Experimental studies on thrombin inhibition in acute- and chronic models of vessel wall injury

Background: The number of percutaneous coronary interventions (PCI) performed worldwide has increased dramatically during the last decade. Complication rates have decreased over time, however the restenosis phenomenon remains an unsolved issue. Thrombin is the key regulator in the pro-coagulant state following PCI. As well as its role in coagulation activation, thrombin also promotes proliferation and migration of smooth muscle cells, which is of pivotal importance in the restenosis process. Objective: The aim of the study was to determine if thrombin inhibition, achieved through antithrombin (AT) or the direct, small molecule thrombin inhibitor melagatran, could favourably alter the acute- and long-term response to deep vessel wall injury in porcine coronary arteries.Methods: In three studies on the acute response to vessel wall injury, the effects of locally delivered AT, and locally or systemically delivered melagatran, were investigated. After local delivery by special local drug delivery catheters, the content of AT in the vessel wall was quantified by immunofluorescence technique using a semi-quantitative method, and that of melagatran was analysed by autoradiography. Autologus platelets were labelled with In111 and infused into the pigs. In addition, in the study with systemically infused melagatran, I125 labelled fibrinogen was injected. A balloon injury was induced to the coronary vessel wall. One hour after balloon injury the pigs were sacrificed, the heart explanted and the radio-activity of the injured vessel segments investigated in a gamma well counter. The number of platelets and amount of fibrin was calculated. In two other studies, the effects of locally delivered AT or high concentration, high purity antithrombin fraction (HCHP-AT), on the long-term response, i.e. development of stenosis, after deep coronary artery wall injury, were investigated. Normo-cholesterolemic Swedish landrace pigs were subjected to a deep balloon or stent injury to the coronary vessel walls and then followed for four weeks, whereafter they were sacrificed. In a sixth study using a double-injury model, hypercholesterolemic Yucatan mini-pigs were treated with a long-term infusion of melagatran after balloon injury or implant of stents in the coronary arteries. The animals were followed for four weeks before being sacrificed. In the three latter studies, the hearts were explanted after sacrifice and the injured vessel segments investigated histo-morphologically. The percent area stenosis, lumen and the different vessel wall areas were determined. In the study with HCHP-AT, an additional quantitative coronary angiogram (QCA) was performed.Results: Local delivery of antithrombin after balloon injury in coronary vessels resulted in detectable amounts of AT in the inner part of the vessel wall, and a significant reduction of platelet deposition as compared to control treatment with pure albumin. It was not possible to deposit any melagatran by local delivery and there was no effect on platelet deposition. Systemically administered melagatran, on the other hand, significantly inhibited both platelet and fibrin deposition in an acute model. There was no favourable effect on vascular wall remodelling or neointima proliferation after coronary vascular wall injury by either local delivery of AT or HCHP-AT nor of long term infusion of melagatran.Conclusions: Thrombin inhibition by either local administration of endogenous antithrombin or systematically administered low molecular weight thrombin inhibitor melagatran, inhibited the acute thrombotic response after vascular wall injury in porcine coronary models, but there were no effects on long-term stenosis/restenosis development.Key words: animal model, antithrombin, fibrinogen, histomorphology, melagatran, percutaneous coronary intervention, platelets, restenosis, stent, thrombin inhibition, thrombosis