Should TPMT genotype and activity be used to monitor 6-mercaptopurine treatment in children with acute lymphoblastic leukaemia?

BACKGROUND AND OBJECTIVE: The activity of thiopurine S-methyltransferase (TPMT), a key enzyme in the metabolism of purine analogues, displays wide inter-subject variability partly due to a genetic polymorphism. Previous studies have suggested adjusting purine analogues dosing according to TPMT activity but measurements are costly and time-consuming. It is still unclear, especially under treatment, whether the simpler TPMT genotyping reliably predicts enzyme activity. Our aim was to study the possible correlation of TPMT genotype with phenotype. METHODS: We determined the genotypic status and TMPT activity, at diagnosis and after 6 months of maintenance therapy, of 118 children with acute lymphoblastic leukaemia (ALL). RESULTS AND DISCUSSION: Eighty-nine per cent of the children had a homozygous wild-type genotype (group 1), 11% had one or two mutant allele(s) (group 2). At both time points, TPMT activity (U/mL peripheral red blood cell) was significantly higher in group 1 than in group 2 (P < 0.001) but inter-group levels overlapped considerably. There was considerable heterogeneity in the percentage increase in TPMT activity after therapy, and little correlation between metabolites ratio [6-methylmercaptopurine derivative/6-thioguanine nucleotides (6-TGN)] and TPMT activity at the end of 6 months' maintenance treatment. These results show that TPMT activity cannot be used as an accurate tool for 6-mercaptopurine monitoring. CONCLUSION: Genotyping at diagnosis identifies patients with a homozygous mutant TPMT and may prevent severe and life-threatening toxicity. ALL treatment monitoring should preferentially be based on repeated determinations of intracellular active metabolites (6-TGN) and methylated metabolites.

Results of 58872 and 58921 trials in acute myeloblastic leukemia and relative value of chemotherapy vs allogeneic bone marrow transplantation in first complete remission: the EORTC Children Leukemia Group report.

The first EORTC (European Organization of Research and Treatment of Cancer) acute myeloblastic leukemia (AML) pilot study (58872) was conducted between January 1988 and December 1991. Out of 108 patients, 78% achieved complete remission (CR), and event-free survival (EFS) and survival rates (s.e., %) at 7 years were 40 (5) and 51% (6%), respectively. It indicated that mitoxantrone could be substituted for conventional anthracyclines in the treatment of childhood AML without inducing cardiotoxicity. The aim of the next EORTC 58921 trial was to compare the efficacy and toxicity of idarubicin vs mitoxantrone in initial chemotherapy courses, further therapy consisting of allogeneic bone marrow transplantation (alloBMT) in patients with an HLA-compatible sibling donor or chemotherapy in patients without a donor. Out of 177 patients, recruited between October 1992 and December 2002, 81% reached CR. Overall 7-year EFS and survival rates were 49 (4) and 62% (4%), respectively. Out of 145 patients who received the first intensification, 39 had a sibling donor. In patients with or without a donor, the 7-year disease-free survival (DFS) rate was 63 (8) and 57% (5%) and the 7-year survival rate was 78 (7) and 65% (5%), respectively. Patients with favorable, intermediate and unfavorable cytogenetic features had a 5-year EFS rate of 57, 45 and 45% and a 5-year survival rate of 89, 67 and 53%, respectively.

Listing of common HLA alleles and haplotypes based on the study of 356 families residing in the Paris, France, area: implications for unrelated hematopoietic stem cell donor selection.

In this study we have identified frequent human leukocyte antigen (HLA)-A, -B, -C,-DRB1, and -DQB1 alleles, frequent HLA-B/C, HLA-DRB1/DQB1 two-allele associations, and the most common HLA-A/B/C/DRB1/DQB1 five-locus haplotypes in a population residing in the Paris, France, area. The study was carried out in 356 families of children awaiting hematopoietic stem-cell transplantation (HSCT), with the selection criterion that haplotypes could be assigned with certainty to both the patient and at least one parent. Parental haplotypes were HLA-A, -B serologically typed, and HLA-C, -DRB1, -DQB1 broadly typed by polymerase chain reaction-sequence-specific oligonucleotide probe. The alleles of the most frequent haplotypes were subsequently defined at a high-resolution level by polymerase chain reaction-sequence-specific primer. The results on the distribution of common alleles and common allele associations demonstrated similarities with the previously published data in Caucasian populations, as expected from the geographic origin of the studied population. More importantly, this study provides the largest listing of common B/C and DRB1/DQB1 associations and of common five-allele haplotypes defined with certainty in a Caucasian population to date. These results can be used to help estimate the likelihood of finding a suitable donor in unrelated HSCT and to delineate search strategies for potential donors.

Late thyroid toxicity in 153 long-term survivors of allogeneic bone marrow transplantation for acute lymphoblastic leukaemia.

The purpose of this study was to identify risk factors for hypothyroidism after bone marrow transplantation (BMT) for high-risk or relapsed acute lymphoblastic leukaemia (ALL) in children. In all, 388 children with acute lymphoblastic leukaemia underwent allogeneic bone marrow transplantation between 1984 and 1994. Overall 5-year survival was 54.6%. Thyroid function was assessed in the 153 patients with more than 5 years of follow-up. In total, 16 patients developed uncompensated hypothyroidism (UH) and 46 compensated hypothyroidism (CH) a median of 2.9 and 2.7 years, respectively, after BMT. Thyroid dysfunction-free survival rates were 73.2% after 5 years and 59.2% after 10 years. Three factors were significantly associated with the onset of hypothyroidism, namely age, bone marrow transplantation in second remission, and single-dose total body irradiation (TBI). Ultrasonography of the thyroid showed nodules in 10 of 35 patients. The median time from BMT to nodule detection was 7.8 years. Cytology (n=5) and surgery (n=4) showed no evidence of thyroid cancer. Four of the 14 patients who received cytoreduction without TBI but with busulphan and cyclophosphamide developed UH (n=2) or CH (n=2). We concluded that children who undergo BMT for ALL are at a high risk of subsequent thyroid dysfunction.

No prognostic effect of additional chromosomal abnormalities in children with acute lymphoblastic leukemia and 11q23 abnormalities.

This study characterized the additional chromosomal abnormalities (ACA) associated with 11q23 rearrangements in 450 infants and children with acute lymphoblastic leukemia (ALL) and examined the impact of these ACA on survival. Overall, 213 (47%) cases had ACA but the incidence varied according to patient age and 11q23 subgroup. Infants and patients with t(4;11)(q21;q23) had the lowest incidence of ACA (50/182 (27%) and 57/216 (26%) respectively), whereas patients with del(11)(q23) had the highest incidence (66/93 (71%)). Del(11)(q23) abnormalities were heterogeneous and occasionally secondary to t(9;22)(q34;q11.2). Thus, patients with del(11)(q23) comprised a separate biological entity, which was clearly distinct from those with an 11q23 translocation. The most frequent specific ACA were trisomy X (n = 38), abnormal 12p (n = 32), abnormal 9p (n = 28) and del(6q) (n = 19). The presence of ACA did not change the 5 year event-free survival estimates among children (56% (95% Cl 46-65%) vs 62% (54-69%)) or infants (22% (15-29%) vs 18% (9-29%)), nor when the different 11q23 subgroups were analyzed separately. This study has conclusively demonstrated that there is no prognostic effect of secondary chromosomal changes in association with 11q23 abnormalities in childhood ALL. However, characterization of these ACA is important to determine their potential role in initiation of MLL driven leukemogenesis.

Value of intravenous 6-mercaptopurine during continuation treatment in childhood acute lymphoblastic leukemia and non-Hodgkin's lymphoma: final results of a randomized phase III trial (58881) of the EORTC CLG.

Between November 1990 and November 1996, EORTC Children Leukemia Group conducted a randomized trial in de novo acute lymphoblastic leukemia and lymphoblastic non-Hodgkin's lymphoma patients using a Berlin-Frankfurt-Munster protocol to evaluate the monthly addition of intravenous 6-mercaptopurine (i.v. 6-MP) (1 g/m(2)) to conventional continuation therapy comprising per oral MTX weekly and 6-MP daily. Only during the first 18 months of the randomization period, 6-MP p.o. was interrupted for 1 week after each i.v. 6-MP. A total of 877 patients was randomized to either no i.v. 6-MP (Arm A) or additional i.v. 6-MP (Arm B). A total of 217 relapses (91 in Group A vs 128 in Group B) and 13 deaths in CR (5 vs 8) were reported; a total of 134 patients (55 vs 79) died. The median follow-up was 7.6 years. At 8 years, the disease-free survival rate was lower (P=0.005) in Arm B (69.1% (s.e.=2.2%)) than in Arm A (77.9% (s.e.=2.0%)), and the hazard ratio was 1.45 (95% CI 1.12-1.89). In conclusion, as delivered in this study, i.v. 6-MP was detrimental to event-free survival.

Acute childhood leukaemia and environmental exposure to potential sources of benzene and other hydrocarbons; a case-control study.

AIM: To analyse the association between potential environmental exposure to hydrocarbons and the risk of acute childhood leukaemia. METHODS: A hospital based multicentre case control study, stratified on centre, age, and sex, with 280 leukaemia cases and 285 controls was carried out. Data were collected by a standardised interview of the mothers. RESULTS: No clear association was seen between maternal occupational exposure to hydrocarbons during pregnancy and leukaemia, or between residential traffic density and leukaemia. There was an association between dwellings neighbouring a petrol station or a repair garage during childhood and the risk of childhood leukaemia (OR 4.0, 95% CI 1.5 to 10.3), with a duration trend. The association, which appeared particularly strong for acute non-lymphocytic leukaemia (OR 7.7, 95% CI 1.7 to 34.3), was not altered by adjustment for potential confounding factors. CONCLUSIONS: Results showed an association between acute childhood leukaemia and dwellings neighbouring auto repair garages and petrol stations, which are benzene emitting sources. These findings could be due to chance, although the strength of the association and the duration trend are arguments for a causal association.

Outcome of childhood acute promyelocytic leukemia with all-trans-retinoic acid and chemotherapy.

PURPOSE: To determine the results of treatment combining all-trans-retinoic acid (ATRA) and chemotherapy (CT) in childhood acute promyelocytic leukemia (APL). PATIENTS AND METHODS: Children (< 18 years) with newly diagnosed APL were included in the APL93 trial, treated by ATRA followed or combined with daunorubicin-cytarabine, and then randomly assigned between no maintenance, intermittent ATRA, continuous CT, or both. RESULTS: Of the 576 patients included in APL93 trial, 31 (5%) were children, including 22 girls (71%) and nine boys (29%). Thirty of the children (97%) obtained complete remission (CR). ATRA syndrome occurred in four children (13%), who all achieved CR, and headaches occurred in 12 children (39%), with signs of pseudotumor cerebri in five children (16%). Seven patients (23%) relapsed. None of the eight patients who received both ATRA and CT for maintenance relapsed. All relapsing patients achieved a second CR. Twenty-two patients remained in first CR after 43+ to 96+ months, six remained in second CR after 17+ to 66+ months, and three patients had died. The 5-year event-free survival (EFS), relapse, and overall survival rates were 71%, 27%, and 90%, respectively. No difference between adults and children included in the APL93 trial was seen for CR rate, 5-year relapse rate, EFS, and overall survival, but significantly better survival was seen in children after adjustment on WBC counts (P =.02) and incidence of microgranular M3 variant (P =.04). CONCLUSION: ATRA combined with CT for induction and also probably for maintenance provides as favorable results in children with APL as in adults and currently constitutes the reference first-line treatment in both age groups.

Incidence of childhood leukaemia and non-Hodgkin's lymphoma in France: National Registry of Childhood Leukaemia and Lymphoma, 1990-1999.

The French National Registry of Childhood Leukaemia and Lymphoma (NRCL) covers the whole French mainland population aged less than 15 years (approximately 11 million children) for all childhood haematopoietic tumours since 1 January 1990, except Hodgkin's disease, which has been registered since 1 January 1999. During the period from 1990 to 1999, 5757 cases of leukaemia, lymphoma and myelodysplastic syndrome were registered in the NRCL, with an average of 2.5 sources per case. The age-standardized incidence rates per million per year were 43.1 for leukaemia (34.3 for acute lymphoblastic leukaemia, 7.1 for acute myeloblastic leukaemia, 0.6 for chronic myeloid leukaemia and 0.5 for chronic myelomonocytic leukaemia), 8.9 for non-Hodgkin's lymphomas and 6.7 for Hodgkin's disease. Down's syndrome was present in 110 cases of acute leukaemia (2.5%) and three cases of non-Hodgkin's lymphoma (0.3%). The incidence of acute lymphoblastic leukaemia showed a typical peak at age 2 years for girls and 3 years for boys. The incidence rates of leukaemia and non-Hodgkin's lymphoma did not show any temporal trends over the 10 year period.

Hematopoietic stem cell transplantation in patients with severe Langerhans cell histiocytosis and hematological dysfunction: experience of the French Langerhans Cell Study Group.

The aim of this study was to assess the results of hematopoietic stem cell transplantation (HSCT) in refractory Langerhans cell histiocytosis (LCH). Among 85 patients with LCH and hematological dysfunction diagnosed in France between 1987 and 2000, eight received HSCT in six institutions. Median age at diagnosis was 0.54 years. The median LCH activity score at diagnosis was 10 (range 3-20). All patients responded poorly to initial chemotherapy. At the time of HSCT, the median activity score was 16.5 (range 7-18). HSCT was autologous in three cases and allogeneic in five cases. The conditioning regimen consisted of TBI in two cases and chemotherapy alone in six cases. Conditioning had to be attenuated in two patients. All patients had persistent active disease after autologous HSCT, which was fatal in two cases and controlled by chemotherapy in one case. After allogeneic HSCT, two patients died from toxicity and three had complete responses; two patients had had no recurrences after 21 months and 7 years of follow-up, while the other patient relapsed and died from sepsis related to splenectomy. HSCT for refractory LCH can thus be highly toxic but can also achieve sustained disease control.

Common genomic HLA haplotypes contributing to successful donor search in unrelated hematopoietic transplantation.

The aim of the study was to identify the most frequent HLA haplotypes in order to optimize donor searches in unrelated hematopoietic stem cell (HSC) transplantation. Pediatric patients from the north of France who underwent initial HLA typing for donor search in our center were included. Patients and family members were broadly typed for HLA class I and II. Patients were further DNA typed at the sequence level for HLA-A, -B, -Cw, -DRB1, and -DQB1 alleles. In 200 of 207 patients HLA haplotypes were assigned by the mode of inheritance. The most common haplotypes were defined based on frequencies over 0.75%. Searches for unrelated donors were completed for 86 patients lacking a family donor. Matching criteria were either the optimal level of 10 alleles or a one-HLA class I mismatch as a second choice. Rates of successful search reach 85% for patients (n=20) who express at least one common five-allele (HLA-A/B/Cw/DRB1/DQB1) haplotype, but also 77% for more patients (n=53) who express at least one of the 20 most frequent three-allele (HLA-A/B/Cw) haplotypes. Success rates are clearly less (39%) in patients lacking these haplotypes. The use of these data to delineate search strategies is discussed.

Clinical heterogeneity in childhood acute lymphoblastic leukemia with 11q23 rearrangements.

To assess the clinical heterogeneity among patients with acute lymphoblastic leukemia (ALL) and various 11q23 abnormalities, we analyzed data on 497 infants, children and young adults treated between 1983 and 1995 by 11 cooperative groups and single institutions. The substantial sample size allowed separate analyses according to age younger or older than 12 months for the various cytogenetic subsets. Infants with t(4;11) ALL had an especially dismal prognosis when their disease was characterized by a poor early response to prednisone (P=0.0005 for overall comparison; 5-year event-free survival (EFS), 0 vs 23+/-+/-12% s.e. for those with good response), or age less than 3 months (P=0.0003, 5-year EFS, 5+/-+/-5% vs 23.4+/-+/-4% for those over 3 months). A poor prednisone response also appeared to confer a worse outcome for older children with t(4;11) ALL. Hematopoietic stem cell transplantation failed to improve outcome in either age group. Among patients with t(11;19) ALL, those with a T-lineage immunophenotype, who were all over 1 year of age, had a better outcome than patients over 1 year of age with B-lineage ALL (overall comparison, P=0.065; 5-year EFS, 88+/-+/-13 vs 46+/-14%). In the heterogeneous subgroup with del(11)(q23), National Cancer Institute-Rome risk criteria based on age and leukocyte count had prognostic significance (P=0.04 for overall comparison; 5-year EFS, 64+/-+/-8% (high risk) vs 83+/-+/-6% (standard risk)). This study illustrates the marked clinical heterogeneity among and within subgroups of infants or older children with ALL and specific 11q23 abnormalities, and identifies patients at particularly high risk of failure who may benefit from innovative therapy.

Immune reconstitution after haematopoietic transplantation with two different doses of pre-graft antithymocyte globulin.

Antithymocyte globulin is widely used before haematopoietic transplantation with HLA-matched unrelated donors or mismatched relatives to prevent rejection and graft-versus-host disease (GVHD). However, optimal dosage is still under debate. Thirty-one consecutive children, mainly with haematological malignancies, were transplanted in a single institution with such donors, selected by HLA-A -B compatibility by serology and DRB1* by DNA typing. Antithymocyte globulin (Thymoglobuline; Sangstat) was infused at days -3, -2, -1. Total dosage varied: 16 patients received a median of 7.5 mg/kg (2.5 to 10.5: low-dose group), and 15 a median of 15.5 mg/kg (14.4 to 19.4: high-dose group). Post-transplant GVHD prophylaxis consisted of cyclosporine, short-course methotrexate and steroids. CD3(+), CD4(+) and CD19(+) cell reconstitution was slower in the high-dose group. Median time to reach 100 CD4(+) cells was 8 months vs 4 months (P = 0.03). Median time to normal CD19(+) cells was 16 months vs 8 months (P = 0.01). CD16(+)CD56(+) and CD8(+) cell reconstitution was similar. Nine patients in the high-dose group and two in the low-dose group experienced life-threatening opportunistic infections (P = 0.009). Although obtained from a limited number of patients, our data suggest that a higher pre-graft dose of antithymocyte globulin may negatively influence immune reconstitution.

Day-care, early common infections and childhood acute leukaemia: a multicentre French case-control study.

We conducted a case-control study to investigate the role of early infections in the aetiology of childhood acute leukaemias. The study included 280 incident cases (240 acute lymphoblastic leukaemia and 40 acute non-lymphoblastic leukaemia) and 288 hospital controls, frequency matched by age, gender, hospital, catchment area of the hospital and ethnic origin. Data were obtained from standardised face-to-face interviews of the mothers. The interviews included questions on early common infections, day-care attendance, breast-feeding, birth order and infantile diseases. Odds ratios were estimated using an unconditional regression model including the stratification variables, parental socio-economic status and perinatal characteristics. Birth order was not associated with childhood leukaemia (acute lymphoblastic or acute non-lymphoblastic). A statistically-significant inverse association was observed between childhood leukaemia and day-care attendance (odds ratio=0.6, 95% Confidence Interval=(0.4-1.0)), repeated early common infections (> or = 4 per year before age two, odds ratio=0.6 (0.4-1.0)), surgical procedures for ear-nose-throat infections before age two (odds ratio=0.5 (0.2-1.0)) and prolonged breast-feeding (> or = 6 months, odds ratio=0.5 (0.2-1.0)). In the multivariate model including day-care attendance, early common infections and breast-feeding, results concerning breast-feeding remained unchanged. A statistically significant interaction between day-care attendance and repeated early common infections was observed. When the interaction was taken into account, the simple effects of day-care and early common infections disappeared (odds ratio=1.1 (0.5-2.3) and odds ratio=0.8 (0.5-1.3), respectively) while the joint effect of day-care attendance and early common infections was negatively associated with childhood leukaemia (odds ratio=0.3 (0.1-0.8)). All the above associations were observed both for acute lymphoblastic leukaemia and acute non-lymphoblastic leukaemia. Our results support Greaves' hypothesis, even though they are not specific of common leukaemia.

The Children Leukemia Group: 30 years of research and achievements.

The EORTC Children Leukemia Group (CLG) is part of the offspring of the EORTC Hemopathies Working Party which in 1978 split into a paediatric and to an 'adult' branch. At that time, the Berlin-Frankfurt-Munster (BFM) designed by H. Riehm for acute lymphoblastic leukaemia (ALL) appeared much more efficacious than all others and the CLG decided to adapt that treatment strategy for its own clinical trials. The main results of these may be summarised as follows:for standard risk patients, the deletion of cyclophosphamide from consolidation and reconsolidation courses does not jeopardise the patient's outcomefor medium- and high-risk patients receiving high-dose methotrexate (MTX), cranial radiotherapy is superfluouswith the dose scheduling of the BFM regimen, E-Coli L-Asparaginase is more efficacious than Erwinia L-Asparaginasethe addition of monthly intravenous (i.v.) 6-mercaptopurine to conventional maintenance chemotherapy is detrimentalthe assessment by quantitative polymerase chain reaction (PCR) of minimal residual disease at completion of induction is feasible in a cooperative setting and can be used as a powerful and independent prognostic factor. The CLG also conducted clinical studies of acute myeloblastic leukaemia. Since 1989, lymphoblastic non-Hodgkin's lymphomas have been treated within the ALL trials. The CLG collaborates with other Groups within the I-BFM Study Group and participants in the meta-analytic studies conducted by the Oxford team by the Oxford Children ALL Collaborative Group.

Chimaeric anti-CD20 monoclonal antibody (rituximab) in post-transplant B-lymphoproliferative disorder following stem cell transplantation in children.

Post-transplant lymphoproliferative disorder (PTLD) after haemopoietic stem cell transplantation is a serious complication that occurs in 8-22% of patients with high-risk factors. We retrospectively investigated tolerance and efficacy of humanized anti-CD20 monoclonal antibody (rituximab) as first-line treatment in 12 children with B-cell PTLD. At diagnosis, eight patients had tumoral involvement. The other four patients had fever, associated with raised Epstein-Barr virus (EBV) viral load and monoclonal gammopathy. Rituximab was given at the dose of 375 mg/m2 once a week by intravenous infusion (1-9 infusions). Only 1/48 infusions was associated with a grade 2 clinical adverse event. Eight out of 12 (66%) patients responded to the treatment and were in complete remission. All patients without tumoral involvement responded to the treatment. A rapid decrease in fever within 1 week was observed in all responders. Non-responders did not show any clinical response during the first week. Tumoral involvement and immunodepression seemed to be more marked in non-responders. Rituximab was an effective and well-tolerated treatment of B-cell PTLD. Early treatment before tumoral involvement seemed to be the most effective approach. Lack of rapid response should lead to intensification of PTLD treatment. Pre-emptive treatment should be considered and evaluated in further longitudinal multicentre studies.

Quantification of busulfan in plasma by liquid chromatography-ion spray mass spectrometry. Application to pharmacokinetic studies in children.

Optimisation of busulfan dosage in patients undergoing bone marrow transplantation is recommended in order to reduce toxic effects associated with high drug exposure. A new method was developed coupling liquid chromatography with mass spectrometry (LC-MS) and was validated for the determination of busulfan concentrations in plasma. Recovery was 86.7%, the limit of detection was 2.5 ng/ml and linearity ranged from 5 to 2500 ng/ml. The correlation between the busulfan concentrations measured by our previously published HPLC-UV method and the new HPLC-MS method was highly significant (P<0.0001). Sample volume was reduced and the method was rapid, sensitive and less expensive than the methods previously used in our laboratory. This method was used to determine the pharmacokinetic parameters of busulfan after the first administration of 1 mg/kg orally, in 13 children receiving the drug as part of the preparative regimen for bone marrow transplantation. Our results were similar to previously reported data. They showed that the apparent oral clearance of busulfan was 0.299+/-0.08 l/h/kg, and that it was significantly higher (P=0.02) in patients below the age of 5 years than in older children.

Possible implication of thiopurine S-methyltransferase in occurrence of infectious episodes during maintenance therapy for childhood lymphoblastic leukemia with mercaptopurine.

6-Mercaptopurine (6-MP) is metabolized by thiopurine S-methyltransferase (TPMT), an enzyme subject to genetic polymorphism. We investigated the relationships between the TPMT locus (TPMT activity and genotype) and the pharmacological response to 6-MP during maintenance therapy of 78 children with acute lymphoblastic leukemia (ALL). For each patient, 6-MP dosage, leukocyte counts and occurrence of infectious episodes were monitored on an 8 week basis. Higher 6-MP dosage was associated with higher TPMT activity (P = 0.03) and higher average leukocyte counts (P < 0.01). Eight patients (10%) carrying a TPMT mutant genotype (one homozygous and seven heterozygous) received lower 6-MP doses (average: 48 vs 65 mg/m2/day; P = 0.02) and had on average lower leukocyte counts (2834 vs 3398 cells/mm3; P = 0.003) than patients carrying the wild-type TPMT genotype. Higher occurrence of infectious episodes graded 2 or 3 was correlated with higher 6-MP dosage (P < 0.01) but no difference was observed between TPMT mutants and TPMT wild-type patients. Patients who received 6-MP dosage above the group median (62 mg/m2/day) or having a TPMT activity above the group median (21.5 nmol/h/ml) had a higher percentage of 8 week periods with infectious episodes requiring treatment (34% vs 17% and 33% vs 19%, respectively) than those with 6-MP dose or TPMT activity below the group median (P < 0.01). In the last 25 patients enrolled in the study, steady-state erythrocyte thioguanine nucleotide (TGN) concentrations were associated with lower leukocyte counts (P= 0.01) but not with a higher occurrence of infectious episodes. In contrast, higher steady-state erythrocyte methylmercaptopurine nucleotide (MeMPN) concentrations were associated with higher 6-MP dosage (P< 0.01) and higher occurrence of infectious episodes (P < 0.001). In conclusion, during maintenance therapy of ALL, children with higher TPMT activity receive a higher 6-MP dosage and may have infectious episodes caused by metabolism of 6-MP into methylmercaptopurine nucleotides.