Cell-permeable. An immunosuppressive agent. Inhibits T and B cell responses in vivo. Inhibitor of tyrosine phosphorylation (tyrosine kinase inhibitor), of dihydroorotate dehydrogenase, an enzyme for de novo pyrimidine synthesis, of TNF-dependent NF-κB activation, and of COX-2. The physiological activity of leflunomide is attributed mainly to its metabolite A77 1726.