Leflunomide acts on:

Leflunomide is metabolized to an active metabolite that acts to inhibit dihydroorotate dehydrogenase, an essential enzyme in the pyrimidine biosynthetic pathway. Its predominant action is to inhibit the proliferation of T lymphocytes. Leflunomide has been shown to control the signs and symptoms of RA and to slow the progression of joint damage as effectively as methotrexate. Leflunomide can be given alone or with methotrexate and is the most frequently employed immunosuppressive agent used to treat patients with Rheumatoid ahritis. Ref: Harrison's principle of internal medicine 17th edition, chapter 302.