A77-1726, leflunomide's active metabolite, inhibits in vivo lymphoproliferation in the popliteal lymph node assay

The new disease-modifying antirheumatic drug (DMARD) leflunomide (Arava(TM) ) recently obtained approval from the Food and Drug Administration (FDA) fo r the treatment of patients with active rheumatoid arthritis. It is conside red to exert its beneficial effects by inhibition of de novo pyrimidine bio synthesis and thereby blocking proliferation of a variety of effector cells . This antiproliferative effect consistently is antagonized completely in v itro by the addition of uridine or cytidine to the cell cultures to repleni sh the nucleotide pools. In the present study we evaluated the effect of A7 7-1726, leflunomide's active metabolite on the in vivo lymphoproliferation that occurs after challenge with allogeneic cells in a local graft-versus-h ost (GvH) reaction in Lewis x Brown-Norway (LBN) F1-hybrid rats by measurin g the enlargement of the popliteal lymph node (PLN) draining the site of al logeneic cell injection. Even a single, oral administration of various conc entrations of A77-1726 on day 0, dose-dependently prevented the localized l ymphoproliferative response and suppressed the lymph node hyperplasia. Mult iple oral administrations resulted in a better suppression of PLN enlargeme nt. This lymphocyte proliferation in vivo was always inhibited, independent ly from the time when the lymph nodes were harvested. Leflunomide's active metabolite also acted therapeutically when it was given during an ongoing l ymphoproliferation as late as days 4 or 5 after challenge. Consistent with the mode of action, that leflunomide inhibits de novo pyrimidine biosynthes is, a complete reversal of the immunosuppression on the lymphoproliferation in vivo in the PLN assay was attempted in this protocol by the addition of exogenous uridine during days -1 to 5. These data suggest that leflunomide and its active metabolite A77-1726 mediate their antiproliferative activit y not only in vitro, but also in vivo in the PLN assay, by decreasing dihyd roorotate-dehydrogenase (DHODH) activity in the lymph node cells, thereby i nhibiting the de novo synthesis of the pyrimidine ribonucleotide uridine-mo nophosphate, and the lymphocytes undergo an arrest of the cell cycle.