(Circulation. 1999;99:2231-2238.)
© 1999 American Heart Association, Inc.
Clinical Investigation and Reports |
Quantifying GPIIb/IIIa Receptor Binding Using 2 Monoclonal Antibodies
Discriminating Abciximab and Small Molecular Weight Antagonists
From the Centre for Cardiovascular Science (M.Q., A.D., M.S., D.C., D.F.), The Royal College of Surgeons, and St. James's Hospital (B.F.), Dublin, Ireland.
Correspondence to Dr Martin Quinn, The Centre for Cardiovascular Science, 123, St. Stephen's Green, Dublin 2, Ireland. E-mail mquinn{at}rcsi.ie
Background—Dosing of glycoprotein (GP) IIb/IIIa receptor antagonists is frequently based on the inhibition of platelet aggregation, which may be influenced by the agonist used or concurrent medications. Here we describe a monoclonal antibody-based technique to quantify total and ligand-occupied GPIIb/IIIa receptors.
Methods and Results—In vitro binding of monoclonal antibodies, LYP18 (Mab1) and 4F8 (Mab2), to the GPIIb/IIIa complex, was characterized using purified receptor and to platelets by flow cytometry. Patients undergoing coronary angioplasty received a single 20 mg dose of the oral GPIIb/IIIa antagonist, xemilofiban, or matching placebo, and antibody binding was compared with inhibition of platelet aggregation. Mab1 and Mab2 were bound to purified GPIIb/IIIa and to unoccupied, inactivated receptor on platelets. Mab2 identified the ß3 subunit, whereas Mab1 was complex-specific. Neither antibody interfered with the other's binding, suggesting that they identified distinct sites. Mab1 identified 53 300±5423 GPIIb/IIIa sites per platelet, whereas Mab2 identified 50 120±5066 sites per platelet. Mab1 binding was inhibited by abciximab in a dose dependent manner (IC50, 0.85±0.1 µg/mL), whereas Mab2 binding was unaffected. In contrast, the 2 small molecular weight antagonists, SC-57101A (IC50, 0.22±0.06 µmol/L) and eptifibatide (IC50, 0.35±0.14 µmol/L) inhibited Mab2 but not Mab1 binding. In patients treated with xemilofiban, Mab1 binding was unaltered but Mab2 binding decreased from 37 930±2061 sites per platelet at baseline to 8318±870 sites per platelet 6 hours after dosing (P<0.0001). Platelet aggregation to adenosine diphosphate (20 µmol/L) fell to 3±3% of baseline in line with the inhibition of Mab2 binding (correlation coefficient 0.8, P<0.0001).
Conclusions—Mab1 and Mab2 bind to GPIIb/IIIa and are differentially displaced by abciximab and small molecular weight antagonists. These antibodies may be used to monitor receptor number and occupancy during administration of a GPIIb/IIIa antagonist.
Key Words: glycoproteins • platelet aggregation inhibitors • abciximab • thrombosis
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