Mechanisms of allergy and clinical immunology: A novel druglike spleen tyrosine kinase binder prevents anaphylactic shock when administered orally

Background: The spleen tyrosine kinase (Syk) is recognized as a potential pharmaceutical target for the treatment of type I hypersensitivity reactions including allergic rhinitis, urticaria, asthma, and anaphylaxis because of its critical position upstream of immunoreceptor signaling complexes that regulate inflammatory responses in leukocytes. Objective: Our aim was to improve the selectivity of anti-Syk therapies by impeding the interaction of Syk with its cellular partners, instead of targeting its catalytic site. Methods: We have previously studied the inhibitory effects of the anti-Syk intracellular antibody G4G11 on FcRI-induced release of allergic mediators. A compound collection was screened by using an antibody displacement assay to identify functional mimics of G4G11 that act as potential inhibitors of the allergic response. The effects of the selected druglike compounds on mast cell activation were evaluated in vitro and in vivo. Results: We discovered compound 13, a small molecule that inhibits FcRI-induced mast cell degranulation in vitro and anaphylactic shock in vivo. Importantly, compound 13 was efficient when administered orally to mice. Structural analysis, docking, and site-directed mutagenesis allowed us to identify the binding cavity of this compound, located at the interface between the 2 Src homology 2 domains and the interdomain A of Syk. Conclusion: We have isolated a new class of druglike compounds that modulate the interaction of Syk with some of its macromolecular substrates implicated in the degranulation pathway in mast cells.