In vitro and in vivo antitumor activity of a novel immunomodulatory drug, leflunomide - Mechanisms of action

Leflunomide, a never immunomodulatory drug, has two biochemical activities: inhibition of tyrosine phosphorylation and inhibition of pyrimidine nucleo tide synthesis. In the present study, we first showed that A77 1726 [N-(4-t rifluoromethylphenyl-2-cyano-3-hydroxycrotoamide)] the active metabolite of leflunomide,was more effective at inhibiting the tyrosine kinase activity of platelet-derived growth factor (PDGF) receptor than that of epidermal gr owth factor (EGF) receptor,and had no effect on the tyrosine kinase activit y of the fibroblast growth factor receptor. In the presence of exogenous ur idine, A77 1726 was more effective at inhibiting the PDGF-stimulated prolif eration of PDGF receptor-overexpressing C6 glioma than the EGF-stimulated p roliferation of EGF receptor-overexpressing A431 cells. In vivo studies dem onstrated that leflunomide treatment strongly inhibited the growth of the C 6 glioma but had only a modest effect on the growth of the A431 tumor. Urid ine co administered with leflunomide did not reverse the antitumor activity of leflunomide on C6 and A431 tumors significantly. Quantitation of nucleo tide levels in the tumor tissue revealed that leflunomide treatment signifi cantly reduced pyrimidine nucleotide levels in the fast-growing C6 glioma b ut had no effect on the relatively slow-growing A431 tumor. Whereas uridine co-administration normalized pyrimidine nucleotide levels, it had minimal effects on the antitumor activity of leflunomide in both tumor models. Immu nohistochemical analysis revealed that leflunomide treatment significantly reduced the number of proliferating cell nuclear antigen-positive cells in C6 glioma, and that uridine only partially reversed this inhibition. These results collectively suggest that the in vivo antitumor effect of leflunomi de is largely independent of its inhibitory effect on pyrimidine nucleotide synthesis. The possibility that leflunomide exerts its antitumor activity by inhibition of tyrosine phosphorylation or by a yet unidentified mode of action is discussed. (C) 1999 Elsevier Science Inc.