Inhibition of Immune Thrombocytopenic Purpura (ITP) by an Orally Bioavailable Inhibitor of Syk Kinase Activity.

Immune Thrombocytopenic Purpura (ITP) is an autoimmune disease characterised by the destruction of platelets by pathogenic autoantibodies. Clearance of platelets by phagocytic cells such as macrophages occurs through activating receptors for the Fc portion of IgG (FcγR). A central tyrosine kinase in this activation pathway is Syk, which is required for FcγR-mediated phagocytosis; thus, we questioned whether inhibition of Syk would ameliorate ITP. R406 is an orally bioavailable Syk-kinase inhibitor which is well tolerated by human volunteers. Using a well-described murine model of ITP, we tested the ability of R788 (the prodrug of R406) to ameliorate thrombocytopenia. Mice injected with an antibody directed to integrin α_IIβ were profoundly thrombocytopenic when platelets were enumerated 24 hr post injection. However, mice pre-treated with 25 or 40 mg/kg R788 were protected from thrombocytopenia. Mice pre-treated with the vehicle alone displayed no protection. It has been demonstrated that chimeric Syk deficient mice display a bleeding diathesis; it was therefore questioned whether treatment of mice would demonstrate prolonged tail vein bleeding time. In independent experiments, mice treated with the with the bioavailable inhibitor R406 had no significant difference in bleeding time compared to untreated mice or mice treated with vehicle alone. Syk is also involved in collagen-induced platelet secretion and aggregation. To further the potential use of this drug in humans, the ability of platelets derived from human volunteers treated with R788 to respond to in vitro collagen-induced aggregation was analysed. Platelets from volunteers treated with the drug displayed normal collagen (as well as ADP)-induced platelet aggregation compared to untreated volunteers or those treated with the vehicle alone. These data suggest that future investigation of Syk inhibition in autoimmune diseases such as ITP is warranted.