title: Quantifying GPIIb/IIIa receptor binding using 2 monoclonal antibodies: discriminating abciximab and small molecular weight antagonists.
abstract: 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.
  
  
  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 beta3 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 microg/mL), whereas Mab2 binding was unaffected. In contrast, the 2 small molecular weight antagonists, SC-57101A (IC50, 0.22+/-0.06 micromol/L) and eptifibatide (IC50, 0.35+/-0.14 micromol/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 micromol/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.
authors.affiliation: Centre for Cardiovascular Science, The Royal College of Surgeons, and St. James's Hospital, Dublin, Ireland. mquinn@rcsi.ie
authors.forename: M
authors.initials: M
authors.lastname: Quinn
authors.forename: A
authors.initials: A
authors.lastname: Deering
authors.forename: M
authors.initials: M
authors.lastname: Stewart
authors.forename: D
authors.initials: D
authors.lastname: Cox
authors.forename: B
authors.initials: B
authors.lastname: Foley
authors.forename: D
authors.initials: D
authors.lastname: Fitzgerald
mesh_terms: D000077284:Abciximab; D000818:Animals; D000911:Antibodies, Monoclonal / Q000378:metabolism* / Q000494:pharmacology*; D001550:Benzamidines / Q000494:pharmacology; D001792:Blood Platelets / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D006801:Humans; D007140:Immunoglobulin Fab Fragments / Q000494:pharmacology*; D051379:Mice; D008970:Molecular Weight; D010974:Platelet Aggregation / Q000187:drug effects; D010975:Platelet Aggregation Inhibitors / Q000494:pharmacology*; D019039:Platelet Glycoprotein GPIIb-IIIa Complex / Q000378:metabolism*
chemical_list: D000911:Antibodies, Monoclonal; D001550:Benzamidines; D007140:Immunoglobulin Fab Fragments; D010975:Platelet Aggregation Inhibitors; D019039:Platelet Glycoprotein GPIIb-IIIa Complex; C091295:xemilofiban; D000077284:Abciximab