FUNCTIONAL RELEVANCE OF THE EXPRESSION OF LIGAND-INDUCED BINDING-SITES IN THE RESPONSE TO PLATELET GP LLB LLLA ANTAGONISTS IN-VIVO/

RGD-containing peptides and other antagonists of the platelet glycopro tein (GP) IIb/IIIa may induce a high-affinity binding site for fibrino gen and the expression of novel epitopes, called ligand-induced bindin g sites (LIBS). The functional relevance of LIES expression in a canin e model of coronary thrombolysis induced by tissue-type plasminogen ac tivator (t-PA) was examined. Ro43-5054 nzoyl)-b-alanyl]-I-a-aspartyl]- 3-phenyl-I-alanine) and Ro44-9883 midinobenzoyl)-I-tyrosyl)-4-piperidi nyl)oxy]acetic acid), antagonists of the GP IIb/IIIa receptor, were ad ministered in increasing doses of 2 to 10 mu g/kg/min, beginning 30 mi n before the infusion of t-PA. LIES expression was determined by the b inding of the monoclonal antibody, D3GP3, to platelets on exposure to Ro43-5054, Ro44-9883 and t-PA. Ro43-5054 was shown to induce LIES, whe reas R044-9883 and t-PA did not. Both drugs abolished platelet aggrega tion in response to U46619 and ADP ex vivo. Reocclusion was prevented with both Ro43-5054 and R044-9883, but neither drug altered reperfusio n 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 hem ostasis between the two drugs. To determine whether the drugs differed in their effect on platelet activation in vivo, urinary 2,3-dinor-thr omboxane (TX) B-2, a major metabolite of TXB2, was determined by gas c hromatography-mass spectrometry. After reperfusion, the urinary 2,3-di nor-TXB2 increased in the Ro43-5054-treated group, similar to control groups (32 +/- 8 and 37 +/- 9 ng/mg creatinine). This increase was blu nted in the Ro44-9883-treated group (9 +/- 3 ng/mg creatinine). GP IIb /IIIa antagonists that do not induce LIES result in a greater suppress ion of platelet activity but not in any discernible functional benefit in vivo.