The Effect of Thiopurine Methyltransferase Expression on Sensitivity to Thiopurine Drugs

• Published in: Molecular pharmacology Vol. 62; no. 1; pp. 102 - 109
• Main Authors: Coulthard, Sally A, Hogarth, Linda A, Little, Margaret, Matheson, Elizabeth C, Redfern, Christopher P F, Minto, Lynne, Hall, Andrew G
• Format: Journal Article
• Language: English
• Published: United States American Society for Pharmacology and Experimental Therapeutics 01.07.2002
• Subjects: Cell Line; Cells, Cultured; Deoxyguanosine - analogs & derivatives; Deoxyguanosine - metabolism; DNA - biosynthesis; DNA - drug effects; Gene Expression - drug effects; Humans; Mercaptopurine - pharmacology; Methyltransferases - biosynthesis; Methyltransferases - genetics; Nucleic Acid Synthesis Inhibitors - pharmacology; Thioguanine - pharmacology; Thionucleosides - metabolism; Transfection
• ISSN: 0026-895X; 1521-0111
• DOI: 10.1124/mol.62.1.102

Although the thiopurine drugs 6-mercaptopurine (6-MP) and 6-thioguanine (6-TG) are well established agents for the treatment of leukemia, controversies remain regarding their main mode of action. Previous evidence has suggested that although 6-TG exerts a cytotoxic effect through incorporation of 6-thioguanine nucleotides into newly synthesized DNA (DNA-TGN), an important component of the mode of action of 6-MP is inhibition of purine de novo synthesis (PDNS) through the production of S -methyl-thioinosine 5′-monophosphate (MeTIMP), not formed in cells exposed to 6-TG. We have shown that thiopurine methyltransferase (TPMT) modulates this effect. By transfection of the human TPMT gene using an inducible system to produce a 3.8-fold increase in TPMT activity in the ecdysone receptor 293 embryonic kidney cell line, we demonstrated a 4.4-fold increase in sensitivity to 6-MP. This was associated with a rise in intracellular levels of MeTIMP but a decrease in levels of DNA-TGN. In contrast, induction of TPMT produced a 1.6-fold decrease in sensitivity to 6-TG, a decrease in levels of DNA-TGN, and an increase in levels of methylated thioguanosine monophosphate. Exposure of cells to equitoxic doses of drug showed similar incorporation of DNA-TGN for 6-TG but for 6-MP significantly reduced DNA-TGN in TPMT-induced compared with uninduced cells. For equitoxic doses of 6-MP, equivalent levels of MeTIMP correlated with equivalent amounts of PDNS. These observations suggest that intracellular TGN levels do not give an accurate reflection of cytotoxic potential in patients treated with 6-MP, because different levels of DNA-TGN may be associated with equitoxic effects.