title: The effect of thiopurine methyltransferase expression on sensitivity to thiopurine drugs.
abstract: 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.
authors.affiliation: Leukaemia Research Fund Molecular Pharmacology Specialist Programme, Cancer Research Unit, Medical School, Newcastle University, Newcastle-upon-Tyne, UK.
authors.forename: Sally A
authors.initials: SA
authors.lastname: Coulthard
authors.forename: Linda A
authors.initials: LA
authors.lastname: Hogarth
authors.forename: Margaret
authors.initials: M
authors.lastname: Little
authors.forename: Elizabeth C
authors.initials: EC
authors.lastname: Matheson
authors.forename: Christopher P F
authors.initials: CP
authors.lastname: Redfern
authors.forename: Lynne
authors.initials: L
authors.lastname: Minto
authors.forename: Andrew G
authors.initials: AG
authors.lastname: Hall
mesh_terms: D002460:Cell Line; D002478:Cells, Cultured; D004247:DNA / Q000096:biosynthesis / Q000187:drug effects; D003849:Deoxyguanosine / Q000031:analogs & derivatives* / Q000378:metabolism; D015870:Gene Expression / Q000187:drug effects*; D006801:Humans; D015122:Mercaptopurine / Q000494:pharmacology*; D008780:Methyltransferases / Q000096:biosynthesis* / Q000235:genetics; D019384:Nucleic Acid Synthesis Inhibitors / Q000494:pharmacology*; D013866:Thioguanine / Q000494:pharmacology*; D013872:Thionucleosides / Q000378:metabolism; D014162:Transfection
chemical_list: D019384:Nucleic Acid Synthesis Inhibitors; D013872:Thionucleosides; C002062:alpha-2'-deoxythioguanosine; D004247:DNA; D015122:Mercaptopurine; D008780:Methyltransferases; C022745:thiopurine methyltransferase; D013866:Thioguanine; D003849:Deoxyguanosine