Specific Inhibition of Syk Suppresses Leukocyte Immune Function and Alleviates Inflammation In Rodent Models of Rheumatoid Arthritis

Abstract 1727

Genetic ablation of Syk in hematopoietic cells blocks various leukocyte immune functions, and protects mice from immune-complex mediated inflammation. These data have helped to identify Syk as an important therapeutic target for immune-mediated diseases. The next step is to test the hypothesis that low level, and specific, pharmacological inhibition of Syk retains the immunomodulatory potential observed with Syk genetic deficiency. With this goal in mind, we provide an update on the development of P505-15, a highly specific and potent small molecule Syk inhibitor which suppresses signaling and activation of primary human and rodent leukocyte immune function. The specificity of P505-15 was tested in a panel of 270 independent purified kinase assays at 300nM. At this concentration, Syk and 8 other kinases were inhibited by ≥ 80%. Subsequent analysis demonstrated a Syk IC₅₀ of 1nM, whereas the next most potently inhibited kinase required an IC₅₀ of 81nM. In a variety of cellular assays we observed potent inhibition of B cell receptor (BCR) induced Syk signaling, but not of Lyn, phorbol 12-myristate 13-acetate (PMA) induced protein kinase C, T cell receptor induced Zap70, or cytokine induced JAK1 (IL6), JAK2 (GM-CSF), or JAK1/3 (IL4) dependent STAT phosphorylation. Consistently, in Ba/F3 cell lines transformed by various kinases, P505-15 only inhibited proliferation of those cells transformed by Syk (IC₅₀ = 0.12μM), and not by Zap70 or JAK family members (IC₅₀ > 6μM). In human whole blood, P505-15 suppressed BCR-induced Syk signaling and cellular activation with IC_50's of 0.383μM and 0.362μM, respectively. FceR1-induced basophil degranulation was similarly suppressed with an IC₅₀ of 0.171μM. Importantly, Syk-independent signaling and cellular activation in human whole blood via PMA (B cell assays) or fMLP (basophil degranulation) was unaffected by this compound at 4μM and 1μM, respectively (the highest concentrations tested), again demonstrating its specificity of action. Oral administration of P505-15 in mice led to a reversible inhibition of Syk, with an IC₅₀ of 0.282μM as determined by an ex vivo whole blood BCR stimulation assay. Finally, we tested the immunomodulatory potential of specific Syk inhibition in vivo using rodent models of rheumatoid arthritis. Oral administration of P505-15 resulted in statistically significant and dose-dependent anti-inflammatory activity in both the mouse collagen antibody-induced arthritis and rat collagen induced arthritis models. In each case, anti-inflammatory effects were achieved at sub-micromolar plasma concentrations in which Syk specificity was maintained. These data support the hypothesis that specific Syk inhibition can modulate immune function in vivo, and provide a therapeutic strategy for the treatment of human inflammatory disease by inhibition of this kinase. P505-15 is currently being evaluated in phase I clinical trials.