MBX · MBX Biosciences, Inc. · Insider Trading

One-third of the shares underlying this option will vest and become exercisable on October 20, 2027, the anniversary of the date of grant, with the remainder vesting monthly over the subsequent two year period, subject to the Reporting Person's continued service on each such vesting date.

The sales reported on this Form 4 represent shares of common stock sold by the Reporting Person to cover tax obligations in connection with the vesting of restricted stock units, pursuant to a mandatory sell-to-cover agreement between the Reporting Person and the Company.

The price reported in Column 4 is a weighted average price. These shares were sold on an aggregate basis for all Company participants, at prices ranging from $65.21 to $66.43, inclusive. The Reporting Person undertakes to provide to the Issuer, any security holder of the Issuer or the staff of the Securities and Exchange Commission, upon request, full information regarding the aggregate number of shares sold at each separate price within the range set forth in this footnote.

The options will vest and become exercisable with 25% of the shares subject to the award vesting on July 13, 2027 and the remainder vesting in 36 equal monthly installments thereafter, subject to continued service on each such vesting date.

The options will vest and become exercisable with 25% of the shares subject to the award vesting on July 13, 2027 and the remainder vesting in 36 equal monthly installments thereafter, subject to continued service on each such vesting date.

Represents the grant of restricted stock units ("RSUs"). 25% of the shares subject to the RSU grant will vest annually over four years, on each July 13, 2027, 2028, 2029 and 2030, subject to continued service on each such vesting date.

Represents the grant of restricted stock units ("RSUs"). 25% of the shares subject to the RSU grant will vest annually over four years, on each July 13, 2027, 2028, 2029 and 2030, subject to continued service on each such vesting date.

The grant will vest in full on the earlier of the one-year anniversary of the grant date or on the date of our next annual meeting of stockholders, subject to the non-employee director's continued services to the Company.

The grant will vest in full on the earlier of the one-year anniversary of the grant date or on the date of our next annual meeting of stockholders, subject to the non-employee director's continued services to the Company.

The grant will vest in full on the earlier of the one-year anniversary of the grant date or on the date of our next annual meeting of stockholders, subject to the non-employee director's continued services to the Company.

The grant will vest in full on the earlier of the one-year anniversary of the grant date or on the date of our next annual meeting of stockholders, subject to the non-employee director's continued services to the Company.

The grant will vest in full on the earlier of the one-year anniversary of the grant date or on the date of our next annual meeting of stockholders, subject to the non-employee director's continued services to the Company.

The grant will vest in full on the earlier of the one-year anniversary of the grant date or on the date of our next annual meeting of stockholders, subject to the non-employee director's continued services to the Company.

The grant will vest in full on the earlier of the one-year anniversary of the grant date or on the date of our next annual meeting of stockholders, subject to the non-employee director's continued services to the Company.

The sales reported on this Form 4 represent the exercise and sale of common stock options by the reporting person pursuant to an approved Rule 10b5-1 trading plan adopted on January 27, 2026.

The sales reported on this Form 4 represent the exercise and sale of common stock options by the reporting person pursuant to an approved Rule 10b5-1 trading plan adopted on January 27, 2026.

The price reported in Column 4 is a weighted average price. These shares were purchased in multiple transactions at prices ranging from $35.00 to $41.26, inclusive. The Reporting Person undertakes to provide to the Issuer, any security holder of the Issuer or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares purchased at each separate price within the range set forth in this footnote.

The sales reported on this Form 4 represent the exercise and sale of common stock options by the reporting person pursuant to an approved Rule 10b5-1 trading plan adopted on January 27, 2026.

25% of the shares underlying this option vested and became exercisable on June 24, 2025, with the remaining 75% vesting in thirty-six (36) substantially equal monthly installments thereafter, subject to the Reporting Person's continued service on each such vesting date.

25% of the shares underlying this option vested and became exercisable on September 12, 2025, with the remaining 75% vesting in thirty-six (36) substantially equal monthly installments thereafter, subject to the Reporting Person's continued service on each such vesting date.

The sales reported on this Form 4 represent the exercise and sale of common stock options by the reporting person pursuant to an approved Rule 10b5-1 trading plan adopted on January 27, 2026.

The price reported in Column 4 is a weighted average price. These shares were purchased in multiple transactions at prices ranging from $35.00 to $41.26, inclusive. The Reporting Person undertakes to provide to the Issuer, any security holder of the Issuer or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares purchased at each separate price within the range set forth in this footnote.

The sales reported on this Form 4 represent shares of common stock sold by the Reporting Person to cover tax obligations in connection with the vesting of restricted stock units, pursuant to a mandatory sell-to-cover agreement between the Reporting Person and the Company.

The price reported in Column 4 is a weighted average price. These shares were sold on an aggregrate basis for all Company participants, at prices ranging from $29.23 to $29.88, inclusive. The Reporting Person undertakes to provide to the Issuer, any security holder of the Issuer or the staff of the Securities and Exchange Commission, upon request, full information regarding the aggregrate number of shares sold at each separate price within the range set forth in this footnote.

The sales reported on this Form 4 represent shares of common stock sold by the Reporting Person to cover tax obligations in connection with the vesting of restricted stock units, pursuant to a mandatory sell-to-cover agreement between the Reporting Person and the Company.

The price reported in Column 4 is a weighted average price. These shares were sold on an aggregate basis for all Company participants, at prices ranging from $29.23 to $29.88, inclusive. The Reporting Person undertakes to provide to the Issuer, any security holder of the Issuer or the staff of the Securities and Exchange Commission, upon request, full information regarding the aggregate number of shares sold at each separate price within the range set forth in this footnote.

The grant will vest 50% upon grant date and the remaining 50% in equal monthly installments over a six-month period from the grant date, subject to the Reporting Person's continued services to the Company.

Represents the grant of restricted stock units (RSUs). The RSUs will vest in full on November 6, 2026, subject to the Reporting Person's continued service on such vesting date.

The price reported in Column 4 is a weighted average price. These shares were purchased in multiple transactions at prices ranging from $28.28 to $28.48, inclusive. The Reporting Person undertakes to provide to the Issuer, any security holder of the Issuer or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares purchased at each separate price within the range set forth in this footnote.

Shares held by the P. Kent Hawryluk Revocable Trust dated January 25, 2011, of which the Reporting Person serves as trustee. The Reporting Person disclaims beneficial ownership except to the extent of any pecuniary interest therein, and this report shall not be deemed an admission that such shares are beneficially owned by him for Section 16 or any other purpose.

25% of the shares underlying this option vested and became exercisable on September 12, 2025, with the remaining 75% vesting in thirty-six (36) equal monthly installments thereafter, subject to the Reporting Person's continued service on each such vesting date.

Represents the grant of restricted stock units (RSUs). The RSUs will vest over a four (4) year period in equal quarterly installments, subject to the Reporting Person's continued service on such vesting date.

The shares underlying this option vest and become exercisable in forty eight (48) equal monthly installments, beginning on March 2, 2026, subject to the Reporting Person's continued service on such vesting date.

The shares underlying this option vest and become exercisable in forty eight (48) equal monthly installments, beginning on March 2, 2026, subject to the Reporting Person's continued service on such vesting date.

Represents the grant of restricted stock units (RSUs). The RSUs will vest over a four (4) year period in equal quarterly installments, subject to the Reporting Person's continued service on such vesting date.

The shares underlying this option vest and become exercisable in forty eight (48) equal monthly installments, beginning on March 2, 2026, subject to the Reporting Person's continued service on such vesting date.

Represents the grant of restricted stock units (RSUs). The RSUs will vest over a four (4) year period in equal quarterly installments, subject to the Reporting Person's continued service on such vesting date.

One-third of the shares underlying this option will vest and become exercisable on January 20, 2027, the anniversary of the date of grant, with the remainder vesting monthly over the subsequent two year period, subject to the Reporting Person's continued service on each such vesting date.

Pursuant to the Company's Non-Employee Director Compensation Policy, these grants were adjusted downward such that the non-employee Director's annual equity compensation in 2025 totals no more than $1,000,000 based on the aggregate grant date fair value as determined in accordance with FASB Topic ASC 718.

The grant will vest in equal monthly installments over a one-year period from November 6, 2025, subject to the non-employee director's continued services to the Company.

Pursuant to the Company's Non-Employee Director Compensation Policy, these grants were adjusted downward such that the non-employee Director's annual equity compensation in 2025 totals no more than $1,000,000 based on the aggregate grant date fair value as determined in accordance with FASB Topic ASC 718.

The grant will vest in equal monthly installments over a one-year period from the date of grant, subject to the non-employee director's continued services to the Company.

1/48th of the shares underlying this option vest in substantially equal monthly installments on each monthly anniversary of November 7, 2022, subject to the Reporting Person's continued service on each such vesting date, until the option vests in full on November 7, 2026. The shares underlying this option became exercisable in full on November 7, 2022, pursuant to an early-exercise provision, subject to an Issuer right of repurchase.

The price reported in Column 4 is a weighted average price. These shares were purchased in multiple transactions at prices ranging from $13.60 to $13.6399, inclusive. The Reporting Person undertakes to provide to the Issuer, any security holder of the Issuer or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares purchased at each separate price within the range set forth in this footnote.

Shares held by the P. Kent Hawryluk Revocable Trust dated January 25, 2011, of which the Reporting Person serves as trustee. The Reporting Person disclaims beneficial ownership except to the extent of any pecuniary interest therein, and this report shall not be deemed an admission that such shares are beneficially owned by him for Section 16 or any other purpose.

Shares held by the Steven L Hoerter Revocable Trust dated November 2, 2018, of which the Reporting Person serves as trustee. The Reporting Person disclaims beneficial ownership except to the extent of any pecuniary interest therein, and this report shall not be deemed an admission that such shares are beneficially owned by him for Section 16 or any other purpose.

The shares reported herein are held of record by Frazier Life Sciences X, L.P. FHMLS X, L.P. is the general partner of Frazier Life Sciences X, L.P. and FHMLS X, L.L.C. is the general partner of FHMLS X, L.P. James N. Topper and Patrick J. Heron are the sole managing members of FHMLS X, L.L.C. and share voting and investment power over the shares held by Frazier Life Sciences X, L.P. Dr. Topper and Mr. Heron disclaim beneficial ownership of such securities except to the extent of their pecuniary interest therein.

The grant will vest in full on the earlier of the one-year anniversary of the grant date or on the date of our next annual meeting of stockholders, subject to the non-employee director's continued services to the Company.

The grant will vest in full on the earlier of the one-year anniversary of the grant date or on the date of our next annual meeting of stockholders, subject to the non-employee director's continued services to the Company.

The grant will vest in full on the earlier of the one-year anniversary of the grant date or on the date of our next annual meeting of stockholders, subject to the non-employee director's continued services to the Company.

The grant will vest in full on the earlier of the one-year anniversary of the grant date or on the date of our next annual meeting of stockholders, subject to the non-employee director's continued services to the Company.

The grant will vest in full on the earlier of the one-year anniversary of the grant date or on the date of our next annual meeting of stockholders, subject to the non-employee director's continued services to the Company.

The grant will vest in full on the earlier of the one-year anniversary of the grant date or on the date of our next annual meeting of stockholders, subject to the non-employee director's continued services to the Company.

The grant will vest in full on the earlier of the one-year anniversary of the grant date or on the date of our next annual meeting of stockholders, subject to the non-employee director's continued services to the Company.

The price reported in Column 4 is a weighted average price. These shares were purchased in multiple transactions at prices ranging from $5.65 to $6.30, inclusive. The Reporting Person undertakes to provide to the Issuer, any security holder of the Issuer or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares purchased at each separate price within the range set forth in this footnote.

One-third of the shares underlying this option will vest and become exercisable on April 7, 2026, the anniversary of the date of grant, with the remainder vesting monthly over the subsequent two year period, subject to the Reporting Person's continued service on each such vesting date.

Represents the weighted average purchase price of the shares of the Issuer's common stock purchased, ranging from a low of $10.25 to a high of $11.46 per share. The Reporting Persons undertake, upon request by the staff of the Securities and Exchange Commission, the Issuer or a security holder of the Issuer, to provide full information regarding the number of shares purchased at each separate price.

These securities are held of record by OrbiMed Private Investments VII, LP ("OPI VII"). OrbiMed Capital GP VII LLC ("GP VII") is the general partner of OPI VII. OrbiMed Advisors LLC ("OrbiMed Advisors") is the managing member of GP VII. By virtue of such relationships, OrbiMed Advisors and GP VII may be deemed to have voting power and investment power over the securities held by OPI VII and as a result, may be deemed to have beneficial ownership over such securities. OrbiMed Advisors exercises voting and investment power through a management committee comprised of Carl L. Gordon, Sven H. Borho, and W. Carter Neild, each of whom disclaims beneficial ownership of the shares held by OPI VII.

This report on Form 4 is jointly filed by OrbiMed Advisors, GP VII, and Genesis GP. Each of the Reporting Persons disclaims beneficial ownership of the securities reported herein for purposes of Rule 16a-1(a) under the Securities Exchange Act of 1934, as amended (the "Exchange Act"), except to the extent of its pecuniary interest therein, if any. OrbiMed Advisors has designated a representative, Carl L. Gordon, a member of OrbiMed Advisors, to serve on the Issuer's board of directors. This report shall not be deemed an admission that any such entity or person is a beneficial owner of such securities for purpose of Section 16 of the Exchange Act, or for any other purpose.