Platelet aggregation inhibition with glycoprotein IIb--IIIa inhibitors.

Inhibitors of the platelet receptor glycoprotein (GP) IIb--IIIa are a novel and potent class of antithrombotic drugs for the management of patients with non-ST-segment elevation acute coronary syndromes (ACS) and those undergoing percutaneous coronary intervention (PCI). Pharmacodynamic studies with three currently approved agents in this class (abciximab [ReoPro, Centocor, Inc., Malvern, Pennsylvania, and Eli Lilly & Company, Indianapolis, Indiana]; eptifibatide [INTEGRILIN, COR Therapeutics, Inc., South San Francisco, California, and Key Pharmaceuticals, Inc., Kenilworth, New Jersey]; and tirofiban HCI [Aggrastat, Merck & Co., Inc., Whitehouse Station, New Jersey]) all sought to identify dosing regimens that would establish and maintain >80 % inhibition of ex vivo platelet aggregation throughout the duration of intravenous infusion. Direct comparison of these prior studies is difficult, however, because the assays used different anticoagulants (sodium citrate [abciximab, tirofiban HCI] or D-phenylalanyl-L-prolyl-L-arginine chloromethyl ketone [PPACK] [eptifibatide]) and different concentrations of the platelet agonist adenosine diphosphate (ADP) (5 micromol [tirofiban HCI] or 20 micromol [abciximab, eptifibatide]). More recent work has attempted to overcome these limitations by using similar assay conditions for all GP IIb--IIIa inhibitors. These studies have indicated that the concentrations of all three agents required to provide the targeted effect for platelet inhibition are considerably higher in the presence of an anticoagulant that does not chelate calcium ions (e.g., heparin or PPACK) than in the presence of calcium-chelating anticoagulant (i.e., sodium citrate). Of the three currently approved GP IIb--IIIa inhibitors, eptifibatide is the only agent whose approved dosing is based on an ex vivo platelet aggregation assay that uses such an anticoagulant. Additionally, Kereiakes et al. have recently reported that the high levels of platelet inhibition (>80 %), using PPACK as an anticoagulant and ADP (20 micromol) as an agonist, are more consistently achieved with the approved dosing regimen of eptifibatide. The antiplatelet effect of abciximab showed more interpatient variability, whereas the median inhibition of ex vivo platelet aggregation with the approved dosing regimen for tirofiban HCl was <80 % at almost all time points during drug infusion.