Sareum Holdings PLC

("Sareum" or the "Company")

Update on toxicology study for SDC-1801

Cambridge, UK, 10 October 2025 - Sareum Holdings plc (AIM: SAR), a clinical-stage biotechnology company developing next-generation kinase inhibitors for autoimmune disease and cancer, announces that it has discontinued its 16-week GLP preclinical toxicology study for SDC-1801 following safety findings observed by the third-party provider of the study. The safety issues were encountered at a higher incidence in the control-group animals which were given an inactive dosing solution compared to those that were actually dosed with SDC-1801. Accordingly, dosing has now been terminated, and the study will be formally closed following completion of scheduled analyses and reporting activities.

The study was designed to support longer-term dosing of SDC-1801 and to investigate the molecule's general toxicology. These are key regulatory requirements ahead of the planned Phase 2 clinical development programme, which is expected to focus initially on psoriasis.

A preliminary assessment of the available data indicates that the findings observed during the study were disproportionately present in control-group animals which were not treated with SDC-1801, and based on the information available, are therefore considered highly unlikely to be related to SDC-1801. Sareum is reviewing the study data with the contractor and expert consultants to determine the cause of the findings and the appropriate next steps.

The Company is also in discussions with alternative providers with the aim of restarting the study as soon as possible. Despite this set back, the Company anticipates being able to complete the full toxicology study, as originally scoped, with its existing cash resources.

As previously reported, the Phase 1 clinical study of SDC-1801 in healthy volunteers met its primary objectives and characterised a pharmacokinetic profile consistent with once-daily dosing. No safety concerns were identified.

Dr Stephen Parker, Executive Chairman of Sareum, said: "While the need to terminate this study is frustrating, the preliminary data strongly suggest that the findings are unlikely to be associated with SDC-1801. Dosing has been terminated out of concern for animal welfare, and we are working closely with our partners and experts to understand the cause and identify a new provider with the aim of restarting the study as quickly as possible with our existing cash resources."

Sareum will provide a further update at the earliest possible opportunity once the review is complete.

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About Sareum

Sareum (AIM: SAR) is a biotechnology company developing next generation kinase inhibitors for autoimmune disease and cancer.

The Company is focused on developing next generation small molecules which modify the activity of the JAK kinase family and have best-in-class potential. Its lead candidate, SDC-1801, simultaneously inhibits TYK2 and JAK1. SDC-1801 is a potential treatment for a range of autoimmune diseases, with a planned initial focus on psoriasis.

Sareum is also developing SDC-1802, a TYK2/JAK1 inhibitor with a potential application for cancer immunotherapy, and has recently initiated a preclinical programme to develop TYK2/JAK1 inhibitors for neuroinflammatory diseases such as multiple sclerosis and Parkinson's disease.

The Company is the license holder for SRA737, a clinical-stage Checkpoint kinase 1 inhibitor that targets cancer cell replication and DNA damage repair mechanisms.

Sareum Holdings plc is based in Cambridge, UK, and is quoted on the AIM market of the London Stock Exchange, trading under the ticker SAR. For further information, please visit the Company's website at www.sareum.com