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.