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

Rhea-AI Filing Summary

Amendment No. 1 to the Schedule 13D reports that Norwest Venture Partners XVI, LP and affiliated reporting persons experienced dilution in their ownership of MBX Biosciences, Inc. common stock as a result of an underwritten public offering that closed on September 26, 2025. The filing states each of the primary reporting persons beneficially owns 2,136,335 shares, representing 4.8% of the class based on 33,607,443 shares outstanding as of August 31, 2025 plus 11,108,055 shares issued in the offering; Ms. Aynechi’s aggregate beneficial ownership is reported as 2,140,989 shares (including 4,654 option shares). The reporting group ceased to be beneficial owners of more than 5% on September 26, 2025. The amendment discloses no transactions in the past 60 days and incorporates a prior joint filing agreement as an exhibit.

Positive

• Timely disclosure of dilution by amendment following the issuer's public offering
• Clear attribution that the change in percentage resulted from the issuer's offering, not from sales or purchases by the reporting persons
• Detailed ownership breakdown showing shared voting and dispositive power and quantified share counts

Negative

• Reported ownership declined below 5%, with stake now 4.8%, reducing prior >5% status
• Dilution from the offering increased the issuer's outstanding shares by 11,108,055, lowering the reporting persons' ownership percentage
• No transactions disclosed in the past 60 days, so the filing reflects passive dilution rather than an active strategic adjustment

Insights

Financial Analyst

TL;DR: The filing documents dilution from a completed public offering; reported ownership falls to 4.8%, a modest but material change for ownership thresholds.

The reporting persons collectively report shared beneficial ownership of 2,136,335 shares, equal to 4.8% of MBX's common stock when including the issuer's newly issued offering shares. This change is explicitly attributed to the issuer's underwritten public offering that closed on September 26, 2025, not to purchases or sales by the reporting persons. For investors, the primary implication is a shift in ownership banding—loss of >5% status—affecting disclosure obligations and potential perception of influence but not indicating active trading or change in intent by the holders.

Corporate Governance Expert

TL;DR: Governance position appears unchanged; dilution reduced reported stake below 5%, and Ms. Aynechi remains a director with a small option position.

The amendment clarifies that shared voting and dispositive power over the disclosed shares remains with the disclosed investment entities and individuals (Genesis XVI, NVP Associates, Crowe, Kossow, Aynechi). Aynechi is identified as an officer of NVP Associates and a director of the issuer and holds 4,654 option shares exercisable within 60 days that are included in her aggregate figure. No recent transactions or new governance arrangements are reported; the filing serves primarily to update ownership percentages after the offering.

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)

Matthew De Dominicis
Norwest Venture Partners, 1300 El Camino Real, Suite 200
Menlo Park, CA, 94024
(650) 321-8000

(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

Norwest Venture Partners XVI, LP

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 2,136,335.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 2,136,335.00

11

Aggregate amount beneficially owned by each reporting person

2,136,335.00

12

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

13

Percent of class represented by amount in Row (11)

4.8 %

14

Type of Reporting Person (See Instructions)

PN

SCHEDULE 13D

CUSIP No. 55287L101

1

Name of reporting person

Genesis VC Partners XVI, LLC

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 2,136,335.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 2,136,335.00

11

Aggregate amount beneficially owned by each reporting person

2,136,335.00

12

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

13

Percent of class represented by amount in Row (11)

4.8 %

14

Type of Reporting Person (See Instructions)

OO

SCHEDULE 13D

CUSIP No. 55287L101

1

Name of reporting person

NVP Associates, LLC

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 2,136,335.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 2,136,335.00

11

Aggregate amount beneficially owned by each reporting person

2,136,335.00

12

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

13

Percent of class represented by amount in Row (11)

4.8 %

14

Type of Reporting Person (See Instructions)

OO

SCHEDULE 13D

CUSIP No. 55287L101

1

Name of reporting person

Jeffrey Crowe

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 2,136,335.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 2,136,335.00

11

Aggregate amount beneficially owned by each reporting person

2,136,335.00

12

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

13

Percent of class represented by amount in Row (11)

4.8 %

14

Type of Reporting Person (See Instructions)

IN

SCHEDULE 13D

CUSIP No. 55287L101

1

Name of reporting person

Jon Erik Kossow

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 2,136,335.00
9	Sole Dispositive Power 0.00
10	Shared Dispositive Power 2,136,335.00

11

Aggregate amount beneficially owned by each reporting person

2,136,335.00

12

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

13

Percent of class represented by amount in Row (11)

4.8 %

14

Type of Reporting Person (See Instructions)

IN

SCHEDULE 13D

CUSIP No. 55287L101

1

Name of reporting person

Tiba Aynechi

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)

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 4,654.00
8	Shared Voting Power 2,136,335.00
9	Sole Dispositive Power 4,654.00
10	Shared Dispositive Power 2,136,335.00

11

Aggregate amount beneficially owned by each reporting person

2,140,989.00

12

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

13

Percent of class represented by amount in Row (11)

4.8 %

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 (this "Amendment No. 1" or this "Schedule 13D/A") amends and supplements the statement on Schedule 13D originally filed with the Securities and Exchange Commission (the "SEC") on September 23, 2024 (the "Statement") by the Reporting Persons. Unless otherwise defined herein, capitalized terms used in this Amendment No. 1 shall have the meanings ascribed to them in the Statement. Unless amended or supplemented below, the information in the Statement remains unchanged. This Amendment No. 1 is being filed to report the dilution in the Reporting Persons' beneficial ownership of the Issuer's common stock resulting from the Issuer's underwritten public offering, which closed on September 26, 2025, and not as a result of any acquisition or disposition of securities by the Reporting Persons.
Item 5.	Interest in Securities of the Issuer
(a)	Rows 11 and 13 of each Reporting Person's cover page to this Schedule 13D/A set forth the aggregate number of shares of common stock and percentages of the shares of common stock beneficially owned by such Reporting Person and are incorporated by reference. The percentage set forth in each row 13 is based upon the sum of (i) 33,607,443 shares of common stock outstanding as of August 31, 2025, as reported in the Issuer's Registration Statement on Form S-1 filed with the Securities and Exchange Commission (the "SEC") on September 22, 2025; (ii) 11,108,055 shares of common stock issued in the Issuer's public offering of equity securities that closed on September 26, 2025; and (iii) with respect to Aynechi, 4,654 shares of common stock issuable upon the exercise of stock options within 60 days. Genesis XVI is the general partner of NVP XVI, NVP Associates is the managing member of Genesis XVI, Crowe and Kossow are Co-Chief Executive Officers of NVP Associates, and Aynechi is an officer of NVP Associates and a director of the Issuer. Each of Genesis XVI, NVP Associates, Crowe, Kossow and Aynechi, may be deemed to share voting and investment authority over the shares held by NVP XVI.
(b)	Rows 7 through 10 of each Reporting Person's cover page to this Schedule 13D/A set forth the number of shares of common stock as to which such Reporting Person has the sole or shared power to vote or direct the vote and sole or shared power to dispose or to direct the disposition and are incorporated by reference.
(c)	No transactions with respect to the securities of the Issuer have been effected during the past sixty days by any Reporting Person. The percentage ownership reported in this amendment reflect an increase in the number of outstanding shares of the Issuer.
(d)	No other person is known to have the right to receive or the power to direct the receipt of dividends from, or any proceeds from the sale of, the securities beneficially owned by any of the Reporting Persons.
(e)	The Reporting Persons ceased to be the beneficial owners of more than five percent of the common stock on September 26, 2025.
Item 7.	Material to be Filed as Exhibits.
	Exhibit 99.1 Joint Filing Agreement (incorporated by reference to Exhibit A to the Reporting Persons' Schedule 13D filed with the SEC 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.

Norwest Venture Partners XVI, LP

Signature:	/s/ Matthew De Dominicis
Name/Title:	By Genesis VC Partners XVI, LLC, its General Partner, By NVP Associates, LLC, its Managing Member, By Matthew De Dominicis, Chief Financial Officer
Date:	09/30/2025

Genesis VC Partners XVI, LLC

Signature:	/s/ Matthew De Dominicis
Name/Title:	By NVP Associates, LLC, its Managing Member, By Matthew De Dominicis, Chief Financial Officer
Date:	09/30/2025

NVP Associates, LLC

Signature:	/s/ Matthew De Dominicis
Name/Title:	By Matthew De Dominicis, Chief Financial Officer
Date:	09/30/2025

Jeffrey Crowe

Signature:	/s/ Matthew De Dominicis
Name/Title:	By Matthew De Dominicis, Attorney-in-fact
Date:	09/30/2025

Jon Erik Kossow

Signature:	/s/ Matthew De Dominicis
Name/Title:	By Matthew De Dominicis, Attorney-in-fact
Date:	09/30/2025

Tiba Aynechi

Signature:	/s/ Matthew De Dominicis
Name/Title:	By Matthew De Dominicis, Attorney-in-fact
Date:	09/30/2025

FAQ

What ownership does Norwest Venture Partners report in MBX (MBX)?

The reporting persons each report beneficial ownership of 2,136,335 shares, representing 4.8% of the outstanding common stock after the offering; Ms. Aynechi's aggregate is 2,140,989 shares including 4,654 option shares.

Why did the reporting persons' ownership percentage change?

The filing states the percentage declined due to an underwritten public offering by MBX that closed on September 26, 2025, which issued 11,108,055 new shares.

Did the reporting persons trade shares in the past 60 days?

No; Item 5(c) explicitly states no transactions by any Reporting Person occurred during the past sixty days.

Do the reporting persons still have voting power over the shares?

Yes; the filing reports shared voting power and shared dispositive power of 2,136,335 shares for the primary reporting entities and individuals.

When did the reporting persons cease to be beneficial owners of more than 5%?

They ceased to be beneficial owners of more than 5% on September 26, 2025, the closing date of the public offering.