Thioguanine

6-Thioguanine is a purine analogue structurally related to 6-mercaptopurine and azathioprine. Thioguanine interferes with several enzymes required for de novo purine synthesis, and its metabolites are incorporated into DNA and RNA, further impeding nucleic acid synthesis. The mechanism of action of thioguanine in psoriasis is not clearly understood; it has been hypothesized to affect the proliferation and trafficking of lymphocytes as well as the proliferation of keratinocytes.

Absorption of orally administered 6-thioguanine is slow and incomplete; only approximately 30% of the oral dose is achieved in the plasma, peak levels being reached after 8 hours. Thioguanine is extensively metabolized prior to excretion. The elimination half-life is on the order of 80 minutes.

Although 6-thioguanine is chiefly used in chemotherapy for acute myelocytic leukemia and other marrow-based malignancies, lower doses are very effective for moderate to severe psoriasis, particularly in patients who cannot tolerate alternative systemic agents such as methotrexate and cyclosporine.

Dose-related myelosuppression is the major adverse effect produced by 6-thioguanine. Patients deficient in thiopurine methyltransferase (TPMT), a cytosolic enzyme required for metabolism of 6-thioguanine, are at heightened risk. Other adverse effects include gastrointestinal complaints and elevations of liver transami-nases. There have been rare reports of more serious he-patotoxicity, including acute hepatitis, acute cholestasis, and hepatic venoocclusive disease.

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