Mercaptopurine (Purinethol) is an analogue of hypo-xanthine and was one of the first agents shown to be active against acute leukemias. It is now used as part of maintenance therapy in acute lymphoblastic leukemia. Mercaptopurine must be activated to a nucleotide by the enzyme HGPRTase. This metabolite is capable of inhibiting the synthesis of the normal purines adenine and guanine at the initial aminotransferase step and inhibiting the conversion of inosinic acid to the nu-cleotides adenylate and guanylate at several steps. Some mercaptopurine is also incorporated into DNA in the form of thioguanine. The relative significance of these mechanisms to the antitumor action of mercap-topurine is not clear.
Resistance to mercaptopurine may be a result of decreased drug activation by HGPRTase or increased in-activation by alkaline phosphatase.
The plasma half-life of an intravenous bolus injection of mercaptopurine is 21 minutes in children and 47 minutes in adults. After oral administration, peak plasma levels are attained within 2 hours. The drug is 20% bound to plasma proteins and does not enter the CSF. Xanthine oxidase is the primary enzyme involved in the metabolic inactivation of mercaptopurine.
Mercaptopurine is used in the maintenance therapy of acute lymphoblastic leukemia. It also displays activity against acute and chronic myelogenous leukemias.
The major toxicities of mercaptopurine are myelo-suppression, nausea, vomiting, and hepatic toxicity.
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