Azathioprine(Azasan, Imuran; BW 57-322) is a drug that suppresses the immune system and is used in organ transplantation and autoimmune disease. Target: Azathioprine is an immunosuppressive antimetabolite pro-drug. It is an imidazolyl derivative of 6-mercaptopurine and many of its biological effects are similar to those of the parent compound. Azathioprine is converted into 6-mercaptopurine in the body where it blocks purine metabolism and DNA synthesis. Thus it most strongly affects proliferating cells, such as the T cells and B cells of the immune system. The main adverse effect of azathioprine is bone marrow suppression, which can be life-threatening, especially in people with a genetic deficiency of the enzyme thiopurine S-methyltransferase. It is also listed by the International Agency for Research on Cancer as a Group 1 carcinogen (carcinogenic to humans) [1-3].

References:
[1]. Patel, A.A., R.A. Swerlick, and C.O. McCall, Azathioprine in dermatology: the past, the present, and the future. J Am Acad Dermatol, 2006. 55(3): p. 369-89.
[2]. Maltzman, J.S. and G.A. Koretzky, Azathioprine: old drug, new actions. J Clin Invest, 2003. 111(8): p. 1122-4.
[3]. Dubinsky, M.C., Azathioprine, 6-mercaptopurine in inflammatory bowel disease: pharmacology, efficacy, and safety. Clin Gastroenterol Hepatol, 2004. 2(9): p. 731-43.