6-Thioguanine dosage

Absorption of thioguanine is incomplete and erratic. It is eliminated mainly by S-methylation. Thioguanine can be administered concurrently with allopurinol without reduction in dosage, unlike mercaptopurine and azathioprine. 

The metabolism of azathioprine is bimodal in humans, with rapid metabolizers clearing the drug four times faster than slow metabolizers. Production of 6-thioguanine is dependent on thiopurine methyltransferase (TPMT), and patients with low or absent TPMT activity (0.3% of the population) are at particularly high risk of myelosuppression by excess concentrations of the parent drug if dosage is not adjusted. 

The usual initial dose is 2 mg/kg daily hy the oral route. If there is no clinical improvement or leukopenia after 4 weeks the dosage is increased to 3 mg/kg daily. In contrast to mercaptopurine, thioguanine may be continued in the usual dose when allopurinol is used to inhibit uric acid formation. 

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