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1. Red blood cells from four children with lymphoblastic leukemia were age fractionated on Percoll density gradients into 'young', 'middle-aged' and 'old' cells. 2. The rates of accumulation of the mercaptopurine (MP) metabolites thioguanine nucleotides (TGNs) and methylmercaptopurine nucleotides (MeMPs) were measured in the cell fractions from the start of MP continuing chemotherapy. 3. TGNs and MeMP metabolites were present in all the red cell fractions after 3 days oral MP. There was no significant difference between the metabolite concentrations measured in either young, middle-aged or old cells (Mann-Whitney P = 1.0 to 0.12). 4. These observations suggest that MP metabolites do not enter red cells at the stem cell level at the start of therapy. 5. With respect to the monitoring of therapy, these results suggest that the concentration of TGNs after 7 to 10 days MP could be used to predict eventual steady-state concentrations using a simple model.

1. 6-Mercaptopurine (6-MP) is used in the continuing chemotherapy of childhood acute lymphoblastic leukaemia. The formation of red blood cell (RBC) 6-thioguanine nucleotide (6-TGN) active metabolites, not the dose of 6-MP, is related to cytotoxicity and prognosis. But there is an apparent sex difference in 6-MP metabolism. Boys require more 6-MP than girls to produce the same range of 6-TGN concentrations. Given the same dose, they experience fewer dose reductions because of cytotoxicity, and have a higher relapse rate. 2. The enzyme hypoxanthine phosphoribosyltransferase (HPRT) catalyses the initial activation step in the metabolism of 6-MP to 6-TGNs, a step that requires endogenous phosphoribosyl pyrophosphate (PRPP) as a cosubstrate. Both HPRT and the enzyme responsible for the formation of PRPP are X-linked. 3. RBC HPRT activity was measured in two populations, 86 control children and 63 children with acute lymphoblastic leukaemia. 6-MP was used as the substrate and the formation of the nucleotide product, 6-thioinosinic acid (TIA) was measured. RBC 6-TGN concentrations were measured in the leukaemic children at a standard dose of 6-MP. 4. There was a 1.3 to 1.7 fold range in HPRT activity when measured under optimal conditions. The leukaemic children had significantly higher HPRT activities than the controls (median difference 4.2 micromol TIA ml(-1) RBCs h(-1), 95% C.I. 3.7 to 4.7, P < 0.0001). In the leukaemic children HPRT activity (range 20.4 to 26.6 micromol TIA ml(-1) RBCs h(-1), median 23.6) was not related to the production of 6-TGNs (range 60 to 1,024 pmol 8 x 10(-8) RBCs, median 323). RBC HPRT was present at a high activity even in those children with low 6-TGN concentrations. 5. When HPRT is measured under optimal conditions it does not appear to be the metabolic step responsible for the observed sex difference in 6-MP metabolism. This may be because RBC HPRT activity is not representative of other tissues but it could equally be because other sex-linked factors are influencing substrate availability.

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. 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. 6-Mercaptopurine dose was related to 6-thioguanine nucleotide concentration (r = 0.4; P less than 0.001; n = 90; y = 18.51 + 0.36 x). Large individual variations around the regression line are observed. Neither 6-mercaptopurine dose nor 6-thioguanine nucleotide concentration was related to the neutrophil count at the time of sampling (day 0) or 7 days later. Both 6-mercaptopurine dose and 6-thioguanine nucleotide concentration correlated with the neutrophil count at day 14 (r = -0.33; P less than 0.01; n = 90 and r = -0.3; P less than 0.01; n = 90 respectively). This delay is compatible with a cytotoxic action on bone marrow stem cells. Excluding children with other, uncontrolled, potentially myelosuppressive influences the correlation between 6-thioguanine nucleotide concentration and neutropenia improved (r = -0.6; P less than 0.001; n = 37). A significant degree of neutropenia was observed by day 14 if the 6-thioguanine nucleotide concentration (day 0) was greater than 210 pmol/8 X 10(8) RBCs. 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.