Novel mechanism of inhibition of cytomegalovirus by the experimental immunosuppressive agent leflunomide

Background Despite progress in antiviral chemotherapy, cytomegalovirus (CMV ) remains a major cause of morbidity and mortality among pharmacologically immunosuppressed organ transplant recipients, frequently engaging the clini cian in a struggle to balance graft preservation with control of CMV diseas e. Leflunomide, an inhibitor of protein kinase activity and pyrimidine synt hesis, is an experimental immunosuppressive agent effective against acute a nd chronic allograft rejection in animal models. Because a number of CMV pr oteins are known to be phosphorylated, we tested the hypothesis that this a gent might exert inhibitory activity against CMV. Methods and results. Plaque assays demonstrated dramatic dose-dependent att enuation of production of multiple clinical CMV isolates in leflunomide-tre ated human fibroblasts and endothelial cells, common targets for CMV infect ion in vivo. As shown by Northern blot analysis and immunohistochemical sta ining, leflunomide neither interferes with transcription of immediate early or late viral genes, nor with expression of corresponding proteins. CMV-sp ecific DNA dot blots and biochemical enzyme assays indicated that, in contr ast to currently approved anti-CMV drugs, leflunomide exerts no inhibitory effect on the accumulation of viral DNA in infected tells, or on viral DNA polymerase activity, Rather, as visualized by transmission electron microsc opy, this agent appears to act at a late stage in virion assembly by preven ting tegument acquisition by viral. nucleocapsids. Finally we have demonstr ated equivalent inhibitory activity of leflunomide against multi-drug-resis tant CMV isolates. Conclusions. These findings imply that leflunomide, an effective immunosupp ressive agent, shows potential to concurrently attenuate a major complicati on of immunosuppression, CMV disease, by a novel mechanism of antiviral act ivity.