Quarterly blood counts are necessary for people on mercaptopurine. Cancers that do not respond to treatment with mercaptopurine do not respond to thioguanine. Allopurinol inhibits xanthine oxidase, the enzyme that breaks down mercaptopurine. The study examined the combined use of two chemotherapy drugs, methotrexate and mercaptopurine. The thiopurines include thioguanine and mercaptopurine. Other treatments may include drugs like Fansidar, mercaptopurine&colon; More commonly used in Europe. Those taking allopurinol (often used to prevent gout) are at risk for mercaptopurine toxicity. Mercaptopurine itself can also be administered directly. Mercaptopurine, or 6-MP is an analog of guanine, and inhibits cellular replication through multiple mechanisms. Mercaptopurine interferes with nucleotide interconversion and glycoprotein synthesis. Mercaptopurine can lower the body's ability to fight off infection. Mercaptopurine was approved for medical use in the United States in 1953.