Associations Between The Proportions Of P2Y12 Receptor Blocked And/or Aspirin-Treated Platelets And The Size Of Aggregatory Responses

Thomas Hoefer1, Nicholas S Kirkby2, Jane A Mitchell2, Timothy D Warner1. 1Barts and the London School of Medicine and Dentistry, London, UK, 2Imperial College London, London, UK.

INTRODUCTION: It is often held that inhibition of platelet aggregation following aspirin ingestion may be negated by the presence of a small minority of uninhibited platelets. Indeed, it is often quoted that more than 95% of platelets are required to be inhibited by aspirin for full anti-thrombotic protection (Fitzgerald et al., 1983; Di Minno et al., 1983). Aspirin is an irreversible inhibitor of platelet cyclooxygenase, while the thienopyridines, notably clopidogrel and prasugrel, are irreversible blockers of platelet P2Y12 receptors. In analogy to studies of the effects of aspirin, we have investigated the relationships between the proportion of P2Y12 inhibited platelets and platelet aggregation responses using as agonists ADP, TRAP-6 amide and collagen.

METHODS: Citrated whole blood was incubated with 3microM prasugrel active metabolite (PAM) and/or 30microM aspirin, or vehicle at 37°C for 4 hours. Inhibitor- and vehicle-treated PRP were then combined in different proportions and tested for responses to 10microM ADP, 3microM TRAP-6 amide, 1microg/ml collagen or 1mM arachidonic acid using light transmission aggregometry (n = 5-8). Data are presented as mean±SEM.

RESULTS: Stimulation of platelets with 1µg/ml collagen produced a robust aggregation response in the absence of aspirin. This remained similar when up to 60% of platelets were treated with aspirin (e.g. treatment of 60% platelets with aspirin caused only 4±1% inhibition of aggregation). Further increase of proportions of inhibited platelets resulted in a steep decrease of responsiveness, with 58±5% inhibition of aggregation when 100% of platelets were treated with aspirin. The relationship of collagen-induced aggregation to proportion of PAM-treated platelets was similar to when treated with aspirin, while that to aspirin+PAM-treated platelets was almost linear. Aggregation in response to ADP or TRAP-6 amide was not sensitive to aspirin, but was linearly related to the proportion of PAM- or aspirin+PAM-treated platelets. However TRAP-6 amide induced aggregation was less PAM sensitive (40±13% inhibition when 100% of platelets were PAM-treated) than aggregation in response to ADP (88±5%).

CONCLUSIONS: These findings demonstrate a complex relationship between proportions of uninhibited platelets and platelet responsiveness. This demands further examination, particularly with regard to at risk patient groups in whom inhibited and uninhibited platelets may co-exist.

Fitzgerald GA, et al. (1983). J Clin Invest 71:676-688

Di Minno G, et al. (1983). Blood 61:1081-1085