In vitro dose response to different GPIIb/IIIa-antagonists:: Inter-laboratory comparison of various platelet function tests

Aims: The aim of this study was to assess the inter- and intra-laboratory variation of the concentration - response to the GPIIb / IIIa-antagonists abciximab and eptifibatide on platelet aggregometry and to compare results with flow cytometric tests as well as the rapid platelet function analyser (RPFA). Methods: In five different laboratory sites, blood from three to five healthy donors was spiked with abciximab or eptifibatide, followed by the assessment of: (1) aggregometry (anticoagulant: sodium citrate 3.18% or hirudin 5 mug/ml); (2) flow cytometry (fibrinogen binding or PAC1-expression), or (3) RPFA. Dose-response curves were established on the basis of a sigmoidal I-max-model [I=(I-max * C-g)/(IC50g+ C-g)]. Results: For citrated blood, aggregation induced by 20 CIM ADP was blocked up to 100% by both CPIIb/IIIa-antagonists, IC50 values varied between 0.11-0.22 mug/ml for eptifibatide and 1.25-2.3 mug/ml for abciximab. I-max of the response to 5 mug/ml collagen ranged from 46% to 100%, and IC50 values varied between 0.28-0.34 mug/ml for eptifibatide and 2.3-3.8 mug/ml for abciximab. In hirudinized blood, IC50 values for eptifibatide were 1.5- to 3-fold higher than those obtained with citrated plasma. Inhibition of PAC1 expression by abciximab (IC50 0.84 mug/ml) showed results similar to those of the RPFA (approx. 1.0 mug/ mi); larger differences between PAC1 and RPFA results were observed for eptifibatide. Based on aggregometry, eptifibatide concentrations for 80% inhibition varied from 0.27 to 0.55 mug/ml, and were considerably less when the RPFA was taken as basis (0.15 or 0.22 mug/ml). A similar pattern was observed for abciximab. Conclusions: We found quite a low inter- and intra-laboratory variation in the in vitro pharmacodynamic characterization of GPIIb /IIIa-antagonists by aggregometry, making results of these tests obtained from different laboratories during clinical trials at least comparable. The RPFA exhibits a higher sensitivity to inhibitory GPIIb/IIIa-effects, in keeping with the "real" inhibition of the activated receptor (PAC1) as assessed with more elaborate flow cytometry. (C) 2001 Elsevier Science Ltd. All rights reserved.