MPM is approved by the Food and Drug Administration to prevent kidney transplant rejection. It is chemically similar to another cytotoxic drug called azathioprine, which has been beneficial in maintaining remission in patients with Wegener's granulomatosis who have been treated successfully with cyclophosphamide. Because MPM is more effective than azathioprine in preventing organ rejection, it may also prove beneficial as a second-line treatment for Wegener's granulomatosis.