title: Methotrexate binds to recombinant thiopurine S-methyltransferase and inhibits enzyme activity after high-dose infusions in childhood leukaemia.
abstract: OBJECTIVE
  Important drugs in the treatment of childhood acute lymphoblastic leukaemia (ALL) are 6-mercaptopurine (6-MP) and methotrexate (MTX). Thiopurine methyltransferase (TPMT) is a polymorphic enzyme causing variability in 6-MP response and toxicity. The aim of this study was to investigate the fluctuation in TPMT enzyme activity over time and the effect of high-dose MTX infusions on TPMT enzyme activity and 6-MP metabolites in paediatric ALL patients.
  
  
  METHODS
  Fifty-three children with ALL treated according to the NOPHO-ALL 2000 protocol were included in the study. TPMT enzyme activity was measured at six different times starting from diagnosis until after the end of maintenance treatment. TPMT and 6-MP metabolites were measured before the initiation of high-dose MTX (HD-MTX) infusions and at 66 h post-infusion. The interaction between MTX and TPMT was investigated in vitro using recombinant TPMT protein and a leukaemic cell line.
  
  
  RESULTS
  Forty percent of TPMT wild-type individuals had deceptively low TPMT enzyme activity according to genotype at the time of diagnosis. TPMT activity had decreased significantly 66 h after the start of HD-MTX infusions (-9.2 %; p = 0.013). MTX bound to recombinant TPMT protein severely inhibiting TPMT enzyme activity (remaining activity 16 %).
  
  
  CONCLUSIONS
  Our results show that TPMT genotyping should be performed in children with ALL, since 40 % of the children in our study who carried the wild-type TPMT gene were at risk of initial underdosing of 6-MP in cases where only TPMT enzyme activity was determined. MTX inhibits the TPMT enzyme activity after HD-MTX infusions due to protein binding.
authors: Department of Physics, Chemistry and Biology, Linköping University, Sweden.
authors: Patricia
authors: P
authors: Wennerstrand
authors: Lars-Göran
authors: LG
authors: Mårtensson
authors: Stefan
authors: S
authors: Söderhäll
authors: Anna
authors: A
authors: Zimdahl
authors: Malin Lindqvist
authors: ML
authors: Appell
mesh_terms: D000293:Adolescent; D000964:Antimetabolites, Antineoplastic / Q000008:administration & dosage; D000971:Antineoplastic Combined Chemotherapy Protocols / Q000008:administration & dosage*; D045744:Cell Line, Tumor; D002648:Child; D002675:Child, Preschool; D004791:Enzyme Inhibitors / Q000008:administration & dosage; D005260:Female; D006801:Humans; D007223:Infant; D007262:Infusions, Intravenous; D008297:Male; D015122:Mercaptopurine / Q000008:administration & dosage; D008727:Methotrexate / Q000008:administration & dosage; D008780:Methyltransferases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D054198:Precursor Cell Lymphoblastic Leukemia-Lymphoma / Q000097:blood* / Q000188:drug therapy; D011994:Recombinant Proteins / Q000378:metabolism
chemical_list: D000964:Antimetabolites, Antineoplastic; D004791:Enzyme Inhibitors; D011994:Recombinant Proteins; D015122:Mercaptopurine; D008780:Methyltransferases; C022745:thiopurine methyltransferase; D008727:Methotrexate
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