Mercaptopurine (Monohydrate) is oral, cell-cycle-phase specific antineoplastic agent used chiefly in the treatment of acute lymphoblastic leukemia. Mercaptopurine (Monohydrate) is a chemical analog of purine, a necessary building block used by cells for synthesizing DNA. Mercaptopurine is also closely related to another antimetabolite, thioguaninean (the two agents have similar indications, adverse effects, and cross-resistance). Mercaptopurine is also the metabolite of azathioprine, a commonly used immunosuppressive agent. It received FDA approval in 1953.it acts as an antimetabolite and is active in the S-phase. Mercaptopurine exhibits a couple of mechanisms of action. It can undergo triphosphorylation and can be incorporated into DNA in place of adenine, leading to a derangement in the DNA message. Mercaptopurine ribonucleotides also can be incorporated into RNA and can halt RNA synthesis. Second, the drug can be metabolized to a nucleotide that inhibits biosynthesis of purines needed for RNA and DNA synthesis. Together these effects lead to cell death.