Mercaptopurine, marketed under the brand name Purinethol among others, is a medication used for cancer and autoimmune diseases. Mercaptopurine competes with hypoxanthine and guanine for the enzyme hypoxanthine-guanine phosphoribosyltransferase (HGPRTase) and is itself converted to thioinosinic acid (TIMP). This intracellular nucleotide inhibits several reactions involving inosinic acid (IMP), including the conversion of IMP to xanthylic acid (XMP) and the conversion of IMP to adenylic acid (AMP) via adenylosuccinate (SAMP). In addition, 6-methylthioinosinate (MTIMP) is formed by the methylation of TIMP. Both TIMP and MTIMP have been reported to inhibit glutamine-5-phosphoribosylpyrophosphate amidotransferase, the first enzyme unique to the de novo pathway for purine ribonucleotide synthesis. Experiments indicate that radiolabeled mercaptopurine may be recovered from the DNA in the form of deoxythioguanosine. Some mercaptopurine is converted to nucleotide derivatives of 6-thioguanine (6-TG) by the sequential actions of inosinate (IMP) dehydrogenase and xanthylate (XMP) aminase, converting TIMP to thioguanylic acid (TGMP). PURINETHOL (mercaptopurine) is indicated for maintenance therapy of acute lymphatic (lymphocytic, lymphoblastic) leukemia as part of a combination regimen. The response to this agent depends upon the particular subclassification of acute lymphatic leukemia and the age of the patient (pediatric or adult).

THIOINOSINE (Methylmercaptopurine riboside, NSC- 40774) is a purine derivative with antineoplastic and anti-angiogenic properties. THIOINOSINE is readily converted in cells to its active form, 6-methylmercaptopurine ribonucleoside 5'- monophosphate (MMPR-P), by the enzyme adenosine kinase. THIOINOSINE inhibits amidophosphoribosyltransferase, the first committed step in de novo purine synthesis, and inhibits fibroblast growth factor-2 (FGF2)-induced cell proliferation. It has been used similarly to Mercaptopurine in the treatment of Leukemia. It has being tested in clinical trials for advanced pancreatic cancer.