Increased concentrations of methylated 6-mercaptopurine metabolites and 6-thioguanine nucleotides in human leukemic cells in vitro by methotrexate.
The effect of methotrexate (MTX) on 6-mercaptopurine (6-MP) metabolism was studied in four human leukemic cell lines in vitro. CCRF-CEM, WI-L2, TBJ, and HL-60 all expressed thiopurine methyltransferase (TPMT) activity. The cells were grown in horse serum-supplemented RPMI 1640 medium to which was added 4 microM of 6-MP or 4 microM of 6-MP and 20 nM of MTX. The presence of MTX resulted in a 2.1-, 1.7-, 2.4- and 8-fold increase in the concentrations of methylmercaptopurine ribonucleotides (MMPRP) in CEM, WI-L2, TBJ, and HL-60 cells, respectively (P < 0.0008). The concentrations of 6-thioguanine nucleotides (6 TGN) increased 1.9-, 1.4-, 2.4- and 1.9-fold in the same cell lines (P < 0.02). The four cell lines differed with respect to the effect of MTX on the consumption of 6-MP from the medium; CEM consumed more 6-MP and WI-L2 less 6-MP from media containing MTX than from media containing 6-MP only (P = 0.005 and 0.02, respectively). MTX did not affect the consumption of 6-MP by TBJ cells (P = 0.17). Media in which HL-60 cells had been grown did not contain detectable amounts of 6-MP at the end of the experiment. The simultaneous increase in methylated 6-MP metabolites and 6-TGN represents a possible explanation for the synergism of MTX and 6-MP; however, the clinical importance of increased MMPRP remains to be elucidated.
Giverhaug
T
T
Department of Pharmacology, Institute of Medical Biology, University of Tromsø, Norway. trudeg@fagmed.uit.no
Loennechen
T
T
Aarbakke
J
J
D003470:Culture Media; D006150:Guanine Nucleotides / Q000737:chemistry / Q000378:metabolism*; D006801:Humans; D015122:Mercaptopurine / Q000378:metabolism*; D008727:Methotrexate / Q000494:pharmacology*; D008745:Methylation; D013866:Thioguanine / Q000737:chemistry*; D014407:Tumor Cells, Cultured
D003470:Culture Media; D006150:Guanine Nucleotides; D015122:Mercaptopurine; D013866:Thioguanine; D008727:Methotrexate