In vitro aspirin has little additional anti-platelet effect in the presenceof prasugrel active metabolite

Philip D. M. Leadbeater1, Paul C. J. Armstrong2, Nicholas S. Kirkby1,2, Melissa V. Chan2, Joseph A. Jakubowski3, Jane A. Mitchell1, Timothy D. Warner1. 1National Heart & Lung Institute, Imperial College, London SW3 6LY, United Kingdom, 2William Harvey Research Institute, Barts & the London School of Medicine, London EC1M 6BQ, United Kingdom, 3Lilly Research Laboratories, Eli Lilly and Company, Indiana 46285, United States.

Dual anti-platelet therapy is commonly used to protect against secondary thrombotic risk with the idea that platelet activation dependent upon thromboxane A2 (TXA2) will be blocked by aspirin and that activation dependent upon P2Y12 receptors will be blocked by thienopyridines. However, blockade of platelet P2Y12 receptors also blocks TXA2–dependent aggregation and platelet production of TXA2. These latter observations have led us to the hypothesis that aspirin may have only little or no additional anti-aggregatory effects in the presence of strong P2Y12 receptor blockade. We have investigated this idea in vitro using combinations of prasugrel-active metabolite (PAM) and aspirin.

Platelet-rich plasma was prepared from citrated whole blood and incubated for 30 minutes at 37°C with vehicle, PAM, aspirin or PAM+aspirin in combination. Incubates were used to determine platelet reactivity by 96-well light transmission aggregometry, ATP+ADP release and platelet TXA2 production in response to a range of agonists (n = 4-6 for all).

PAM (3μM) inhibited (P < 0.05, two-way ANOVA with Bonferroni post-test) the platelet production of TXA2 induced by arachidonic acid (AUC: control, 187±41; PAM, 58±18), collagen (control, 171±29; PAM, 91±18) and epinephrine (control, 13±2; PAM, 6±2) and the ATP+ADP release in response to U46619 (control, 0.98±0.11; PAM, 0.20±0.04), TRAP-6 amide (control, 0.95±0.11; PAM, 0.66±0.08) and collagen (control, 0.56±0.15; PAM, 0.30±0.07). Addition of aspirin further reduced the production of TXA2 but not the release of ATP+ADP. In full agonist concentration response curves, PAM inhibited platelet aggregation induced by all agonists with no extra inhibition following from addition of aspirin (30μM), with the exception of collagen for which a weak additional effect was noted (P < 0.05, two-way ANOVA with Bonferroni post-test; AUC: control, 143±3; PAM, 48±4; PAM+aspirin, 38±2).

We have shown in vitro that PAM alone inhibits P2Y12 and TXA2-driven platelet aggregation and secondary mediator release with little additional effect of aspirin. Given the increased risk of bleeding events associated with aspirin, its overall benefit as an additional anti-platelet therapy to potent thienopyridines warrants further investigation through appropriate clinical studies.