Background: GP IIb/IIIa inhibitors have primarily been used short-term e.g., during PTCA. They failed to show clinical benefit during long-term therapy. One reason might be the absence of a method to monitor inhibitor activity. This study compared platelet aggregometry, the rapid platelet function analyzer (RPFA) test, single platelet counting, and flow cytometric determination of receptor occupancy to measure GP IIb/IIIa-receptor inhibitor activity.
Methods: Increasing doses of abciximab, tirofiban, and eptifibatide were added to whole blood in vitro. Whole blood was used for the RPFA, for single platelet counting and flow cytometry. Platelet rich plasma was prepared for aggregometry.
Results: The correlation between aggregometry and RPFA results was linear for abciximab and eptifibatide. Tirofiban was a stronger inhibitor with the RPFA (IC(50) 7.7nM) than with aggregometry (IC(50) 19.6nM). The single platelet counting technique showed that even supratherapeutic concentrations of all three inhibitors could not completely suppress microaggregation. Abciximab concentrations that were equipotent to tirofiban with aggregometry were less potent with regards to the inhibition of microaggregation. This difference was more pronounced with TRAP induced microaggregation than with ADP. The flow cytometric receptor occupancy test showed that occupancy was 95% with 5 microg/ml abciximab and almost 97% with 10 microg/ml. Tirofiban reached a maximum receptor occupancy of 56%, eptifibatide 64%.
Conclusions: While aggregometry is time consuming the RPFA provides results fast and with little variability. There is still a discrepancy between aggregometry and RPFA results for tirofiban. The single platelet counting technique detects the inhibition of microaggregation the relevance of which for the clinical outcome is not known. The flow cytometric receptor occupancy assay is best suited for abciximab.