A derivative of isoxazole used for its immunosuppressive and anti-inflammatory properties. As a prodrug, leflunomide is converted to an active metabolite, A77 1726, which blocks dihydroorotate dehydrogenase, a key enzyme of de novo pyrimidine synthesis, thereby preventing the expansion of activated T lymphocytes. This agent also inhibits various protein tyrosine kinases, such as protein kinase C (PKC), thereby inhibiting cell proliferation. (NCI04)