Mestag's New Drug Aims to Build Immune Forts Inside Tumors

CAMBRIDGE, UK – May 19, 2026 – In a significant move that could reshape cancer therapy, clinical-stage biotech firm Mestag Therapeutics announced today it has dosed the first patient in a clinical trial for a drug designed to fundamentally re-engineer a tumor's environment. The Phase I study, named STARLYS, will test MST-0312, a novel antibody therapy aimed at patients with advanced solid tumors, some of the most challenging cancers to treat.

This first-in-human trial marks a critical milestone for a new class of therapeutics that don't just attack cancer cells directly or unmask them for the immune system, but instead seek to force the tumor itself to build the very structures needed for a robust immune assault. The study will evaluate MST-0312 both as a standalone treatment and in combination with the blockbuster PD-1 inhibitor, Keytruda (pembrolizumab).

Engineering an Immune Stronghold

The central challenge in modern oncology is that many tumors are immunologically "cold." They lack the necessary immune cells to be detected and destroyed, rendering powerful immunotherapies like checkpoint inhibitors ineffective for a large portion of patients. Mestag's approach with MST-0312 aims to solve this by turning these cold tumors "hot."

The drug is a bispecific antibody engineered to perform a highly specific, two-step function. First, it targets a protein called Fibroblast Activation Protein (FAP), which is abundant on cancer-associated fibroblasts (CAFs)—cells that help create a supportive, immunosuppressive structure for the tumor. Once anchored to these fibroblasts, the second part of MST-0312 activates a receptor known as the lymphotoxin-beta receptor (LTBR).

Activating this receptor is the key to the entire strategy. It is designed to trigger a biological cascade that induces the formation of Tertiary Lymphoid Structures (TLS) and their associated High Endothelial Venules (HEV) directly within the tumor. In essence, TLS are like pop-up immune system training grounds—organized aggregates of T-cells, B-cells, and dendritic cells that can recruit, educate, and activate an army of cancer-fighting lymphocytes. The HEVs act as gateways, allowing these immune cells to travel from the bloodstream into the newly formed TLS.

An extensive body of published research has already correlated the natural presence of these structures in tumors with significantly better survival rates and improved responses to therapy. The STARLYS trial represents a pioneering effort to create them on demand.

Dr. Elena Garralda MD PhD, the coordinating Principal Investigator of the STARLYS trial and a leading oncology researcher at Vall d’Hebron Institute of Oncology in Barcelona, Spain, highlighted the potential of this approach. “Published data show that the presence of TLS is associated with significantly improved outcomes for patients,” she stated. “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.”

A New Front Against Hard-to-Treat Cancers

The STARLYS trial is structured to provide a wealth of information about this novel mechanism. As an adaptive, multi-arm study, it will assess the safety, tolerability, and anti-tumor activity of MST-0312. By testing the drug both alone and alongside Keytruda, researchers hope to see if it can not only work on its own but also make tumors that were previously resistant to checkpoint inhibitors newly vulnerable to them.

The initial patient cohorts will focus on advanced solid tumors in so-called "barrier organs"—the lung, gut, bladder, breast, and skin. These sites are believed to be particularly receptive to the formation of TLS, providing a strategic starting point for evaluating the drug's biological effect.

“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.”

The first patient was dosed at the START Madrid-CIOCC Hospital in Spain, a moment of considerable anticipation for the clinical team.

“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,” commented Dr. Emiliano Calvo MD PhD, a Principal Investigator of the study.

Mestag's Strategic Play in a Crowded Field

For Mestag Therapeutics, the launch of the STARLYS trial is a pivotal moment that could validate its entire scientific platform, which is centered on a new understanding of fibroblast immunology. The company was founded by SV Health Investors and is backed by a syndicate of top-tier life science investors, including GV (Google Ventures), Forbion, Northpond Ventures, and the venture arm of Johnson & Johnson.

This strong financial and strategic backing underscores the high level of confidence in Mestag's novel approach. The company has already forged significant partnerships with pharmaceutical giants, including a target discovery collaboration with MSD (Merck) and a prior licensing deal with Johnson & Johnson's Janssen subsidiary. These agreements signal that the broader industry sees significant value in Mestag's specialized fibroblast-immune RAFT Platform.

While other companies are exploring ways to modulate the tumor microenvironment, Mestag's FAP-targeted, conditional LTBR activation is a highly differentiated strategy. By ensuring the immune-stimulating signal is delivered specifically within the tumor via FAP-expressing cells, MST-0312 is designed to maximize local efficacy while potentially minimizing the systemic side effects that can plague other immunotherapies. The success of this trial could position Mestag as a leader in overcoming one of the most significant hurdles in cancer treatment and could pave the way for a new pillar of immunotherapy.

The journey through clinical trials is long and fraught with uncertainty, but for patients with advanced cancers and limited options, the STARLYS trial represents a tangible and scientifically innovative new form of hope.