Mestag Therapeutics has secured $40 million as it prepares to translate its fibroblast-focused cancer strategy into the clinic. The financing announced Tuesday will fund the Phase I STARLYS study of MST-0312, a bispecific antibody designed to alter immune activity inside tumours.The Cambridge, UK-based biotech was built around research suggesting that fibroblasts — connective-tissue cells long viewed mainly as structural components — play an important role in shaping immune responses in diseased tissue. The idea has caught the attention of some big-name pharma companies; both Johnson & Johnson and Merck & Co. have struck deals with Mestag, the latter valued at up to $1.9 billion.Organised immune-cell clustersMST-0312 is designed to exploit this fibroblast-immune interaction. The antibody binds fibroblast activation protein (FAP), which is highly expressed on cancer-associated fibroblasts, while simultaneously activating the lymphotoxin-β receptor (LTBR), a signalling pathway involved in forming lymphoid tissues. By activating LTBR only where FAP is present, the company hopes to promote the formation of immune cell hubs known as tertiary lymphoid structures (TLSs) that attract and train immune cells to attack cancer.Mestag says an extensive body of clinical data suggest the presence of TLS along with specialised blood vessels called high endothelial venules (HEVs) in solid tumours improves patient survival and enhances response to therapy.Roche was working on an FAP-LTBR bispecific called RO7567132 (RG6221) in Phase I testing for advanced solid tumours, but the programme was scrapped earlier this year as the Swiss pharma diversifies into new therapeutic areas."Our programme, designed entirely independently from Roche's approach, uses a different format and in our hands displays functional differentiation in human in vitro testing which we believe may make it more potent in clinic," Mestag CEO Susan Hill told FirstWord.Mid-year study startThe STARLYS study of MST-0312 is expected to begin enrolling people with solid tumours in mid-2026. The company plans to focus initially on cancers arising in so-called barrier organs, such as skin, lung or gut, that are known to be associated with active TLS biology.The financing was backed by existing investors including SV Health Investors, Johnson & Johnson Innovation – JJDC, Forbion, GV and Northpond Ventures, and brings Mestag's total capital committed since its 2021 founding to more than $95 million.The biotech also appointed Lindsey Rolfe as chief medical officer and Pascal Merchiers as chief development officer as it prepares to begin clinical development.