THROMBIN INHIBITION IN AN EX-VIVO MODEL OF PORCINE HEART XENOGRAFT HYPERACUTE REJECTION

Prominent components of vascularized xenograft rejection such as plate let activation and microvascular thrombosis may be dependent upon thro mbin generation in vivo. To study potential therapeutic benefits of a synthetic low-molecular-weight thrombin inhibitor, SDZ MTH 958, in hyp eracute porcine heart rejection by human blood ex vivo, a working mode l of hyperacute rejection of porcine hearts by fresh, heparinized (6 u /ml) human blood with or without 1 mu M 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 reje cted hearts were analyzed by immunohistopathology. Control porcine hea rts (n=8) underwent a rapid and consistent decline in cardiac output, ceasing function by 60 min. Experimental cardiac output values of 14 m l/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 incr eased consistently in control samples when compared with plasma sample s containing SDZ MTH 958. Immunohistopathological examination confirme d diminished fibrin deposition, reduced leukocyte adherence to endothe lium, impaired diapedesis and less tissue necrosis in the hearts perfu sed with SDZ MTH 958. SDZ MTH 958, in this xenoperfusion model, prolon ged survival, enhanced function of the explanted organ, and improved h istological features at the time of rejection. Effective and specific antagonism of thrombin may be useful as an adjunct therapy to compleme nt inhibition for xenograft rejection.