About mercaptopurineantimetabolite (thiopurine ),immunosuppressent, antineoplastic.mechanism of action of mercaptopurinein the body drug is converted into 6- thioinosine-5`-monophosphate. it inhibits the conversion of inosine monophosphate to adenine monophosphate and guanine monophosphate which is an important step in the purine synthesis. this inhibits the dna & rna synthesis of cancer cells.