Childhood leukaemia: A relationship between intracellular 6-mercaptopurine metabolites and neutropenia. Commentary. Author's reply

1 6-Mercaptopurine is extensively used in the treatment of childhood lymphoblastic leukaemia to prolong the duration of remission achieved with other drugs. The response to remission maintenance therapy varies widely. 2 We investigated the relationship between red blood cell 6-thioguanine nucleotide, a metabolite of 6-mercaptopurine, and myelosuppression in 22 children with acute lymphoblastic leukaemia in remission. The peripheral neutrophil count was used as an index of myelosuppression. 3 6-Mercaptopurine dose was related to 6-thioguanine nucleotide concentration (r = 0.4; P 210 pmol/8 x 10 8 RBCs. 7 The assay of 6-thioguanine nucleotide may highlight those individuals with pharmacokinetic resistance. Two children on continuous high dose 6-mercaptopurine, had low red blood cell 6-thioguanine nucleotide concentrations and neutropenia was not observed.