Are children with lymphoblastic leukaemia given enough 6-mercaptopurine?

Thiopurines' Metabolites and Drug Toxicity: A Meta-Analysis

Many questions remain unanswered regarding therapeutic drug monitoring (TDM) utility with thiopurines. This study aims to establish a relationship between thiopurines' metabolites and drug toxicity. We performed a systematic review with inclusion of studies evaluating the relationship between thiopurines' metabolites and drug toxicity. Meta-analysis of mean difference (MD), correlations and odds ratio (OR) was performed. We identified 21,240 records, 72 of which were eligible for meta-analysis. Levels of 6-thioguanine nucleotides (6-TGN) were higher in patients with leukopenia (MD 127.06 pmol/8 × 10⁸ RBC) and gastrointestinal intolerance (MD 201.46 pmol/8 × 10⁸ RBC), and lower in patients with hepatotoxicity (MD -40.6 pmol × 10⁸ RBC). We established a significant correlation between 6-TGN and leukocytes (r = -0.21), neutrophils (r = -0.24) and alanine aminotransferase levels (r = -0.24). OR for leukopenia in patients with elevated 6-TGN was 4.63 (95%CI 2.24; 9.57). An optimal cut-off of 135 pmol/8 × 10⁸ RBC for leukopenia was calculated (sensitivity 75.4%; specificity 46.4%). 6-methylmercaptopurine ribonucleotides (6-MMPR) were significantly associated with hepatotoxicity (MD 3241.2 pmol/8 × 10⁸ RBC; OR 4.28; 95%CI 3.20; 5.71). Levels of 6-MMPR measured in the first 8 weeks of treatment were associated with leukopenia. We conclude that TDM could be used to prevent thiopurines' toxicity. As optimal metabolites level may vary according to indication, physicians may adapt posology to decrease toxicity without compromising efficacy.

Pharmacogenetics and blood dyscrasias

Pharmacogenetic differences in the handling of and response to drugs can markedly alter the risk of severe idiosyncratic adverse drug reactions, including neutropenia, agranulocytosis and aplastic anaemia. Inherited deficiencies of drug metabolizing enzymes can shunt the metabolism of drugs to metabolites which are directly toxic (e.g. 6-mercaptopurine metabolism to 6-thioguanine nucleotides) or towards electrophilic metabolites which can kill cells and/or lead to a host immune response (e.g. sulphonamide metabolism to hydroxylamine metabolites). Defects in detoxification pathways (e.g. glutathione conjugation) similarly can predispose patients to adverse outcomes. The advent of molecular screening tools to define individual (rather than population) risk may lead to the use of clinical laboratory tests to identify/predict idiosyncratic adverse drug reactions.

Pharmacokinetic studies in children with cancer

We reviewed the current status of our knowledge of pharmacokinetics and pharmacodynamics of some anti-neoplastic drugs, used in the treatment of childhood cancer. Extrapolation of data from pharmacokinetic studies in adults to the paediatric population is often not feasible. Specific studies in children are needed. Of all reviewed anti-neoplastic drugs methotrexate appears to be most extensively studied. Methotrexate pharmacokinetics is correlated with toxicity and response to therapy, and it has been shown that individualized adaptive dosing of methotrexate is correlated with a better response to therapy without increasing toxicity in children with ALL and osteosarcoma. Of most of the other reviewed anti-neoplastic drugs it is demonstrated that pharmacokinetics is correlated with toxicity, and of some drugs a relationship of pharmacokinetics with response to therapy is demonstrated as well. In case of cytarabine, etoposide, and teniposide, individualized dosing also appears to be feasible. However, there is no evidence that this strategy improves response to therapy. Specifically data on pharmacokinetic and pharmacodynamic correlations and effect of pharmacokinetically guided, individualized dosing are important for the design of optimal cancer chemotherapy for individual patients. Unfortunately for a considerable number of anti-neoplastic drugs these specific data are lacking in children and future research is needed.

Pharmacogenetic determinants of outcome in acute lymphoblastic leukaemia

Present day paediatric co-operative group acute lymphoblastic leukaemia (ALL) protocols cure approximately 80% of patients, a result achieved largely through the use of risk-stratified therapies that employ multiple chemotherapy agents. These risk-based therapies utilize host and leukaemia traits to select the most appropriate therapy. However, these risk-stratified approaches predict therapy response imperfectly and an important fraction of patients experience relapse or therapy-related toxicity. Pharmacogenetics, the study of genetic variations in drug-processing genes and individual responses to drugs, may enable the improved identification of patients at higher risk for either disease relapse or chemotherapy-associated side effects. While the impact of genetic variation in the thiopurine-S-methyltransferase gene on ALL treatment outcome and toxicity has been extensively studied, the role of other polymorphisms remains less well known. This review summarizes current research on the impact of genetic variation in drug-processing genes in paediatric ALL and reviews important methodological and statistical issues presently challenging the field of pharmacogenetics.

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.

Therapeutic drug monitoring of antimetabolic cytotoxic drugs

Therapeutic drug monitoring is not routinely used for cytotoxic agents. There are several reasons, but one major drawback is the lack of established therapeutic concentration ranges. Combination chemotherapy makes the establishment of therapeutic ranges for individual drugs difficult, the concentration-effect relationship for a single drug may not be the same as that when the drug is used in a drug combination. Pharmacokinetic optimization protocols for many classes of cytotoxic compounds exist in specialized centres, and some of these protocols are now part of large multicentre trials. Nonetheless, methotrexate is the only agent which is routinely monitored in most treatment centres. An additional factor, especially in antimetabolite therapy, is the existence of pharmacogenetic enzymes which play a major role in drug metabolism. Monitoring of therapy could include assay of phenotypic enzyme activities or genotype in addition to, or instead of, the more traditional measurement of parent drug or drug metabolites. The cytotoxic activities of mercaptopurine and fluorouracil are regulated by thiopurine methyltransferase (TPMT) and dihydropyrimidine dehydrogenase (DPD), respectively. Lack of TPMT functional activity produces life-threatening mercaptopurine myelotoxicity. Very low DPD activity reduces fluorouracil breakdown producing severe cytotoxicity. These pharmacogenetic enzymes can influence the bioavailability, pharmacokinetics, toxicity and efficacy of their substrate drugs.

Reduced thiopurine methyltransferase activity and development of side effects of azathioprine treatment in patients with rheumatoid arthritis

OBJECTIVE

To investigate thiopurine enzyme activities for their possible value in predicting the development of azathioprine (AZA)-related toxicity in patients with rheumatoid arthritis (RA).

METHODS

Patients with longstanding RA (n = 33) were enrolled in a study of treatment with AZA. Before the initiation of AZA treatment and at months 1 and 6 of treatment, we measured activities of the purine key enzymes hypoxanthine guanine phosphoribosyltransferase, 5'-nucleotidase, purine nucleoside phosphorylase, and thiopurine methyltransferase (TPMT). Controls included patients with early RA (n = 24) and healthy volunteers (n = 42).

RESULTS

Fourteen of the 33 patients rapidly developed severe side effects, most frequently gastrointestinal (GI) intolerance. Compared with the other groups, the group with adverse effects had significantly lower TPMT activities (P = 0.004). Seven of 8 patients with reduced ("intermediate") baseline TPMT levels developed toxicity, resulting in a significant relationship (P = 0.005) between toxicity and "intermediate" TPMT activity. Compared with "high" activity, baseline intermediate TPMT activity gave a relative risk of 3.1 (95% confidence interval 1.6-6.2) for the development of severe toxicity with AZA treatment.

CONCLUSION

In RA patients, inherited intermediate TPMT activity seems predictive for the development of severe side effects of AZA. Clinicians should consider measuring TPMT prior to treatment initiation to improve the safety of AZA use. We hypothesize that GI intolerance may also be related to a thiopurine metabolic imbalance.

Pharmacokinetics of mercaptopurine: plasma drug and red cell metabolite concentrations after an oral dose

Measurement of red cell 6-mercaptopurine (MP) derived 6-thioguanine nucleotide (TGN) and methylmercaptopurine metabolites (MeMPs) can be used to monitor therapy in children who are administered MP for childhood lymphoblastic leukemia. Red cell TGNs are not influenced by the time of blood sampling in relation to the last MP dose. The purpose of this study was to find out whether the same is true for the MeMPs. Plasma MP and red cell MP metabolite pharmacokinetics were studied in seven children immediately before and for 4 hours after a protocol standardized dose of MP. Duplicate blood samples were taken, one was processed immediately whereas one was left at an ambient temperature for 24 hours. The variation in TGN and MeMP metabolites over the 0- to 4-hour period (10 time points per child) was within the error of the assays used. The coefficients of variation for the TGNs ranged from 2.7% to 7% and for the MeMPs, 4% to 10.7%. There was no difference in the TGN and MeMP concentrations measured when the blood samples were left for 24 hours. If a child takes a MP tablet immediately before a clinic appointment, it has no major influence on MeMP measurements.

Should 6-thioguanine nucleotides be monitored in heart transplant recipients given azathioprine?

The commonly used immunosuppressive regimen after orthotopic heart transplantation consists of cyclosporine (CsA), azathioprine (AZA), and steroids. Although AZA therapy is generally regarded as unproblematic, its use can be associated with severe side effects, particularly myelosuppression. Since AZA is a prodrug, which must first be metabolized to its active metabolites, AZA therapy, in contrast to CsA therapy, cannot be controlled by measuring blood levels of this drug. Because of the myelosuppressive properties of the AZA metabolites, the 6-thioguanine nucleotides (6-TGN), the white blood cell count is usually monitored in patients on AZA therapy, and AZA is discontinued if neutropenia appears. In a group of 20 consecutive heart recipients, 6-TGN concentrations ranged from < 30 to 2,211 pmol/8 x 10(8) red blood cells (RBCs); levels < or = 450 pmol/8 x 10(8) RBCs were not associated with AZA-induced myelosuppression. Three cases of neutropenia were experienced, two of them with a fatal outcome. One patient died in septicemia owing to total myelosuppression. In this case an excessively high erythrocyte 6-TGN concentration (2,211 pmol/8 x 10(8) RBCs) was associated with a complete deficiency of thiopurine methyltransferase (TPMT), one of the main AZA detoxifying enzymes. The second patient, who had high RBC TPMT activity, developed neutropenia during rehabilitation, and AZA was withdrawn. Coincidentally, in this case the CsA blood level was only 132 g/L, and the RBC 6-TGN level was very low (maximum 46 pmol/8 x 10(8) RBCs). This patient rapidly developed cardiogenic shock with clinical signs of acute rejection and was given a second transplant on an emergency basis, but finally died from rejection of the second graft. Retrospectively, it was determined that neutropenia in this patient was not related to AZA toxicity. A high 6-TGN level (698 pmol/8 x 10(8) RBCs) was also seen in a third patient with mild neutropenia, who required allopurinol, an inhibitor of xanthine oxidase, the other major detoxifying enzyme for AZA. In this patient AZA therapy could be individually adapted by RBC 6-TGN monitoring. Based on our experience, we suggest that RBC 6-TGN monitoring allows for better individualization of treatment with AZA and may help avoid fatal complications.

Metabolism of intravenously administered high-dose 6-mercaptopurine with and without allopurinol treatment in patients with non-Hodgkin lymphoma

PURPOSE

We investigated the metabolism of high dose 6 mercaptopurine (HD-6MP) infusions and its influence on the metabolism by allopurinol, an inhibitor of xanthine oxidase, the enzyme that catabolizes 6MP into thioxanthine and thiouric acid.

PATIENTS AND METHODS

Nine patients (aged 2-11 years) with non-Hodgkin lymphoma (NHL) were treated with HD-6MP (1300 mg/m(2).24h) within a therapeutic window after diagnosis. Four patients received oral allopurinol (200 mg/m(2).day) to prevent urate nephropathy, and five did not. Plasma and RBC were isolated before and 4, 20, 24, 28, and 48h after the start of the infusion. All measurements were performed with HPLC.

RESULTS

Considerable variations were found in the plasma levels of 6MP, thioxanthine, and thiouric acid and of RBC-MeTIN levels. 6MP-riboside was not detectable, and MeMP and MeMPR levels were <1.3 muM in the plasma. In general, 6MP, thioxanthine, and MeMP levels in plasma were higher, and thiouric acid plasma levels and RBC-MeTIN levels were lower in the patients treated with allopurinol compared to those who did not receive allopurinol.

CONCLUSIONS

6MP is extensively metabolized in patients with NHL treated with HD-6MP. Thiopurine methylation, at the levels of nucleotide, nucleoside, and base, is an important metabolic pathway after HD-6MP. Co-administration of allopurinol can result in both a decreased catabolism and anabolism of 6MP compared to treatment with HD-6MP alone. This observation may have consequences for the therapeutic efficacy and toxic effects of 6MP in combination with allopurinol.

The accumulation of mercaptopurine metabolites in age fractionated red blood cells

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.

Purine enzyme activities in peripheral blood mononuclear cells: comparison of a new non-radiochemical high-performance liquid chromatography procedure and a radiochemical thin-layer chromatography procedure

Purine enzyme activities are usually assayed by radiochemical procedures and often TLC is part of the separation method. In screening patients with rheumatic diseases, these procedures have shown disadvantages like a relatively large coefficient of variation (C.V.) and time-instability. We describe a non-radiochemical reversed-phase HPLC micro-method with UV detection for measurement of activities of purine 5'-nucleotidase (5'NT; EC 3.1.3.5), purine nucleoside phosphorylase (PNP; EC 2.4.2.1) and hypoxanthine guanine phosphoribosyltransferase (HGPRT; EC 2.4.2.8) in human peripheral blood mononuclear cells (PBMC). The HPLC procedure is compared with the radiochemical TLC procedure by testing both with a 5'NT and a PNP assay. Reproducibility is tested with 14 healthy controls in each procedure. Short-term and long-term time-stability is tested by comparing enzyme activities measured immediately after preparation of the PBMC (week 0) with those found after freezing and storage at -20 degrees C for a maximum of 10 weeks. The HPLC procedure is preferable to the radiochemical TLC procedure because it shows significantly better reproducibility and better time-stability and in addition is non-radiochemical and less time-consuming.

Thiopurine methyltransferase activity in a French population: h.p.l.c. assay conditions and effects of drugs and inhibitors

1. Thiopurine methyltransferase (TPMT) is a cytosolic enzyme involved in the catabolism of thiopurine drugs, which are used to treat cancer patients and organ transplant recipients. Because TPMT activity is polymorphic and under genetic control, large interindividual variations in the immunosuppressive activity and toxicity of these drugs may, at least in part, be inherited. 2. We have developed a specific h.p.l.c. method for measuring 6-methyl mercaptopurine formed from 6-mercaptopurine (6-MP) in red blood cell lysates during the TPMT assay procedure. In blinded assays of 55 samples from adult blood donors, the results of the h.p.l.c. method correlated with those of the radiochemical reference method (r = 0.83, P < 0.001). 3. Using this h.p.l.c. assay, we tested the effect of known inhibitors of TPMT activity (syringic acid, p-anisic acid and tropolone) in vitro and showed that they were highly inhibitory. We also found that drugs often administered concomitantly with 6-MP (prednisone, prednisolone, 6-methylprednisolone, cyclophosphamide, methotrexate, and trimethoprim-sulphamethoxazole) had little or no effect on TPMT activity in vitro. 4. In a group of 300 French individuals, TMPT activity was highly variable, ranging from 4.7 to 35.3 nmol h-1 ml-1 of packed red blood cells (nmol h-1 ml-1 PRBC) with a mean value of 19.3 +/- 4.9. TMPT activity was not influenced by sex. 5. This sensitive and reproducible h.p.l.c. assay for TPMT activity in red blood cells may prove useful for prospective clinical studies designed to optimise dosage regimens of thiopurine drugs (detection limit for 6-methyl mercaptopurine is 5 ng ml-1, intra- and inter-assay variations are 6.8 and 8.2%, respectively).

Mercaptopurine metabolism and risk of relapse in childhood lymphoblastic leukaemia

Many months' treatment with daily oral mercaptopurine is an important part of therapy for nearly all children with lymphoblastic leukaemia (ALL). Even when prescribed the same dose based on body surface area, patients have widely different intracellular concentrations of drug metabolites. Whether this variation matters in terms of disease control is not yet clear. To find out, we followed up a large group of children with ALL in whom mercaptopurine-derived thioguanine nucleotides in the red cells were measured during treatment with an identical dose of mercaptopurine early in first remission. 172 unselected children (100 boys, 72 girls) were recruited between 1980 and 1992. At median follow-up of 5 years from diagnosis 42 (24%) had relapsed; 30 had erythrocyte thioguanine nucleotide concentrations below the group median (284 pmol per 8 x 10(8) erythrocytes) and 12 had values above the median. The actuarial relapse-free survival at 5 years was 63% in the below-median group and 84% in the above-median group (difference 21% [95% CI 3-39%], p = 0.0018). Multivariate analysis showed that erythrocyte thioguanine nucleotide concentration was independent of other prognostic variables including age, leukaemia immunophenotype, white-blood-cell count at diagnosis, trial protocol, and sex. Whatever the cause, in childhood ALL variable formation of intracellular mercaptopurine metabolites seems to be clinically important. Therapeutic schedules that include long-term daily oral mercaptopurine might be more effective if such metabolites are monitored.

The importance of methylthio-IMP for methylmercaptopurine ribonucleoside (Me-MPR) cytotoxicity in Molt F4 human malignant T-lymphoblasts

The importance of methyl-thioIMP (Me-tIMP) formation for methylmercaptopurine ribonucleoside (Me-MPR) cytotoxicity was studied in Molt F4 cells. Cytotoxicity of Me-MPR is caused by Me-tIMP formation with concomitant inhibition of purine de novo synthesis. Inhibition of purine de novo synthesis resulted in decreased purine nucleotide levels and enhanced 5-phosphoribosyl-1-pyrophosphate (PRPP) levels, with concurrent increased pyrimidine nucleotide levels. The Me-tIMP concentration increased proportionally with the concentration of Me-MPR. High Me-tIMP concentration also caused inhibition of PRPP synthesis. Maximal accumulation of PRPP thus occurred at low Me-MPR concentrations. As little as 0.2 microM Me-MPR resulted already after 2 h in maximal inhibition of formation of adenine and guanine nucleotides, caused by inhibition of purine de novo synthesis by Me-tIMP. Under these circumstances increased intracellular PRPP concentrations could be demonstrated, resulting in increased levels of pyrimidine nucleotides. So, in Molt F4 cells, formation of Me-tIMP from Me-MPR results in cytotoxicity by inhibition of purine de novo synthesis.

Altered mercaptopurine metabolism, toxic effects, and dosage requirement in a thiopurine methyltransferase-deficient child with acute lymphocytic leukemia

Thiopurine methyltransferase deficiency, inherited as an autosomal codominant trait, is associated with aberrant mercaptopurine metabolism leading to excessive cellular accumulation of 6-thioguanine nucleotides, the active metabolites of mercaptopurine. We describe a case of severe thiopurine methyltransferase deficiency (activity less than 1 U/8 x 10(8) erythrocytes) in a young girl with acute lymphocytic leukemia. The level of 6-thioguanine nucleotide in the patient's erythrocytes was seven times the population median value, and she had intolerable hematologic toxic effects during postremission therapy with a standard dosage of mercaptopurine (75 mg/m2 per day). Subsequent therapy with 6% of this dosage (10 mg/m2 three times weekly) yielded erythrocytic 6-thioguanine nucleotide concentrations consistently above the population median but not associated with prohibitively toxic effects. This case demonstrates that thiopurine methyltransferase deficiency does not absolutely contraindicate mercaptopurine therapy, and it also provides insight into the mechanism of excessive toxic effects of mercaptopurine sometimes observed in children with acute lymphocytic leukemia.

Genetic variation in response to 6-mercaptopurine for childhood acute lymphoblastic leukaemia

6-mercaptopurine (6-MP) can be inactivated by S-methylation, which is catalysed by thiopurine methyltransferase (TPMT). An alternative metabolic route leads to the formation of cytotoxic 6-thioguanine nucleotides (6-TGN). To investigate whether these two pathways compete with each other to affect the therapeutic response to 6-MP, 6-TGN concentrations and TPMT enzymatic activity were measured in erythrocytes (RBC) from 95 children on long-term 6-MP therapy for lymphoblastic leukaemia (ALL). RBC TPMT activities were also measured in 130 control children and 104 long-term survivors of ALL no longer on treatment. The 95 children on 6-MP showed wide interindividual differences in RBC 6-TGN concentrations at the full protocol dose of 75 mg/m2, and RBC 6-TGN concentrations correlated negatively with RBC TPMT activity. Children with 6-TGN concentrations below the group median had higher TPMT activities and a higher subsequent relapse rate. 50 of the 104 long-term survivors had been treated with "gentle" low-dose protocols, and this subgroup contained an excess of children with lower TPMT activities compared with normal controls. These results indicate that genetically determined TPMT activity may be a substantial regulator of the cytotoxic effect of 6-MP, an effect which in turn could be important in influencing the outcome of therapy for childhood ALL.

6-Thioguanine nucleotide accumulation in red blood cells during maintenance chemotherapy for childhood acute lymphoblastic leukemia, and its relation to leukopenia

In the present study of 12 boys and 19 girls 2-16 years of age (median, 7 years) on oral 6-mercaptopurine (6MP) and methotrexate (MTX) maintenance therapy (MT) for non-B-cell acute lymphoblastic leukemia (ALL), we found that (a) during MT, 6-thioguanine nucleotides (6TGN) (the major cytotoxic metabolite of 6MP) accumulate in the erythrocytes (E-6TGN); (b) for patients receiving an unchanged dose of 6MP, no significant correlation could be demonstrated between the mean E-6TGN (mE-6TGN) and the dose of 6MP (r = -0.11, P = 0.28) (31 patients); (c) among 21 patients receiving 50-75 mg/m2 6MP, a variation of up to 3 orders of magnitude in mE-6TGN could be demonstrated, with the interindividual coefficient of variation (CV) in mE-6TGN for these patients being 0.31; (d) the median intraindividual CV in E-6TGN at an unchanged dose of 6MP was 0.11 (range, 0.04-0.18); and (e) the degree of myelodepression as measured by the mean white cell count was related to mE-6TGN (r = -0.55, P = 0.0006). These results indicate that E-6TGN could be a useful parameter for monitoring 6MP maintenance chemotherapy, although this needs to be explored in prospective studies.

Prognostic significance of hepatotoxicity during maintenance chemotherapy for childhood acute lymphoblastic leukaemia

In a population-based study of 115 children with non-B-cell acute lymphoblastic leukaemia, we analysed the relation of the degree of leukopenia and risk of relapse to the degree of hepatotoxicity (as measured by serum aminotransferase (AT] during oral methotrexate (MTX) and 6-mercaptopurine (6MP) maintenance chemotherapy (MT). Hepatotoxicity was calculated as a mean of all AT-measurements (mATMT). Lack of hepatotoxicity was defined as a mATMT less than or equal to 40 IUl-1. A highly significant correlation was demonstrated between the mean AT during the first, second, and third year of MT (r greater than 0.70, P less than 0.00001). mATMT was not related to the mean WBC during MT (r = -0.03, P = 0.36), but was related to the rise in WBC following cessation of therapy (r = 0.24, P = 0.06). Patients with recurrent disease had significantly lower mATMT than patients staying in remission (P = 0.03 for both over-all and haematological relapse risk). Patients with a mATMT greater than 40IUl-1 had a lower risk of relapse than patients with a mATMT less than or equal to 40IUl-1 (4.5 year CCR: 0.70 and 0.50, P = 0.06; and 4.5 year haematological remission: 0.83 and 0.63, P = 0.03). The favourable outcome for patients with hepatoxicity could be demonstrated for all risk groups.

Analysis of 6-mercaptopurine, 6-thioguanine nucleotides, and 6-thiouric acid in biological fluids by high-performance liquid chromatography

We present a rapid, sensitive and specific high-performance liquid chromatographic method for the analysis of 6-mercaptopurine (6MP), 6-thioguanine nucleotides (6TGN) and 6-thiouric acid (6TU), with excellent chromatographic separation of the thiopurines. Thiopurines in plasma and erythrocytes were extracted by mercuricellulose and re-eluted with beta-mercaptoethanol. For quantitative detection a reverse phase column (Lichrosorb RP-18 4 mm X 30 cm) and a UV detector were used. Detection limits were 20 nmol/l for 6MP in plasma, 250 nmol/l for 6TGN in erythrocytes, and 10 nmol/l and 15 nmol/l, respectively, for 6MP and 6TU in urine diluted 1:100 with beta-mercaptoethanol. Within-run and between-run variations were less than 10%. Recovery of 6MP added to plasma, and 6TGN monophosphate added to haemolysed erythrocytes were 91% and 81%, respectively.

Clinical pharmacology of cancer chemotherapy in children

The pharmacokinetics of most anticancer drugs are highly variable in children, and are commonly different when children are compared to adults. Several recent studies have demonstrated that variability in systemic exposure due to interpatient pharmacokinetic variability, may be related to the probability of oncolytic effects or toxicity for some anticancer drugs. This review has exemplified differences in the clinical pharmacology of several anticancer drugs, when children are compared to adults. Such age-related differences in the pharmacokinetics and pharmacodynamics of these drugs, together with biologic differences between pediatric and adult cancers, provide the rationale for systematically conducting pediatric phase I through IV studies of anticancer drugs and denote the risks of relying on adult trials to identify new therapeutic strategies for childhood cancers.

Pharmacologic considerations in the treatment of acute lymphoblastic leukemia

Great strides have been made in the chemotherapy of childhood ALL over the past three decades. Principles and concepts learned from early successes in treating this disease subsequently have been applied to the treatment of other pediatric cancers and adult malignancies. Because of the prominent role of chemotherapy in the treatment of ALL, a clear understanding of the clinical pharmacology of the antileukemic drugs is essential. This article reviews issues relating to the clinical pharmacology of the anticancer drugs that are commonly used in the treatment of acute lymphoblastic leukemia.

Pharmacogenetics of methylation: relationship to drug metabolism

Pharmacogenetics is the study of inherited variation in drug response. Genetic differences in drug metabolism are the most common causes for inherited variations in drug response or adverse reactions to medications. Methyl conjugation is an important pathway in the biotransformation of many drugs. Experiments performed during the past decade showed that individual variations in the activities of enzymes that catalyze S-methylation, O-methylation and N-methylation are under genetic control in human tissue. These inherited variations are responsible for individual differences in metabolism, effect, and toxicity of drugs that undergo methyl conjugation. The approach used to study the pharmacogenetics of methylation may also be applicable to the study of inherited variations in other pathways of drug metabolism.