Comparison of GP IIB/IIIA inhibitors and their activity as measured by aggregometry, flow cytometry, single platelet counting, and the rapid plateletfunction analyzer

Background: GP IIb/IIIa inhibitors have primarily been used short-term e.g. , during PTCA. They failed to show clinical benefit during long-term therap y. One reason might be the absence of a method to monitor inhibitor activit y. This study compared platelet aggregometry, the rapid platelet function a nalyzer (RPFA) test, single platelet counting, and flow cytometric determin ation of receptor occupancy to measure GP IIb/IIIa-receptor inhibitor activ ity. Methods: Increasing doses of abciximab, tirofiban, and eptifibatide were ad ded 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 f or abciximab and eptifibatide. Tirofiban was a stronger inhibitor with the RPFA (IC50 7.7 nM) than with aggregometry (IC50 19.6 nM). The single platel et counting technique showed that even supratherapeutic concentrations of a ll three inhibitors could not completely suppress microaggregation. Abcixim ab concentrations that were equipotent to tirofiban with aggregometry were less potent with regards to the inhibition of microaggregation. This differ ence was more pronounced with TRAP induced microaggregation than with ADP. The flow cytometric receptor occupancy test showed that occupancy was 95% w ith 5 mug/ml abciximab and almost 97% with 10 mug/ml. Tirofiban reached a m aximum 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 agg regometry and RPFA results for tirofiban. The single platelet counting tech nique 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.