Mechanism of the antiproliferative action of leflunomide. A77 1726, the active metabolite of leflunomide, does not block T-cell receptor-mediated signal transduction but its antiproliferative effects are antagonized by pyrimidine nucleosides.

Background : Leflunomide, a novel immunosuppressive drug, prolongs experimental graft survival effectively and has been well tolerated in patients with rheumatoid arthritis. A77 1726, the active metabolite of leflunomide, inhibits lymphocyte proliferation in vitro. This study was conducted in Jurkat T cells to investigate the effects of A77 1726 on signal transduction pathways initiated by ligands of the T-cell receptor CD3 complex and to evaluate the effects of A77 1726 on nucleotide biosynthesis. Methods : Tritiated thymidine incorporation and cell counts quantitated cell proliferation. Spectrofluorescence of Indo/AM dye measured intracellular Ca 2+ mobilization. A luciferase assay quantitated interleukin-2 gene promoter activity in stimulated cells transfected with an interleukin-2 promoter-luciferase gene construct. Pyrimidine and purine nucleosides were used to assess antagonism of the antiproliferative activity of A77 1726. Results : (1) A77 1726 dose-dependently inhibited the proliferation of Jurkat T cells (inhibitory concentration of 50% = 6 μmol/L) ; (2) A77 1726 did not decrease mobilization of intracellular Ca 2+ stimulated by phytohemagglutinin or anti-CD3 monoclonal antibody ; (3) A77 1726 did not inhibit interleukin-2 gene promoter activity in cells stimulated with ionomycin plus phorbol myristate acetate ; (4) inhibition of cell proliferation by A77 1726 was antagonized by addition of uridine, cytidine, or 2'-deoxycytidine ; (5) addition of uridine 24 hours after treatment with A77 1726 antagonized inhibition of proliferation ; (6) A77 1726 was not antagonized by 2'-deoxyuridine, thymidine, adenosine, or guanosine. Conclusions : (1) A77 1726 inhibited Jurkat T-cell proliferation without inhibiting T-cell receptor-mediated signal transduction events, including tyrosine kinase-dependent intracellular Ca 2+ mobilization and activation of the interleukin-2 gene promoter ; (2) the antiproliferative effects of A77 1726 on Jurkat T cells are primarily due to interruption of de novo pyrimidine nucleotide biosynthesis. These data provide evidence for a novel in vitro mechanism of the antiproliferative action of this immunosuppressant.