SecProbe.io

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 VIA EDGAR September 22, 2025 United States Securities and Exchange Commission Division of Corporation Finance Office of Life Sciences 100 F. Street, N.E. Washington, D.C. 20549   Attention: Tyler Howes

 Re:
 MBX Biosciences, Inc.

 Acceleration Request for Registration Statement on Form S-1

 File No. 333-290447

 Dear Ladies and Gentlemen, Pursuant to Rule 461 under the Securities Act of 1933, as amended (the “ Act ”), MBX Biosciences, Inc. (the “ Company ”) hereby requests that the effective date of the above-referenced registration statement (the “ Registration Statement ”) be accelerated to September 24, 2024 at 4:00 p.m. Eastern Time, or as soon thereafter as practicable, unless we or our outside counsel, Goodwin Procter LLP, request by telephone that such Registration Statement be declared effective at some other time. In making this acceleration request, the Company acknowledges that it is aware of its responsibilities under the Act. Once the Registration Statement is effective, please orally confirm the event with our counsel, Goodwin Procter LLP, by calling Edwin M. O’Connor at (212) 813-8853. If you have any questions regarding this request, please contact Edwin M. O’Connor of Goodwin Procter LLP at (212) 813-8853.

 Sincerely,

 MBX BIOSCIENCES, INC.

   /s/ P. Kent Hawryluk P. Kent Hawryluk

 Chief Executive Officer

 cc:
 Richard Bartram, MBX Biosciences, Inc.

 Mitchell S. Bloom, Esq., Goodwin Procter LLP

 Edwin M. O’Connor, Esq., Goodwin Procter LLP

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 VIA EDGAR J.P. Morgan Securities LLC 383 Madison Avenue New York, New York 10179 Jefferies LLC 520 Madison Avenue New York, New York 10022 September 22, 2025 United States Securities and Exchange Commission Division of Corporation Finance Office of Life Sciences 100 F. Street, N.E. Washington, D.C. 20549 Attention: Tyler Howes

 Re:
 MBX Biosciences, Inc.

 Acceleration Request for Registration Statement on Form S-1 File No. 333-290447 Ladies and Gentlemen: Pursuant to Rule 461 under the Securities Act of 1933, as amended (the “ Act ”), we, as representatives of the several underwriters, hereby join in the request of MBX Biosciences, Inc. (the “ Company ”) for acceleration of the effective date of the above-referenced Registration Statement on Form S-1, as amended, so that it becomes effective at 4:00 p.m. Eastern time on September 24, 2025, or as soon thereafter as practicable, or at such other time as the Company or its outside counsel, Goodwin Procter LLP, request by telephone that such Registration Statement be declared effective. Pursuant to Rule 460 under the Act, we, as representatives of the several underwriters, wish to advise you that there will be distributed to each underwriter or dealer, who is reasonably anticipated to participate in the distribution of the security, as many copies of the preliminary prospectus as appears to be reasonable to secure adequate distribution of the preliminary prospectus. We, the undersigned, as representatives of the several underwriters, have complied and will comply, and we have been informed by the participating underwriters that they have complied and will comply, with the requirements of Rule 15c2-8 under the Securities Exchange Act of 1934, as amended.

 Very truly yours,

 J.P. MORGAN SECURITIES LLC

 JEFFERIES LLC

 As representatives of the several underwriters listed in Schedule 1 to the Underwriting Agreement

 J.P. MORGAN SECURITIES LLC

 By:

 /s/ David Ke

 Name: David Ke

 Title: Managing Director

 JEFFERIES LLC

 By:

 /s/ Matthew Kim

 Name: Matthew Kim

 Title: Managing Director

 [ Signature Page to Underwriters’ Acceleration Request ]

<DOCUMENT>
<TYPE>TEXT-EXTRACT
<SEQUENCE>2
<FILENAME>filename2.txt
<TEXT>
 August 13, 2025

P. Kent Hawryluk
Chief Executive Officer
MBX Biosciences, Inc.
11711 N. Meridian Street, Suite 300
Carmel, IN 46032

 Re: MBX Biosciences, Inc.
 Draft Registration Statement on Form S-1
 Submitted August 8, 2025
 CIK No. 0001776111
Dear P. Kent Hawryluk:

 This is to advise you that we do not intend to review your registration
statement.

 We request that you publicly file your registration statement and
non-public draft
submission on EDGAR at least two business days prior to the requested effective
date and
time. Please refer to Rules 460 and 461 regarding requests for acceleration. We
remind you
that the company and its management are responsible for the accuracy and
adequacy of their
disclosures, notwithstanding any review, comments, action or absence of action
by the staff.

 Please contact Tyler Howes at 202-551-3370 with any questions.

 Sincerely,

 Division of
Corporation Finance
 Office of Life
Sciences
cc: Edwin O Connor, Esq.
</TEXT>
</DOCUMENT>

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 VIA EDGAR

J.P. Morgan Securities LLC

 383 Madison Avenue

New York, New York 10179

 Jefferies LLC

520 Madison Avenue

 New York, New York 10022

September 10, 2024

 United States Securities and Exchange
Commission

 Division of Corporation Finance

 Office of Life
Sciences

 100 F. Street, N.E.

 Washington, D.C. 20549

Attention: Christine Torney, Angela Connell, Jimmy McNamara, Alan Campbell

Re:
 MBX Biosciences, Inc.

Acceleration Request for Registration Statement on Form S-1

File No. 333-281764

Ladies and Gentlemen:

 Pursuant to Rule 461
under the Securities Act of 1933, as amended (the “Act”), we, as representatives of the several underwriters, hereby join in the request of MBX Biosciences, Inc. (the “Company”) for acceleration of the effective
date of the above-referenced Registration Statement on Form S-1, as amended, so that it becomes effective at 4:00 p.m. Eastern time on September 12, 2024, or as soon thereafter as practicable, or at such
other time as the Company or its outside counsel, Goodwin Procter LLP, request by telephone that such Registration Statement be declared effective.

Pursuant to Rule 460 under the Act, we, as representatives of the several underwriters, wish to advise you that there will be distributed to
each underwriter or dealer, who is reasonably anticipated to participate in the distribution of the security, as many copies of the preliminary prospectus as appears to be reasonable to secure adequate distribution of the preliminary prospectus.

 We, the undersigned, as representatives of the several underwriters, have complied and will comply, and we have been informed by the
participating underwriters that they have complied and will comply, with the requirements of Rule 15c2-8 under the Securities Exchange Act of 1934, as amended.

Very truly yours,

J.P. MORGAN SECURITIES LLC

JEFFERIES LLC

As representatives of the several underwriters listed in Schedule 1 to the Underwriting Agreement

J.P. MORGAN SECURITIES LLC

By:

/s/ David Ke

Name: David Ke

Title: Managing Director

JEFFERIES LLC

By:

/s/ Matthew Kim

Name: Matthew Kim

Title: Managing Director

 [Signature Page to
Underwriters’ Acceleration Request]

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 VIA EDGAR

September 10, 2024

 United States Securities and Exchange
Commission

 Division of Corporation Finance

 Office of Life
Sciences

 100 F. Street, N.E.

 Washington, D.C. 20549

Attention: Christine Torney, Angela Connell, Jimmy McNamara, Alan Campbell

Re:

MBX Biosciences, Inc.

Acceleration Request for Registration Statement on Form S-1

File No. 333-281764

 Dear Ladies and Gentlemen,

Pursuant to Rule 461 under the Securities Act of 1933, as amended (the “Act”), MBX Biosciences, Inc. (the
“Company”) hereby requests that the effective date of the above-referenced registration statement (the “Registration Statement”) be accelerated to September 12, 2024 at 4:00 p.m. Eastern Time, or as soon
thereafter as practicable, unless we or our outside counsel, Goodwin Procter LLP, request by telephone that such Registration Statement be declared effective at some other time. In making this acceleration request, the Company acknowledges that it
is aware of its responsibilities under the Act.

 Once the Registration Statement is effective, please orally confirm the event with our
counsel, Goodwin Procter LLP, by calling Edwin M. O’Connor at (212) 813-8853.

 If you have
any questions regarding this request, please contact Edwin M. O’Connor of Goodwin Procter LLP at (212) 813-8853.

Sincerely,

MBX BIOSCIENCES, INC.

/s/ P. Kent Hawryluk

P. Kent Hawryluk

Chief Executive Officer

cc:

Richard Bartram, MBX Biosciences, Inc.

Mitchell S. Bloom, Esq., Goodwin Procter LLP

 Edwin M. O’Connor, Esq., Goodwin Procter LLP

Daniel Hughes, Esq., Goodwin Procter LLP

August 30, 2024
P. Kent Hawryluk
President and Chief Executive Officer
MBX Biosciences, Inc.
11711 N. Meridian Street, Suite 300
Carmel, Indiana 46032
Re:MBX Biosciences, Inc.
Registration Statement on Form S-1
Filed August 23, 2024
File No. 333-281764
Dear P. Kent Hawryluk:
            We have reviewed your registration statement and have the following comments.
            Please respond to this letter by amending your registration statement and providing the
requested information. If you do not believe a comment applies to your facts and circumstances
or do not believe an amendment is appropriate, please tell us why in your response.
            After reviewing any amendment to your registration statement and the information you
provide in response to this letter, we may have additional comments.
Registration Statement on Form S-1
Prospectus summary
MBX 2109: Potential treatment for chronic hypoparathyroidism, page 4
1.Please revise the second paragraph of this section discussing treatments for HP to reflect
your disclosure elsewhere in the prospectus that Yorvipath has been approved by the FDA
for the treatment of HP. Please similarly revise your risk factor discussion of
Yorvipath and the left column of the graphic on page 132, as needed.
Our company and team, page 6
2.Please limit the discussion of your existing stockholders to those stockholders who appear
in your Principal Stockholders section.

August 30, 2024
Page 2
            We remind you that the company and its management are responsible for the accuracy
and adequacy of their disclosures, notwithstanding any review, comments, action or absence of
action by the staff.
            Refer to Rules 460 and 461 regarding requests for acceleration. Please allow adequate
time for us to review any amendment prior to the requested effective date of the registration
statement.
            Please contact Christine Torney at 202-551-3652 or Angela Connell at 202-551-3426 if
you have questions regarding comments on the financial statements and related matters. Please
contact Jimmy McNamara at 202-551-7349 or Alan Campbell at 202-551-4224 with any other
questions.
Sincerely,
Division of Corporation Finance
Office of Life Sciences
cc:Edwin O’Connor

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 Goodwin Procter LLP

100 Northern Avenue

 Boston, MA 02210

goodwinlaw.com

 +1 617 570 1000

 August 30, 2024

 VIA
EDGAR AND FEDERAL EXPRESS

 United States Securities and Exchange Commission

Division of Corporation Finance

 Office of Life Sciences

100 F. Street, N.E.

 Washington, D.C. 20549

Attention: Christine Torney, Angela Connell, Jimmy McNamara, Alan Campbell

Re:
 MBX Biosciences, Inc.

 
 Registration Statement on Form S-1

 
 Filed August 23, 2024

 
 File No. 333-281764

Dear Ladies and Gentlemen:

 This letter is
submitted on behalf of MBX Biosciences, Inc. (the “Company”), in response to the comments of the staff of the Division of Corporation Finance (the “Staff”) of the U.S. Securities and Exchange Commission (the
“Commission”) with respect to the Company’s Registration Statement on Form S-1 filed on August 23, 2024 (the “Registration Statement”), as set forth in the
Staff’s letter, dated August 30, 2024, addressed to P. Kent Hawryluk (the “Comment Letter”). The Company is concurrently publicly filing Amendment No. 1 to the Registration Statement on Form S-1 (the “Amendment No. 1”), which includes changes to reflect the response to the Staff’s comment and other updates.

For reference purposes, the text of the Comment Letter has been reproduced herein with responses below each numbered comment. For your
convenience, we have italicized the reproduced Staff comments from the Comment Letter. Unless otherwise indicated, page references in the descriptions of the Staff’s comments refer to the Registration Statement, and page references in the
responses refer to the Amendment No. 1. All capitalized terms used and not otherwise defined herein shall have the meanings set forth in the Amendment No. 1.

Registration Statement on Form S-1 Business

Prospectus summary

 MBX 2109: Potential treatment for
chronic hypoparathyroidism, page 4

1.
 Please revise the second paragraph of this section discussing treatments for HP to reflect your disclosure
elsewhere in the prospectus that Yorvipath has been approved by the FDA for the treatment of HP. Please similarly revise your risk factor discussion of Yorvipath and the left column of the graphic on page 132, as needed.

RESPONSE: The Company acknowledges the Staff’s comment and respectfully advises the Staff that it has revised the disclosure on pages 4, 36 and
132 of the Amendment No. 1 in response to the Staff’s comment.

 Our company and team, page 6

2.
 Please limit the discussion of your existing stockholders to those stockholders who appear in your Principal
Stockholders section.

 RESPONSE: The Company acknowledges the Staff’s comment and respectfully advises the Staff that
it has revised the disclosure on pages 7 and 127 of the Amendment No. 1 in response to the Staff’s comment.

 [Signature Page
Follows]

 Division of Corporation Finance

Office of Life Sciences

 United States Securities and Exchange
Commission

 August 30, 2024

  Page
 2

 If you should have any questions concerning the enclosed matters, please contact the
undersigned at (212) 813-8853.

Sincerely,

/s/ Edwin M. O’Connor

Edwin M. O’Connor, Esq.

 Enclosures

 cc:

P. Kent Hawryluk, MBX Biosciences, Inc.

 Richard Bartram,
MBX Biosciences, Inc.

 Mitchell S. Bloom, Esq., Goodwin Procter LLP

Daniel Hughes, Esq., Goodwin Procter LLP

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 Goodwin Procter LLP

 100 Northern Avenue

Boston, MA 02210

 goodwinlaw.com

+1 617 570 1000

 August 26, 2024

FOIA CONFIDENTIAL TREATMENT REQUEST

 The entity
requesting confidential treatment is

 MBX Biosciences, Inc.

11711 N. Meridian Street, Suite 300

 Carmel, Indiana 46032

Telephone: (317) 659-0200

CERTAIN PORTIONS OF THIS LETTER HAVE BEEN OMITTED FROM THE VERSION FILED VIA EDGAR. CONFIDENTIAL TREATMENT HAS BEEN REQUESTED WITH RESPECT
TO THE OMITTED PORTIONS. INFORMATION THAT WAS OMITTED IN THE EDGAR VERSION HAS BEEN NOTED IN THIS LETTER WITH A PLACEHOLDER IDENTIFIED BY THE MARK “[***].”

VIA EDGAR, FACSIMILE AND FEDERAL EXPRESS

 United
States Securities and Exchange Commission

 Division of Corporation Finance

Office of Life Sciences

 100 F Street, N.E.

Washington, D.C. 20549

 Attention: Christine Torney, Angela
Connell, Jimmy McNamara, Alan Campbell

 RE: MBX Biosciences, Inc.

Registration Statement on Form S-1

File No. 333-281764

CIK No. 0001776111

 Rule 83 Confidential
Treatment Request by MBX Biosciences, Inc.

 Dear Ladies and Gentlemen:

On behalf of MBX Biosciences, Inc. (the “Company”), in response to comments from the staff (the “Staff”) of the Securities
and Exchange Commission (the “Commission”) received by letter dated April 17, 2024 (the “Comment Letter”) relating to the Company’s Registration Statement on Form
S-1, originally confidentially submitted to the Commission on March 22, 2024, amended by way of confidential submission on April 26, 2024, further amended by way of confidential submission on
July 11, 2024, and subsequently publicly filed by the Company with the Commission on August 23, 2024 (File No. 333-281764) (the “Registration Statement”), we submit this
supplemental letter to address comment 14 of the Comment Letter.

 1

CONFIDENTIAL TREATMENT REQUESTED BY MBX BIOSCIENCES, INC.

 Securities and Exchange Commission

August 26, 2024

  Page
 2

 Because of the commercially sensitive nature of information contained herein, this submission is accompanied
by the Company’s request for confidential treatment for selected portions of this letter. The Company has concurrently filed a separate letter with the Office of Freedom of Information and Privacy Act Operations in connection with the
confidential treatment request, pursuant to Rule 83 of the Commission’s Rules on Information and Requests, 17 C.F.R. § 200.83. For the Staff’s reference, we have enclosed a copy of the Company’s letter to the Office of Freedom of
Information and Privacy Act Operations.

 We confirm on behalf of the Company that, prior to circulating copies of the preliminary prospectus in connection
with the offering, the Company will file a pre-effective amendment to the Registration Statement that will include all information other than information that may be excluded in reliance upon Rule 430A of
Regulation C, and the actual price range to be included in such amendment which will comply with the Staff’s interpretation regarding the parameters of a bona fide price range.

The Company expects to reflect the Stock Split (as defined below) in a pre-effective amendment to the Registration
Statement that includes the actual price range; however, all dollar amounts and per share amounts in this letter are pre-Stock Split, and therefore, consistent with the Registration Statement.

The Company respectfully requests that the bracketed information contained in this letter be treated as confidential information pursuant to Rule 83
promulgated by the Commission, 17 C.F.R. §200.8, and that the Commission provide timely notice to Richard Bartram before it permits any disclosure of the bracketed information in this letter.

For the convenience of the Staff, we have recited the prior comment from the Staff in the Comment Letter in italicized type and have followed the comment with
the Company’s response.

 14. Once you have an estimated offering price or range, please explain to us how you determined the fair value of the
common stock underlying your equity issuances and the reasons for any differences between the recent valuations of your common stock leading up to the IPO and the estimated offering price. This information will help facilitate our review of your
accounting for equity issuances including stock compensation. Please discuss with the staff how to submit your response.

 2

CONFIDENTIAL TREATMENT REQUESTED BY MBX BIOSCIENCES, INC.

 Securities and Exchange Commission

August 26, 2024

  Page
 3

 Preliminary IPO Price Range

The Company advises the Staff that it estimates a preliminary price range of approximately $[***] to $[***] per share (the “Preliminary Price
Range”) for its initial public offering (“IPO”), before giving effect to a reverse stock split that the Company plans to implement prior to effectiveness of the Registration Statement (the “Stock Split”)
resulting in a midpoint of the Preliminary Price Range of $[***] per share (the “Midpoint Price”). The actual price range to be included in a subsequent amendment to the Registration Statement (which will comply with the
Staff’s interpretation regarding the parameters of a bona fide price range) has not yet been determined and remains subject to adjustment based on factors outside of the Company’s control. However, the Company believes that the
foregoing Preliminary Price Range will not be subject to significant change.

 Determining the Fair Value of Common Stock Prior to the IPO

As there has been no public market for the Company’s common stock, par value $0.0001 (“Common Stock”) to date, the estimated fair value
of its Common Stock has been determined by the Company’s board of directors (the “Board”) as of the date of each equity award with input from management, considering the Company’s most recently available third-party
valuations of its Common Stock, and the Board’s assessment of additional objective and subjective factors that it believed were relevant and which may have affected the fair value from the date of the most recent valuation through the date of
the grant. These third-party valuations were performed in accordance with the guidance outlined in the American Institute of Certified Public Accountants’ Accounting and Valuation Guide, Valuation of Privately-Held-Company Equity Securities
Issued as Compensation (the “Practice Aid”).

 The Company’s most recent third-party valuations of its Common Stock estimated fair
value were as follows:

 Date of Third-Party Valuation

Date of Board
Approval

Estimated Fair Market
Value of Common Stock
per Share

 November 7, 2022

November 7, 2022

$
0.27
(1)

 August 15, 2023

August 15, 2023

$
0.65

 January 31, 2024

January 31, 2024

$
0.76

 March 22, 2024

April 24, 2024

$
0.87

 August 2, 2024

August 2, 2024

$
0.87

(1)
 On November 7, 2022, the Board, with the assistance of a third-party valuation firm, determined the fair
market value of the Common Stock to be $0.27. The fair market value of the Common Stock was subsequently reassessed as of November 7, 2022, solely for financial reporting purposes, to appropriately bifurcate upside and downside scenarios in
accordance with the Practice Aid. As a result, the fair value of the Common Stock as of November 7, 2022 was determined to be $0.34, as compared to the $0.27 determined by the originally performed valuation for tax reporting purposes. This
revaluation resulted in recognition of additional stock-based compensation expense. The exercise price for stock options granted from November 7, 2022 through April 19, 2023 was derived from the originally performed valuation.

 3

CONFIDENTIAL TREATMENT REQUESTED BY MBX BIOSCIENCES, INC.

 Securities and Exchange Commission

August 26, 2024

  Page
 4

 The following table summarizes by grant date the number of shares subject to awards granted between
January 18, 2023 through the date of this letter, the per share exercise price of the awards and the fair value of Common Stock underlying the awards on each grant date:

Grant date

Number of shares
subject options
granted

Per share exercise
price of options

Fair value per
common share on
grant date

Estimated fair
value per option

 January 18, 2023

35,000

$
0.27

$
0.34
(1)

$
0.25
(2)

 April 19, 2023

1,404,250

$
0.27

$
0.34
(1)

$
0.25
(2)

 August 15, 2023

17,057,250

$
0.65

$
0.65

$
0.50
(2)

 November 2, 2023

2,571,360

$
0.65

$
0.65

$
0.50
(2)

 January 31, 2024

3,675,056

$
0.76

$
0.76

$
0.58
(2)

 April 24, 2024

2,205,455

$
0.87

$
0.87

$
0.71
(2)

 August 2, 2024

4,298,500

$
0.87

$
0.87

$
0.73
(2)

(1)
 On November 7, 2022, the Board, with the assistance of a third-party valuation firm, determined the fair
market value of the Common Stock to be $0.27. The fair market value of the Common Stock was subsequently reassessed as of November 7, 2022, solely for financial reporting purposes, to appropriately bifurcate upside and downside scenarios in
accordance with the Practice Aid. As a result, the fair value of the Common Stock as of November 7, 2022 was determined to be $0.34, as compared to the $0.27 determined by the originally performed valuation for tax reporting purposes. This
revaluation resulted in recognition of additional stock-based compensation expense. The exercise price for stock options granted from November 7, 2022 through April 19, 2023 was derived from the originally performed valuation.

(2)
 The per share estimated fair value of options reflects the weighted-average fair value of options granted on
each grant date determined using the Black-Scholes option-pricing model.

 The third-party valuations were performed in accordance with
the guidance outlined in the Practice Aid. The Common Stock valuations were prepared using an option pricing method (“OPM”), a hybrid method of OPM and probability-weighted expected return method (“PWERM”) or the
current value method (the “Current Value Method”). The OPM, the hybrid method and the Current Value method use market approaches to estimate the Company’s enterprise value.

The OPM treats common stock and preferred stock as call options on the total equity value of a company, with exercise prices based on the value thresholds at
which the allocation among the various holders of a company’s securities changes. Under this method, the common stock has value only if the funds available for distribution to stockholders exceeded the value of the preferred stock liquidation
preferences at the time of the liquidity event, such as a strategic sale or a merger.

 4

CONFIDENTIAL TREATMENT REQUESTED BY MBX BIOSCIENCES, INC.

 Securities and Exchange Commission

August 26, 2024

  Page
 5

 The PWERM is a scenario-based methodology that estimates the fair value of common stock based upon an
analysis of future values for the company, assuming various outcomes. The common stock value is based on the probability-weighted present value of expected future investment returns considering each of the possible outcomes available as well as the
rights of each class of stock. The future value of the common stock under each outcome is discounted back to the valuation date at an appropriate risk-adjusted discount rate and probability weighted to arrive at an indication of value for the common
stock. A discount for lack of marketability (“DLOM”) of the common stock is then applied to arrive at an indication of value for the common stock.

The hybrid method is a hybrid between the PWERM and OPM, estimating the probability-weighted value across multiple scenarios, but using the OPM to estimate
the allocation of value within one or more of those scenarios. When using the hybrid method, the third party valuations considered two future-event scenarios: an IPO scenario and an alternative exit scenario. The IPO scenario estimated an equity
value based on the guideline public company method under a market approach. The guideline public companies considered for this scenario consist of biopharmaceutical companies with recently completed initial public offerings. The value of the shares
under an initial public offering event scenario was determined according to the PWERM and the OPM scenario, using an appropriate time to a liquidity event, was used to estimate the fair value of the share class assuming the initial public offering
event does not occur. In certain instances, the Precedent Transaction (“Backsolve”) Method was used to determine value of shares under an IPO scenario. The resulting share values under each scenario are weighted by their respective
probabilities. The equity value for the alternative exit scenario was estimated using an enterprise value based on the price of a recently issued preferred securities and using the OPM method for allocation of value. The valuation utilized an option
pricing model to quantify or attribute value to the economic rights of convertible preferred stock as compared to the common stock, such as liquidation preferences, dividend provisions, and participation rights after liquidation preferences. This
method was determined to be the most appropriate valuation methodology based on the Company’s stage of development and other relevant factors. In determining the estimated fair value of the Common Stock, the Board also considered the fact that
the Company’s stockholders could not freely trade the Common Stock in the public markets. Accordingly, discounts were applied to reflect the lack of marketability of the Common Stock based on the weighted-average expected time to liquidity.

 The current value method, once the fair value of the enterprise is established, allocates value to the various series of preferred and common stock based
on their respective seniority, liquidation preferences or conversion values, whichever is greatest.

 5

CONFIDENTIAL TREATMENT REQUESTED BY MBX BIOSCIENCES, INC.

 Securities and Exchange Commission

August 26, 2024

  Page
 6

 In addition to considering the results of independent third-party valuations, the Committee considered
various objective and subjective factors to determine the thresholds for the options of common stock as of each grant date, including:

•

 the prices of the Company’s convertible preferred stock sold to investors in arm’s length transactions
and the rights, preferences and privileges of the convertible preferred stock relative to those of the Common Stock;

•

 the Company’s stage of development and business strategy and the material risks related to the business and
industry;

•

 the progress of the Company’s research and development programs, including the status of preclinical studies
and clinical trials for its drug candidates;

•

 the Company’s results of operations and financial position, including levels of available capital resources;

•

 the valuation of publicly traded companies in the life sciences and biotechnology sectors, as well as recently
completed mergers and acquisitions of peer companies;

•

 the lack of marketability of the Common Stock as a private company;

•

 the likelihood of achieving a liquidity event for the holders of Common Stock, such as an initial public offering
or a sale of the Company, given prevailing market conditions;

•

 trends and developments in the industry;

•

 external market conditions affecting the life sciences and biotechnology industry sectors; and

•

 the economy in general.

The assumptions underlying these valuations are highly complex and subjective, and represent management’s best estimates, which involved inherent
uncertainties and the application of management’s judgment. As a result, if the Company uses significantly different assumptions or estimates, the fair value of the Common Stock and the Company’s stock-based compensation expense could be
materially different.

 Once a public trading market for the Common Stock has been established in connection with the completion of this offering, it will
no longer be necessary for the Board or the Committee to estimate the fair value

CORRESP
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 Goodwin Procter LLP

100 Northern Avenue

 Boston, MA 02210

goodwinlaw.com

 +1 617 570 1000

 August 23, 2024

 VIA
EDGAR AND FEDERAL EXPRESS

 United States Securities and Exchange Commission

Division of Corporation Finance

 Office of Life Sciences

100 F. Street, N.E.

 Washington, D.C. 20549

Attention: Christine Torney, Angela Connell, Jimmy McNamara, Alan Campbell

Re:
 MBX Biosciences, Inc.

Amendment No. 2 to Draft Registration Statement on Form S-1

Submitted July 11, 2024

CIK 0001776111

 Dear Ladies and
Gentlemen:

 This letter is submitted on behalf of MBX Biosciences, Inc. (the “Company”), in response to the comments of
the staff of the Division of Corporation Finance (the “Staff”) of the U.S. Securities and Exchange Commission (the “Commission”) with respect to the Company’s Draft Registration Statement on Form S-1, originally confidentially submitted on March 22, 2024 (the “Draft Registration Statement”), and resubmitted on April 26, 2024 ( “Amendment No. 1 to the
Draft Registration Statement”) and July 11, 2024 (“Amendment No. 2 to the Draft Registration Statement”), as set forth in the Staff’s letter, dated July 22, 2024, addressed to P. Kent
Hawryluk (the “Comment Letter”). The Company is concurrently publicly filing the Registration Statement on Form S-1 (the “Registration Statement”), which includes changes to
reflect the response to the Staff’s comment and other updates.

 For reference purposes, the text of the Comment Letter has been
reproduced herein with responses below each numbered comment. For your convenience, we have italicized the reproduced Staff comments from the Comment Letter. Unless otherwise indicated, page references in the descriptions of the Staff’s
comments refer to Amendment No. 2 to the Draft Registration Statement, and page references in the responses refer to the Registration Statement. All capitalized terms used and not otherwise defined herein shall have the meanings set forth in
the Registration Statement.

 Amendment No. 2 to Draft Registration Statement on Form S-1

 Business

 Phase 1 clinical development and
results, page 135

1.
 We note your discussion of the results of your Phase 1 trial of MBX 2109 and the separate Phase 1 trial of
TransCon PTH. Please revise your disclosure to briefly describe the limitations of cross-study comparisons as opposed to head-to-head comparisons.

 RESPONSE: The Company acknowledges the Staff’s comment and respectfully advises the Staff that it has revised the
disclosure on page 136 of the Registration Statement in response to the Staff’s comment.

 If you should have any questions concerning
the enclosed matters, please contact the undersigned at (212) 813-8853.

Sincerely,

/s/ Edwin M. O’Connor

Edwin M. O’Connor, Esq.

 Enclosures

cc:

 P. Kent Hawryluk, MBX Biosciences, Inc.

Richard Bartram, MBX Biosciences, Inc.

 Mitchell S.
Bloom, Esq., Goodwin Procter LLP

 Daniel Hughes, Esq., Goodwin Procter LLP

July 22, 2024
P. Kent Hawryluk
President and Chief Executive Officer
MBX Biosciences, Inc.
11711 N. Meridian Street, Suite 300
Carmel, Indiana 46032
Re:MBX Biosciences, Inc.
Amendment No. 2 to Draft Registration Statement on Form S-1
Submitted July 11, 2024
CIK No. 0001776111
Dear P. Kent Hawryluk:
            We have reviewed your amended draft registration statement and have the following
comment.
            Please respond to this letter by providing the requested information and either submitting
an amended draft registration statement or publicly filing your registration statement on EDGAR.
If you do not believe this comment applies to your facts and circumstances or do not believe an
amendment is appropriate, please tell us why in your response.
            After reviewing the information you provide in response to this letter and your amended
draft registration statement or filed registration statement, we may have additional comments.
Amendment No. 2 to Draft Registration Statement on Form S-1
Business
Phase 1 clinical development and results, page 135
1.We note your discussion of the results of your Phase 1 trial of MBX 2109 and the separate
Phase 1 trial of TransCon PTH. Please revise your disclosure to briefly describe the
limitations of cross-study comparisons as opposed to head-to-head comparisons.
            Please contact Christine Torney at 202-551-3652 or Angela Connell at 202-551-3426 if
you have questions regarding comments on the financial statements and related matters. Please
contact Jimmy McNamara at 202-551-7349 or Alan Campbell at 202-551-4224 with any other
questions.

July 22, 2024
Page 2
Sincerely,
Division of Corporation Finance
Office of Life Sciences
cc:Edwin O’Connor

United States securities and exchange commission logo
May 7, 2024
P. Kent Hawryluk
President and Chief Executive Officer
MBX Biosciences, Inc.
11711 N. Meridian Street, Suite 300
Carmel, Indiana 46032
Re:MBX Biosciences, Inc.
Amendment No. 1 to Draft Registration Statement on Form S-1
Submitted April 26, 2024
CIK No. 0001776111
Dear P. Kent Hawryluk:
            We have reviewed your amended draft registration statement and have the following
comments.
            Please respond to this letter by providing the requested information and either submitting
an amended draft registration statement or publicly filing your registration statement on
EDGAR. If you do not believe a comment applies to your facts and circumstances or do not
believe an amendment is appropriate, please tell us why in your response.
            After reviewing the information you provide in response to this letter and your amended
draft registration statement or filed registration statement, we may have additional
comments. Unless we note otherwise, any references to prior comments are to comments in our
April 17, 2024 letter.
Amendment No. 1 to Draft Registration Statement on Form S-1
Prospectus Summary
Overview, page 1
1.We note your response to prior comment 3 and re-issue in part. Please revise the
Overview section of the Prospectus Summary to clarify that developing drug candidates is
an "uncertain process" and that you have not yet demonstrated the ability to gain
regulatory approvals.
2.We note your response to comment 5 and re-issue in part. Please revise your comparison
of the preclinical results of MBX 4291 and tirzepatide to clarify that MBX 4291’s results
in clinical trials may not reflect your findings in preclinical studies.

 FirstName LastNameP. Kent Hawryluk
 Comapany NameMBX Biosciences, Inc.
 May 7, 2024 Page 2
 FirstName LastName
P. Kent Hawryluk
MBX Biosciences, Inc.
May 7, 2024
Page 2
3.We note your response to prior comment 17 and revised disclosure. Please further revise
your Summary disclosure comparing MBX 4291 and tirzepatide to reflect (i) your
statements on page 144 indicating that it appears that less frequent dosing of MBX 4291
would require a higher dose than tirzepatide; (ii) the content of the graphic on page 144
showing that the concentration of the active component of MBX 4291 was significantly
lower than the concentration of tirzepatide in the duration comparison; and (iii) that the
study supporting the potential duration of MBX 4291 was conducted separately from
studies evaluating its effects.
MBX 2109: Potential best-in-class treatment for chronic hypoparathyroidism, page 3
4.We note your response to comment 9 and re-issue in part. Please provide balancing
disclosure when Orphan Drug Designation is first introduced in the prospectus that it does
not shorten the development time or regulatory review time of a product candidate and
does not provide any guarantee of approval in the regulatory review or approval process.
Our company and team, page 6
5.We note your response to comment 8 and re-issue in part. Please revise to clarify, if
true, Dr. DiMarchi is not an employee of your company. Please also disclose the number
of hours per week, if any, that Dr. DiMarchi is required to devote to your company. Please
also revise your disclosure on page 2 to clearly state whether you currently have any
independent discovery capabilities or whether you are currently reliant on Dr. DiMarchi's
discovery capabilities.
Business
Our solution: MBX 4291, page 141
6.Please revise your narrative description of the graphic at the top of page 144 to disclose
the range of concentrations of the (i) active component of MBX 4291 and (ii) tirzepatide,
respectfully.
            Please contact Christine Torney at 202-551-3652 or Angela Connell at 202-551-3426 if
you have questions regarding comments on the financial statements and related matters. Please
contact Jimmy McNamara at 202-551-7349 or Alan Campbell at 202-551-4224 with any other
questions.
Sincerely,
Division of Corporation Finance
Office of Life Sciences
cc:       Edwin O’Connor

United States securities and exchange commission logo
April 17, 2024
P. Kent Hawryluk
President and Chief Executive Officer
MBX Biosciences, Inc.
11711 N. Meridian Street, Suite 300
Carmel, Indiana 46032
Re:MBX Biosciences, Inc.
Draft Registration Statement on Form S-1
Submitted March 22, 2024
CIK No. 0001776111
Dear P. Kent Hawryluk:
            We have reviewed your draft registration statement and have the following comments.
            Please respond to this letter by providing the requested information and either submitting
an amended draft registration statement or publicly filing your registration statement on
EDGAR. If you do not believe a comment applies to your facts and circumstances or do not
believe an amendment is appropriate, please tell us why in your response.
            After reviewing the information you provide in response to this letter and your amended
draft registration statement or filed registration statement, we may have additional comments.
Draft Registration Statement on Form S-1
Cover Page
1.Please disclose, if accurate, that the closing of this offering is contingent upon a Nasdaq
listing, or otherwise advise. Please ensure the disclosure is consistent with your
underwriting agreement.
Prospectus Summary
Overview, page 1
2.Please revise your Prospectus Summary to define or explain briefly scientific or technical
terms. By way of example, we note the following terms:
•Peak-to-trough
•Prodrug
•Receptor antagonist
•Fatty acylation

 FirstName LastNameP. Kent Hawryluk
 Comapany NameMBX Biosciences, Inc.
 April 17, 2024 Page 2
 FirstName LastName
P. Kent Hawryluk
MBX Biosciences, Inc.
April 17, 2024
Page 2
•Hypercalcemic
•Hypocalcemic
3.We note your disclosure regarding your pipeline of novel candidates with "defined
regulatory pathways" and "large market opportunities." Please revise to reflect your
statements (i) on page 17 that developing product candidates is an “uncertain process”, (ii)
on page 16 that you have not yet demonstrated an ability to obtain regulatory approvals
and (iii) on page 30 that your estimates as to prevalence may not be accurate and that
published literature includes estimates which are lower than your estimates.
4.Please remove references throughout your prospectus to potential "best-in-class" and
“first-in-class” when describing your product candidates as these descriptions imply an
expectation of regulatory approval and are inappropriate given the length of time and
uncertainty with respect to securing such approval. In addition, please remove claims that
you are able to design and develop novel peptide therapeutics that have high or enhanced
"potency" and you can improve efficacy and tolerability. Please also remove any similar
disclosures regarding the current potency or efficacy of your product candidates as these
statements appear to be premature given your current stage of development.
5.We note your disclosure here and elsewhere stating that preclinical studies demonstrated
that MBX 4291 showed a “similar efficacy profile” as tirzepatide. As safety and efficacy
determinations are solely within the FDA’s authority, please remove these references to
efficacy. You may compare the performances of MBX 4291 and tirzepatide in preclinical
studies without concluding as to efficacy. Please also clarify that MBX 4291's results in
clinical trials may not reflect your findings in preclinical studies.
6.Please revise your Prospectus Summary, where appropriate, to reflect your disclosure
elsewhere in the prospectus that TransCon PTH was granted a marketing authorization in
the EU in November 2023 and that you could potentially be precluded from gaining
approval for MBX 2109 in the EU until 2035. Please also revise to reflect that an NDA for
TransCon PTH is currently under review by the FDA.
Our Platform, page 2
7.We note that you characterize your PEP platform as "leading" and "world-class." Please
revise to provide the basis for these statements.
8.We note your disclosure here, and elsewhere, regarding your co-founder Dr. DiMarchi’s
global recognition. Please revise to clarify, if true, Dr. DiMarchi is not a director or
employee of your company. Please also disclose the number of hours per week, if any,
that Dr. DiMarchi is required to devote to your company. With reference to your
disclosure on page 22, please also revise to state whether you have any independent
discovery capabilities or whether you are reliant on Dr. DiMarchi's discovery capabilities.

 FirstName LastNameP. Kent Hawryluk
 Comapany NameMBX Biosciences, Inc.
 April 17, 2024 Page 3
 FirstName LastName
P. Kent Hawryluk
MBX Biosciences, Inc.
April 17, 2024
Page 3
MBX 2109: Potential best-in-class treatment for chronic hypoparathyroidism, page 3
9.We note your disclosure on page 4 that the FDA has granted Orphan Drug Designation to
MBX 2109 for the treatment of HP. Please briefly describe the significance of having
obtained orphan drug designation. In addition, please revise your reference to orphan drug
designation to clarify that such a designation neither shortens the development time or
regulatory review time of a product candidate, nor does it provide any guarantee of
approval in the regulatory review or approval process.
Our Pipeline, page 3
10.We note your inclusion of a row in your pipeline table for "Additional Obesity Programs."
However, none of these programs appear to be discussed in your prospectus. Accordingly,
please remove this row from your pipeline table.
Our company and team, page 6
11.We note your disclosure regarding raising funding from “leading” healthcare investors.
Please clarify that prospective investors should not rely on the named investors’
investment decisions, that these investors may have different risk tolerances and, if true,
that the shares purchased by these investors were acquired at a discount to the IPO price.
Our Strategy, page 7
12.We note your disclosure here, and elsewhere, regarding your strategy to “rapidly advance”
MBX 2109 and MBX 1416 through clinical development. Please revise these statements
and any other similar statements to remove any implication that you will be successful in
advancing your product candidates in a rapid or accelerated manner, as such statements
are speculative. In this regard, we note your disclosure on page 17 that developing product
candidates, including conducting preclinical studies and clinical trials, is a very time-
consuming, expensive and uncertain process that takes years to complete.
Critical Accounting Policies and Significant Judgments and Estimates
Determination of the Fair Value of Common Stock, page 114
13.Please revise disclosures to explain the specific event(s) or factor(s) that resulted in an
increase in the initial valuation of the common stock fair value from $0.27 per share to
$0.34 per share.
14.Once you have an estimated offering price or range, please explain to us how you
determined the fair value of the common stock underlying your equity issuances and the
reasons for any differences between the recent valuations of your common stock leading
up to the IPO and the estimated offering price. This information will help facilitate our
review of your accounting for equity issuances including stock compensation. Please
discuss with the staff how to submit your response.

 FirstName LastNameP. Kent Hawryluk
 Comapany NameMBX Biosciences, Inc.
 April 17, 2024 Page 4
 FirstName LastNameP. Kent Hawryluk
MBX Biosciences, Inc.
April 17, 2024
Page 4
Business
Ongoing Avail Phase 2 clinical trial, page 134
15.Please revise this section to reflect your disclosure on pages 29-30 indicating that you
have added new sites to this trial following slow enrollment at the originally selected sites
and that you were unable to enroll sites in the EU.
Our solution: MBX 4291
Preclinical studies, page 141
16.Please define CPS in the graphic on page 141.
17.Please revise to clarify the number of mice dosed with the MBX 4291 active drug and
tirzepatide, as well as the number of non-human primates used to assess the conversion of
MBX 4191. We also note your claims here and throughout the prospectus indicating that
MBX 4291 demonstrated extended duration as compared to tirzepatide in a preclinical
study that may support once-monthly dosing. However, the preclinical study on page 142
appears to indicate that observation of the concentration of tirzepatide ceased after one
week. In addition, it appears that you are relying on the results of MBX 4291 active drug
to support your claim that the decline in exposure is flatter than the more rapid reduction
in tirzepatide exposure. However, your disclosure elsewhere in the prospectus indicates
that you are developing MBX 4291 as a prodrug and the prodrug decline in exposure
appears to track tirzepatide's. Please advise and revise your disclosure accordingly. To the
extent observation of tirzepatide ceased after one week, please remove or revise your
claims that MBX 4291 has demonstrated an extended duration as compared to tirzepatide.
License agreement
Indiana University Research and Technology Corporation Exclusive License Agreement, page
145
18.Please revise to provide the percentage of the sublicensing revenue, or a range not
exceeding 10 percentage points.
Exhibits
19.Please revise to either (i) clearly disclose that Exhibit 10.6 also contains amendments to
the original agreement or (ii) separately number the amendments to the license agreement.
20.When available, please file the Senior Executive Cash Incentive Bonus Plan as an exhibit
to your registration statement.
General
21.Please ensure the writing is legible in the visual depictions throughout your draft
registration statement. For example only, certain text on the y-axis on pages 127 and 133
is not legible.

 FirstName LastNameP. Kent Hawryluk
 Comapany NameMBX Biosciences, Inc.
 April 17, 2024 Page 5
 FirstName LastName
P. Kent Hawryluk
MBX Biosciences, Inc.
April 17, 2024
Page 5
22.Please supplementally provide us with copies of all written communications, as defined in
Rule 405 under the Securities Act, that you, or anyone authorized to do so on your behalf,
present to potential investors in reliance on Section 5(d) of the Securities Act, whether or
not they retain copies of the communications.
            Please contact Christine Torney at 202-551-3652 or Angela Connell at 202-551-3426 if
you have questions regarding comments on the financial statements and related matters. Please
contact Jimmy McNamara at 202-551-7349 or Alan Campbell at 202-551-4224 with any other
questions.
Sincerely,
Division of Corporation Finance
Office of Life Sciences
cc:       Edwin O’Connor