Vol. 286, Issue 2, 945-951, August 1998

Functional Relevance of the Expression of Ligand-Induced Binding Sites in the Response to Platelet GP IIb/IIIa Antagonists In Vivo.¹

Natalie P. Murphy, Domenico Pratico and Desmond J. Fitzgerald

Centre for Cardiovascular Science, Department of Clinical Pharmacology Royal College of Surgeons in Ireland, St. Stephens Green, Dublin, Ireland (N.M., D.F.), and Center for Experimental Therapeutics, University of Pennsylvania, Philadelphia, PA (D.P.)

RGD-containing peptides and other antagonists of the platelet glycoprotein (GP) IIb/IIIa may induce a high-affinity binding site for fibrinogen and the expression of novel epitopes, called ligand-induced binding sites (LIBS). The functional relevance of LIBS expression in a canine model of coronary thrombolysis induced by tissue-type plasminogen activator (t-PA) was examined. Ro43-5054 (N-[N-[N-(p-amidinobenzoyl)-b-alanyl]-l-a-aspartyl]-3-phenyl-l-alanine) and Ro44-9883 ([1-(N-(p-amidinobenzoyl)-l-tyrosyl)-4-piperidinyl)oxy]acetic acid), antagonists of the GP IIb/IIIa receptor, were administered in increasing doses of 2 to 10 µg/kg/min, beginning 30 min before the infusion of t-PA. LIBS expression was determined by the binding of the monoclonal antibody, D3GP3, to platelets on exposure to Ro43-5054, Ro44-9883 and t-PA. Ro43-5054 was shown to induce LIBS, whereas Ro44-9883 and t-PA did not. Both drugs abolished platelet aggregation in response to U46619 and ADP ex vivo. Reocclusion was prevented with both Ro43-5054 and Ro44-9883, but neither drug altered reperfusion times (49 ± 8 and 55 ± 39 min). Both drugs increased the rate of bleeding compared with t-PA alone, but there was no difference in hemostasis between the two drugs. To determine whether the drugs differed in their effect on platelet activation in vivo, urinary 2,3-dinor-thromboxane (TX) B₂, a major metabolite of TXB₂, was determined by gas chromatography-mass spectrometry. After reperfusion, the urinary 2,3-dinor-TXB₂ increased in the Ro43-5054-treated group, similar to control groups (32 ± 8 and 37 ± 9 ng/mg creatinine). This increase was blunted in the Ro44-9883-treated group (9 ± 3 ng/mg creatinine). GP IIb/IIIa antagonists that do not induce LIBS result in a greater suppression of platelet activity but not in any discernible functional benefit in vivo.