c)	Citizenship
	(d)	Title of Class of Securities Common Stock
	(e)	CUSIP Number 55287L101
Item 3.	If This Statement is Filed Pursuant to Rule 13d-1(b), or 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 Rule 240.13d-1(b)(1)(ii)(E);
	(f)	[ ]	An employee benefit plan or endowment fund in accordance with Rule 240.13d-1(b)(1)(ii)(F);
	(g)	[ ]	A parent holding company or control person in accordance with Rule 240.13d-1(b)(1)(ii)(G);
	(h)	[ ]	A savings association 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)	[ ]	Group, in accordance with Rule 240.13d-1(b)(1)(ii)(J).
		If this statement is filed pursuant to Rule 13d-1(c), check this box [X]
Item 4.	Ownership.
Provide the following information regarding the aggregate number and percentage of the class of securities of the issuer identified in Item 1.
	(a)	Amount Beneficially Owned:
		1,790,122
	(b)	Percent of Class:
		5.62%
	(c)	Number of shares as to which such person has:
		(i)	sole power to vote or to direct the vote	0
		(ii)	shared power to vote or to direct the vote	1,790,122
		(iii)	sole power to dispose or to direct the disposition of	0
		(iv)	shared power to dispose or to direct the disposition of	1,790,122
Item 5.	Ownership of Five Percent or Less of Class.
	If this statement is being filed to report the fact that as of the date hereof the reporting person has ceased to be the beneficial owner of more than five percent of the class of securities, check the following: [ ]
Item 6.	Ownership of More than Five Percent on Behalf of Another Person.
	Not Applicable
Item 7.	Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on by the Parent Holding Company.
	Not Applicable.
Item 8.	Identification and Classification of Members of the Group. Not Applicable.
Item 9.	Notice of Dissolution of Group. Not Applicable.
Item 10.	Certification. By signing below I certify that, to the best of my knowledge and belief, the securities referred to above 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.

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.

	By: Wellington Biomedical Innovation Master Investors (Cayman) II, L.P.

	By: Wellington Alternative Investments LLC Its General Partner
	By: /s/ Matthew N. Shea
	Name: Matthew N. Shea
	Title: Authorized Person
	Date: September 20,2024

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549

SCHEDULE 13G

Under the Securities Exchange Act of 1934

(Amendment No. _ )*

SIGNATURE

UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

(Amendment No. _ )^*

SEC Form SC 13G filed by MBX Biosciences Inc.