Mestag's FAP-targeted lymphotoxin agonist aims to engineer tertiary lymphoid structures in cold tumors

Mestag Therapeutics, a Cambridge, UK-based clinical-stage biotech, has dosed the first patient in the [STARLYS trial](https://www.globenewswire.com/news-release/2026/05/19/3297161/0/en/Mestag-Therapeutics-Announces-the-First-Patient-Dosed-with-MST-0312-in-the-Phase-I-STARLYS-Trial.html), a Phase I study evaluating [MST-0312](https://app.allsci.com/clinical-trial/ASC-CT-0000001155857-1.0-1774289584), a fibroblast-activated protein (FAP)-targeted lymphotoxin-beta agonist bispecific antibody, in patients with advanced solid tumors. The first patient was dosed at START Madrid-CIOCC Hospital in Spain. MST-0312 is designed to activate lymphotoxin-beta receptor (LTBR) within the tumor microenvironment, with the goal of inducing tertiary lymphoid structures (TLS) and associated high endothelial venules (HEV) — organized immune aggregates whose presence in tumors has been correlated with improved survival and treatment response across multiple solid tumor types. The adaptive, open-label study will evaluate the drug as monotherapy and in combination with pembrolizumab (Keytruda, Merck) across tumors arising in barrier organs, specifically lung, gut, bladder, breast, and skin. The trial's design reflects an interest in both immunologically cold and warm tumors, with pharmacodynamic endpoints focused on TLS and HEV formation alongside conventional safety, tolerability, pharmacokinetics, and anti-tumor activity assessments. Specific enrollment numbers and a trial completion date were not disclosed in the announcement. ## Scientific and clinical context The premise underlying MST-0312's development is that durable anti-tumor immunity depends not only on the presence of immune cells in tumors, but on their spatial organization. TLS — ectopic lymphoid aggregates comprising T cells, B cells, and dendritic cells — function analogously to secondary lymphoid organs, supporting local immune priming and effector cell activation. Published data cited by Mestag, including work from Teillaud et al. in Nature Reviews Cancer (2024) and Schumacher et al. in Science (2022), describe TLS as a hallmark of effective anti-tumor immunity. Separate clinical correlative studies across gastrointestinal, lung, and other cancers have associated TLS presence with extended survival and enhanced response to checkpoint inhibition. The current immunotherapy landscape has expanded substantially with PD-1 and PD-L1 inhibitors, yet a large proportion of patients with solid tumors — particularly those with microsatellite-stable colorectal cancer, immunologically cold TNBC, and post-checkpoint-progression NSCLC — derive limited benefit. The mechanistic gap is partly structural: without organized lymphoid infrastructure in the tumor, even pharmacologically active immune cells may fail to mount a coordinated response. MST-0312's approach of actively constructing that infrastructure, rather than simply releasing existing immune brakes, represents a departure from the dominant checkpoint inhibitor paradigm. The FAP-targeting element of MST-0312's design is relevant here. FAP is expressed on cancer-associated fibroblasts, which are enriched in the tumor stroma and have been implicated in immune exclusion. By directing LTBR agonism specifically to FAP-expressing stromal cells, [MST-0312 aims to localize TLS-inducing signals](https://app.allsci.com/news/ASC-NR-0000000463049-1.0-1773317174?query=mestag) within the tumor microenvironment rather than driving systemic lymphoid activation — a design choice with potential implications for both efficacy and tolerability. The selection of barrier organ tumors as the initial STARLYS cohort is grounded in biological reasoning: tissues such as gut, lung, and skin are embryologically conditioned for lymphoid structure formation, and TLS induction may be more tractable in these contexts than in other tumor types. ## Competitive landscape The TLS-induction space remains early, but Mestag is not alone in pursuing it. Several programs are targeting components of the lymphotoxin pathway or broader stromal remodeling strategies to reshape tumor immunity, though most remain preclinical or in very early clinical evaluation. The LTBR agonism approach is relatively novel at the clinical stage, and MST-0312 appears to be among the first FAP-targeted LTBR agonists to enter human trials. More broadly, the solid tumor immunotherapy field has seen diverging mechanistic strategies. Iovance Biotherapeutics' lifileucel (Amtagvi), a tumor-infiltrating lymphocyte therapy approved for melanoma in 2024, addresses immune cell quantity and quality rather than spatial organization. Checkpoint combinations — including nivolumab plus ipilimumab and enfortumab vedotin plus pembrolizumab in urothelial cancer — have expanded response rates but have not resolved the cold tumor problem. The pembrolizumab combination arm within STARLYS reflects an intent to test whether TLS induction can convert cold tumors into checkpoint-responsive ones, a hypothesis with direct clinical relevance given pembrolizumab's established role across several of the targeted indications. Mestag's broader pipeline includes the M402 program in inflammatory disease and earlier discovery-stage assets developed through its fibroblast-immune RAFT Platform, including a collaboration with MSD. The company previously licensed a target to Johnson & Johnson under a 2021 agreement with Janssen Biotech. Investors include Johnson & Johnson Innovation – JJDC, Forbion, GV, and Northpond Ventures. The STARLYS trial's adaptive, multi-arm design is intended to generate mechanistic and clinical data across tumor contexts simultaneously, with TLS and HEV formation as pharmacodynamic readouts that could inform patient selection criteria for later-stage development. Whether MST-0312 can reliably induce TLS in human tumors, and whether induced TLS translates into clinical benefit, are the central questions the trial is designed to begin answering. *** This article was generated with AI assistance and reviewed and edited by the AllSci editorial team Explore more at AllSci News: [https://allsci.com/news/](https://allsci.com/news/) --- Spot something wrong? [Report an issue with this article](https://newsgen-prod.reframedata.com/feedback/mst-0312-phase-i-trial-mestags-fap-targeted)