Thioguanine
Dosage Form: tablets
TABLOID®
brand Thioguanine
40-mg Scored Tablets
 CAUTION
TABLOID brand Thioguanine is a potent drug. It should not be used unless a diagnosis of acute nonlymphocytic leukemia has been adequately established and the responsible physician is knowledgeable in assessing response to chemotherapy.
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Thioguanine Description
TABLOID brand Thioguanine was synthesized and developed by Hitchings, Elion, and associates at the Wellcome Research Laboratories. It is one of a large series of purine analogues which interfere with nucleic acid biosynthesis, and has been found active against selected human neoplastic diseases.
Thioguanine, known chemically as 2-amino-1,7-dihydro-6H-purine-6-thione, is an analogue of the nucleic acid constituent guanine, and is closely related structurally and functionally to PURINETHOL® (mercaptopurine). Its structural formula is:
TABLOID brand Thioguanine is available in tablets for oral administration. Each scored tablet contains 40 mg Thioguanine and the inactive ingredients acacia, lactose monohydrate, magnesium stearate, potato starch, and stearic acid.
Thioguanine - Clinical Pharmacology
Clinical studies have shown that the absorption of an oral dose of Thioguanine in humans is incomplete and variable, averaging approximately 30% of the administered dose (range: 14% to 46%). Following oral administration of 35S-6-Thioguanine, total plasma radioactivity reached a maximum at 8 hours and declined slowly thereafter. Parent drug represented only a very small fraction of the total plasma radioactivity at any time, being virtually undetectable throughout the period of measurements.
The oral administration of radiolabeled Thioguanine revealed only trace quantities of parent drug in the urine. However, a methylated metabolite, 2-amino-6-methylthiopurine (MTG), appeared very early, rose to a maximum 6 to 8 hours after drug administration, and was still being excreted after 12 to 22 hours. Radiolabeled sulfate appeared somewhat later than MTG but was the principal metabolite after 8 hours. Thiouric acid and some unidentified products were found in the urine in small amounts. Intravenous administration of 35S-6-Thioguanine disclosed a median plasma half-disappearance time of 80 minutes (range: 25 to 240 minutes) when the compound was given in single doses of 65 to 300 mg/m2. Although initial plasma levels of Thioguanine did correlate with the dose level, there was no correlation between the plasma half-disappearance time and the dose.
Thioguanine is incorporated into the DNA and the RNA of human bone marrow cells. Studies with intravenous 35S-6-Thioguanine have shown that the amount of Thioguanine incorporated into nucleic acids is more than 100 times higher after 5 daily doses than after a single dose. With the 5-dose schedule, from one-half to virtually all of the guanine in the residual DNA was replaced by Thioguanine. Tissue distribution studies of 35S-6-Thioguanine in mice showed only traces of radioactivity in brain after oral administration. No measurements have been made of Thioguanine concentrations in human cerebrospinal fluid (CSF), but observations on tissue distribution in animals, together with the lack of CNS penetration by the closely related compound, mercaptopurine, suggest that Thioguanine does not reach therapeutic concentrations in the CSF.
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