Azathioprine immunosuppressant drug. Tablet of the immunosuppressant drug azathioprine. Marketed under the name Imuran, azathioprine works by inhibiting purine synthesis, which is essential for cell proliferation. It was used in cancer chemotherapy, but this use has dwindled as more powerful drugs have been developed. However, it is still widely used to suppress the immune system to prevent rejection after organ transplants and to treat autoimmune disorders such as rheumatoid arthritis and Crohn's disease.