title: Methylated metabolites of 6-mercaptopurine are associated with hepatotoxicity.
abstract: BACKGROUND
  Hepatotoxicity with elevation of aminotransferase levels is common during combined methotrexate and thiopurine therapy. However, the mechanism of hepatotoxicity induced by these drugs remains obscure. We have investigated the relationship of a rise in aminotransferase levels to erythrocyte levels of methotrexate and its polyglutamates, 6-thioguanine nucleotides, the major cytotoxic metabolites of 6-mercaptopurine, and methylated metabolites of 6-mercaptopurine generated by thiopurine methyltransferase in competition with the formation of 6-thioguanine nucleotides.
  
  
  METHODS
  Weighted means of alanine aminotransferase levels and metabolites of methotrexate and 6-mercaptopurine were calculated from a total of 929 blood samples from 43 patients with acute lymphoblastic leukemia during maintenance therapy with methotrexate and 6-mercaptopurine.
  
  
  RESULTS
  Weighted means of aminotransferase levels were significantly related to the average doses of 6-mercaptopurine (r(s) = 0.60, P <.001), erythrocyte levels of methylated metabolites of 6-mercaptopurine (r(s) = 0.63, P <.001), and thiopurine methyltransferase activity (r(s) = 0.32, P =.03). Weighted means of aminotransferase levels were negatively correlated with erythrocyte levels of 6-thioguanine nucleotides and were not related to the average doses of methotrexate or erythrocyte levels of methotrexate and its polyglutamates.
  
  
  CONCLUSIONS
  It was found that 6-mercaptopurine and its methylated metabolites are correlated with the degree of hepatotoxicity during methotrexate and 6-mercaptopurine maintenance therapy. Further studies are needed to explore whether aminotransferase levels could be used as a surrogate marker for methylated metabolites of 6-mercaptopurine and thus to evaluate 6-mercaptopurine treatment compliance.
authors.affiliation: The Pediatric Clinic II, The University Hospital of Copenhagen, Rigshospitalet, Copenhagen, Denmark. Ulrikka@dadlnet.dk
authors.forename: Ulrikka
authors.initials: U
authors.lastname: Nygaard
authors.forename: Nina
authors.initials: N
authors.lastname: Toft
authors.forename: Kjeld
authors.initials: K
authors.lastname: Schmiegelow
mesh_terms: D000410:Alanine Transaminase / Q000187:drug effects / Q000378:metabolism*; D000964:Antimetabolites, Antineoplastic / Q000008:administration & dosage; D000971:Antineoplastic Combined Chemotherapy Protocols / Q000008:administration & dosage / Q000009:adverse effects* / Q000378:metabolism; D056486:Chemical and Drug Induced Liver Injury; D015331:Cohort Studies; D004305:Dose-Response Relationship, Drug; D004334:Drug Administration Schedule; D004789:Enzyme Activation / Q000187:drug effects; D005260:Female; D006801:Humans; D008099:Liver / Q000187:drug effects; D008111:Liver Function Tests; D008297:Male; D020714:Maximum Tolerated Dose; D015122:Mercaptopurine / Q000008:administration & dosage*; D008727:Methotrexate / Q000008:administration & dosage*; D054198:Precursor Cell Lymphoblastic Leukemia-Lymphoma / Q000175:diagnosis / Q000188:drug therapy; D011237:Predictive Value of Tests; D011336:Probability; D018570:Risk Assessment; D012720:Severity of Illness Index
chemical_list: D000964:Antimetabolites, Antineoplastic; D015122:Mercaptopurine; D000410:Alanine Transaminase; D008727:Methotrexate