Frazier Funds Report 11.7% Stake in MBX Biosciences (5.22M Shares)

Frazier-affiliated investment vehicles disclosed their holdings in MBX Biosciences, Inc. Frazier Life Sciences X, L.P. directly holds 5,219,440 shares of common stock, representing 11.7% of the class based on the filing's share-count assumptions.

Accepted by SEC EDGAR 09/30/2025 - 05:01 PM EDT Learn about SEC filing dates

Rhea-AI Filing Summary

Frazier-affiliated investment vehicles disclosed their holdings in MBX Biosciences, Inc. Frazier Life Sciences X, L.P. directly holds 5,219,440 shares of common stock, representing 11.7% of the class based on the filing's share-count assumptions. Frazier Life Sciences Public Fund, L.P. holds 1,103,920 shares (2.5%), and Frazier Life Sciences Public Overage Fund, L.P. holds 325,653 shares (0.7%). The filing details pre-IPO purchases of preferred stock that converted at a 12.0221-to-1 ratio at IPO, IPO purchases at $16.00 per share, and a post-IPO purchase on September 26, 2025 by FLS X of 666,666 shares at $18.00 per share. The filing clarifies voting and dispositive power allocations among related entities and identifies exhibits, including an investors' rights agreement and a transactions exhibit.

Positive

• Material ownership disclosed: Frazier Life Sciences X, L.P. holds 5,219,440 shares (11.7%), a clearly quantified, material stake.
• Detailed funding and conversion disclosure: Pre-IPO preferred purchases, the 12.0221-to-1 conversion ratio, IPO purchases at $16.00, and a subsequent $18.00 purchase on September 26, 2025 are itemized.
• Governance transparency: The filing specifies which entities and individuals share voting and dispositive power, and attaches an investors' rights agreement as an exhibit.

Negative

• None.

Insights

Financial Analyst

TL;DR: Frazier-affiliated funds hold a material equity stake (11.7%) via Frazier Life Sciences X, reflecting concentrated insider-aligned ownership.

The filing documents that Frazier Life Sciences X, L.P. owns 5,219,440 shares (11.7%), acquired through conversion of preferred shares and additional IPO and secondary purchases, including a 666,666-share purchase at $18.00 on September 26, 2025. The disclosure quantifies other affiliated holdings (1,103,920 shares and 325,653 shares) and explains the 12.0221-to-1 conversion used to translate pre-IPO preferred stakes into common stock. For investors, the material stake size is important because it represents a significant ownership block explicitly disclosed in the Schedule 13D.

Corporate Governance Expert

TL;DR: Reporting clarifies voting and dispositive power across related Frazier entities and names individuals who share control.

The statement explains governance relationships: FHMLS X, L.L.C.'s members (Patrick J. Heron and James N. Topper) share voting and investment power over the FLS X shares. Several funds have general partners and investment committees described, with attribution rules noted. The filing also includes executed signatures and references to an amended investors' rights agreement. These disclosures improve transparency about who holds control and how voting influence is structured among the reporting persons.

FAQ

AI-generated questions and answers. How Rhea-AI works. Not financial advice.

How many MBX shares does Frazier Life Sciences X, L.P. own and what percent is that of the class?

Frazier Life Sciences X, L.P. directly holds 5,219,440 shares, which the filing states represents 11.7% of MBX's outstanding common stock under the filing's assumptions.

What conversion ratio converted Frazier's pre-IPO preferred shares into MBX common stock?

The filing states a conversion on a 12.0221-to-1 basis for Series A, B and C preferred shares into common stock at the IPO.

Did any Frazier entity purchase MBX shares after the IPO?

Yes. The filing discloses that on September 26, 2025 Frazier Life Sciences X purchased 666,666 shares at $18.00 per share.

What other Frazier-affiliated holdings in MBX are disclosed?

Frazier Life Sciences Public Fund, L.P. holds 1,103,920 shares (2.5%) and Frazier Life Sciences Public Overage Fund, L.P. holds 325,653 shares (0.7%) as reported in the filing.

Who among the named individuals shares voting power over the largest Frazier stake?

The filing states that Patrick J. Heron and James N. Topper are the members of FHMLS X, L.L.C. and therefore share voting and investment power over the FLS X shares.

AI-generated analysis. How Rhea-AI works. Not financial advice.

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SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. 2)

MBX BIOSCIENCES, INC.

(Name of Issuer)

Common Stock, $0.0001 par value per share

(Title of Class of Securities)

55287L101

(CUSIP Number)

Steve R. Bailey
601 Union Street, Suite 3200,
Seattle, WA, 98101
(206) 621-7200

(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

Frazier Life Sciences Public Fund, 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)

WC

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 1,103,920.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 1,103,920.00

11

Aggregate amount beneficially owned by each reporting person

1,103,920.00

12

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

13

Percent of class represented by amount in Row (11)

2.5 %

14

Type of Reporting Person (See Instructions)

PN

Comment for Type of Reporting Person:

The Percent of Class is calculated based on (i) 33,593,866 shares of the Issuer's Common Stock outstanding as of August 4, 2025, as reported in the Issuer's Quarterly Report on Form 10-Q, filed with the SEC on August 7, 2025 and (ii) 11,108,055 shares of the Issuer's Common Stock that were sold on September 26, 2025, as reported in the Issuer's Prospectus dated September 24, 2025, filed with the SEC on September 25, 2025.

SCHEDULE 13D

CUSIP No. 55287L101

1

Name of reporting person

FHMLSP, 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 1,103,920.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 1,103,920.00

11

Aggregate amount beneficially owned by each reporting person

1,103,920.00

12

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

13

Percent of class represented by amount in Row (11)

2.5 %

14

Type of Reporting Person (See Instructions)

PN

Comment for Type of Reporting Person:

The Percent of Class is calculated based on (i) 33,593,866 shares of the Issuer's Common Stock outstanding as of August 4, 2025, as reported in the Issuer's Quarterly Report on Form 10-Q, filed with the SEC on August 7, 2025 and (ii) 11,108,055 shares of the Issuer's Common Stock that were sold on September 26, 2025, as reported in the Issuer's Prospectus dated September 24, 2025, filed with the SEC on September 25, 2025.

SCHEDULE 13D

CUSIP No. 55287L101

1

Name of reporting person

FHMLSP, L.L.C.

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 1,103,920.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 1,103,920.00

11

Aggregate amount beneficially owned by each reporting person

1,103,920.00

12

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

13

Percent of class represented by amount in Row (11)

2.5 %

14

Type of Reporting Person (See Instructions)

OO

Comment for Type of Reporting Person:

The Percent of Class is calculated based on (i) 33,593,866 shares of the Issuer's Common Stock outstanding as of August 4, 2025, as reported in the Issuer's Quarterly Report on Form 10-Q, filed with the SEC on August 7, 2025 and (ii) 11,108,055 shares of the Issuer's Common Stock that were sold on September 26, 2025, as reported in the Issuer's Prospectus dated September 24, 2025, filed with the SEC on September 25, 2025.

SCHEDULE 13D

CUSIP No. 55287L101

1

Name of reporting person

Frazier Life Sciences Public Overage Fund, 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)

WC

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 325,653.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 325,653.00

11

Aggregate amount beneficially owned by each reporting person

325,653.00

12

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

13

Percent of class represented by amount in Row (11)

0.7 %

14

Type of Reporting Person (See Instructions)

PN

Comment for Type of Reporting Person:

The Percent of Class is calculated based on (i) 33,593,866 shares of the Issuer's Common Stock outstanding as of August 4, 2025, as reported in the Issuer's Quarterly Report on Form 10-Q, filed with the SEC on August 7, 2025 and (ii) 11,108,055 shares of the Issuer's Common Stock that were sold on September 26, 2025, as reported in the Issuer's Prospectus dated September 24, 2025, filed with the SEC on September 25, 2025.

SCHEDULE 13D

CUSIP No. 55287L101

1

Name of reporting person

FHMLSP Overage, 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 325,653.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 325,653.00

11

Aggregate amount beneficially owned by each reporting person

325,653.00

12

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

13

Percent of class represented by amount in Row (11)

0.7 %

14

Type of Reporting Person (See Instructions)

PN

Comment for Type of Reporting Person:

The Percent of Class is calculated based on (i) 33,593,866 shares of the Issuer's Common Stock outstanding as of August 4, 2025, as reported in the Issuer's Quarterly Report on Form 10-Q, filed with the SEC on August 7, 2025 and (ii) 11,108,055 shares of the Issuer's Common Stock that were sold on September 26, 2025, as reported in the Issuer's Prospectus dated September 24, 2025, filed with the SEC on September 25, 2025.

SCHEDULE 13D

CUSIP No. 55287L101

1

Name of reporting person

FHMLSP Overage, L.L.C.

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 325,653.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 325,653.00

11

Aggregate amount beneficially owned by each reporting person

325,653.00

12

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

13

Percent of class represented by amount in Row (11)

0.7 %

14

Type of Reporting Person (See Instructions)

OO

Comment for Type of Reporting Person:

The Percent of Class is calculated based on (i) 33,593,866 shares of the Issuer's Common Stock outstanding as of August 4, 2025, as reported in the Issuer's Quarterly Report on Form 10-Q, filed with the SEC on August 7, 2025 and (ii) 11,108,055 shares of the Issuer's Common Stock that were sold on September 26, 2025, as reported in the Issuer's Prospectus dated September 24, 2025, filed with the SEC on September 25, 2025.

SCHEDULE 13D

CUSIP No. 55287L101

1

Name of reporting person

Frazier Life Sciences X, 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)

WC

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 5,219,440.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 5,219,440.00

11

Aggregate amount beneficially owned by each reporting person

5,219,440.00

12

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

13

Percent of class represented by amount in Row (11)

11.7 %

14

Type of Reporting Person (See Instructions)

PN

Comment for Type of Reporting Person:

The Percent of Class is calculated based on (i) 33,593,866 shares of the Issuer's Common Stock outstanding as of August 4, 2025, as reported in the Issuer's Quarterly Report on Form 10-Q, filed with the SEC on August 7, 2025 and (ii) 11,108,055 shares of the Issuer's Common Stock that were sold on September 26, 2025, as reported in the Issuer's Prospectus dated September 24, 2025, filed with the SEC on September 25, 2025.

SCHEDULE 13D

CUSIP No. 55287L101

1

Name of reporting person

FHMLS X, 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 5,219,440.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 5,219,440.00

11

Aggregate amount beneficially owned by each reporting person

5,219,440.00

12

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

13

Percent of class represented by amount in Row (11)

11.7 %

14

Type of Reporting Person (See Instructions)

PN

Comment for Type of Reporting Person:

The Percent of Class is calculated based on (i) 33,593,866 shares of the Issuer's Common Stock outstanding as of August 4, 2025, as reported in the Issuer's Quarterly Report on Form 10-Q, filed with the SEC on August 7, 2025 and (ii) 11,108,055 shares of the Issuer's Common Stock that were sold on September 26, 2025, as reported in the Issuer's Prospectus dated September 24, 2025, filed with the SEC on September 25, 2025.

SCHEDULE 13D

CUSIP No. 55287L101

1

Name of reporting person

FHMLS X, L.L.C.

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 5,219,440.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 5,219,440.00

11

Aggregate amount beneficially owned by each reporting person

5,219,440.00

12

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

13

Percent of class represented by amount in Row (11)

11.7 %

14

Type of Reporting Person (See Instructions)

OO

Comment for Type of Reporting Person:

The Percent of Class is calculated based on (i) 33,593,866 shares of the Issuer's Common Stock outstanding as of August 4, 2025, as reported in the Issuer's Quarterly Report on Form 10-Q, filed with the SEC on August 7, 2025 and (ii) 11,108,055 shares of the Issuer's Common Stock that were sold on September 26, 2025, as reported in the Issuer's Prospectus dated September 24, 2025, filed with the SEC on September 25, 2025.

SCHEDULE 13D

CUSIP No. 55287L101

1

Name of reporting person

Frazier Life Sciences XI, 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)

WC

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

11

Aggregate amount beneficially owned by each reporting person

3,000.00

12

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

13

Percent of class represented by amount in Row (11)

0.0 %

14

Type of Reporting Person (See Instructions)

PN

Comment for Type of Reporting Person:

The Percent of Class is calculated based on (i) 33,593,866 shares of the Issuer's Common Stock outstanding as of August 4, 2025, as reported in the Issuer's Quarterly Report on Form 10-Q, filed with the SEC on August 7, 2025 and (ii) 11,108,055 shares of the Issuer's Common Stock that were sold on September 26, 2025, as reported in the Issuer's Prospectus dated September 24, 2025, filed with the SEC on September 25, 2025.

SCHEDULE 13D

CUSIP No. 55287L101

1

Name of reporting person

FHMLS XI, 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 3,000.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 3,000.00

11

Aggregate amount beneficially owned by each reporting person

3,000.00

12

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

13

Percent of class represented by amount in Row (11)

0.0 %

14

Type of Reporting Person (See Instructions)

PN

Comment for Type of Reporting Person:

The Percent of Class is calculated based on (i) 33,593,866 shares of the Issuer's Common Stock outstanding as of August 4, 2025, as reported in the Issuer's Quarterly Report on Form 10-Q, filed with the SEC on August 7, 2025 and (ii) 11,108,055 shares of the Issuer's Common Stock that were sold on September 26, 2025, as reported in the Issuer's Prospectus dated September 24, 2025, filed with the SEC on September 25, 2025.

SCHEDULE 13D

CUSIP No. 55287L101

1

Name of reporting person

FHMLS XI, L.L.C.

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

11

Aggregate amount beneficially owned by each reporting person

3,000.00

12

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

13

Percent of class represented by amount in Row (11)

0.0 %

14

Type of Reporting Person (See Instructions)

OO

Comment for Type of Reporting Person:

The Percent of Class is calculated based on (i) 33,593,866 shares of the Issuer's Common Stock outstanding as of August 4, 2025, as reported in the Issuer's Quarterly Report on Form 10-Q, filed with the SEC on August 7, 2025 and (ii) 11,108,055 shares of the Issuer's Common Stock that were sold on September 26, 2025, as reported in the Issuer's Prospectus dated September 24, 2025, filed with the SEC on September 25, 2025.

SCHEDULE 13D

CUSIP No. 55287L101

1

Name of reporting person

James N. Topper

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 5,219,440.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 5,219,440.00

11

Aggregate amount beneficially owned by each reporting person

5,219,440.00

12

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

13

Percent of class represented by amount in Row (11)

11.7 %

14

Type of Reporting Person (See Instructions)

IN

Comment for Type of Reporting Person:

The Aggregate Amount represents 5,219,440 shares of Common Stock held directly by Frazier Life Sciences X, L.P. The Percent of Class is calculated based on (i) 33,593,866 shares of the Issuer's Common Stock outstanding as of August 4, 2025, as reported in the Issuer's Quarterly Report on Form 10-Q, filed with the SEC on August 7, 2025 and (ii) 11,108,055 shares of the Issuer's Common Stock that were sold on September 26, 2025, as reported in the Issuer's Prospectus dated September 24, 2025, filed with the SEC on September 25, 2025.

SCHEDULE 13D

CUSIP No. 55287L101

1

Name of reporting person

Patrick J. Heron

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 4,655.00
8	Shared Voting Power 5,219,440.00
9	Sole Dispositive Power 4,655.00
10	Shared Dispositive Power 5,219,440.00

11

Aggregate amount beneficially owned by each reporting person

5,224,095.00

12

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

13

Percent of class represented by amount in Row (11)

11.7 %

14

Type of Reporting Person (See Instructions)

IN

Comment for Type of Reporting Person:

The Aggregate Amount represents (i) 4,655 shares of Common Stock that are issuable upon the exercise of options held directly by Patrick J. Heron, and (ii) 5,219,440 shares of Common Stock held directly by Frazier Life Sciences X, L.P. The Percent of Class is calculated based on (i) 33,593,866 shares of the Issuer's Common Stock outstanding as of August 4, 2025, as reported in the Issuer's Quarterly Report on Form 10-Q, filed with the SEC on August 7, 2025, (ii) 11,108,055 shares of the Issuer's Common Stock that were sold on September 26, 2025, as reported in the Issuer's Prospectus dated September 24, 2025, filed with the SEC on September 25, 2025.and (iii) 4,655 shares of Common Stock that are issuable upon the exercise of options held directly by Patrick J. Heron

SCHEDULE 13D

Item 1.	Security and Issuer
(a)	Title of Class of Securities: Common Stock, $0.0001 par value per share
(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. 2 ("Amendment No. 2") to Schedule 13D amends the statement on Schedule 13D filed on September 23, 2024 (the "Original Schedule 13D") as amended on August 20, 2025 (the "Prior Amendment", and together with the Original Schedule 13D and this Amendment No. 2, the "Schedule 13D"). Except as otherwise specified in this Amendment No. 2, all items in the Original Schedule 13D, as amended by the Prior Amendment, are unchanged. All capitalized terms used in this Amendment No. 2 and not otherwise defined herein have the meanings ascribed to such terms in the Original Schedule 13D, as amended by the Prior Amendment.
Item 2.	Identity and Background
(a)	The entities and persons filing this statement (collectively, the "Reporting Persons") are: Frazier Life Sciences Public Fund, L.P. ("FLSPF") FHMLSP, L.P. FHMLSP, L.L.C. Frazier Life Sciences Public Overage Fund, L.P. ("FLSPOF") FHMLSP Overage, L.P. FHMLSP Overage, L.L.C. Frazier Life Sciences X, L.P. ("FLS X") FHMLS X, L.P. FHMLS X, L.L.C. Frazier Life Sciences XI, L.P. ("FLS XI") FHMLS XI, L.P. FHMLS XI, L.L.C. James N. Topper ("Topper") Patrick J. Heron ("Heron" and together with Topper, the "Members")
Item 3.	Source and Amount of Funds or Other Consideration
	Item 3 of the Schedule 13D is hereby amended and restated as follows: Prior to the Issuer's initial public offering (the "IPO"), in a series of private transactions, (i) FLS X purchased from the Issuer 18,922,852 shares of Series A Preferred Stock ("Series A Stock") for an aggregate approximate purchase price of $13,000,000 and 28,297,265 shares of Series B Preferred Stock ("Series B Stock") for an aggregate approximate purchase price of $25,065,525, (ii) FLSPF purchased from the Issuer 7,497,087 shares of Series C Preferred Stock ("Series C Stock"), for an aggregate approximate purchase price of $7,722,000, and (iii) FLSPOF purchased from the Issuer 2,211,650 shares of Series C Stock for an aggregate approximate purchase price of $2,278,000. Upon closing of the IPO, the shares of Series A Stock, Series B Stock and Series C Stock held by FLS X, FLSPF and FLSPOF automatically converted into shares of Common Stock of the Issuer on a 12.0221-to-1 basis (the "Conversion), resulting in (i) FLS X holding a total of 3,927,774 shares of Common Stock, (ii) FLSPF holding a total of 623,608 shares of Common Stock, and (iii) FLSPOF holding a total of 183,965 shares of Common Stock, in each case on an as-converted basis following the Conversion. In addition, at the time of the IPO, (i) FLS X purchased an aggregate of 625,000 shares of Common Stock of the Issuer at the IPO price of $16.00 per share, (ii) FLSPF purchased an aggregate of 480,312 shares of Common Stock of the Issuer at the IPO price of $16.00 per share, (iii) FLSPOF purchased an aggregate of 141,688 shares of Common Stock of the Issuer at the IPO price of $16.00 per share and (iv) FLS XI purchased an aggregate of 3,000 shares of Common Stock of the Issuer at the IPO price of $16.00 per share (items (i)-(iv) together, the "IPO Purchases"). On September 26, 2025, FLS X purchased an aggregate of 666,666 shares of Common Stock of the Issuer at the price of $18.00 per share The working capital of FLSPF, FLSPOF, FLS X and FLS XI was the source of the funds for the purchase of the FLSPF Shares, the FLSPOF Shares, the FLS X Shares and the FLS XI Shares. No part of the purchase price of the FLSPF Shares, the FLSPOF Shares, the FLS X Shares or the FLS XI Shares was represented by funds or other consideration borrowed or otherwise obtained for the purpose of acquiring, holding, trading or voting the FLSPF Shares, the FLSPOF Shares, the FLS X Shares or the FLS XI Shares.
Item 5.	Interest in Securities of the Issuer
(a)	The information contained in Rows 7, 8, 9, 10, 11, and 13 of each Reporting Person's cover page to this Schedule 13D (including the footnotes thereto) is incorporated by reference into this Item 5. FLSPF directly holds 1,103,920 shares of the Issuer's Common Stock (the "FLSPF Shares"). FHMLSP, L.P. is the general partner of FLSPF and the general partner of FHMLSP, L.P. is FHMLSP, L.L.C., which is managed by an investment committee of four that acts by majority vote. Accordingly, no members of such committee are attributed beneficial ownership of the securities directly held by FLSPF. FLSPOF directly holds 325,653 shares of the Issuer's Common Stock (the "FLSPOF Shares"). FHMLSP Overage, L.P. is the general partner of FLSPOF and the general partner of FHMLSP Overage, L.P. is FHMLSP Overage, L.L.C., which is managed by an investment committee of four that acts by majority vote. Accordingly, no members of such committee are attributed beneficial ownership of the securities directly held by FLSPOF. FLS X directly holds 5,219,440 shares of the Issuer's Common Stock (the "FLS X Shares"). FHMLS X, L.P. is the general partner of FLS X and FHMLS X, L.L.C. is the general partner of FHMLS X, L.P. Heron and Topper are the members of FHMLS X, L.L.C. and therefore share voting and investment power over the FLS X Shares. FLS XI directly holds 3,000 shares of the Issuer's Common Stock (the "FLS XI Shares"). FHMLS XI, L.P. is the general partner of FLS XI and the general partner of FHMLS XI, L.P. is FHMLS XI, L.L.C., which is managed by an investment committee of three that acts by majority vote. Accordingly, no members of such committee are attributed beneficial ownership of the securities directly held by FLS XI. Except as specifically stated herein, the filing of this Schedule 13D shall not be construed as an admission that any Reporting Person or any of the foregoing is, for the purposes of Section 13(d) and/or Section 13(g) of the Act or otherwise, the beneficial owner of any securities covered by this Schedule 13D or a member of a "group" with any other person.
(b)	The information contained in Rows 7, 8, 9, 10, 11, and 13 of each Reporting Person's cover page to this Schedule 13D (including the footnotes thereto) is incorporated by reference into this Item 5.
(c)	Except as set forth on Exhibit 10.2 of this Schedule 13D, none of the Reporting Persons has effected any transactions relating to the shares of Common Stock during the past 60 days.
Item 7.	Material to be Filed as Exhibits.
	Exhibit 10.1 Second Amended and Restated Investors' Rights Agreement (incorporated by reference to Exhibit 4.1 to the Issuer's Registration Statement on Form S-1 filed with the Commission on August 23, 2024) Exhibit 10.2 Transactions Relating to the Ordinary Shares of the Issuer During the Past Sixty (60) Days Exhibit 99.1 Joint Filing Agreement

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.

Frazier Life Sciences Public Fund, L.P.

Signature:	/s/ Steve R. Bailey
Name/Title:	By Steve R. Bailey, CFO of FHMLSP, L.L.C., GP of FHMLSP, L.P., GP of Frazier Life Sciences Public Fund, L.P.
Date:	09/30/2025

FHMLSP, L.P.

Signature:	/s/ Steve R. Bailey
Name/Title:	By Steve R. Bailey, CFO of FHMLSP, L.L.C., GP of FHMLSP, L.P.
Date:	09/30/2025

FHMLSP, L.L.C.

Signature:	/s/ Steve R. Bailey
Name/Title:	By Steve R. Bailey, CFO of FHMLSP, L.L.C.
Date:	09/30/2025

Frazier Life Sciences Public Overage Fund, L.P.

Signature:	/s/ Steve R. Bailey
Name/Title:	By Steve R. Bailey, CFO of FHMLSP Overage, L.L.C., GP of FHMLSP Overage, L.P., GP of Frazier Life Sciences Public Overage Fund, L.P.
Date:	09/30/2025

FHMLSP Overage, L.P.

Signature:	/s/ Steve R. Bailey
Name/Title:	By Steve R. Bailey, CFO of FHMLSP Overage, L.L.C., GP of FHMLSP Overage, L.P.
Date:	09/30/2025

FHMLSP Overage, L.L.C.

Signature:	/s/ Steve R. Bailey
Name/Title:	By Steve R. Bailey, CFO of FHMLSP Overage, L.L.C.
Date:	09/30/2025

Frazier Life Sciences X, L.P.

Signature:	/s/ Steve R. Bailey
Name/Title:	By Steve R. Bailey, CFO of FHMLS X, L.L.C., GP of FHMLS X, L.P., GP of Frazier Life Sciences X, L.P.
Date:	09/30/2025

FHMLS X, L.P.

Signature:	/s/ Steve R. Bailey
Name/Title:	By Steve R. Bailey, CFO of FHMLS X, L.L.C., GP of FHMLS X, L.P.
Date:	09/30/2025

FHMLS X, L.L.C.

Signature:	/s/ Steve R. Bailey
Name/Title:	By Steve R. Bailey, CFO of FHMLS X, L.L.C.
Date:	09/30/2025

Frazier Life Sciences XI, L.P.

Signature:	/s/ Steve R. Bailey
Name/Title:	By Steve R. Bailey, CFO of FHMLS XI, L.L.C., GP of FHMLS XI, L.P., GP of Frazier Life Sciences XI, L.P.
Date:	09/30/2025

FHMLS XI, L.P.

Signature:	/s/ Steve R. Bailey
Name/Title:	By Steve R. Bailey, CFO of FHMLS XI, L.L.C., GP of FHMLS XI, L.P.
Date:	09/30/2025

FHMLS XI, L.L.C.

Signature:	/s/ Steve R. Bailey
Name/Title:	By Steve R. Bailey, CFO of FHMLS XI, L.L.C.
Date:	09/30/2025

James N. Topper

Signature:	/s/ Steve R. Bailey
Name/Title:	By Steve R. Bailey, Attorney-in-Fact for James N. Topper, pursuant to a Power of Attorney, a copy of which was filed with the SEC on July 31, 2017
Date:	09/30/2025

Patrick J. Heron

Signature:	/s/ Steve R. Bailey
Name/Title:	By Steve R. Bailey, Attorney-in-Fact for Patrick J. Heron, pursuant to a Power of Attorney, a copy of which was filed with the SEC on July 31, 2017
Date:	09/30/2025