Arcus Biosciences (NYSE: RCUS) has registered a first-in-human trial of AB102, an oral small-molecule antagonist of MRGPRX2 (Mas-related G-protein-coupled receptor X2), marking the company's first inflammation program to reach clinical testing. The move signals a deliberate expansion beyond Arcus's established oncology pipeline into immunology, targeting a receptor implicated in non-IgE-dependent mast cell activation — a mechanism distinct from the biology addressed by most approved biologics in atopic dermatitis and chronic spontaneous urticaria.
The Phase I study (NCT07662057) is a double-blind, randomized, placebo-controlled ascending dose trial enrolling approximately 130 healthy volunteers aged 18 to 55. Structured as a sequential single ascending dose (SAD) followed by multiple ascending dose (MAD) design, the trial is scheduled to begin in Q3 2026, with primary completion anticipated by December 2026. Primary endpoints focus on characterizing AB102's pharmacokinetic profile — including plasma exposure (AUC), peak concentration (Cmax), time to peak (Tmax), terminal half-life, and accumulation ratios across repeated dosing — alongside adverse event monitoring over up to 49 days. Participants with a history of atopy, drug hypersensitivity, or recent cannabis use are excluded, a design choice consistent with the drug's mechanism and the need for a clean immunological baseline in this drug safety tolerability study.
MRGPRX2 is a non-canonical GPCR expressed on human mast cells that mediates pseudoallergic degranulation — the release of histamine, tryptase, and inflammatory mediators — in response to a range of endogenous peptides, neuropeptides, and exogenous compounds, independently of IgE receptor signaling. Arcus has reported that AB102 fully blocks MRGPRX2-dependent degranulation and transcriptional activation in both LAD2 and primary skin mast cells, and inhibits all common human MRGPRX2 variants. The company has also presented data suggesting that a MRGPRX2-specific activation signature is enriched in atopic dermatitis skin lesions and wheals from chronic spontaneous urticaria, providing the disease-biology rationale for the intended indications.