Pharmacokinetics, dose adjustments, and 6-mercaptopurine/methotrexate drug interactions in two patients with thiopurine methyltransferase deficiency.
Two children with acute lymphoblastic leukaemia (ALL) were found to be thiopurine methyltransferase (TPMT)-deficient by both genotype and phenotype. They were monitored with haematological parameters and red blood cell concentrations of 6-thioguanine nucleotides (E-6TGN) and methotrexate (E-MTX, including MTX polyglutamates), in relation to the doses of 6-mercaptopurine (6MP) and methotrexate (MTX), during their maintenance chemotherapy. Both patients developed severe pancytopenia at the standard protocol dose of 6MP. Even at 25% and 5%, respectively, of the protocol dose of 6MP, they achieved E-6TGN values several-fold above the population median, but without unacceptable bone-marrow toxicity. Their high E-6TGN values had only a minor influence on their E-MTX values and their tolerance to oral MTX, but severe pancytopenia followed high-dose MTX infusions. Due to the risk of fatal myelosuppression we recommend up-front determination of TPMT activity in patients treated with 6MP or azathioprine.
Andersen
J B
JB
Laboratory of Paediatric Haematology and Oncology, The Juliane Marie Centre, State University Hospital, Copenhagen, Denmark.
Szumlanski
C
C
Weinshilboum
R M
RM
Schmiegelow
K
K
D000971:Antineoplastic Combined Chemotherapy Protocols / Q000008:administration & dosage* / Q000009:adverse effects / Q000493:pharmacokinetics*; D002648:Child; D002675:Child, Preschool; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D005500:Follow-Up Studies; D006801:Humans; D008297:Male; D015122:Mercaptopurine / Q000008:administration & dosage; D008727:Methotrexate / Q000008:administration & dosage; D008780:Methyltransferases / Q000172:deficiency*; D054198:Precursor Cell Lymphoblastic Leukemia-Lymphoma / Q000150:complications / Q000175:diagnosis / Q000188:drug therapy*
D015122:Mercaptopurine; D008780:Methyltransferases; C022745:thiopurine methyltransferase; D008727:Methotrexate