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

Rhea-AI Filing Summary

Frazier Life Sciences-related funds report holdings in MBX Biosciences following IPO conversions and purchases. Frazier Life Sciences X, L.P. holds 4,552,774 shares (13.6% of 33,593,866 outstanding), Frazier Life Sciences Public Fund, L.P. holds 1,103,920 shares (3.3%), Frazier Life Sciences Public Overage Fund, L.P. holds 325,653 shares (1.0%), and FLS XI holds 3,000 shares. These positions arose from pre-IPO preferred-stock purchases that converted at a 12.0221-to-1 ratio plus additional IPO purchases at $16.00 per share. The filers state purchases were funded from each fund's working capital and correct prior attributions of beneficial ownership for certain investment committee members.

Positive

• Frazier Life Sciences X holds a material 13.6% stake (4,552,774 shares), clearly disclosed
• Purchases funded from funds' working capital with no borrowings used to acquire the reported shares

Negative

• Filing corrects prior overstatements of beneficial ownership attribution for certain investment committee members, indicating earlier disclosure errors

Insights

Financial Analyst

TL;DR: Frazier-affiliated funds hold a material 13.6% position via Frazier Life Sciences X, which is significant for MBX shareholder composition.

The filing discloses that Frazier Life Sciences X directly holds 4,552,774 shares, representing 13.6% of MBX's outstanding common stock based on 33,593,866 shares. Positions originate from large private preferred purchases converted at a 12.0221-to-1 ratio and from $16.00 IPO purchases. Funding is from fund working capital with no borrowings for these purchases. For investors, a 13.6% holder can influence corporate governance discussions and voting outcomes; this concentration is material to ownership structure and potential strategic dialogue.

Corporate Governance Expert

TL;DR: The amendment corrects prior beneficial-ownership attributions and clarifies voting/dispositive power allocation across entities.

The Schedule 13D/A clarifies that certain investment committee members are not attributed beneficial ownership of shares held by the funds, correcting earlier overstatements. It explains general-partner and manager relationships that determine voting and dispositive power for each reporting entity. These clarifications reduce ambiguity about who holds control rights and ensure SEC disclosure alignment, which matters for governance transparency though it does not change the underlying share counts disclosed.

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, $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)

08/18/2025

(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)

3.3 %

14

Type of Reporting Person (See Instructions)

PN

Comment for Type of Reporting Person:

The Percent of Class is calculated based on 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.

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)

3.3 %

14

Type of Reporting Person (See Instructions)

PN

Comment for Type of Reporting Person:

The Percent of Class is calculated based on 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.

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)

3.3 %

14

Type of Reporting Person (See Instructions)

OO

Comment for Type of Reporting Person:

The Percent of Class is calculated based on 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.

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)

1.0 %

14

Type of Reporting Person (See Instructions)

PN

Comment for Type of Reporting Person:

The Percent of Class is calculated based on 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.

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)

1.0 %

14

Type of Reporting Person (See Instructions)

PN

Comment for Type of Reporting Person:

The Percent of Class is calculated based on 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.

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)

1.0 %

14

Type of Reporting Person (See Instructions)

OO

Comment for Type of Reporting Person:

The Percent of Class is calculated based on 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.

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 4,552,774.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 4,552,774.00

11

Aggregate amount beneficially owned by each reporting person

4,552,774.00

12

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

13

Percent of class represented by amount in Row (11)

13.6 %

14

Type of Reporting Person (See Instructions)

PN

Comment for Type of Reporting Person:

The Percent of Class is calculated based on 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.

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 4,552,774.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 4,552,774.00

11

Aggregate amount beneficially owned by each reporting person

4,552,774.00

12

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

13

Percent of class represented by amount in Row (11)

13.6 %

14

Type of Reporting Person (See Instructions)

PN

Comment for Type of Reporting Person:

The Percent of Class is calculated based on 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.

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 4,552,774.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 4,552,774.00

11

Aggregate amount beneficially owned by each reporting person

4,552,774.00

12

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

13

Percent of class represented by amount in Row (11)

13.6 %

14

Type of Reporting Person (See Instructions)

OO

Comment for Type of Reporting Person:

The Percent of Class is calculated based on 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.

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 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.

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 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.

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 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.

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 4,552,774.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 4,552,774.00

11

Aggregate amount beneficially owned by each reporting person

4,552,774.00

12

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

13

Percent of class represented by amount in Row (11)

13.6 %

14

Type of Reporting Person (See Instructions)

IN

Comment for Type of Reporting Person:

The Aggregate Amount represents 4,552,774 shares of Common Stock held directly by Frazier Life Sciences X, L.P. The Percent of Class is calculated based on 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.

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 4,323.00
8	Shared Voting Power 4,552,774.00
9	Sole Dispositive Power 4,323.00
10	Shared Dispositive Power 4,552,774.00

11

Aggregate amount beneficially owned by each reporting person

4,557,097.00

12

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

13

Percent of class represented by amount in Row (11)

13.6 %

14

Type of Reporting Person (See Instructions)

IN

Comment for Type of Reporting Person:

The Aggregate Amount represents (i) 4,323 shares of Common Stock that are issuable upon the exercise of options held directly by Patrick J. Heron, and (ii) 4,552,774 shares of Common Stock held directly by Frazier Life Sciences X, L.P. The Percent of Class is calculated based on 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.

SCHEDULE 13D

CUSIP No. 55287L101

1

Name of reporting person

Albert Cha

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 0.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 0.00

11

Aggregate amount beneficially owned by each reporting person

0.00

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)

IN

SCHEDULE 13D

CUSIP No. 55287L101

1

Name of reporting person

James Brush

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 0.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 0.00

11

Aggregate amount beneficially owned by each reporting person

0.00

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)

IN

SCHEDULE 13D

CUSIP No. 55287L101

1

Name of reporting person

Daniel Estes

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 0.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 0.00

11

Aggregate amount beneficially owned by each reporting person

0.00

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)

IN

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. 1 ("Amendment No. 1") to Schedule 13D amends the statement on Schedule 13D filed on September 23, 2024 (the "Original Schedule 13D" and this Amendment No. 1, the "Schedule 13D"). Except as otherwise specified in Amendment No. 1, all items in the Original Schedule 13D are unchanged. All capitalized terms used in this Amendment No. 1 and not otherwise defined herein have the meanings ascribed to such terms in the Original Schedule 13D.
Item 3.	Source and Amount of Funds or Other Consideration
	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"). As of the date of this filing, (i) FLS X holds 4,552,774 shares of the Issuer's Common Stock (the "FLS X Shares"); (ii) FLSPF holds 1,103,920 shares of the Issuer's Common Stock (the "FLSPF Shares"); (iii) FLSPOF holds 325,653 shares of the Issuer's Common Stock (the "FLSPOF Shares"); and (iv) FLS XI holds 3,000 shares of the Issuer's Common Stock (the "FLS XI Shares"). 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, and this Schedule 13D corrects any previous attribution to, and overstatement of beneficial ownership by, such members as reflected in any prior Statements. 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, and this Schedule 13D corrects any previous attribution to, and overstatement of beneficial ownership by, such members as reflected in any prior Statements. FLS X directly holds 4,552,774 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, and this Schedule 13D corrects any previous attribution to, and overstatement of beneficial ownership by, such members as reflected in any prior Statements. 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.
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 99.1 Joint Filing Agreement (incorporated by reference to Exhibit A to Schedule 13D filed on September 23, 2024).

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:	08/20/2025

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:	08/20/2025

FHMLSP, L.L.C.

Signature:	/s/ Steve R. Bailey
Name/Title:	By Steve R. Bailey, CFO of FHMLSP, L.L.C.
Date:	08/20/2025

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:	08/20/2025

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:	08/20/2025

FHMLSP Overage, L.L.C.

Signature:	/s/ Steve R. Bailey
Name/Title:	By Steve R. Bailey, CFO of FHMLSP Overage, L.L.C.
Date:	08/20/2025

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:	08/20/2025

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:	08/20/2025

FHMLS X, L.L.C.

Signature:	/s/ Steve R. Bailey
Name/Title:	By Steve R. Bailey, CFO of FHMLS X, L.L.C.
Date:	08/20/2025

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:	08/20/2025

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:	08/20/2025

FHMLS XI, L.L.C.

Signature:	/s/ Steve R. Bailey
Name/Title:	By Steve R. Bailey, CFO of FHMLS XI, L.L.C.
Date:	08/20/2025

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:	08/20/2025

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:	08/20/2025

Albert Cha

Signature:	/s/ Steve R. Bailey
Name/Title:	By Steve R. Bailey, Attorney-in-Fact for Albert Cha, pursuant to a Power of Attorney, a copy of which was filed with the SEC on August 16, 2021
Date:	08/20/2025

James Brush

Signature:	/s/ Steve R. Bailey
Name/Title:	By Steve R. Bailey, Attorney-in-Fact for James Brush, pursuant to a Power of Attorney, a copy of which was filed with the SEC on August 16, 2021
Date:	08/20/2025

Daniel Estes

Signature:	/s/ Steve R. Bailey
Name/Title:	By Steve R. Bailey, Attorney-in-Fact for Daniel Estes, pursuant to a Power of Attorney, a copy of which was filed with the SEC on April 18, 2022
Date:	08/20/2025

FAQ

How many MBX (MBX) shares does Frazier Life Sciences X, L.P. hold?

Frazier Life Sciences X, L.P. holds 4,552,774 shares, representing 13.6% of MBX common stock based on 33,593,866 shares outstanding.

What created Frazier's MBX holdings reported in this Schedule 13D/A?

Holdings arose from pre-IPO purchases of Series A, B and C preferred that converted at a 12.0221-to-1 ratio plus additional IPO purchases at $16.00 per share.

Were the reported MBX purchases financed with borrowed funds?

No. The filing states the purchases were funded from each fund's working capital and no part of the purchase price was borrowed to acquire the shares.

Does this amendment change prior ownership attributions?

Yes. The amendment corrects prior attributions by clarifying that certain investment committee members are not attributed beneficial ownership of shares held by the funds.

What is the total number of MBX shares used to calculate percentages?

Percentages are calculated using 33,593,866 shares of MBX common stock outstanding as of August 4, 2025, per the issuer's quarterly report.