Malononitrilamides synergistically prevent acute and treat ongoing skin allograft rejection with cyclosporine.

The low molecular weight malononitrilamides (MNAs), a new class of immunosuppressive agents, belong to the derivatives of leflunomide's active metabolite, A771726. They have been shown to bind specifically to dehydroorotate dehydrogenase and inhibit de novo pyrimidine biosynthesis, thereby blocking T- and B-cell proliferation and strongly suppressing IgM and IgG antibody production. Here we evaluated their efficacy together with cyclosporine (CyA) in rat skin allotransplantation models, using different strain combinations. Monotherapy of transplanted animals in these models with the MNAs HMR 1279 and HMR 1715 resulted in a significant and dose-dependent prolongation of the graft survival time. Even a short-term application showed efficacy in the prevention of acute rejection. The MNAs were also effective when treatment was started at the time of expected rejection crisis, demonstrating strong therapeutic activity to reverse ongoing acute rejection, whereas CyA was ineffective for the treatment of ongoing allograft rejection episodes. Combination therapy of MNAs with CyA proved to be very effective for the prevention of acute skin graft rejection. Interestingly, whereas CyA alone was unable to treat ongoing acute rejection episodes, comedication of MNAs and CyA, even after a short-term application, was synergistically effective and significantly suppressed ongoing allogeneic skin graft rejection. These results demonstrate that MNAs are potent and well tolerated immunosuppressants with a potential comparable to that of CyA, but they are superior to CyA in their ability to reverse acute rejection episodes. They represent powerful rescue drugs and demonstrate synergistic activity with CyA to prevent acute and treat ongoing skin allograft rejection.

Regulation of alloreactivity in the popliteal lymph node assay by the new immunosuppressants: malononitrilamides.

Malononitrilamides (MNAs) represent a new class of low molecular weight immunosuppressants and have been shown to prevent and reverse ongoing acute allograft rejection and effectively prolong xenograft survival in rodents. MNAs were also found to be potent inhibitors of B and T cell-mediated autoimmune processes and mediate their effects by binding specifically to dihydro-orotate dehydrogenase (DHODH), inhibiting de novo pyrimidine biosynthesis, thereby blocking T and B cell proliferation and strongly suppressing the IgM and IgG antibody production. Here we evaluated the effects of the MNAs (HMR 1279 and HMR 1715) on the in vivo lymphoproliferation that occurs after challenge with allogeneic cells in a local graft-versus-host reaction in Lewis x Brown Norway F1 hybrid rats by measuring the enlargement of the popliteal lymph nodes (PLN) draining the site of allogeneic cell injection. Oral administration of the MNAs dose-dependently prevented the localized lymphoproliferative response in the PLN assay and suppressed the lymph node hyperplasia. The MNAs even acted therapeutically when they were given during an ongoing alloreactivity as late as days 4 or 5 after challenge. Consistent with the mode of action, a complete reversal of the immunosuppression on the lymphoproliferation in vivo was attempted in this protocol by the addition of exogenous uridine during days 0-5. These data suggest the HMR 1279 and HMR 1715 mediate their antiproliferative and immunosuppressive effects in the PLN assay in vivo by decreasing the activity of DHODH in the lymph node cells and thereby inhibiting pyrimidine biosynthesis.