Specific Pharmacological Targeting of the Syk Kinase Activity in Platelets: A Novel, Safe Anti-Thrombotic Strategy

Studies of the Syk −/− mouse have implicated spleen tyrosine kinase (Syk), a signaling protein with both kinase and scaffolding activities, in platelet signaling following engagement of GPVI and αIIbβ3 by collagen and fibrinogen, respectively. The present study was designed to determine whether specific inhibition of the kinase activity of Syk, without targeting the Syk scaffolding function, affected in vivo arterial thrombosis. In preliminary experiments, blood from wild-type and Syk−/− mice was perfused through collagen-coated capillaries under arterial shear rates to study ex vivo thrombosis. While blood from wild-type mice formed robust thrombi (37±4.7 μm³/μm²), none was observed in Syk^−/− mice. Thrombi intermediate in size (16±3.9 μm³/μm²) developed in Syk^+/− mice. To achieve specific pharmacological targeting of the kinase activity of Syk, P142-76, a potent (IC₅₀ = 4 nM) and selective Syk kinase inhibitor was utilized. P142-76 was screened against a broad panel of 139 purified kinases at 50 nM. While Syk kinase was inhibited by 92%, all other kinases retained more than 70% of their activity. In washed human platelets, P142-76 inhibited convulxin (CVX)-induced phosphorylation of LAT (linker for activation of T-cells; IC₅₀ = 111 nM) and intracellular calcium increases (IC₅₀ = 31 nM). The GPVI/Syk-specificity of P142-76 activity was confirmed by its inability to inhibit intracellular calcium increases induced by the PAR1 thrombin receptor agonist TRAP. P142-76 also inhibited CVX-induced aggregation of both human washed platelets (IC₅₀ = 87 nM) and platelet-rich plasma (IC₅₀ = 2.5 μM). Considering the controversial data in respect to the participation of GPVI in arterial thrombosis in murine models, the dependence of arterial thrombosis on Syk function was studied in vivo in pigs. Cross-species activity of P142-76 was confirmed in vitro (CVX-induced PRP aggregation IC₅₀= 350 nM; 5 μM P142-76 completely inhibited thrombosis triggered by collagen in the perfusion chamber assay). At a plasma concentration which abolished ex vivo CVX-induced but not ADP-induced pig platelet aggregation, P142-76 significantly inhibited the deposition of [¹¹¹In]-labeled platelets in a carotid artery crush swine thrombosis model, without compromising primary hemostasis.

To clarify further the contribution of the kinase activity of Syk to arterial thrombosis, effects of P142-76 on human blood were evaluated in real time in the collagen-coated perfusion chamber. Low concentrations of P142-76 (0.3 μM) affected thrombus stability, while increasing concentrations (1–5 μM) delayed and then completely inhibited thrombus formation. Furthermore, P142-76 destabilized pre-formed thrombi, indicating a critical role for Syk in conferring strength to platelet-platelet interactions, i.e. αIIbβ3-mediated cohesion. Our data indicate that the kinase activity of Syk acts in arterial thrombosis through at least two distinct mechanisms. First, Syk kinase confers stability to platelet-platelet interactions downstream of αIIbβ3. Second, it initiates thrombus formation on collagen surfaces. This dual activity of the kinase activity of Syk makes it a preferred target for inhibition of arterial thrombosis, as it does not compromise primary hemostasis.