Pharmacogenetically based dosing of thiopurines in childhood acute lymphoblastic leukemia: influence on cure rates and risk of second cancer

Utilization of Thiopurine Metabolites and Allopurinol in Pediatric Acute Lymphoblastic Leukemia: Consideration for an Algorithmic Approach

Persistently elevated absolute neutrophil counts during maintenance for acute lymphoblastic leukemia is a risk factor for relapse and may be related to wild-type thiopurine methyltransferase activity and overly efficient shunting of 6-mercaptopurine to hepatotoxic metabolites (6-methylmercaptopurine nucleotides), leading to low 6-thioguanine nucleotides. 6-mercaptopurine is also metabolized by xanthine oxidase, and therefore allopurinol, an inhibitor of xanthine oxidase, allows for increased 6-thioguanine nucleotides and decreased 6-methylmercaptopurine nucleotide. Here, we report our experience with allopurinol for persistently elevated absolute neutrophil count or hepatotoxicity and suggest an algorithmic approach for checking thiopurine metabolites and initiating allopurinol in acute lymphoblastic leukemia maintenance.

No association between relapse hazard and thiopurine methyltransferase geno- or phenotypes in non-high risk acute lymphoblastic leukemia: a NOPHO ALL2008 sub-study

PURPOSE

6-mercaptopurine(6MP)/methotrexate maintenance therapy is essential to reduce relapse of childhood acute lymphoblastic leukemia (ALL). Common germline variants in TPMT cause low activity of thiopurine methyltransferase (TPMT) and higher 6MP metabolite (TGN) levels. Higher levels of TGNs incorporated into DNA (DNA-TG) and low TPMT activity have previously been associated with a lower relapse risk. We explored if TPMT geno- or phenotype was associated with DNA-TG levels and relapse rate in NOPHO ALL2008.

METHODS

TPMT genotype, repeated phenotyping, and DNA-TG measurements were collected in 918 children with non-high risk ALL (NOPHO ALL2008 maintenance therapy study). Maintenance therapy started with 6MP at 50 and 75 mg/m² for TPMT heterozygous and wildtype patients and was adjusted to a target WBC of 1.5 - 3.0 × 10⁹/L.

RESULTS

Of 918 patients, 78 (8.5%) were TPMT heterozygous and 903 had at least one TPMT measurement (total 3063). Mean TPMT activities were higher with wildtype than heterozygous TPMT (N = 752, 16.6 versus 9.6 U/mL ery., p < 0.001). The 5-year cumulative incidence of relapse was 6.4% and 6.0% for TPMT heterozygous and wildtype patients, and there was no association between genotype and relapse rate (N = 918, hazard ratio = 1.01, 95% confidence interval [CI] 0.40 - 2.54, p = 0.98). Although TPMT heterozygous patients had higher DNA-TG (N = 548, median 760.9 [interquartile range (IQR) 568.7 - 890.3] versus 492.7 [IQR 382.1 - 634.6] fmol/µg, p < 0.001), TPMT activity was not associated with relapse rate (N = 813; hazard ratio = 0.98 per one U/mL ery. increase in TPMT activity, 95% CI 0.91 - 1.06, p = 0.67).

CONCLUSION

TPMT geno- and phenotype were not associated with relapse in non-high risk NOPHO ALL2008.

Promising genes and variants to reduce chemotherapy adverse effects in acute lymphoblastic leukemia

Almost two decades ago, the sequencing of the human genome and high throughput technologies came to revolutionize the clinical and therapeutic approaches of patients with complex human diseases. In acute lymphoblastic leukemia (ALL), the most frequent childhood malignancy, these technologies have enabled to characterize the genomic landscape of the disease and have significantly improved the survival rates of ALL patients. Despite this, adverse reactions from treatment such as toxicity, drug resistance and secondary tumors formation are still serious consequences of chemotherapy, and the main obstacles to reduce ALL-related mortality. It is well known that germline variants and somatic mutations in genes involved in drug metabolism impact the efficacy of drugs used in oncohematological diseases therapy. So far, a broader spectrum of clinically actionable alterations that seems to be crucial for the progression and treatment response have been identified. Although these results are promising, it is necessary to put this knowledge into the clinics to help physician make medical decisions and generate an impact in patients' health. This review summarizes the gene variants and clinically actionable mutations that modify the efficacy of antileukemic drugs. Therefore, knowing their genetic status before treatment is critical to reduce severe adverse effects, toxicities and life-threatening consequences in ALL patients.

Effects of TPMT, NUDT15, and ITPA Genetic Variants on 6-Mercaptopurine Toxicity for Pediatric Patients With Acute Lymphoblastic Leukemia in Yunnan of China

Background: 6-Mercaptopurine (6-MP) is the cornerstone of current antileukemia regimen and contributes greatly to improve the survival of pediatric acute lymphoblastic leukemia (ALL) patients. However, 6-MP dose-related toxicities limit its application. TPMT, NUDT15, and ITPA are pharmacogenetic markers predicting 6-MP-related toxicities, but their genetic polymorphisms differ from those of ethnic populations. In Yunnan province, a multiethnic region of China, we had no genetic data to predict 6-MP toxicities. In this study, we evaluated the most common variants involved in 6-MP metabolism-TPMT ^*3C (rs1142345), NUDT15 c.415C>T (rs116855232), and ITPA c.94C>A (rs1127354) variants-in our cohort of pediatric ALL patients. Methods: A total of 149 pediatric ALL patients in the Affiliated Children's Hospital of Kunming Medical University (Yunnan Children's Medical Center) from 2017 to 2019 were enrolled in this retrospective study. We assessed the TPMT ^*3C (rs1142345), NUDT15 c.415C>T (rs116855232), and ITPA c.94C>A (rs1127354) frequencies and evaluated association between genotypes and 6-MP toxicities, 6-MP dose, and event-free survival (EFS) in these ALL patients. Results: The allele frequencies of TPMT ^*3C (rs1142345), NUDT15 c.415C>T (rs116855232), and ITPA c.94C>A (rs1127354) were 1.34%, 14.43%, and 18.79%, respectively. Only NUDT15 c.415C>T (rs116855232) was strongly associated with 6-MP toxicity and 6-MP tolerable dose. NUDT15 c.415C>T was related to leukopenia, p = 0.008, OR = 2.743 (95% CI: 1.305-5.768). The T allele was significantly correlated with 6-MP tolerable dose, dose of NUDT15 c.415C>T wild genotype CC 39.80 ± 1.32 mg/m², heterozygotes CT 35.20 ± 2.29 mg/m², and homozygotes TT 18.95 ± 3.95 mg/m². 6-MP tolerable dose between CC and TT had a significant difference, p = 0.009. Between CC and CT, and CT and TT, they had no significant difference. EFS showed no significant difference among NUDT15 c.415C>T genotypes. Conclusion: NUDT15 c.415C>T (rs116855232) was an optimal predictor for 6-MP toxicity and tolerable dose in pediatric ALL patients from Yunnan province, a multiethnic region in China, and would play an important role in precise therapy for ALL.

The Role of TPMT, ITPA, and NUDT15 Variants during Mercaptopurine Treatment of Swedish Pediatric Patients with Acute Lymphoblastic Leukemia

OBJECTIVE

To evaluate the roles of thiopurine methyltransferase (TPMT), inosine triphosphatase (ITPA), and Nudix hydrolase 15 (NUDT15) in 6-mercaptopurine (6-MP) sensitivity during treatment of pediatric patients with acute lymphoblastic leukemia (ALL).

STUDY DESIGN

The study included 102 pediatric patients with ALL subject to the Nordic society Of Paediatric Haematology and Oncology (NOPHO) ALL-2000 and ALL-2008 protocols. Episodes of neutropenia and febrile neutropenia, TPMT sequence variants, as well as 6-MP end doses, were collected retrospectively from medical records. TPMT, ITPA, and NUDT15 sequence variants were analyzed using pyrosequencing.

RESULTS

TPMT variants were associated with a reduced risk of neutropenia and febrile neutropenia during the maintenance II period (P = .019 and P < .0001, respectively). In addition, a NUDT15 variant was associated with a lower end dose of 6-MP (P = .0097), but not with neutropenia and febrile neutropenia. ITPA variants were not associated with an increased risk of neutropenia, febrile neutropenia, nor lower end dose of 6-MP. However, when analyzing the entire treatment period, ITPA variants were associated with a decreased risk of febrile neutropenia.

CONCLUSIONS

White blood cell count-based dose adjustments are regularly performed for known TPMT- deficient patients and results in a reduced risk of neutropenia and febrile neutropenia. Also in NUDT15-deficient patients dose adjustments are performed as indicated by low end dose of 6-MP. ITPA-deficient patients had a decreased risk of febrile neutropenia when analyzing the entire treatment period. Our data suggest that NUDT15 plays an important role in 6-MP treatment and the results should be confirmed in larger cohorts. Future studies should also follow up whether white blood cell count-based dose adjustments affect the risk of relapse.

Children with low-risk acute lymphoblastic leukemia are at highest risk of second cancers

BACKGROUND

The improved survival rates for childhood acute lymphoblastic leukemia (ALL) may be jeopardized by the development of a second cancer, which has been associated with thiopurine therapy.

PROCEDURE

We retrospectively analyzed three sequential Nordic Society of Paediatric Haematology and Oncology's protocols characterized by increasing intensity of thiopurine-based maintenance therapy. We explored the risk of second cancer in relation to protocols, risk group, thiopurine methyltransferase (TPMT) activity, ALL high hyperdiploidy (HeH), and t(12;21)[ETV6/RUNX1].

RESULTS

After median 9.5 years (interquartile range, 5.4-15.3 yrs) of follow-up, 40 of 3,591 patients had developed a second cancer, of whom 38 had non-high-risk B-cell precursor ALL. Patients with standard-risk ALL, who received the longest maintenance therapy, had the highest adjusted hazard of second cancer (hazard ratio [HR], intermediate vs. standard risk: 0.16, 95% CI: 0.06-0.43, P < 0.001; HR, high vs. standard risk: 0.09, 95% CI: 0.02-0.49, P = 0.006); no significant effects of protocol, age, or white blood cell count at diagnosis, ALL HeH, or t(12;21)[ETV6/RUNX1] were observed. A subset analysis on the patients with standard-risk ALL did not show an increased hazard of second cancer from either HeH or t(12;21) (adjusted HR 2.02, 95% CI: 0.69-5.96, P = 0.20). The effect of low TPMT low activity was explored in patients reaching maintenance therapy in clinical remission (n = 3,368); no association with second cancer was observed (adjusted HR 1.43, 95% CI: 0.54-3.76, P = 0.47).

CONCLUSIONS

The rate of second cancer was generally highest in patients with low-risk ALL, but we could not identify a subset at higher risk than others.

Thiopurine S-methyltransferase polymorphisms in acute lymphoblastic leukemia, inflammatory bowel disease and autoimmune disorders: influence on treatment response

The thiopurine S-methyltransferase (TPMT) gene encodes for the TPMT enzyme that plays a crucial role in the metabolism of thiopurine drugs. Genetic polymorphisms in this gene can affect the activity of the TPMT enzyme and have been correlated with variability in response to treatment with thiopurines. Advances in the pharmacogenetics of TPMT allowed the development of dosing recommendations and treatment strategies to optimize and individualize prescribing thiopurine in an attempt to enhance treatment efficacy while minimizing toxicity. The influence of genetic polymorphisms in the TPMT gene on clinical outcome has been well-documented and replicated in many studies. In this review, we provide an overview of the evolution, results, conclusions and recommendations of selected studies that investigated the influence of TPMT pharmacogenetics on thiopurine treatment in acute lymphoblastic leukemia, inflammatory bowel disease and autoimmune disorders. We focus mainly on prospective studies that explored the impact of individualized TPMT-based dosing of thiopurines on clinical response. Together, these studies demonstrate the importance of preemptive TPMT genetic screening and subsequent dose adjustment in mitigating the toxicity associated with thiopurine treatment while maintaining treatment efficacy and favorable long-term outcomes. In addition, we briefly address the cost-effectiveness of this pharmacogenetics approach and its impact on clinical practice as well as the importance of recent breakthrough advances in sequencing and genotyping techniques in refining the TPMT genetic screening process.

Meta-analysis of the impact of thioprine S-methyltransferase polymorphisms on the tolerable 6-mercaptopurine dose considering initial dose and ethnic difference

A meta-analysis was conducted to decide whether to reduce an initial 6-mercaptopurine (6-MP) dose in TPMT heterozygote in the case of an initial 6-MP dose of <75 mg/m²/d and to compare the tolerable 6-MP dose among different ethnic groups. The study was undertaken according to Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. The differences in mean values of the tolerable 6-MP dose were calculated by using Comprehensive Meta-Analysis version 3. The results of the meta-analysis indicated that the tolerable 6-MP dose was significantly lower in the TPMT heterozygote group (difference in mean values =11.729, 95% confidence interval =7.617-15.842, P<0.001) even when the initial 6-MP dose was <75 mg/m²/d. The TPMT*3C allele-dominant ethnic group (Asian) needed less reduction in mean 6-MP dose in comparison to the TPMT*3A allele-dominant ethnic group (Caucasian, Mediterranean, South American) (difference in mean values =8.884 vs 15.324). In conclusion, the initial 6-MP dose needs to be reduced in TPMT heterozygote when compared to the wild-type, and ethnic difference might influence the tolerable 6-MP dose in TPMT heterozygotes.

Maintenance therapy of childhood acute lymphoblastic leukemia revisited-Should drug doses be adjusted by white blood cell, neutrophil, or lymphocyte counts?

BACKGROUND

6-Mercaptopurine (6MP) and methotrexate (MTX) based maintenance therapy is a critical phase of childhood acute lymphoblastic leukemia treatment. Wide interindividual variations in drug disposition warrant frequent doses adjustments, but there is a lack of international consensus on dose adjustment guidelines.

PROCEDURE

To identify relapse predictors, we collected 28,255 data sets on drug doses and blood counts (median: 47/patient) and analyzed erythrocyte (Ery) levels of cytotoxic 6MP/MTX metabolites in 9,182 blood samples (median: 14 samples/patient) from 532 children on MTX/6MP maintenance therapy targeted to a white blood cell count (WBC) of 1.5-3.5 × 10⁹ /l.

RESULTS

After a median follow-up of 13.8 years for patients in remission, stepwise Cox regression analysis did not find age, average doses of 6MP and MTX, hemoglobin, absolute lymphocyte counts, thrombocyte counts, or Ery levels of 6-thioguanine nucleotides or MTX (including its polyglutamates) to be significant relapse predictors. The parameters significantly associated with risk of relapse (N = 83) were male sex (hazard ratio [HR] 2.0 [1.3-3.1], P = 0.003), WBC at diagnosis (HR = 1.04 per 10 × 10⁹ /l rise [1.00-1.09], P = 0.048), the absolute neutrophil count (ANC; HR = 1.7 per 10⁹ /l rise [1.3-2.4], P = 0.0007), and Ery thiopurine methyltransferase activity (HR = 2.7 per IU/ml rise [1.1-6.7], P = 0.03). WBC was significantly related to ANC (Spearman correlation coefficient, r_s  = 0.77; P < 0.001), and only a borderline significant risk factor for relapse (HR = 1.28 [95% CI: 1.00-1.64], P = 0.046) when ANC was excluded from the Cox model.

CONCLUSIONS

This study indicates that a low neutrophil count is likely to be the best hematological target for dose adjustments of maintenance therapy.

Metabolism, Biochemical Actions, and Chemical Synthesis of Anticancer Nucleosides, Nucleotides, and Base Analogs

Nucleoside, nucleotide, and base analogs have been in the clinic for decades to treat both viral pathogens and neoplasms. More than 20% of patients on anticancer chemotherapy have been treated with one or more of these analogs. This review focuses on the chemical synthesis and biology of anticancer nucleoside, nucleotide, and base analogs that are FDA-approved and in clinical development since 2000. We highlight the cellular biology and clinical biology of analogs, drug resistance mechanisms, and compound specificity towards different cancer types. Furthermore, we explore analog syntheses as well as improved and scale-up syntheses. We conclude with a discussion on what might lie ahead for medicinal chemists, biologists, and physicians as they try to improve analog efficacy through prodrug strategies and drug combinations.

The pharmacogenomics of drug resistance to protein kinase inhibitors

Dysregulation of growth factor cell signaling is a major driver of most human cancers. This has led to development of numerous drugs targeting protein kinases, with demonstrated efficacy in the treatment of a wide spectrum of cancers. Despite their high initial response rates and survival benefits, the majority of patients eventually develop resistance to these targeted therapies. This review article discusses examples of established mechanisms of drug resistance to anticancer therapies, including drug target mutations or gene amplifications, emergence of alternate signaling pathways, and pharmacokinetic variation. This reveals a role for pharmacogenomic analysis to identify and monitor for resistance, with possible therapeutic strategies to combat chemoresistance.

Prophylactic Trimethoprim-Sulfamethoxazole Does Not Affect Pharmacokinetics or Pharmacodynamics of Methotrexate

Trimethoprim-sulfamethoxazole (TMP/SMX) is used as prophylaxis against Pneumocystis jiroveci during chemotherapy. Many groups recommend withholding TMP/SMX during high-dose methotrexate (HDMTX) for concerns that it will delay methotrexate clearance. We compared methotrexate exposure following HDMTX (NCT00549848) in 424 patients including 783 courses that were given concurrently and 602 courses that were not given concurrently with TMP/SMX. Among 176 patients (555 courses) on the low-risk arm (HDMTX=2.5 g/m/24 h), there was no difference in clearance (110.7 [1.8%] vs. 108.2 [0.9%] mL/min/m, P=0.3) nor in 42 hour methotrexate concentration (0.37 [5.1%] vs. 0.40 (5.0%) μM, P=0.23). Among 248 patients (830 courses) on the standard/high-risk arm (HDMTX ~5 g/m/24 h), there was slightly higher clearance (95.5 [1.4%] vs. 91.2 [0.8%] mL/min/m, P=0.005) in those receiving TMP/SMX, with no difference in the 42 hour methotrexate concentration (0.59 [4.1%] vs. 0.66 [4.2%] μM, P=0.06). There was no difference in neutrophil counts based on TMP/SMX during HDMTX (P=0.83). TMP/SMX also did not have a significant impact on myelosuppression of low-dose methotrexate (40 mg/m) given during continuation therapy among 230 patients enrolled on a prior study (NCT00137111). Thus, we found no evidence for an interaction between methotrexate and TMP/SMX given prophylactically.

Genotyping NUDT15 can predict the dose reduction of 6-MP for children with acute lymphoblastic leukemia especially at a preschool age

The pharmacokinetics among children has been altered dynamically. The difference between children and adults is caused by immaturity in things such as metabolic enzymes and transport proteins. The periods when these alterations happen vary from a few days to some years after birth. We hypothesized that the effect of gene polymorphisms associated with the dose of medicine could be influenced by age. In this study, we analyzed 51 patients with childhood acute lymphoblastic leukemia (ALL) retrospectively. We examined the associations between the polymorphism in NUDT15 and clinical data, especially the dose of 6-mercaptopurine (6-MP). Ten of the patients were heterozygous for the variant allele in NUDT15. In patients under 7 years old with NUDT15 variant allele, the average administered dose of 6-MP was lower than that for the patients homozygous for the wild-type allele (P=0.04). Genotyping of NUDT15 could be a beneficial to estimate the tolerated dose of 6-MP for patients with childhood ALL, especially at a preschool age in Japan. Furthermore, the analysis with stratification by age might be useful in pharmacogenomics among children.

Shorter Maintenance Therapy in Childhood Acute Lymphoblastic Leukemia: The Experience of the Prospective, Randomized Brazilian GBTLI ALL-93 Protocol

AIM

Maintenance therapy is an important phase of the childhood ALL treatment, requiring 2-year long therapy adherence of the patients and families. Weekly methotrexate with daily 6-mercaptopurine (6MP) constitutes the backbone of maintenance therapy. Reduction in the maintenance therapy could overweight problems related with poverty of children with ALL living in limited-income countries (LIC).

OBJECTIVE

To compare, prospectively, the EFS rates of children with ALL treated according to two maintenance regimens: 18 vs. 24 months duration.

MATERIALS AND METHODS

From October 1993 to September 1999, 867 consecutive untreated ALL patients <18 years of age were treated according to the Brazilian Cooperative Group for Childhood ALL Treatment (GBTLI) ALL-93 protocol. Risk classification was based exclusively on patient's age and leukocyte count (NCI risk group) and clinical extra medullary involvement of the disease. Data were analyzed by the intention-to-treat approach.

RESULTS

Fourteen patients (1.6%) were excluded: wrong diagnosis (n = 7) and previous corticosteroid (n = 7). Of the 853 eligible patients, 421 were randomly allocated, at study enrollment, to receive 18-month (group 1) and 432 to receive 24-month (group 2) maintenance therapy. Complete remission rate was achieved in 96% of the patients (817/853). Twenty-eight patients (3.4%) died during the induction phase. Thirty-four patients (4.0%) were lost to follow-up. The overall EFS was 66.1 ± 1.7% at 15 years. No difference was seen according to maintenance: EFS_15y was 65.8 ± 2.3% (group 1) and 66.3 ± 2.3% (group 2; p = 0.79). No difference between regimens was detected after stratifying the analyses according to factors associated with adverse prognosis in this study (age group <1 year or >10 years and high WBC at diagnosis). Overall death in remission rate was 6.85% (56 patients). Deaths during maintenance were 13 in group 1 and 12 in group 2, all due to infection. Over 15 years of follow-up, two patients both from group 2 presented a second malignancy (Hodgkin's disease and thyroid carcinoma) after 8.3 and 11 years off therapy, respectively.

CONCLUSION

Six-month reduction of maintenance therapy in ALL children treated according to the GBTLI ALL-93 protocol provided the same overall outcome as 2-year duration regimen.

Inherited genetic variation in childhood acute lymphoblastic leukemia

Although somatically acquired genomic alterations have long been recognized as the hallmarks of acute lymphoblastic leukemia (ALL), the last decade has shown that inherited genetic variations (germline) are important determinants of interpatient variability in ALL susceptibility, drug response, and toxicities of ALL therapy. In particular, unbiased genome-wide association studies have identified germline variants strongly associated with the predisposition to ALL in children, providing novel insight into the mechanisms of leukemogenesis and evidence for complex interactions between inherited and acquired genetic variations in ALL. Similar genome-wide approaches have also discovered novel germline genetic risk factors that independently influence ALL prognosis and those that strongly modify host susceptibility to adverse effects of antileukemic agents (eg, vincristine, asparaginase, glucocorticoids). There are examples of germline genomic associations that warrant routine clinical use in the treatment of childhood ALL (eg, TPMT and mercaptopurine dosing), but most have not reached this level of actionability. Future studies are needed to integrate both somatic and germline variants to predict risk of relapse and host toxicities, with the eventual goal of implementing genetics-driven precision-medicine approaches in ALL treatment.

A case of recurrent pancytopenia in a patient with acute promyelocytic leukemia on maintenance chemotherapy and concomitant methyltetrahydrofolate reductase and thiopurine S-methyltransferase mutation - review of literature

Pharmacogenetics is a study of how genetic variation of an individual affects the drug response. We report a case of recurrent pancytopenia resulting from maintenance chemotherapy in a patient with acute promyelocytic leukemia and two pharmacogenetic mutations, namely, methylene tetrahydrofolate reductase C677T homozygous mutation and thiopurine methyltransferase mutation.

Thiopurine dose intensity and treatment outcome in childhood lymphoblastic leukaemia: the influence of thiopurine methyltransferase pharmacogenetics

The impact of thiopurine methyltransferase (TPMT) genotype on thiopurine dose intensity, myelosuppression and treatment outcome was investigated in the United Kingdom childhood acute lymphoblastic leukaemia (ALL) trial ALL97. TPMT heterozygotes had significantly more frequent cytopenias and therefore required dose adjustments below target levels significantly more often than TPMT wild‐type patients although the average dose range was similar for both genotypes. Event‐free survival (EFS) for patients heterozygous for the more common TPMT*1/*3A variant allele (n = 99, 5‐year EFS 88%) was better than for both wild‐type TPMT*1/*1 (n = 1206, EFS 80%, P = 0·05) and TPMT*1/*3C patients (n = 17, EFS 53%, P = 0·002); outcomes supported by a multivariate Cox regression analysis. Poor compliance without subsequent clinician intervention was associated with a worse EFS (P = 0·02) and such non‐compliance may have contributed to the poorer outcome for TPMT*1/*3C patients. Patients prescribed escalated doses had a worse EFS (P = 0·04), but there was no difference in EFS by dose intensity or duration of cytopenias. In contrast to reports from some USA and Nordic trials, TPMT heterozygosity was not associated with a higher rate of second cancers. In conclusion, TPMT*1/*3A heterozygotes had a better EFS than TPMT wild‐type patients. Thiopurine induced cytopenias were not detrimental to treatment outcome.

Pharmacogenetics of cancer drugs

The variability in treatment outcomes among patients receiving the same therapy for seemingly similar tumors can be attributed in part to genetics. The tumor's (somatic) genome largely dictates the effectiveness of the therapy, and the patient's (germline) genome influences drug exposure and the patient's sensitivity to toxicity. Many potentially clinically useful associations have been discovered between common germline genetic polymorphisms and outcomes of cancer treatment. This review highlights the germline pharmacogenetic associations that are currently being used to guide cancer treatment decisions, those that are most likely to someday be clinically useful, and associations that are well known but their roles in clinical management are not yet certain. In the future, germline genetic information will likely be available from tumor genetic analyses, creating an efficient opportunity to integrate the two genomes to optimize treatment outcomes for each individual cancer patient.

Myelotoxicity after high-dose methotrexate in childhood acute leukemia is influenced by 6-mercaptopurine dosing but not by intermediate thiopurine methyltransferase activity

PURPOSE

Through enhancement of 6-mercaptopurine (6MP) bioavailability and inhibition of purine de novo synthesis, high-dose methotrexate (HD-MTX) may increase incorporation into DNA of 6-thioguanine nucleotides, the cytotoxic metabolites of 6MP. Patients with intermediate activity of thiopurine methyltransferase (TPMT(IA)) have higher cytosol 6-thioguanine nucleotide levels. We investigated toxicity following HD-MTX during MTX/6MP maintenance therapy in relation to 6MP and TPMT.

METHODS

Using linear mixed models, we explored myelo- and hepatotoxicity in relation to 6MP dosage and TPMT phenotype following 1,749 HD-MTX courses to 411 children with acute lymphoblastic leukemia on maintenance therapy.

RESULTS

The degree of myelosuppression following HD-MTX was similar for patients with TPMT(IA) and patients with high TPMT activity (TPMT(HA)), when HD-MTX started with same blood counts and 6MP doses. However, since TPMT(IA) had lower blood counts at initiation of HD-MTX compared with TPMT(HA) patients (median WBC 2.8 vs. 3.3 × 10⁹/L, P = 0.01; median ANC 1.4 vs. 1.7 × 10⁹/L, P = 0.02), TPMT(IA) continued to have lower WBC and ANC levels compared with TPMT(HA) during all 28 days after HD-MTX [relative difference 9 % (95 % CI 2-17), P = 0.02 and 21 % (95 % CI 6-39), P = 0.005]. Still, the fractional decrease in WBC and ANC levels after HD-MTX did not differ between TPMT(IA) and TPMT(HA) patients (P = 0.47; P = 0.38). The degree of leukopenia, neutropenia, thrombocytopenia and rise in aminotransferases were all significantly related to 6MP dose (P < 0.001 for all analyses).

CONCLUSION

For both TPMT(IA) and TPMT(HA) patients, dose of 6MP prior to HD-MTX should be guided by pre-HD-MTX blood counts, but not by TPMT activity.

Mercaptopurine/Methotrexate maintenance therapy of childhood acute lymphoblastic leukemia: clinical facts and fiction

The antileukemic mechanisms of 6-mercaptopurine (6MP) and methotrexate (MTX) maintenance therapy are poorly understood, but the benefits of several years of myelosuppressive maintenance therapy for acute lymphoblastic leukemia are well proven. Currently, there is no international consensus on drug dosing. Because of significant interindividual and intraindividual variations in drug disposition and pharmacodynamics, vigorous dose adjustments are needed to obtain a target degree of myelosuppression. As the normal white blood cell counts vary by patients' ages and ethnicity, and also within age groups, identical white blood cell levels for 2 patients may not reflect the same treatment intensity. Measurements of intracellular levels of cytotoxic metabolites of 6MP and MTX can identify nonadherent patients, but therapeutic target levels remains to be established. A rise in serum aminotransferase levels during maintenance therapy is common and often related to high levels of methylated 6MP metabolites. However, except for episodes of hypoglycemia, serious liver dysfunction is rare, the risk of permanent liver damage is low, and aminotransferase levels usually normalize within a few weeks after discontinuation of therapy. 6MP and MTX dose increments should lead to either leukopenia or a rise in aminotransferases, and if neither is experienced, poor treatment adherence should be considered. The many genetic polymorphisms that determine 6MP and MTX disposition, efficacy, and toxicity have precluded implementation of pharmacogenomics into treatment, the sole exception being dramatic 6MP dose reductions in patients who are homozygous deficient for thiopurine methyltransferase, the enzyme that methylates 6MP and several of its metabolites. In conclusion, maintenance therapy is as important as the more intensive and toxic earlier treatment phases, and often more challenging. Ongoing research address the applicability of drug metabolite measurements for dose adjustments, extensive host genome profiling to understand diversity in treatment efficacy and toxicity, and alternative thiopurine dosing regimens to improve therapy for the individual patient.