Paper Details

ABSTRACT

Mast cells play a central role in immune surveillance and drive symptoms in systemic mastocytosis, chronic spontaneous urticaria, and other mast cell–driven diseases. In systemic mastocytosis, mast cells accumulate in the skin and extracutaneous tissues, including the bone marrow, liver, spleen, and gastrointestinal tract, leading to symptoms such as hives, abdominal pain, fatigue, and an increased risk of anaphylaxis. Despite recent advances with KIT-targeted therapies, significant unmet need remains. Current approaches require prolonged treatment to alleviate symptoms, do not completely eliminate clonal mast cells, and affect other KIT-expressing cell populations leading to toxicities. Because MRGPRX2 expression is restricted to mast cells, including those that accumulate in systemic mastocytosis, it represents an attractive target for selective mast cell elimination.

We have developed a Cytotoxicity Targeting Chimera (CyTAC™) platform designed to bind target-expressing cells and, when combined with a platform antibody, eliminate those cells . Here, we describe the identification of an MRGPRX2-targeting CyTAC that potently and selectively binds MRGPRX2 on mast cells. When combined with an anti-CD3 bispecific T cell–engaging platform antibody, the MRGPRX2 CyTAC is designed to eliminate target mast cells through effector T cell–mediated killing. Importantly, this mechanism of action does not rely on receptor antagonism for therapeutic activity but depends on triggering T cell engager activity provided by the bispecific CD3 antibody, representing a key differentiator from other MRGPRX2-directed agents.

Chemistry research identified a potent and selective MRGPRX2 CyTAC with 2 nM potency in a T cell activation reporter assay. We designed a CD3 bispecific platform antibody with detuned binding affinity to prevent T cell activation in the absence of the CyTAC and target cells. The combination of the MRGPRX2 CyTAC and CD3 bispecific platform antibody, STX-0640, is a two-component agent designed to deplete mast cells. STX-0640 effectively killed LAD2 mast cells, a human mastocytosis-derived mast cell line, in co-culture with T cells in a dose-dependent manner (EC50 = 0.7 nM) without inducing mast cell degranulation. Mast cell depletion was also demonstrated in a human skin explant model in which STX-0640 treatment triggered mast cell depletion without the addition of exogenous effector cells demonstrating endogenous immune engagement.

In vivo, the MRGPRX2 CyTAC combined with a mouse surrogate CD3 bispecific antibody achieved 79–92% depletion of peritoneal mast cells in 7/7 human MRGPRX2 knock-in (hMRGPRX2) mice. Because mast cells accumulate in the skin in systemic mastocytosis, tissue-resident mast cell depletion was also assessed in vivo. Deep skin-resident mast cell depletion (90–99%) and improved skin inflammation were observed in 9/9 hMRGPRX2 mice in an oxazolone-induced atopic dermatitis model. In vivo, STX-0640 consistently achieved profound mast cell depletion, reducing mast cells by 79–99% depending on tissue compartment.

Importantly, STX-0640 was more rapid, effective, and selective than a c-KIT antibody in depleting mast cells in vivo. In the atopic dermatitis model, average mast cell depletion was 96% with STX-0640 compared with 28% with the c-KIT antibody. In addition, STX-0640 reduced peritoneal mast cells by 79–92% within 2 days, whereas the c-KIT antibody failed to measurably reduce mast cells over 8 days. Unlike the c-KIT antibody, which reduced peritoneal eosinophils by approximately 90%, STX-0640 selectively depleted mast cells without affecting eosinophils, supporting the highly selective expression of MRGPRX2 on mast cells.

STX-0640 was generally well tolerated in mouse and non-human primate studies, with no cytokine increases observed in any animals, at and above therapeutically relevant exposures. Translational studies are underway to further characterize MRGPRX2 expression in skin and bone marrow mast cells from patients with systemic mastocytosis and to evaluate STX-0640-mediated depletion of primary patient-derived mast cells.