Vol. 281, Issue 3, 1178-1185, 1997

Interaction of a Thrombin Inhibitor and a Platelet GP IIb/IIIa Antagonist In Vivo: Evidence That Thrombin Mediates Platelet Aggregation and Subsequent Thromboxane A₂ Formation During Coronary Thrombolysis¹

Domenico Pratico², Natalie P. Murphy and Desmond J. Fitzgerald

Centre for Cardiovascular Science, Dept. of Clinical Pharmacology, Royal College of Surgeons in Ireland, St. Stephens Green, Dublin, Ireland

We examined the effect of a specific thrombin inhibitor, Ro 46-6240, alone and combined with an antagonist of the platelet GP IIb/IIIa, Ro44-9883, on the response to tissue-type plasminogen activator in a canine model of thrombolysis. Platelet activity was determined by measuring the excretion of 2,3-dinor-thromboxane (TX)B₂, an enzymatic metabolite of TXA₂. Ro 46-6240 administered before tissue-type plasminogen activator induced a dose-dependent prolongation of the activated partial thromboplastin time and prothrombin time. The time to reperfusion decreased dose-dependently (P < .01) to 10 ± 6 min vs. 52 ± 5 min in controls. Ro 46-6240 also prevented reocclusion, which occurred in every case in control experiments. Urinary excretion of 2,3-dinor-TXB₂ increased from 3 ± 1 to 37 ± 9 ng/mg creatinine in controls after reperfusion. This increase was reduced in a dose-dependent fashion by Ro 46-6240, such that at the highest dose, urinary 2,3-dinor-TXB₂ after reperfusion was 5.6 ± 1 ng/mg creatinine. Similar functional and biochemical effects were seen when a subthreshold dose of Ro 46-6240 was combined with Ro 44-9883. At the dose used, Ro 44-9883 alone abolished platelet aggregation ex vivo but failed to modify the response to tissue-type plasminogen activator or the excretion of 2,3-dinor-TXB₂ after reperfusion (51 ± 6 ng/mg creatinine, n = 3). However, the combination of Ro 44-9883 and Ro 46-6240 reduced the time to reperfusion (40 ± 8 vs. 68 ± 15 min; n = 7, P < .05), prevented reocclusion and abolished the rise in urinary 2,3-dinor-TXB₂ (5 ± 1 ng/mg creatinine, n = 4). These findings suggest that thrombin mediates platelet activation during coronary thrombolysis. The increased platelet activity results in platelet aggregation and a subsequent increase in TXA₂ formation.