Hirudin, heparin, and placebo during deep arterial injury in the pig. The in vivo role of thrombin in platelet-mediated thrombosis.

Abstract

Three dosages (0.3, 0.7, and 1.0 mg/kg) of recombinant hirudin, a specific inhibitor of thrombin, were compared with heparin (50 units/kg) and placebo for reducing thrombus formation in the carotid arteries of 50 pigs after deep injury by balloon dilatation. Each drug was administered as a bolus followed immediately by a continuous infusion of the same dose per hour. Major end points were quantitative indium-111-labeled platelet and iodine-125-labeled fibrinogen deposition and the incidence of mural thrombosis. This study showed that heparin, at a dose that prolonged the activated partial thromboplastin time (APTT) to twice the control time, did not prevent mural thrombosis or significantly reduce platelet deposition compared with placebo but did reduce fibrinogen deposition. Recombinant hirudin markedly reduced platelet and fibrinogen deposition in a dose-related manner and totally eliminated mural thrombosis at an APTT of two to three times that of control. Platelet deposition (x 16/cm2, mean±SEM) in areas of deep arterial injury for the placebo, heparin, and 0.3, 0.7, and 1.0 mg/kg hirudin groups was 54+±21, 33+9, 22+f6, 8±1, and 7±1, respectively; electron microscopy showed a single layer (or less) of platelets at the two highest hirudin dosages. The incidence of macroscopic mural thrombosis was 76% with placebo, 57% with heparin, 46% with 0.3 mg/kg hirudin; there were no thrombi with 0.7 or 1.0 mg/kg hirudin (p<0.01). The APTT is a valuable index of the plasma hirudin concentration (r=0.89, p<0.001) and correlated inversely with quantitative platelet (r= -0.67, p<0.0001) and fibrinogen deposition (r= -0.42, p=0.005). The thrombin time is overly sensitive and plays no role in monitoring heparin or hirudin therapy for arterial thrombosis. Heparin and all dosages of hirudin abolished platelet aggregation to thrombin and prolonged the bleeding time to a similar degree; neither test reflects the in vivo antithrombotic efficacy of heparin or hirudin. Thrombin plays a critical role in arterial thrombosis. Heparin in conventional doses does not significantly reduce arterial platelet thrombosis. Hirudin produces a dose-dependent reduction in platelet and fibrinogenfibrin deposition, which correlates with the prolongation of the APTT and the hirudin plasma level. Hirudin totally eliminates macroscopic mural thrombus formation at dosages that prolong the APTT to at least twice control and is a promising therapeutic agent for platelet-rich arterial thrombi. (Circulation 1990;82:1476-1484 Ba alloon dilatation produces mechanical disruption of the arterial wall with injury into the media that releases tissue thromboplastin (tissue factor) and exposes very thrombogenic deep arterial structures (mainly smooth muscle cells and

Dissolution of Mural Thrombus by Specific Thrombin Inhibition With r-Hirudin

“…Growth of thrombus induced by preformed, fresh mural thrombus was lowest in pigs treated with the specific thrombin inhibitor r-hirudin at a dose that prevented macroscopic mural thrombus after deep injury 13,14 and increased with decreasing antithrombin activity, confirming our previous results. 9 Thrombus size in animals treated with aspirin was not significantly reduced at these rheological conditions, a finding that is consistent with previous results.…”

“…9,22 Thrombin plays a central role in the formation, growth, maintenance, and consolidation of thrombus. Direct thrombin inhibition with hirudin, but not heparin, blocks these processes and leads to profound inhibition of thrombus 14,[23][24][25] and even to dissolution of preformed thrombus, as in our study. In vivo, in the pig with a carotid artery crush injury, 90% of the preformed, half-hour-old thrombus dissolved within 1 hour of hirudin administration.…”

“…38 In the present study, dissolution of platelet-rich thrombus occurred at a dose of r-hirudin that completely abolished platelet thrombus formation in previous studies. 9,14 In the hirudin group, the mean aPTT ratios were 3.3, which were significantly lower those than in the high-dose heparin group (Ͼ12 times control). In patients receiving r-hirudin for 3 to 5 days, thrombus dissolution in aortocoronary vein grafts occurred at aPTT levels that were two to three times those of control.…”

In Vivo Dynamic Real-Time Monitoring and Quantification of Platelet-Thrombus Formation

“…The effect of these commonly used drugs on platelet deposition is well known. 21 Our model confirmed the weak antiplatelet effects of aspirin, 22 the modest effect of heparin, and the potent effect of hirudin but, importantly, showed that compared with flow measurements, monitoring continuous isotope-labeled platelet deposition can more sensitively quantify thrombus formation, even before changes in flow or total occlusion occur.…”

Prolonged Thrombin Inhibition Reduces Restenosis After Balloon Angioplasty in Porcine Coronary Arteries

“…When higher doses (aPTT Ͼ2 times control) were used, acute mural thrombosis was totally abolished and platelet-vessel wall interaction was reduced to a monolayer. 9,10 Further evidence supporting the hypothesis of thrombin as a major mediator of restenosis comes from studies using hirudin 13 and recombinant tick anticoagulant peptide (rTAP) in the cholesterol-fed rabbit femoral artery model. 30 TAP, by binding and inhibiting factor Xa, also inhibits thrombin generation at the initiation of the coagulation cascade.…”