Prognostic Importance of 6-Mercaptopurine Dose Intensity in Acute Lymphoblastic Leukemia

Thiopurine methyltransferase polymorphisms in a multiracial asian population and children with acute lymphoblastic leukemia

The purpose of this study was to determine the frequency of thiopurine methyltransferase (TPMT) polymorphisms in a multiracial Asian population and to assess its relevance in the management of childhood acute lymphoblastic leukemia (ALL). Six hundred unrelated cord blood samples from 200 Chinese, Malay, and Indian healthy newborns were collected at the National University Hospital, Singapore; an additional 100 children with ALL were analyzed for five of the commonly reported TPMT variant alleles using polymerase chain reaction/restriction fragment length polymorphism and allele-specific polymerase chain reaction-based assays. In the cord blood study, the TPMT*3C variant was detected in all three ethnic groups; Chinese, Malays, and Indians had allele frequencies of 3%, 2.3%, and 0.8%, respectively. The TPMT*3A variant was found only among the Indians at a low allele frequency of 0.5%. The TPMT*6 variant was found in one Malay sample. Among the children with ALL, two white and one Chinese were heterozygous for the TPMT*3A variant and showed intermediate sensitivity to 6-mercaptopurine during maintenance therapy. Three Chinese patients and one Malay patient were heterozygous for the TPMT*3C variant. Mercaptopurine sensitivity could be validated in only one out of four TPMT*3C heterozygous patients. The overall allele frequency of the TPMT variants in this multiracial population was 2.5%. The TPMT*3C was the most common variant allele; TPMT*3A and TPMT*6 were rare. These results support the feasibility of performing TPMT genotyping in all children diagnosed with acute leukemia to minimize toxicity from thiopurine chemotherapy.

Thiopurine methyltransferase activity in a Spanish population sample: decrease of enzymatic activity in multiple sclerosis patients

The present study was performed in order to obtain the thiopurine methyltransferase (TPMT) activity frequency distribution histogram in a Spanish population. A total of 3640 Spanish clinical laboratory samples were evaluated, which included 1249 patients with Crohn's disease, 589 with ulcerative colitis, 348 with multiple sclerosis (MS), 487 with several autoimmune diseases different from the above-mentioned diseases and 967 a donor group. We have measured the TPMT activity in red blood cells (RBCs) by a radiochemical method, using S-adenosyl-L-[methyl-3H]methionine as methyl donor. The different groups present in their entirety a normal distribution histogram and a wide range of TPMT activity from 0 to 41 U/ml RBCs. The differences found between the Spanish population TPMT activity frequency distribution histogram and the pattern previously described in a North American population were not due to azathioprine treatment or gender. The effect of autoimmune diseases on TPMT activity was evaluated: the enzymatic activity was similar in the donor group (19.9 +/- 6.3 U/ml RBCs) and in the patients with Crohn's disease (20.0 +/- 5.8 U/ml RBCs) and ulcerative colitis (19.7 +/- 6.1 U/ml RBCs); however, it decreased significantly (p<0.0001) in MS patients (17.1 +/- 6.1 U/ml RBCs) with respect to the donor group. In conclusion, our results show that the Spanish population TPMT distribution is closer to that of the Jewish population of Israel than to North American populations, and that in MS the enzymatic activity of TPMT decreases significantly. This observation may take into account the usage of azathioprine as therapeutic agent in Spanish MS patients.

Recent advances in the pharmacogenomics of thiopurine methyltransferase

The thiopurine drugs (6-mercaptopurine, 6-thioguanine and azathioprine) are commonly used cytotoxic agents and immunosuppressants. One important route for the metabolism of these agents is methylation, mediated by thiopurine methyltransferase (TPMT). It is now well established that inter-individual variation in sensitivity to thiopurines can be due to the presence of common genetic polymorphisms affecting the TPMT gene. More recently variations in the number of tandem repeats in the 5' promoter region have been shown to influence TPMT expression in vitro. In this article, we review recent advances in the understanding of the range of inter-individual variation that may be involved in the open reading frame or promoter region of the TPMT gene. We also review the data which have been published regarding the influence such variations may have on both the clinical efficacy and toxicity of the thiopurine drugs.

Effect of methotrexate polyglutamates on thioguanine nucleotide concentrations during continuation therapy of acute lymphoblastic leukemia with mercaptopurine

Methotrexate is widely administered with mercaptopurine, a prodrug requiring activation into thioguanine nucleotides (TGN) to exert antileukemic effects. In vitro, methotrexate enhances TGN formation, but in vivo, such enhancement has yet to be demonstrated. We investigated whether TGN concentrations were related to methotrexate concentrations in children with acute lymphoblastic leukemia who received a weekly intravenous methotrexate (40 mg/m(2)) dose combined with daily mercaptopurine (75 mg/m(2)). A total of 141 erythrocyte TGN concentrations were measured with erythrocyte methotrexate polyglutamates (MTX-PG) concentrations in 87 patients. Average TGN concentrations ranged from 137 to 958 pmol/8 x 10(8) cells (median 389), average total MTX-PG concentrations (MTX- PG(1-7)) from 0.60 to 97.7 pmol/10(9)cells (median 29), and average long chain polyglutamate concentrations (MTX-PG(5-7)) from 0 to 8.35 pmol/10(9) cells (median 2.43). Higher TGN concentrations correlated with higher MTX-PG(5-7) concentrations (P = 0.002). These data support the practice of administering methotrexate with mercaptopurine during continuation therapy of acute lymphoblastic leukemia.

Pharmacogenomics: the inherited basis for interindividual differences in drug response

It is well recognized that most medications exhibit wide interpatient variability in their efficacy and toxicity. For many medications, these interindividual differences are due in part to polymorphisms in genes encoding drug metabolizing enzymes, drug transporters, and/or drug targets (e.g., receptors, enzymes). Pharmacogenomics is a burgeoning field aimed at elucidating the genetic basis for differences in drug efficacy and toxicity, and it uses genome-wide approaches to identify the network of genes that govern an individual's response to drug therapy. For some genetic polymorphisms (e.g., thiopurine S-methyltransferase), monogenic traits have a marked effect on pharmacokinetics (e.g., drug metabolism), such that individuals who inherit an enzyme deficiency must be treated with markedly different doses of the affected medications (e.g., 5%-10% of the standard thiopurine dose). Likewise, polymorphisms in drug targets (e.g., beta adrenergic receptor) can alter the sensitivity of patients to treatment (e.g., beta-agonists), changing the pharmacodynamics of drug response. Recognizing that most drug effects are determined by the interplay of several gene products that govern the pharmacokinetics and pharmacodynamics of medications, pharmacogenomics research aims to elucidate these polygenic determinants of drug effects. The ultimate goal is to provide new strategies for optimizing drug therapy based on each patient's genetic determinants of drug efficacy and toxicity. This chapter provides an overview of the current pharmacogenomics literature and offers insights for the potential impact of this field on the safe and effective use of medications.

HPLC Determination of Thiopurine Nucleosides and Nucleotides in Vivo in Lymphoblasts following Mercaptopurine Therapy

Background: Mercaptopurine is a prodrug requiring intracellular activation to thiopurine nucleotides to exert antileukemic effect. We developed a reversed-phase liquid chromatographic assay for the quantification of mercaptopurine, thioguanine, and methylmercaptopurine nucleoside and nucleotide concentrations in the target tissue, the leukemic lymphoblast.

Methods: Leukemic blasts were isolated from peripheral blood and bone marrow by a standard Ficoll-hypaque procedure. Proteins were removed by ultrafiltration in the presence of dithiothreitol. Thiopurine ribonucleotides were converted into their respective ribonucleosides by treatment of ultrafiltrate with acid phosphatase. Thiopurine nucleosides and bases were measured by direct injection of ultrafiltrate into the chromatographic system. Thiopurine nucleotide concentrations were calculated by subtracting the thiopurine nucleoside concentrations measured after treatment with acid phosphatase from those measured after direct injection of ultrafiltrate in the chromatographic system. Analytes were separated on a C18 Supelco column with ammonium phosphate-methanol eluent coupled with ultraviolet detection.

Results: CVs for intra- and interday precision were 1.1–14% (median, 4.9%), and recovery of added analyte was 89–126% (median, 105%) at low and high concentrations of analytes, except for mercaptopurine riboside. The median signal for each of the five metabolites in lymphoblast samples was 98% (range, 80–106%) of that in water. Detection limits for thiopurine bases and nucleosides ranged from 0.5 to 4.5 pmol/5 × 106 cells.

Conclusions: This method is suitable for measurement of thiopurine metabolite concentrations in lymphoblasts in children with acute lymphoblastic leukemia following a single dose of intravenous mercaptopurine.

The thiopurine S-methyltransferase gene locus -- implications for clinical pharmacogenomics

Thiopurine methyltransferase catalyzes the S-methylation of azathioprine (AZA), 6-mercapto-purine (6-MP) and thioguanine, medications widely used to treat malignancies, rheumatic diseases, dermatologic conditions, inflammatory bowel disease and solid organ transplant rejection. TPMT activity exhibits a genetic polymorphism in 10% of Caucasians, with 1/300 individuals having complete deficiency. Patients with intermediate or deficient TPMT activity are at risk for excessive toxicity, including fatal myelosuppression, after receiving standard doses of thiopurine medications. The molecular basis for low TPMT activity has been elucidated, leading to the development of assays for the three signature mutations, which account for the majority of mutant alleles. TPMT genotype is correlated with erythrocyte and leukemia blast cell TPMT activity and associated with a risk of toxicity after thiopurine therapy. Recent studies defined target starting doses for mercaptopurine based on TPMT genotypes. This polymorphism is one of the best models for the translation of genomic information to guide patient therapeutics.

Continuing therapy for childhood acute lymphoblastic leukaemia: clinical and cellular pharmacology of methotrexate, 6-mercaptopurine and 6-thioguanine

Across the world, therapy with 6-mercaptopurine (6-MP) and methotrexate (MTX) forms the basis of the continuing therapy of childhood acute lymphoblastic leukaemia (ALL). In this review, the pharmacological determinants of the sensitivity of human leukaemia cell lines and lymphoblasts derived from children with ALL will be discussed. In addition, clinical pharmacological studies of 6-MP and MTX in relation to the continuing therapy with childhood ALL will be reviewed. For 6-MP in vitro, prolonged exposure times to relatively high extracellular drug concentrations are necessary for cytotoxicity, and these concentrations are much higher than those achieved during continuing therapy for childhood ALL. For MTX, plasma concentrations are achieved during continuing therapy that would be cytotoxic to human leukaemia cells during prolonged exposures in vitro. For both MTX and 6-MP, wide inter- and intrapatient variation in plasma pharmacokinetic parameters has been described. For 6-MP and MTX, cellular pharmacological studies have been largely restricted to erythrocytes as a surrogate of the possible effects in leukaemic blasts. Although measures of the pharmacology of 6-MP and MTX in erythrocytes has been related to prognosis in many studies, 6-MP systemic exposure and the dose intensity of 6-MP and MTX actually received by children during this phase of therapy seems to be the most important determinant of efficacy. Further studies will be needed to determine the importance of pharmacokinetic variability during continuing therapy as a determinant of outcome for children with ALL. In this respect, minimal residual disease status during this phase of treatment may prove to be a useful pharmacodynamic endpoint.

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.

Pharmacogenetics and cancer therapy

Pharmacogenetics is the study of how genetic variations affect drug response. These variations can affect a patient's response to cancer drugs, for which there is usually a fine line between a dosage that has a therapeutic effect and one that produces toxicity. Gaining better insight into the genetic elements of both the patient and the tumour that affect drug efficacy will eventually allow for individualized dosage determination and fewer adverse effects.

Determination of thiopurine methyltransferase activity in human erythrocytes by high-performance liquid chromatography: comparison with the radiochemical method

The current article describes a new assay to measure thiopurine methyltransferase (TPMT) activity from red blood cells. This method is based on the measurement of the reaction product 6-methylmercaptopurine (6-MMP) by high-performance liquid chromatography (HPLC). 6-MMP is extracted by ethyl acetate with recoveries of 85%, 80%, 80%, and 92% for 50, 250, 500, and 1,000 ng/100 microL packed red blood cells, respectively. 6-MMP was identified and measured by a Zorbax CN column installed in an HPLC system. The chromatograms were resolved using a mobile phase consisting of 40 mmol/L sodium phosphate buffer (pH 3) and methanol in a gradient from 1% to 20% of methanol. Under these conditions 6-MMP is well resolved from substrates (6-mercaptopurine and S-adenosyl-L-methionine) and endogenous peaks. When the TPMT activity from 20 patients was measured by the HPLC-linked assay and the classic radiochemical method, a linear correlation was obtained between both procedures ( y = 0.99x + 0.33; x-axis, radiochemical assay; y-axis, HPLC-linked assay; r = 0.98). In conclusion, the current report describes a new, reliable, safe, and nonradioactive method to measure TPMT activity that is shorter and simpler than the previously described ones.

Preponderance of thiopurine S-methyltransferase deficiency and heterozygosity among patients intolerant to mercaptopurine or azathioprine

PURPOSE

To assess thiopurine S-methyltransferase (TPMT) phenotype and genotype in patients who were intolerant to treatment with mercaptopurine (MP) or azathioprine (AZA), and to evaluate their clinical management.

PATIENTS AND METHODS

TPMT phenotype and thiopurine metabolism were assessed in all patients referred between 1994 and 1999 for evaluation of excessive toxicity while receiving MP or AZA. TPMT activity was measured by radiochemical analysis, TPMT genotype was determined by mutation-specific polymerase chain reaction restriction fragment length polymorphism analyses for the TPMT*2, *3A, *3B, and *3C alleles, and thiopurine metabolites were measured by high-performance liquid chromatography.

RESULTS

Of 23 patients evaluated, six had TPMT deficiency (activity < 5 U/mL of packed RBCs [pRBCs]; homozygous mutant), nine had intermediate TPMT activity (5 to 13 U/mL of pRBCs; heterozygotes), and eight had high TPMT activity (> 13.5 U/mL of pRBCs; homozygous wildtype). The 65.2% frequency of TPMT-deficient and heterozygous individuals among these toxic patients is significantly greater than the expected 10% frequency in the general population (P <.001, chi(2)). TPMT phenotype and genotype were concordant in all TPMT-deficient and all homozygous-wildtype patients, whereas five patients with heterozygous phenotypes did not have a TPMT mutation detected. Before thiopurine dosage adjustments, TPMT-deficient patients experienced more frequent hospitalization, more platelet transfusions, and more missed doses of chemotherapy. Hematologic toxicity occurred in more than 90% of patients, whereas hepatotoxicity occurred in six patients (26%). Both patients who presented with only hepatic toxicity had a homozygous-wildtype TPMT phenotype. After adjustment of thiopurine dosages, the TPMT-deficient and heterozygous patients tolerated therapy without acute toxicity.

CONCLUSION

There is a significant (> six-fold) overrepresentation of TPMT deficiency or heterozygosity among patients developing dose-limiting hematopoietic toxicity from therapy containing thiopurines. However, with appropriate dosage adjustments, TPMT-deficient and heterozygous patients can be treated with thiopurines, without acute dose-limiting toxicity.

The role of prognostic features in the treatment of childhood acute lymphoblastic leukemia

Acute lymphoblastic leukemia (ALL) is the most common cancer in children and is among the most curable of the pediatric malignancies. Many clinical, biological, genetic, and molecular features have been identified as having prognostic significance in the outcome of patients with ALL. The standard features are age and WBC at diagnosis, with infants (less than one year), adolescents (greater than nine years), and children with WBC above 50,000/microl being at higher risk. Certain chromosomal abnormalities are also strong predictors; in particular, the Philadelphia chromosome and MLL gene rearrangements (especially in infants) are adverse features, while TEL-AML1 is favorable. It is important to note, however, that even the most important known predictors explain only a small proportion of the variability in outcome. These features are currently used to tailor the intensity of treatment so that the toxicity of treatment can be minimized and cure rates can continue to improve. This article reviews time-honored prognostic features, recent advances, and future directions in this field.

6-mercaptopurine dosage and pharmacokinetics influence the degree of bone marrow toxicity following high-dose methotrexate in children with acute lymphoblastic leukemia

Through inhibition of purine de novo synthesis and enhancement of 6-mercaptopurine (6MP) bioavailability high-dose methotrexate (HDM) may increase the incorporation into DNA of 6-thioguanine nucleotides (6TGN), the cytoxic metabolites of 6MP. Thus, coadministration of 6MP could increase myelotoxicity following HDM. Twenty-one children with standard risk (SR) and 25 with intermediate risk (IR) acute lymphoblastic leukemia (ALL) were studied. During consolidation therapy they received either three courses of HDM at 2 week intervals without concurrent oral 6MP (SR-ALL) or four courses of HDM given at 2 week intervals with 25 mg/m2 of oral 6MP daily (IR-ALL). During the first year of maintenance with oral 6MP (75 mg/m2/day) and oral MTX (20 mg/m2/week) they all received five courses of HDM at 8 week intervals. In all cases, HDM consisted of 5,000 mg of MTX/m2 given over 24 h with intraspinal MTX and leucovorin rescue. Erythrocyte levels of 6TGN (E-6TGN) and methotrexate (E-MTX) were, on average, measured every second week during maintenance therapy. When SR consolidation (6MP: 0 mg), IR consolidation (6MP: 25 mg/m2), and SR/IR maintenance therapy (6MP: 75 mg/m2) were compared, white cell and absolute neutrophil count (ANC) nadir, lymphocyte count nadir, thrombocyte count nadir, and hemoglobin nadir after HDM decreased significantly with increasing doses of oral 6MP. Three percent of the HDM courses given without oral 6MP (SR consolidation) were followed by an ANC nadir <0.5 x 10(9)/l compared to 50% of the HDM courses given during SR/IR maintenance therapy. Similarly, only 13% of the HDM courses given as SR-ALL consolidation induced a thrombocyte count nadir <100 x 10(9)/l compared to 58% of the HDM courses given during maintenance therapy. The best-fit model to predict the ANC nadir following HDM during maintenance therapy included the dose of 6MP prior to HDM (beta = -0.017, P= 0.001), the average ANC level during maintenance therapy (beta = 0.82, P = 0.004), and E-6TGN (beta = -0.0029, P= 0.02). The best-fit model to predict the thrombocyte nadir following HDM during maintenance therapy included only mPLATE (beta = 0.0057, P = 0.046). In conclusion, the study indicates that reductions of the dose of concurrently given oral 6MP could be one way of reducing the risk of significant myelotoxicity following HDM during maintenance therapy of childhood ALL.

Long-term results of Total Therapy studies 11, 12 and 13A for childhood acute lymphoblastic leukemia at St Jude Children's Research Hospital

We present the long-term results of three consecutive clinical trials (Total Therapy studies 11, 12 and 13A) conducted for children with newly diagnosed acute lymphoblastic leukemia (ALL) between 1984 and 1994. In study 11 (1984-1988), the overall event-free survival rates (+/-1 s.e.) were 71.8 +/- 2.4% and 69.3 +/- 2.4%, and the cumulative risks of isolated central nervous system (CNS) relapse 5.6 +/- 1.2% and 5.9 +/- 1.3%, at 5 and 10 years, respectively. In study 12 (1988-1991), event-free survival rates were 67.6 +/- 3.4% and 61.5+/- 9.0%, and isolated CNS relapse rates were 10.4 +/- 2.3% and 10.4 +/- 2.3%, respectively. Early intensive intrathecal therapy in study 13A (1991-1994) has yielded a very low 5-year isolated CNS relapse rate of 1.2 +/- 0.9%, boosting the 5-year event-free survival rate to 76.9 +/- 3.3%. Factors consistently associated with an adverse prognosis included male sex, infant or adolescent age group, leukocyte count >100 x 10(9)/l, nonhyperdiploidy karyotype and poor early response to treatment. Risk classification based on age and leukocyte count had prognostic significance in B-lineage but not T-lineage ALL. Early therapeutic interventions or modifications for patients with specific genetic abnormalities or persistent minimal residual leukemia may further improve long-term results.

Long-term results of three randomized trials (58831, 58832, 58881) in childhood acute lymphoblastic leukemia: a CLCG-EORTC report. Children Leukemia Cooperative Group

We present here the long-term results of three randomized clinical trials conducted on children with newly diagnosed acute lymphoblastic leukemia (ALL) between 1983 and 1998 by the Children Leukemia Cooperative Group (CLCG) from EORTC. In study 58831/32, the overall event-free survival (EFS) rates (+/- s.e.) at 6 and 10 years were 66% +/- 1.8% and 65% +/- 1.8%, respectively, and the risk of isolated central nervous system (CNS) relapse was 6% +/- 1% and 7% +/- 1%, respectively. In patients with a standard risk of relapse the omission of cyclophosphamide had no adverse effect on disease-free survival rates at 10 years (trial 58831). In medium- and high-risk patients the omission of radiotherapy did not increase the risk of CNS or systemic relapse (trial 58832). In study 58881 (1989-1998) the overall EFS rate at 8 years was 68.4% +/- 1.2% and the risk of isolated CNS relapse was 4.2%+/-0.5%. In this trial which adressed three randomized questions, the following results were obtained: the combination of cytarabine at high doses with methotrexate at high doses during interval therapy did not improve prognosis. The addition of 6-mercaptopurine iv during maintenance increased the risk of late relapse. E. coli asparaginase was more toxic and has a higher efficacy than Erwinia asparaginase. Leukocyte counts >100 x 10(9)/l, specific genetic abnormalities, a poor initial response to steroids or a high level of minimal residual disease at early time points were consistently associated with an adverse prognosis in the 58881 trial.

Pharmacogenetics: a tool for individualizing antineoplastic therapy

This article reviews the clinical relevance of pharmacogenetics in cancer chemotherapy, with emphasis on drugs for which genetic differences in enzyme metabolism have been demonstrated to affect patient outcome. About 10% of children with leukaemia are intolerant to mercaptopurine (6-mercaptopurine) because of genetic defects in mercaptopurine inactivation by thiopurine S-methyltransferase. However, mercaptopurine dose intensity, a critical factor for outcome in patients deficient in thiopurine S-methyltransferase, can be maintained by means of thiopurine S-methyltransferase phenotyping or genotyping. Patients with reduced fluorouracil (5-fluorouracil) catabolism are more likely to be exposed to severe toxicity. The measurement of dihydropyrimidine dehydrogenase activity in patients cannot be considered fully predictive, and the role of dihydropyrimidine dehydrogenase gene variants in this syndrome has yet to be clarified. With regard to irinotecan, patients with Gilbert's syndrome phenotype have reduced inactivation of the active topoisomerase I inhibitor 7-ethyl-10-hydroxycamptothecin (SN-38) caused by a mutation in the UDP-glucuronosyltransferase 1A1 gene promoter. This subset of patients is more likely to be exposed to irinotecan toxicity and could be identified by genotyping for gene promoter variants. Finally, the experience with amonafide represents a model for dose individualization approaches that use simple phenotypic probes.

Genetic polymorphism of thiopurine methyltransferase and its clinical relevance for childhood acute lymphoblastic leukemia

Thiopurine methyltransferase (TPMT) catalyses the S-methylation of thiopurines, including 6-mercaptopurine and 6-thioguanine. TPMT activity exhibits genetic polymorphism, with about 1/300 inheriting TPMT deficiency as an autosomal recessive trait. If treated with standard doses of thiopurines, TPMTdeficient patients accumulate excessive thioguanine nucleotides in hematopoietic tissues, leading to severe hematological toxicity that can be fatal. However, TPMT-deficient patients can be successfully treated with a 10- to 15-fold lower dosage of these medications. The molecular basis for altered TPMT activity has been defined, with rapid and inexpensive assays available for the three signature mutations which account for the majority of mutant alleles. TPMT genotype correlates well with in vivo enzyme activity within erythrocytes and leukemic blast cells and is clearly associated with risk of toxicity. The impact of 6-mercaptopurine dose intensity is also being clarified as an important determinate of event-free survival in childhood leukemia. In addition, there are emerging data that TPMT genotype may influence the risk of secondary malignancies, including brain tumors and acute myelogenous leukemia. Ongoing studies aim to clarify the influence of TPMT on thiopurine efficacy, acute toxicity, and risk for delayed toxicity. Together, these advances hold the promise of improving the safety and efficacy of thiopurine therapy.

Intensification with intermediate-dose intravenous methotrexate is effective therapy for children with lower-risk B-precursor acute lymphoblastic leukemia: A Pediatric Oncology Group study

PURPOSE

To determine whether early intensification with 12 courses of intravenous (IV) methotrexate (MTX) and IV mercaptopurine (MP) is superior to 12 courses of IV MTX alone for prevention of relapse in children with lower-risk B-lineage acute lymphoblastic leukemia (ALL).

PATIENTS AND METHODS

Six hundred fifty-one eligible patients were entered onto the study. Vincristine, prednisone, and asparaginase were used for remission induction therapy. Patients were randomized to receive intensification with IV MTX 1,000 mg/m(2) plus IV MP 1,000 mg/m(2) (regimen A) or IV MTX 1,000 mg/m(2) alone (regimen C). Twelve courses were administered at 2-week intervals. Triple intrathecal therapy was used for CNS prophylaxis. Continuation therapy included standard oral MP, weekly MTX, and triple intrathecal therapy every 12 weeks for 2 years.

RESULTS

Six hundred forty-five patients (99.1%) achieved remission. Three hundred twenty-five were assigned to regimen A and 320 to regimen C. The estimated 4-year overall continuous complete remission for patients treated with regimen A is 82.1% (SE = 2.4%) and for regimen C is 82.2% (SE = 2.6%; P =.5). No significant difference in overall outcome was shown by sex or race. Serious grade 3/4 neurotoxicity, principally characterized by seizures, was observed in 7.6% of patients treated with either regimen.

CONCLUSION

Intensification with 12 courses of IV MTX is an effective therapy for prevention of relapse in children with B-precursor ALL who are at lower risk for relapse but may be associated with an increased risk for neurotoxicity. Prolonged infusions of MP combined with IV MTX did not provide apparent advantage.

Racial differences in the survival of childhood B-precursor acute lymphoblastic leukemia: a Pediatric Oncology Group Study

PURPOSE

We conducted a historic cohort study to test the hypothesis that, after adjustment for biologic factors, African-American (AA) children and Spanish surname (SS) children with newly diagnosed B-precursor acute lymphoblastic leukemia had lower survival than did comparable white children.

PATIENTS AND METHODS

From 1981 to 1994, 4,061 white, 518 AA, and 507 SS children aged 1 to 20 years were treated on three successive Pediatric Oncology Group multicenter randomized clinical trials.

RESULTS

AA and SS patients were more likely to have adverse prognostic features at diagnosis and lower survival than were white patients. The 5-year cumulative survival rates were (probability +/- SE) 81.9% +/- 0.6%, 68.6% +/- 2.1%, and 74.9% +/- 2.0% for white, AA, and SS children, respectively. Adjusting for age, leukocyte count, sex, era of treatment, and leukemia blast cell ploidy, we found that AA children had a 42% excess mortality rate compared with white children (proportional hazards ratio [PHR] = 1.42; 95% confidence interval [CI], 1.12 to 1. 80), and SS children had a 33% excess mortality rate compared with white children (PHR = 1.33; 95% CI, 1.19 to 1.49).

CONCLUSION

Clinical presentation, tumor biology, and deviations from prescribed therapy did not explain the differences in survival and event-free survival that we observed, although differences seem to be diminishing over time with improvements in therapy. The disparity in outcome for AA and SS children is most likely related to variations in chemotherapeutic response to therapy and not to compliance. Further improvements in outcome may require individualized dosing based on specific pharmacogenetic profiles, especially for AA and SS children.

Acute lymphoblastic leukemia in children

As the overall long-term event-free survival rate in children with acute lymphoblastic leukemia approaches 80%, emphasis is being placed on risk-directed therapy so that patients are neither overtreated nor undertreated. It has become apparent that a risk assignment system based on primary genetic abnormalities is inadequate by itself. For example, leukemias with the MLL-AF4 or BCR-ABL fusion gene are, in fact, heterogeneous diseases. Many require allogeneic hematopoietic stem-cell transplantation; some, if the patient is of favorable age and has a low presenting leukocyte count, can be cured with chemotherapy alone. Measurement of early responses to therapy and extent of minimal residual disease can greatly improve the accuracy of risk assessment. Consideration of the variable effects of therapy on the prognostic significance of specific genetic abnormalities is also important. Therefore, TEL-AML1 fusion confers a favorable prognosis in some protocols of chemotherapy but not in others. Studies to identify genetic polymorphisms with pharmacokinetic and pharmacodynamic significance promise to guide further refinement of treatment strategies. This will allow maximization of anticancer effects without induction of unacceptable toxicity in individual patients.

Mercaptopurine therapy intolerance and heterozygosity at the thiopurine S-methyltransferase gene locus

BACKGROUND

Patients with acute lymphoblastic leukemia are often treated with 6-mercaptopurine, and those with homozygous deficiency in thiopurine S-methyltransferase (TPMT) enzyme activity have an extreme sensitivity to this drug as a result of the accumulation of higher cellular concentrations of thioguanine nucleotides. We studied the metabolism, dose requirements, and tolerance of 6-mercaptopurine among patients with different TPMT phenotypes.

METHODS

We compared, by use of statistical modeling, 6-mercaptopurine pharmacology and tolerance in 180 patients who achieved remission on St. Jude Children's Research Hospital Protocol Total XII composed of weekly methotrexate (40 mg/m(2)) and daily oral 6-mercaptopurine (75 mg/m(2)) given for 2.5 years, interrupted every 6 weeks during the first year for treatment with either high-dose methotrexate or teniposide plus cytarabine. Statistical tests were two-sided.

RESULTS

Erythrocyte concentrations of thioguanine nucleotides (pmol/8 x 10(8) erythrocytes) were inversely related to TPMT enzyme activity (P<.01), with averages (+/- standard deviations) of 417 (+/-179), 963 (+/-752), and 3565 (+/-1282) in TPMT homozygous wild-type (n = 161), heterozygous (n = 17), and homozygous-deficient (n = 2) patients, respectively. There was complete concordance between TPMT genotype and phenotype in a subset of 28 patients for whom TPMT genotype was determined. There were no sex differences in thioguanine nucleotide concentrations (P =.24), TPMT enzyme activity (P =.22), or average weekly prescribed dose of 6-mercaptopurine (P=.49). The cumulative incidence of 6-mercaptopurine dose reductions due to toxicity was highest among patients homozygous for mutant TPMT (100%), intermediate among heterozygous patients (35%), and lowest among wild-type patients (7%) (P<.001), with average (+/- standard deviation) final weekly 6-mercaptopurine doses of 72 (+/-60), 449 (+/-160), and 528 (+/-90) mg/m(2), respectively. Lowering doses of 6-mercaptopurine in TPMT heterozygotes and in deficient patients allowed administration of full protocol doses of other chemotherapy while maintaining high thioguanine nucleotide concentrations.

CONCLUSION

We conclude that genetic polymorphism in TPMT is an important determinant of mercaptopurine toxicity, even among patients who are heterozygous for this trait.

Pharmacogenomics: translating functional genomics into rational therapeutics

Genetic polymorphisms in drug-metabolizing enzymes, transporters, receptors, and other drug targets have been linked to interindividual differences in the efficacy and toxicity of many medications. Pharmacogenomic studies are rapidly elucidating the inherited nature of these differences in drug disposition and effects, thereby enhancing drug discovery and providing a stronger scientific basis for optimizing drug therapy on the basis of each patient's genetic constitution.

Molecular diagnostics in the treatment of leukemia

The molecular characterization of childhood leukemias directly affects our treatment strategies. Acute lymphoblastic leukemia patients with the TEL-AML1 fusion have a favorable prognosis, whereas those with the E2A-PBX1 fusion require more intensive therapy to obtain a good outcome. Acute lymphoblastic leukemia patients whose leukemic lymphoblasts contain the MLL-AF4 or the BCR-ABL fusion are often candidates for allogeneic hematopoietic stem cell transplantation during first remission. Among acute myeloid leukemia patients, AML1-ETO and CBFbeta-MYH11 fusions are associated with a favorable response, especially when the chemotherapy regimen includes high-dose cytarabine. Patients with acute promyelocytic leukemia who carry the PML-RAR alpha fusion respond to all-trans retinoic acid and have an excellent outcome after treatment with all-trans retinoic acid in combination with anthracyclines. Several novel therapeutic agents targeted to molecular lesions of leukemic cells are under investigation.