UNITED STATES
SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

Current Report

Pursuant to Section 13 or 15(d) of

the Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): December 29, 2025

MEREO BIOPHARMA GROUP PLC

(Exact name of registrant as specified in its charter)

4th Floor, One Cavendish Place,

London, W1G 0QF

United Kingdom

(Address of principal executive offices, including zip code)

+44-333-023-7300

(Registrant’s telephone number, including area code)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

☐ Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

☐ Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

☐ Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

☐ Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

Securities registered pursuant to Section 12(b) of the Act:

*Not for trading, but only in connection with the listing of the American Depositary Shares on The Nasdaq Stock Market LLC.

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§240.12b-2 of this chapter).

Emerging growth company ☐

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act. ☐

On December 29, 2025, Mereo BioPharma Group plc (“Mereo” or the “Company”) issued a press release announcing the results from the Phase 3 Orbit and Cosmic studies evaluating setrusumab (UX143) in pediatric and young adult patients with osteogenesis imperfecta (OI). A copy of the press release is attached hereto as Exhibit 99.1.

The information contained in this Item 7.01, including Exhibit 99.1 shall not be deemed “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), or otherwise subject to the liability of that section, nor shall it be deemed incorporated by reference in any filing under the Securities Act of 1933, as amended (the “Securities Act”) or the Exchange Act, except as shall be expressly set forth by reference in such a filing.

Item 8.01 Other Events.

On December 29, 2025, the Company announced the results from the Phase 3 Orbit and Cosmic studies evaluating setrusumab (UX143) in pediatric and young adult patients with osteogenesis imperfecta (OI). Neither study achieved statistical significance against the primary endpoints of reduction in annualized clinical fracture rate compared to placebo or bisphosphonates, respectively. Both studies achieved their secondary endpoints of improvements in bone mineral density (BMD) against comparators (placebo and bisphosphonates) with strong statistical significance. There was no change in the safety profile observed.

ORBIT and COSMIC bone mineral density (BMD) improvements

In the Orbit study, participants experienced statistically significant and substantial improvements in BMD compared to placebo, at levels consistent with the treatment effect observed in Phase 2 studies. These BMD changes were not accompanied by a corresponding reduction in annualized fracture rates and there was a low fracture rate in the placebo group.

In the pediatric Cosmic study, patients had a substantially higher baseline fracture rate compared to the patients enrolled in Orbit. In this younger patient population, meaningful improvements in BMD were associated with a reduction in annualized fracture rate for setrusumab treated patients over bisphosphonate treated patients, though the reduction did not meet statistical significance.

Additional analyses on the data across both studies are being conducted, including in other bone health and clinical endpoints beyond fractures.

Forward Looking Statements

This Current Report on Form 8-K contains “forward-looking statements.” All statements other than statements of historical fact contained herein are forward-looking statements within the meaning of Section 27A of the Securities Act, and Section 21E of the Exchange Act. Forward-looking statements usually relate to future events and anticipated revenues, earnings, cash flows or other aspects of the Company’s operations or operating results. Forward-looking statements are often identified by the words “believe,” “expect,” “anticipate,” “plan,” “intend,” “foresee,” “should,” “would,” “could,” “may,” “estimate,” “outlook” and similar expressions, including the negative thereof. The absence of these words, however, does not mean that the statements are not forward-looking. These forward-looking statements are based on Mereo’s current expectations, beliefs and assumptions concerning future developments and business conditions and their potential effect on Mereo. While management believes that these forward-looking statements are reasonable as and when made, there can be no assurance that future developments affecting Mereo will be those that it anticipates. All of Mereo’s forward-looking statements involve known and unknown risks and uncertainties some of which are significant or beyond its control and assumptions that could cause actual results to differ materially from Mereo’s historical experience and its present expectations or projections. Such risks and uncertainties include, among others, the uncertainties inherent in the clinical development process; Mereo’s reliance on third parties to conduct and provide funding for its clinical trials; Mereo’s dependence on enrollment of patients in its clinical trials; and Mereo’s dependence on its key executives. You should carefully consider the foregoing factors and the other risks and uncertainties that affect Mereo’s business, including those described in the “Risk Factors” section of its latest Annual Report on Form 10-K, as well as discussions of potential risks, uncertainties, and other important factors in Mereo’s subsequent filings with the Securities and Exchange Commission. Mereo wishes to caution you not to place undue reliance on any forward-looking statements, which speak only as of the date hereof. Mereo undertakes no obligation to publicly update or revise any of our forward-looking statements after the date they are made, whether as a result of new information, future events or otherwise, except to the extent required by law.

Item 9.01 Financial Statements and Exhibits.

The following exhibit relating to Item 7.01 shall be deemed to be furnished, and not filed:

(d) Exhibits

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, as amended, the registrant has duly caused this report to be signed on its behalf by the undersigned, hereunto duly authorized.