THROMBIN INHIBITION IN DISCORDANT XENOGRAFT REJECTION

Microvascular thrombosis and the associated platelet and endothelial c ell activation are prominent observations in xenograft rejection. This pathological picture could be related to the excessive generation of thrombin in the context of either inflammation or putative inter-speci es molecular incompatibilities between activated coagulation factors a nd their natural anticoagulants. Relatively selective thrombin inhibit ion with the serine protease inhibitor SDZ MTH 958 (MTH-958) are indep endent of heparinoids and anti-thrombin III. MTH-958 has been shown to significantly prolong porcine cardiac function during perfusion with human blood in an ex vivo model. The aim of this study was to validate the role of thrombin generation in a rodent model of discordant xenog raft rejection in vivo. The effect of thrombin inhibition with MTH-958 was tested in both hyperacute rejection (HAR) and delayed xenograft r ejection (DXR) after decomplementation with cobra venom factor (CVF) i n normal Lewis (Lew) rats and intrinsic C6 deficiency in PVG (C6-/PVG) recipient rats, Recipient rats received heterotopic guinea pig cardia c xenografts and were treated with titrated doses of MTH-958 until the time of graft rejection. Plasma samples at selected time points were examined to confirm effective thrombin inhibition, and rejected grafts were analyzed by immunohistology. MTH-958 significantly improved graf t survival in HAR albeit the extent of prolongation was not marked, bu t the agent failed to prolong survival in CVF-treated Lew rats. In C6- /PVG rats receiving MTH-958, a significantly reduced graft survival ti me was observed when compared with C6-/PVG controls. The grafts from M TH-958-treated animals showed dense deposits of C3, IgM, and IgG with fibrin levels similar to controls. The thrombin antagonist tested coul d prolong xenograft survival during HAR but had no benefit in DXR. The relative non-specificity of the serine protease inhibitor MTH-958 wit h the potential activation of alternative pathway of complement via th e inhibition of factor I could account for the failure to prolong xeno graft survival in DXR. The pathogenetic significance of thrombin gener ation in this situation remains to be determined by the use of more se lective and pharmacologically acceptable anti-thrombin agents.