Prominent components of vascularized xenograft rejection such as platelet activation and microvascular thrombosis may be dependent upon thrombin generation in vivo. To study potential therapeutic benefits of a synthetic low-molecular-weight thrombin inhibitor, SDZ MTH 958, in hyperacute porcine heart rejection by human blood ex vivo, a working model of hyperacute rejection of porcine by fresh, heparinized (6 microM/ml) human blood with or without 1 microM SDZ MTH 958 was used. Thrombin-antithrombin complexes (TAT) and prothrombin fragment F1.2 levels as markers of thrombin activation were determined, and biopsies from rejected hearts were analyzed by immunohistopathology. Control porcine hearts (n=8) underwent a rapid and consistent decline in cardiac output, ceasing function by 60 min. Experimental cardiac output values of 14 ml/g (SEM 1.2) were significantly higher than seen in controls (5 ml/g SEM 0.6) after 5 min of cardiac work, and prolonged survival times up to 120 min were noted (P<0.05). Activity of SDZ MTH 958 was confirmed by functional assays throughout perfusion. Levels of TAT and F1.2 increased consistently in control samples when compared with plasma samples containing SDZ MTH 958. Immunohistopathological examination confirmed diminished fibrin deposition, reduced leukocyte adherence to endothelium, impaired diapedesis and less tissue necrosis in the hearts perfused with SDZ MTH 958. SDZ MTH 958, in this xenoperfusion model, prolonged survival, enhanced function of the explanted organ, and improved histological features at the time of rejection. Effective and specific antagonism of thrombin may be useful as an adjunct therapy to complement inhibition for xenograft rejection