Activated Clotting Time as an Appropriate Test to Compare Heparin and Direct Thrombin Inhibitors Such as Hirudin or Ro 46-6240 in Experimental Arterial Thrombosis

BACKGROUND: Specific thrombin inhibitors are considered to be more potent antithrombotics than heparin. However, the relation between the systemic anticoagulation generated by thrombin inhibitors and their antithrombotic effect is not well defined. In a guinea pig carotid thrombosis model, the activated clotting time (ACT), the activated partial thromboplastin time (aPTT), and thrombin-generation tests were evaluated for their ability to predict the arterial antithrombotic effect of direct thrombin inhibitors such as hirudin and Ro 46-6240 compared with heparin. METHODS AND RESULTS: Thrombosis of the carotid artery was induced by subendothelial damage in guinea pigs, and the subsequent cyclic flow variations were monitored. The effects of pretreatment with intravenous heparin, hirudin, and Ro 46-6240 were tested. After doubling the baseline aPTT, 1 IU.kg-1.min-1 heparin was inactive, whereas either hirudin or Ro 46-6240 (30 micrograms.kg-1.min-1) prevented thrombus formation by 80%. Heparin (10 IU.kg-1.min-1) induced the same antithrombotic effect but with indefinite aPTT prolongation. However, for similar prolongation of the ACT, the three compounds had equivalent antithrombotic effects. Thrombin generation was predictive of the antithrombotic effect of the thrombin inhibitors but not of heparin. CONCLUSIONS: The arterial antithrombotic effect of direct thrombin inhibitors, when compared with those of heparin, should be evaluated by the ACT and not the aPTT or thrombin-generation assays. For a "therapeutic" aPTT prolongation, thrombin inhibitors induce higher systemic anticoagulation than does heparin and thus might unduly have higher bleeding liability.