An inhibitor of purine synthesis and interconversion; rapidly converted to 6-mercaptopurine ribonucleoside-5'-monophosphate, which inhibits PRPP amidotransferase, the conversion of IMP to adenylosuccinic acid and xanthylic acid, and blocks AMP formation in vitro; inhibits growth of sarcoma 180, adenocarcinoma E 0771, and adenocarcinoma 755 tumors and reduces the size of leukemia L1210 subcutaneous growths in mice when administered at a dose of 30 mg/kg; decreases delayed-type hypersensitivity and thyroid inflammation in a guinea pig model of thyroiditis when administered pre- or post-disease onset