Rajan V. Nair

Chronic rejection in the form of graft vascular disease (GVD) continues to plague clinical transplantation of vascularized organs. The histopathology of this lesion is characterized by neointimal hyperplasia, smooth muscle cell proliferation, and obliterative arteriopathy. Due to the lack of effective medical therapy for preventing or reversing these chronic vascular changes, retransplantation remains the final resort in treatment. Some of the newer immunosuppressive agents, including the new isoxazole derivative leflunomide (LFM), have shown efficacy in preventing chronic rejection in animal models of transplantation. Although its mechanism of action remains incompletely elucidated, previous work using lymphocytes in vitro suggests that the drug might act as a tyrosine kinase inhibitor, an inhibitor of de novo pyrimidine biosynthesis, or both. In order to elucidate whether the efficacy of LFM in vivo is attributable not only to anti-proliferative effects on the recipient immune system but also to direct effects on mesenchymal cells in the donor organ, we examined the effects of LFM on a transformed 9E11G murine smooth muscle cell (M-SMC) line in vitro. We demonstrate here that the active metabolite of LFM, A77 1726, dose-dependently inhibits the constitutive and growth-factor stimulated proliferation of M-SMC in vitro. Furthermore, the anti-proliferative effect of the drug can be reversed by the addition of uridine to the culture medium. These results suggest that inhibition of uridine biosynthesis appears to be a mechanism by which LFM exerts anti-proliferative effects on both lymphocytes and smooth muscle cells, and this dual action may be responsible for its efficacy in preventing GVD in vivo.

During the past 5 years, there has been a concerted effort to identify new immunosuppressive agents for organ transplantation that have greater efficacy and fewer side effects than current therapies (corticosteroids, azathioprine, cyclosporine, anti-T cell antibodies). These new drugs can be generally classified according to their varying structures or mechanisms of action: xenobiotic molecules (FK506, rapamycin, cyclosporine analogues); antimetabolites (mycophenolate mofetil, mizoribine, brequinar sodium); and those agents with novel or incompletely defined mechanisms of action (leflunomide, 15-deoxyspergualin). Many of the newer agents offer better therapeutic indexes, and some even show promise in the treatment of two of the major obstacles currently facing cardiac allograft transplantation: antibody-mediated accelerated humoral rejection and chronic vascular rejection (also known as accelerated graft coronary artery disease). With the increasing shortage of donor organs in recent years, there has been a resurgence of interest in xenotransplantation; some of the new immunosuppressants demonstrate efficacy in prolonging xenograft survival. Most of these agents will probably find their niches as components of low-dose combination regimens designed to maximize effective immunosuppression and minimize drug toxicities and opportunistic infections.