1. Leflunomide is a medication of DMARD（disease-modifying antirheumatic drugs）type, used for severe RA(rheumatoid arthritis) and psoriatic arthritis. 2. Mechanism of Action: Leflunomide, an isoxazol-like immunosuppressant with anti-proliferation activity, exerts its function by inhibiting the activity of dihydroorate dehydrogenase, and in turn affect the pyrimidine synthesis in activated lymphocytes. It is suggested by in vivo and in vitro experiment that this product has an anti-inflammation effect. The in vivo activity of leflunomide is mainly created by its active metabolite A771726.