Effect of phosphodiesterase inhibitors on platelet function

Phosphodiesterase enzymes (PDEs) play a pivotal role in regulating platelet activity by modulating intracellular levels of cAMP and cGMP. Modulation of PDE-2, -3 and -5 activity by suitable inhibitors has been found to reduce platelet activity, and thus thrombus formation. Our aim was to study Ibudilast effects on platelet activation, degranulation and aggregation. Therefore, we used the nonspecific PDE inhibitors IBMX as well as the PDE-5 inhibitor Sildenafil as controls. Platelet agonists collagen-related peptide (CRP-A), adenosine diphosphate (ADP) and thrombin receptor activator peptide (TRAP6) were used to induce distinct activation pathways. PDE inhibition was quantified by western blot analysis. Platelet activity was assessed using flow cytometry, light transmission aggregometry and in vitro thrombus formation. Inhibition of all platelet PDEs by IBMX substantially reduced platelet activation and aggregation in response to all tested platelet agonists. Ibudilast preferentially inhibits PDE-3 in platelets. Ibudilast decreased platelet activation and aggregation induced by ADP and TRAP, but not CRP-A. Sildenafil alone induced no reduction in PDE activity, platelet activation or aggregation. However, the combination of Sildenafil and Ibudilast had an additive effect on platelet activation. Interestingly, all tested PDE inhibitors demonstrated a significant effect on platelet-dependent thrombus formation. In conclusion, the effect of PDE inhibitors on platelet function is influenced by two primary factors: the pharmacological target of the inhibitor and the cAMP/cGMP interaction with the activation pathways induced. Platelet activation by ADP via P₂Y₁₂ and TRAP via PAR1 showed a greater response to PDE inhibitors than platelet activation by CRP via GPVI.