Effect of Ca²⁺ Chelation on the Platelet Inhibitory Ability of the GPIIb/IIIa Antagonists Abciximab, Eptifibatide and Tirofiban

Summary

Objective. Enhanced GPIIb/IIIa binding and inhibition of platelet aggregation of eptifibatide by the reduction of ionized plasma calcium concentrations have been reported. The present study compared the importance of Ca²⁺ chelation on the in vitro platelet inhibitory profiles of the GPIIb/IIIa antagonists abciximab, eptifibatide and tirofiban. Methods and Results. Turbidimetric platelet aggregation dose response curves of the various GPIIb/IIIa antagonists were performed using platelet rich plasma (PRP) anticoagulated with either trisodium citrate, or the non-chelating anticoagulant, PPACK. The concentrations of antagonist that resulted in 50% inhibition of TRAP-induced (10 M) platelet aggregation (IC₅₀) were measured in the presence of either citrate or PPACK. In addition, the influence of Ca²⁺ chelation on the binding properties (relative affinity, on- and off-rates) of abciximab for the GPIIb/IIIa receptor on platelets was measured. For all three agonists, the IC₅₀ concentrations were lower for platelets treated with citrate than PPACK, but the degree of difference varied among the agents. The mean TRAP IC₅₀ values for citrate and PPACK were 88.2 ± 12.2 nM and 126.1 ± 28.4 nM for abciximab (1.4 fold enhancement; p = 0.0007), 75.9 ± 13.3 nM and 142.6 ± 32.6 nM for tirofiban (1.9-fold enhancement; p = 0.001), and 260.2 ± 62.5 nM and 810.3 ± 182.5 nM for eptifibatide (3.1-fold enhancement; p = 0.001). A similar shift in effective inhibitor concentrations for abciximab was observed with ADP (10 M). The relative affinities (EC₅₀), on- and off-rates of abciximab for the platelet GPIIb/IIIa receptor in the presence of trisodium citrate and PPACK were equivalent. Conclusions. These data confirm previous observations that Ca²⁺ chelation afforded by citrate decreases the effective inhibitor concentrations of GPIIb/IIIa antagonists, as assessed by turbidimetric platelet aggregation. However, the extent of decrease was less for abciximab and tirofiban, compared to eptifibatide.

Keywords

Platelet aggregation - anticoagulants - GPIIb/IIIa antagonists - Ca²⁺ chelation

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