On May 19, 2026 Mestag Therapeutics, a clinical-stage biotech company harnessing fibroblast immunology for the benefit of patients with inflammatory disease and cancer, reported the dosing of the first patient in a Phase 1 clinical trial evaluating MST-0312 in patients with selected advanced solid tumors. The first in human trial, named the STARLYS trial, is an adaptive, modular multi-part, multi-arm open-label study designed to evaluate the safety, tolerability, pharmacodynamics, pharmacokinetics and anti-tumor activity of MST-0312 alone and in combination with pembrolizumab.
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MST-0312 is a bispecific antibody designed to activate lymphotoxin-beta receptor (LTBR) in the tumor microenvironment to induce the formation of tertiary lymphoid structures (TLS) and associated high endothelial venules (HEV) in tumor tissue. TLS are a hallmark of effective anti-tumor immunity consisting of aggregates of T, B and dendritic cells, associated with the recruitment, education, and activation of immune cells to drive anti-tumor immune responsesi,ii. Patients with TLS and HEV in their tumors show significantly improved response to treatment and extended survival outcomes compared to patients whose tumors lack these structuresiii,iv,v,vi. The STARLYS trial will initially evaluate MST-0312 in tumors formed in barrier organs (lung, gut, bladder, breast and skin), which are believed to be particularly sensitive to TLS formation.
"Dosing the first patient with MST-0312 is a significant milestone in developing this potential new therapeutic class," said Dr. Lindsey Rolfe, MBChB, Chief Medical Officer of Mestag. "Our carefully designed study evaluates monotherapy and combination therapy in immunologically ‘cold’ and ‘warm’ tumors, generating multiple mechanistic and clinical insights to inform future development. This important step reflects the scientific rigor and dedication of the Mestag team, as we advance novel therapies for patients."
Dr. Emiliano Calvo MD PhD, a Principal Investigator of the study and Director of Clinical Research at START Madrid-CIOCC (Centro Integral Oncológico Clara Campal) Hospital in Madrid, Spain where the first patient was dosed, said "MST-0312 is an exciting new investigative approach for the treatment of solid tumors and we are thrilled to have dosed the first patient in the STARLYS trial."
Dr. Elena Garralda MD PhD, Co-Director of the Clinical Research Program and Head of Early Drug Development at Vall d’Hebron Institute of Oncology in Barcelona, Spain, and coordinating Principal Investigator of the STARLYS trial, added, "Published data show that the presence of TLS is associated with significantly improved outcomes for patients. Despite recent therapeutic advances, many patients with solid tumors derive limited benefit from current therapies. MST-0312 is designed to address this unmet need by inducing TLS and reshaping anti-tumor immunity. I look forward to working with the STARLYS investigators to evaluate MST-0312 in the clinic."
(Press release, Mestag Therapeutics, MAY 19, 2026, View Source [SID1234665844])