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CORRESP
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CORRESP

 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.

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CONFIDENTIAL TREATMENT REQUESTED BY MBX BIOSCIENCES, INC.

 Securities and Exchange Commission

August 26, 2024

  Page
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 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.

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CONFIDENTIAL TREATMENT REQUESTED BY MBX BIOSCIENCES, INC.

 Securities and Exchange Commission

August 26, 2024

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 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.

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CONFIDENTIAL TREATMENT REQUESTED BY MBX BIOSCIENCES, INC.

 Securities and Exchange Commission

August 26, 2024

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 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.

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CONFIDENTIAL TREATMENT REQUESTED BY MBX BIOSCIENCES, INC.

 Securities and Exchange Commission

August 26, 2024

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 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.

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CONFIDENTIAL TREATMENT REQUESTED BY MBX BIOSCIENCES, INC.

 Securities and Exchange Commission

August 26, 2024

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 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