COMBINATION THERAPY WITH BREQUINAR SODIUM AND CYCLOSPORINE SYNERGISTICALLY PROLONGS HAMSTER-TO-RAT CARDIAC XENOGRAFT SURVIVAL

The accelerated rejection of hamster cardiac xenografts by Lewis rat r ecipients is due primarily to a humoral immune response. Traditional i mmunosuppressive agents, including cyclosporine, FK 506, and rapamycin , have not been effective in prolonging the rejection of xenografts in this model. We have recently examined the ability of the combination of cyclosporine and a new immunosuppressive agent, Brequinar sodium, t o prevent the rejection of hamster xenografts. Prolongation of xenogra ft survival by treatment with Brequinar sodium alone (3 mg/kg/day) was minimally effective, with a median survival of 5.5 days. Conversely, a combination of Brequinar sodium (3 mg/kg/day) and cyclosporine (10 m g/kg/day), was associated with a significant prolongation of median gr aft survival (> 100 days). A quantitative analysis of the dose-effect relationship of Brequinar sodium and cyclosporine demonstrated a poten t synergistic interaction for this combination therapy (combination in dex < 1). A sharp increase occurred in the binding of immunoglobulin M antibodies at day 4 after transplantation in the serum of untreated a nd cyclosporine-treated Lewis recipients of cardiac xenografts, wherea s a clear decrease occurred in anti-hamster immunoglobulin M antibody production during this period of time in the animals treated by combin ed immunosuppression. Inhibition of anti-donor immunoglobulin M antibo dy production, as evaluated by the direct enzyme-linked immunosorbent assay and 3-[4,5-dimethylthiazol-2-yl]-2,5 diphenyltetrazolium bromide measurements, extended for more than 100 days after transplantation i n the combination therapy group. The demonstration of anti-hamster imm unoglobulin M antibody deposition on the vascular endothelium of cardi ac grafts by immunofluorescence staining correlated with the results o bserved in the enzyme-linked immunosorbent assay. Cessation of treatme nt after 120 days after transplantation resulted in subsequent rejecti on of the cardiac xenografts and was associated with increased product ion of rat immunoglobulin M and G anti-hamster antibodies. The results of these experiments demonstrate that the combination of low doses of Brequinar sodium and cyclosporine is highly effective in prolonging c ardiac xenograft survival. The prolongation of graft survival is assoc iated with effective suppression of rat anti-hamster antibody producti on. We believe that the combination of the two drugs with different me chanisms of action provides for broader suppression of the T and B cel l interactions that mediate the xenograft reaction and a higher level of efficiency for preventing graft rejection.