Increased Frequency of Expression of Elevated Dihydrofolate Reductase in T-Cell Versus B-Precursor Acute Lymphoblastic Leukemia in Children

Metabolic inhibitor screening identifies dihydrofolate reductase as an inducer of the tumor immune escape mediator CD24

Immune checkpoint inhibitors have improved the clinical management of some cancer cases, yet patients still fail to respond to immunotherapy. Dysregulated metabolism is a common feature of many cancers, and metabolites are known to modulate functions in cancer cells. To identify potential metabolic pathways involved in anti-tumor immune response, we employed a metabolic inhibitor-based drug screen in human lung cancer cell lines and examined expression changes in a panel of immune regulator genes. Notably, pharmacologic inhibition of dihydrofolate reductase (DHFR) downregulated cancer cell expression of cluster of differentiation 24 (CD24), an anti-phagocytic surface protein. Genetic modulation of DHFR resulted in decrease of CD24 expression, whereas tetrahydrofolate, the product of DHFR, enhanced CD24 expression. DHFR inhibition and the consequent CD24 decrease enhanced T cell-mediated tumor cell killing, whereas replenishment of DHFR or CD24 partially mitigated the immune-mediated tumor cell killing that resulted from methotrexate treatment in cancer cells. Moreover, publicly available clinical data analyses further revealed the link between DHFR, CD24, and the antitumor immune response in lung cancer patients. Our study highlights a novel connection between folate metabolism and the anti-tumor immune response and partially interprets how DHFR inhibitors lead to clinical benefits when combined with cancer immunotherapy agents.

Racial and ethnic disparities in hematologic malignancies

Racial and ethnic disparities in patients with solid malignancies have been well documented. Less is known about these disparities in patients with hematologic malignancies. With the advent of novel chemotherapeutics and targeted molecular, cellular, and immunologic therapies, it is important to identify differences in care that may lead to disparate outcomes. This review provides a critical appraisal of the empirical research on racial and ethnic disparities in incidence, survival, and outcomes in patients with hematologic malignancies. The review focuses on patients with acute myeloid leukemia, acute lymphocytic leukemia, multiple myeloma, non-Hodgkin lymphoma, Hodgkin lymphoma, myeloproliferative neoplasms, and myelodysplastic syndrome. The review discusses possible causes of racial and ethnic disparities and also considers future directions for studies to help decrease disparities.

Evolution of modern treatment of childhood acute leukemia and cancer: adventures and battles in the 1970s and 1980s

This article summarizes the adventures and explorations in the 1970s and 1980s in the treatment of children with leukemia and cancer that paved the way for the current success in childhood cancers. Indeed, these were adventures and bold steps into unchartered waters. Because childhood leukemia the most common of the childhood cancers, success in childhood leukemia was pivotal in the push toward cure of all childhood cancers. The success in childhood leukemia illustrates how treatment programs were designed using clinical- and biology-based risk factors seen in the patients.

DNA variants in region for noncoding interfering transcript of dihydrofolate reductase gene and outcome in childhood acute lymphoblastic leukemia

PURPOSE

Dihydrofolate reductase (DHFR) is the major target of methotrexate, a key component in childhood acute lymphoblastic leukemia (ALL) treatment. We recently reported an association of DHFR promoter polymorphisms with ALL outcome. Lower event-free survival correlated with haplotype *1, defined by A(-317) and C(-1610) alleles. Haplotype *1 was also associated higher DHFR expression.

EXPERIMENTAL DESIGN

Here, we analyzed adjacent 400-bp region participating in DHFR regulation as both a major promoter and a noncoding minor transcript.

RESULTS

Six polymorphisms were identified, of which five were single nucleotide polymorphisms and one was length polymorphism composed of variable number of 9-bp elements and 9-bp insertion/deletion. Haplotype analysis including all promoter polymorphisms revealed diversification of haplotype *1 into five subtypes (*1a-*1e). DNA variations of major promoter/noncoding transcript region and haplotype *1 subtypes were subsequently analyzed for the association with ALL outcome. Lower event-free survival was associated with an A allele of G(308)A polymorphism (P = 0.02) and with *1b haplotype (P = 0.01). This association was particularly striking in high-risk patients (P = 0.001) and was subsequently confirmed in independent patient cohort (P = 0.02). Haplotype *1b was the only haplotype *1 subtype associated with higher mRNA levels.

CONCLUSIONS

The study provides a new insight into DHFR regulatory variations predisposing to an event in ALL patients.

Acquired variation outweighs inherited variation in whole genome analysis of methotrexate polyglutamate accumulation in leukemia

Methotrexate polyglutamates (MTXPGs) determine in vivo efficacy in acute lymphoblastic leukemia (ALL). MTXPG accumulation differs by leukemic subtypes, but genomic determinants of MTXPG variation in ALL remain unclear. We analyzed 3 types of whole genome variation: leukemia cell gene expression and somatic copy number variation, and inherited single nucleotide polymorphism (SNP) genotypes and determined their association with MTXPGs in leukemia cells. Seven genes (FHOD3, IMPA2, ME2, RASSF4, SLC39A6, SMAD2, and SMAD4) displayed all 3 types of genomic variation associated with MTXPGs (P < .05 for gene expression, P < .01 for copy number variation and SNPs): 6 on chromosome 18 and 1 on chromosome 10. Increased chromosome 18 (P = .002) or 10 (P = .036) copy number was associated with MTXPGs even after adjusting for ALL subtype. The expression of the top 7 genes in leukemia cells accounted for more variation in MTXPGs (46%) than did the expression of the top 7 genes in normal HapMap cell lines (20%). The top 7 inherited SNPs in patients accounted for approximately the same degree of variation (17%) in MTXPGs as did the top 7 SNP genotypes in HapMap cell lines (20%). We conclude that acquired genetic variation in leukemia cells has a stronger influence on MTXPG accumulation than inherited genetic variation.

Resistance to chemotherapy in cancer: a complex and integrated cellular response

Inherent and acquired resistance pathways account for the high rate of failure in cancer chemotherapy. The mechanisms or pathways mediating resistance may be classified as pharmacokinetic (i.e. alter intratumour drug exposue) or pharmacodynamic (i.e. failure to elicit cytotoxicity). More often than not, the resistant phenotype is characterised by alterations in multiple pathways. Consequently, the pathways may act synergistically or generate a broad spectrum of resistance to anticancer drugs. There has been a great deal of systematic characterisation of drug resistance in vitro. However, translating this greater understanding into clinical efficacy has rarely been achieved. This review explores the phenomenon of drug resistance in cancer and highlights the gap between in vitro and in vivo observations. This gap presents a major obstacle in overcoming drug resistance and restoring sensitivity to chemotherapy.

DNA variants in the dihydrofolate reductase gene and outcome in childhood ALL

Dihydrofolate reductase (DHFR) is the major target of methotrexate (MTX), a key component in childhood acute lymphoblastic leukemia (ALL) treatment. A total of 15 polymorphisms in DHFR promoter were analyzed, and 3 sites (C-1610G/T, C-680A, and A-317G) were identified as sufficient to define observed haplotypes (tag single nucleotide polymorphisms [tagSNPs]). These polymorphisms were investigated for association with treatment response in 277 children with ALL. Lower event-free survival (EFS) was associated with homozygosity for the allele A-317 and C-1610 (P=.03 and .02), and with the haplotype *1, defined by both C-1610 and A-317 alleles (P=.03). The haplotype *1 conferred higher transcriptional activity (P<.01 compared with haplotypes generating minimal luciferase expression). Quantitative mRNA analysis showed higher DHFR levels for particular haplotype *1 carriers (P<.01). The analysis combining haplotype *1 with thymidylate synthase (TS) and cyclin D1 (CCND1) genotypes previously shown to affect ALL outcome showed that the number of event-predisposing genotypes was associated with increasingly lower EFS (P<.001). In conclusion, DHFR promoter polymorphisms are associated with worse ALL outcome, likely due to a higher DHFR expression. Combined effects among genes of the folate cycle can further accentuate differences in the response to the treatment.

High-dose compared with intermediate-dose methotrexate in children with a first relapse of acute lymphoblastic leukemia

High-dose methotrexate (MTX) has been extensively used for treatment of acute lymphoblastic leukemia (ALL). To determine the optimal dose of MTX in childhood relapsed ALL, the ALL Relapse Berlin-Frankfurt-Münster (ALL-REZ BFM) Study Group performed this prospective randomized study. A total of 269 children with a first early/late isolated (n = 156) or combined (n = 68) bone marrow or any isolated extramedullary relapse (n = 45) of precursor B-cell (PBC) ALL (excluding very early marrow relapse within 18 months after initial diagnosis) were registered at the ALL-REZ BFM90 trial and randomized to receive methotrexate infusions at either 1 g/m(2) over 36 hours (intermediate dose, ID) or 5 g/m(2) over 24 hours (high dose, HD) during 6 (or 4) intensive polychemotherapy courses. Intensive induction/consolidation therapy was followed by cranial irradiation, and by conventional-dose maintenance therapy. Fifty-five children received stem-cell transplants. At a median follow-up of 14.1 years, the 10-year event-free survival probability was .36 (+/- .04) for the ID group (n = 141), and .38 (+/- .04) for the HD group (n = 128, P = .919). The 2 groups did not differ in terms of prognostic factors and other therapeutic parameters. In conclusion, methotrexate infusions at 5 g/m(2) per 24 hours, compared with 1 g/m(2) per 36 hours, are not associated with increased disease control in relapsed childhood PBC acute lymphoblastic leukemia.

Human reduced folate carrier: translation of basic biology to cancer etiology and therapy

This review attempts to provide a comprehensive overview of the biology of the physiologically and pharmacologically important transport system termed the "reduced folate carrier" (RFC). The ubiquitously expressed RFC has unequivocally established itself as the major transport system in mammalian cells and tissues for a group of compounds including folate cofactors and classical antifolate therapeutics. Loss of RFC expression or function may have potentially profound pathophysiologic consequences including cancer. For chemotherapeutic antifolates used for cancer such as methotrexate or pemetrexed, synthesis of mutant RFCs or loss of RFC transcripts and proteins results in antifolate resistance due to incomplete inhibition of cellular enzyme targets and insufficient substrate for polyglutamate synthesis. Since RFC was first cloned in 1994, tremendous advances have been made in understanding the complex transcriptional and posttranscriptional regulation of RFC, in identifying structurally and functionally important domains and amino acids in the RFC molecule as a prelude to establishing the mechanism of transport, and in characterizing the molecular defects in RFC associated with loss of transport in antifolate resistant cell line models. Many of the insights gained from laboratory models of RFC portend opportunities for modulating carrier expression in drug resistant tumors, and for designing a new generation of agents with improved transport by RFC or substantially enhanced transport by other folate transporters over RFC. Many of the advances in the basic biology of RFC in cell line models are now being directly applied to human cancers in the clinical setting, most notably pediatric acute lymphoblastic leukemia and osteogenic sarcoma.

DHFR and MSH3 co-amplification in childhood acute lymphoblastic leukaemia, in vitro and in vivo

The MSH3 and dihydrofolate reductase (DHFR) genes, located on chromosome 5, share a common promoter but are divergently transcribed. Dysregulation of the mismatch repair (MMR) pathway has been found to occur in cell line models due to co-amplification of MSH3 as a coincident effect of DHFR amplification, acquired as a mechanism generating resistance to methotrexate (MTX). The increased levels of MSH3 perturbed MutSalpha function resulting in hypermutability and increased resistance to thiopurines, drugs whose cytotoxic effects are triggered by MutSalpha. The relevance of this phenomenon in clinical samples is unknown but is extremely pertinent in childhood acute lymphoblastic leukaemia (ALL) in which children are exposed for prolonged periods to both MTX and thiopurines such that a single amplification event involving both the DHFR and the MSH3 genes may cause chemotherapeutic resistance to both agents. Thus, we have generated a leukaemic cell line (PreB697) and a normal human lymphoblastoid cell line (TK6) that are resistant to a pharmacologically relevant dose of MTX and show that while increased DHFR levels result in MTX resistance, the associated increased levels of MSH3 are insufficient to perturb MutSalpha functionality, in terms of MMR capacity or 6-thioguanine sensitivity. In addition, we show that although low-level DHFR amplification occurs alone in a significant number of samples, both at disease onset and relapse, co-amplification of both MSH3 and DHFR is rarely found in primary ALL samples, even after prolonged MTX therapy and is not at a sufficiently high level to perturb MMR function.

Acute lymphoblastic leukaemia: a model for the pharmacogenomics of cancer therapy

The use of combination chemotherapy to cure acute lymphoblastic leukaemia (ALL) in children emerged in the 1980s as a paradigm for curing any disseminated cancer, and many of the therapeutic principles were subsequently applied to the treatment of other disseminated human cancers. Similarly, elucidation of the pharmacogenomics of ALL and its translation into new chemotherapeutic approaches might serve as a model for optimizing the treatment of other human cancers. Germline polymorphisms and gene-expression patterns in ALL cells have been linked to the toxicity and efficacy of chemotherapy for ALL and are beginning to emerge as useful clinical diagnostics.

PHARMACOGENOMICS OF ACUTE LEUKEMIA

Over the past four decades, treatment of acute leukemia in children has made remarkable progress, from this disease being lethal to now achieving cure rates of 80% for acute lymphoblastic leukemia and 45% for acute myeloid leukemia. This progress is largely owed to the optimization of existing treatment modalities rather than the discovery of new agents. However, the annual number of patients with leukemia who experience relapse after initial therapy remains greater than that of new cases of most childhood cancers. The aim of pharmacogenetics is to develop strategies to personalize medications and tailor treatment regimens to individual patients, with the goal of enhancing efficacy and safety through better understanding of the person's genetic makeup. In this review, we summarize recent pharmacogenomic studies related to the treatment of pediatric acute leukemia. These include work using candidate-gene approaches, as well as genome-wide studies using haplotype mapping and gene expression profiling. These strategies illustrate the promise of pharmacogenomics to further advance the treatment of human cancers, with childhood leukemia serving as a paradigm.

Homocysteine: exploring its potential as a pharmacodynamic biomarker of antifolate chemotherapy

For decades it has been well known that elevated levels of homocysteine are harmful to humans on the basis of clinical observations derived from classical model diseases, such as inherited metabolic disorders. This group of diseases includes classical homocystinuria and several other inherited diseases affecting the so-called 'transsulfuration pathways'. Homocysteine lies in a metabolic checkpoint that interconnects one-carbon-transferring reactions with metabolism of sulfur-containing amino acids, since every molecule of 5-methyltetrahydrofolate derived either from plasma or generated from other folate species must be demethylated to liberate the reduced tetrahydrofolate. This unidirectional mechanism operates in every cell and has no alternative in eukaryotic cells. Antifolates are a group of anticancer agents targeting various metabolic steps within folate metabolism. They exert an indirect influence on the rate of appearance/disappearance of homocysteine from cellular and plasma compartments. Recently, it has been postulated that homocysteine may be a marker of the 'pharmacodynamic effect' of methotrexate, but studies attesting to this role are only now emerging. Here, we explore the genetic disease of folate and homocysteine metabolism and discuss the links between these model disorders with pharmacology and pharmacogenetics of folate antagonists used in the clinic. We outline possible ways of how homocysteine may be used as a biomarker of antifolate therapy.

Pharmacogenetics of methotrexate

Methotrexate (MTX) has proven efficient in the treatment of a number of malignancies, as well as non-malignant disorders characterized by a rapid cellular growth. Yet some patients might develop resistance, while others could have toxic side effects. MTX achieves its cytotoxicity through the inhibition of folate-dependent enzymes, suggesting that the genes controlling their activity or the levels of folate cofactors can modulate drug efficacy and, thus, the sensitivity of a patient to MTX. Indeed, several studies, conducted mostly in leukemia and rheumatoid arthritis patients, have addressed the potential for tailoring MTX therapy based on a patient's genetics. Several genetic variants have been shown to have a predictive role, among which the most frequently studied are those of methylenetetrahydrofolate reductase and thymidylate synthase genes. The other candidates, as well as gene-gene interactions, which may be even more important for the prediction of disease outcomes than the individual gene effects, are also briefly discussed.

CNS-directed therapy in young children with T-lineage acute lymphoblastic leukemia: High-dose methotrexate versus cranial irradiation

BACKGROUND

Prophylactic use of cranial radiation therapy (CRT) in young children with acute lymphoblastic leukemia (ALL) is associated with significant long-term morbidity. Therefore, current treatment protocols for pediatric B-precursor ALL have abandoned prophylactic CRT in favor of intrathecal chemotherapy, combined with either high-dose methotrexate infusions (HD-MTX) or intensive systemic chemotherapy. In contrast, prophylactic CRT continues to be used in children with T-lineage ALL (T-ALL), who historically have had an inferior prognosis. We conducted a retrospective cohort study to determine the effect on survival of substituting HD-MTX for CRT in young children with T-ALL, a group that faces a high risk of long-term sequelae from CRT.

PROCEDURE

Twenty-six children, diagnosed with T-ALL between the ages of 1 and 5 years, were treated on the same high-risk leukemia protocol. Central nervous system (CNS) directed therapy consisted of either CRT (1,800 cGy) or HD-MTX (three doses of 8 g/m2), depending on the treatment era in which patients were diagnosed.

RESULTS

Of the 24 patients who entered remission, 12 received CRT and 12 received HD-MTX. Five-year event-free survival (EFS) (+/-SE) was 92 +/- 8% in the HD-MTX group versus 75 +/- 13% in the CRT group (P=0.23). Five-year overall survival (OS) was 100% in the HD-MTX group versus 75 +/- 13% in the CRT group (P=0.07). There were no CNS recurrences in the HD-MTX group. One patient treated with CRT developed a brain tumor.

CONCLUSIONS

The use of HD-MTX instead of CRT as CNS-directed therapy in very young children with T-ALL does not compromise survival, while avoiding the adverse long-term effects of cranial irradiation.

Reduced folate carrier and dihydrofolate reductase expression in acute lymphocytic leukemia may predict outcome: a Children's Cancer Group Study

PURPOSE

Methotrexate is a major component of current treatment regimens for children with acute lymphocytic leukemia (ALL). Potential mechanisms of methotrexate resistance include impaired drug uptake, decreased drug retention, and dihydrofolate reductase (DHFR) amplification. The purpose of this study was to assess whether reduced folate carrier (RFC) and DHFR expression in untreated leukemic blasts correlated with outcome.

METHODS

Quantitative real-time RT-PCR was used to measure RFC and DHFR mRNA expression in leukemic blasts from 40 newly diagnosed patients with ALL obtained in a blinded fashion from Children's Cancer Group studies.

RESULTS

Low RFC expression at diagnosis correlated significantly with an unfavorable event free survival. Surprisingly, low, not high, DHFR expression correlated significantly with an unfavorable event-free survival. Proliferative cell nuclear antigen (PCNA) expression demonstrated a weak inverse relationship between sample PCNA and DHFR or RFC expression, suggesting that DHFR and RFC expression may be markers for factors other than drug resistance.

CONCLUSIONS

These results suggest that impaired transport may be an important mechanism of intrinsic methotrexate resistance in ALL, and DHFR expression also may be an important prognostic factor in ALL. Additional studies are necessary to clarify the mechanism for the correlation of low DHFR expression with poor outcome.

Membrane transport of folates

The chapter reviews the current understanding of the transport mechanisms for folates in mammalian cells--their molecular identities and organization, tissue expression, regulation, structures, and their kinetic and thermodynamic properties. This encompasses a variety of diverse processes. Best characterized is the reduced folate carrier, a member of the SLC19 family of facilitative carriers. But other facilitative organic anion carriers (SLC21), largely expressed in epithelial tissues, transport folates as well. In addition to these bi-directional carrier systems are the membrane-localized folate receptors alpha and beta, that mediate folate uptake unidirectionally into cells via an endocytotic process. There are also several transporters, typified by the family of multidrug resistance-associated proteins, that unidirectionally export folates from cells. There are transport activities for folates, that function optimally at low pH, related in part to the reduced folate carrier, with at least one activity that is independent of this carrier. The reduced folate carrier-associated low-pH route mediates intestinal folate transport. This review considers how these different transport processes contribute to the generation of transmembrane folate gradients and to vectorial flows of folates across epithelia. The role of folate transporters in mouse development, as assessed by homologous deletion of folate receptors and the reduced folate carrier, is described. Much of the focus is on antifolate cancer chemotherapeutic agents that are often model surrogates for natural folates in transport studies. In particular, antifolate transport mediated by the reduced folate carrier is a major determinant of the activity of, and resistance to, these agents. Finally, many of the key in vitro findings on the properties of antifolate transporters are now beginning to be extended to patient specimens, thus setting the stage for understanding response to these drugs in the clinical setting at the molecular level.

Decreased expression of the reduced folate carrier and folypolyglutamate synthetase is the basis for acquired resistance to the pemetrexed antifolate (LY231514) in an L1210 murine leukemia cell line

Pemetrexed (LY231514) is a new-generation antifolate that, in its polyglutamyl forms, is a potent inhibitor of thymidylate synthase and glycinamide ribonucleotide formyltransferase (GAR transformylase). This study explored the mechanisms of resistance to pemetrexed in L1210 murine leukemia cells using chemical mutagenesis with 5-formyltetrahydrofolate (5-formylTHF) as the growth substrate. A cell line, MTA-13, was identified that was 8.5-fold resistant to pemetrexed with comparable cross-resistance to ZD1694 (Tomudex) and lesser cross-resistance (5-fold) to ZD9331 [(2S)-2-(O-fluoro-p-[N-(2,7-dimethyl-4-oxo-3,4-dihydro-quinazolin-6-ylmethyl)-N-(prop-2-ynyl)amino]benzamido)-4-(tetrazol-5-yl)-butyric acid], DDATHF (dideazatetrahydrofolate) (3.5-fold), and methotrexate (MTX) (2.7-fold) but comparable sensitivity to trimetrexate. Influx of pemetrexed, MTX, and 5-formylTHF into MTA-13 cells was decreased by 56, 47, and 38% compared to wild-type cells. Folate receptor expression was negligible in both cell lines. Net drug uptake declined within 15min to a slower, constant rate over the next 45min, reflecting the rate of accumulation of pemetrexed polyglutamate derivatives. This rate in the MTA-13 line was half that of the wild-type cells. Accumulation of 50nM [3H]pemetrexed, 25nM [3H]5-formylTHF, or 50nM [3H]DDATHF after 3 days was decreased to 35, 46, and 56% the level of L1210 cells. The reduced folate carrier (RFC) message and protein were decreased by 50%, and folypolyglutamate synthetase (FPGS) message was decreased by 65% in MTA-13 cells. No mutations were detected in either protein by DNA sequence analysis. There was a slight decrease (approximately 25%) in thymidylate synthase mRNA, without mutations in the protein, and there was no change in GAR transformylase message. The data indicate that resistance to pemetrexed in the MTA-13 cell line was due to changes in both RFC and FPGS expression, two proteins that act in tandem to regulate polyglutamation of folates and antifolates in cells, resulting in cellular depletion of these active pemetrexed congeners.

Role of the E45K-reduced folate carrier gene mutation in methotrexate resistance in human leukemia cells

Resistance to the antifolate methotrexate (MTX) can cause treatment failure in childhood acute lymphoblastic leukemia (ALL). This may result from defective MTX accumulation due to alterations in the human reduced folate carrier (hRFC) gene. We have identified an hRFC gene point mutation in a transport-defective CCRF-CEM human T-ALL cell line resulting in a lysine to glutamic acid substitution at codon 45 (E45K), which has been identified in other antifolate-resistant sublines (JBC 273:30 189, 1998; JBC 275:30 855, 2000). To characterize the role of this mutation in MTX resistance, transfection experiments were performed using hRFC-null CCRF-CEM cells. E45K transfectants demonstrated an initial rate of MTX influx that was approximately 0.5-fold that of CCRF-CEM cells, despite marked protein overexpression. Cytotoxicity studies revealed partial reversal of MTX and raltitrexed resistance in E45K transfectants, while trimetrexate resistance was significantly increased. Kinetic analysis indicated only minor differences in MTX kinetics between wild-type and E45K hRFCs, however, K(i)s for folic acid and 5-formyltetrahydrofolate were markedly reduced for E45K hRFC. This was paralleled by increased folic acid transport and reduced synthesis of MTX polyglutamates. Collectively, the results demonstrate that expression of E45K hRFC leads to increased MTX resistance due to decreased membrane transport and, secondarily, from alterations in binding affinities and transport of folate substrates. However, despite these findings, we could find no evidence of this mutation in 121 childhood ALL samples, suggesting that it does not contribute to clinical MTX resistance in this disease.

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.

Relationship of p15 and p16 gene alterations to elevated dihydrofolate reductase in childhood acute lymphoblastic leukaemia

The downstream effects of p15 and p16 gene deletions and loss of transcripts on dihydrofolate reductase (DHFR) were examined in 63 B-precursor (BP) acute lymphoblastic leukaemia (ALL) samples. p15 and/or p16 gene deletions were seen in 6% and 8%, respectively, of BP-ALL samples; however, losses of p15 and/or p16 transcripts were seen in 26 out of 63 (41%) samples. Loss of p15 transcripts (36.5%) exceeded that for p16 (17.5%). For the 26 BP-ALLs that lacked p15 and/or p16 transcripts, only six (23%) exhibited low levels of DHFR by flow cytometry assay with Pt430, a fluorescent anti-folate. Conversely, 18 out of 37 (49%) BP-ALL samples with intact p15 and/or p16 genes and transcripts showed low levels of DHFR (P = 0.04). In p15- and p16-null K562 cells transfected with a tetracycline-inducible p15 cDNA construct, induction of p15 transcripts and protein was accompanied by decreased growth rates, decreased S-phase fraction, decreased retinoblastoma protein phosphorylation, and markedly reduced levels of DHFR transcripts and protein. Collectively, our results suggest that losses of p15 and/or p16 gene expression result in elevated levels of DHFR in BP-ALL in children. However, additional downstream factors undoubtedly also contribute to elevated levels of this enzyme target.

mRNA expression levels of methotrexate resistance-related proteins in childhood leukemia as determined by a standardized competitive template-based RT-PCR method

Drug resistance of leukemic blasts is correlated to event-free survival and might be predicted by mRNA expression of drug resistance-related proteins. Methotrexate (MTX) is an important component in the treatment of childhood leukemia. Mechanisms of MTX resistance include (1) decreased transport via the reduced folate carrier (RFC), (2) altered levels of target enzymes, eg dihydrofolate reductase (DHFR) and thymidylate synthase (TS), (3) decreased ratio of folylpolyglutamate synthetase (FPGS)/folylpolyglutamate hydrolase (FPGH). We designed competitive templates for each of these genes to measure mRNA expression by quantitative RT-PCR and normalized the expression to that of beta-actin. T-lineage acute lymphoblastic leukemia (T-ALL), relatively MTX resistant compared to common/preB-ALL, displayed higher mRNA levels of DHFR and TS (three- and four-fold higher, respectively; P < 0.001), while FPGS expression was lower (three-fold, P = 0.006) compared to common/preB-ALL. The ratio of (DHFR x FPGH)/(RFC x FPGS) was more discriminating between T-ALL and c/preB-ALL (eight-fold higher; P < 0.001) than either target independently. Acute myeloid leukemia (AML) cells, considered MTX resistant, expressed two-fold lower levels of FPGS mRNA compared to c/preB-ALL (P = 0.04). The ratio of FPGH/FPGS was more discriminating between AML and c/preB-ALL (four-fold higher; P = 0.001) than either target independently. For the total group of 79 leukemic samples, mRNA expression of DHFR varied 549-fold and paralleled TS mRNA expression (r = 0.80; P < 0.001). Although variations in mRNA expression resembled variations in functional activity, no direct correlations were found for RFC (58-fold variation in mRNA expression), FPGS (95-fold) and FPGH (178-fold). In conclusion, differences in mRNA expression of MTX resistance parameters between leukemic subtypes as detected by competitive RT-PCR are in line with known differences in MTX resistance.

Methotrexate resistance in relapsed childhood acute lymphoblastic leukaemia

Treatment failure in childhood acute lymphoblastic leukaemia (ALL) might be associated with methotrexate (MTX) resistance. Little is known about MTX resistance in relapsed ALL. In this study, we determined ex vivo MTX resistance in precursor-B ALL at relapse (rALL) and determined possible defects in MTX membrane transport and polyglutamylation. Using the in situ thymidylate synthase inhibition assay, 21 rALL samples were threefold more MTX resistant than 63 initial precursor-B ALL samples, both after short-term and after continuous MTX exposure (P < or = 0.01). [3H]-MTX membrane transport did not differ between eight rALL and 25 precursor-B ALL samples. Incubation for 24 h with 1 microM [3H]-MTX resulted in a trend towards a lower accumulation of MTX in 20 relapsed than in 83 initial samples of precursor-B ALL samples (906 vs. 1364 pmol/109 cells; P = 0.07). Accumulation of long-chain MTX polyglutamates (MTX-Glu4-6) did not differ between relapsed and newly diagnosed samples (746 and 889 pmol/109 cells; P = 0.1). Activities of the enzymes involved in polyglutamylation (folylpolyglutamate synthetase and folylpolyglutamate hydrolase) did not differ between rALL and untreated c/pre-B-ALL. This study demonstrates that leukaemic cells of children with relapsed precursor-B ALL are relatively MTX resistant, but that this MTX resistance is not associated with major impairments in MTX uptake or polyglutamylation.

Modulation of dihydrofolate reductase gene expression in methotrexate-resistant human leukemia CCRF-CEM/E cells by antisense oligonucleotides

An increase in the cellular levels of dihydrofolate reductase (DHFR) is one of the most common mechanisms of tumor resistance to methotrexate (MTX), an antimetabolite that is widely used in the treatment of a variety of human malignancies. The MTX-resistant phenotype generally occurs as a consequence of DHFR gene amplification which in turn is responsible for DHFR gene overexpression. We have designed antisense oligodeoxynucleotides (aODNs) against the DHFR mRNA and tested their in vitro effect on human leukemia CCRF-CEM/E cells, overexpressing the DHFR gene about 20-fold in comparison with the CCRF-CEM/S parental cell line. An aODN complementary to a region encompassing the AUG translation start (DHFR1) of DHFR mRNA and a mixture of two aODNs complementary to the 5' untranslated region (DHFR2+DHFR3) have been used. A DHFR1 scrambled-sequence ODN and a fully degenerated ODN were the controls. All ODNs had a phosphodiester backbone. DHFR1 and the relevant scrambled ODN were also capped with two phosphorothioate derivatives at both the 5' and 3' ends in order to increase ODN stability against serum nucleases. ODNs were vehiculated with a cationic lipid, N-[1-(dioleoyloxy)propyl]-N,N,N-trimethylammonium methyl sulfate (DOTAP), known to enhance ODN cell uptake and biological activity. The effects of ODNs on DHFR gene expression were studied after a 4 day treatment by measuring both DHFR mRNA levels, using a semi-quantitative reverse transcription polymerase chain reaction method, and DHFR protein levels by flow cytometry. A marked reduction in DHFR mRNA levels (79.7 and 74.2%, respectively) was observed with both DHFR1 and DHFR2+DHFR3 aODNs, associated with a lower decrease in DHFR enzyme (44.8 and 61%, respectively). aODN effects on MTX cytotoxicity in CCRF-CEM/E cells were also assessed. No marked enhancement of in vitro MTX cytotoxicity was observed following co-exposure of cells with aODNs and the tested concentrations of the antifol (0.05 and 0.5 microM), indicating that no substantial reversal of the MTX-resistant phenotype was induced by the study aODNs.

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.

Management of chronic lymphocytic leukaemia

Chronic lymphocytic leukaemia (CLL) is the most common form of adult leukaemia in Western countries. The diagnosis requires mature-appearing lymphocytes in the peripheral blood to >5 x 10(9)/L. The immunophenotype typically includes B cell antigens CD19, CD20 and CD23, low expression of surface immunoglobulin and CD5+, with other T cell antigens absent. Bone marrow biopsy, although not required for diagnosis, must show at least 30% lymphocytes. Cytogenetic abnormalities are frequent in patients with CLL, and may be associated with poor prognosis. Clinically, most patients are asymptomatic at presentation, with incidental lymphadenopathy and/or hepatosplenomegaly in the routine physical examination. Infections by opportunistic pathogens are the major cause of death. Aggressive transformation occurs in 10% of patients with CLL, most commonly prolymphocytic leukaemia (PLL) and Richter's syndrome. PLL de novo must be differentiated from PLL of an aggressive transformation. The incidences of autoimmune diseases and solid or haemopoietic secondary malignancies are increased in patients with CLL. Clinical stage is the strongest prognostic factor in CLL. There is no indication for early intervention. The current recommendation to start treatment includes disease-related symptoms, massive and/or progressive hepatosplenomegaly or lymphadenopathy, increasing bone marrow failure, autoimmune disease, and recurrent infections. Alkylating agents (e.g. chlorambucil) and nucleoside analogues (e.g. fludarabine) are the most active agents for CLL. Fludarabine induces higher response rates, but no improvement in overall survival has been observed. Fludarabine is the drug of choice for the majority of patients with CLL. Chlorambucil may be helpful for elderly patients with poor performance, and for patients who do not tolerate fludarabine. No drug combination is better than single agents. For patients refractory to initial treatment, referral to a clinical trial is the best choice. Other salvage therapy includes retreatment with the same initial agent (chlorambucil or fludarabine) if initial response was observed, or fludarabine for patients refractory to chlorambucil. Promising new approaches include cycle-active agents, nelarabine, biological therapy such as anti-CD52 monoclonal antibody, bone marrow transplantation, including the use of submyeloablative preparative regimens ('minitransplant') to induce graft-versus-leukaemia effect, and gene therapy. Prophylactic antibacterials and intravenous immunoglobulin should not be used routinely during supportive care. Epoetin may be helpful for patients who have anaemia without obvious cause. Assessment of response to therapy in CLL has been updated by the National Cancer Institute Working Group, and these guidelines are used worldwide for clinical trials.

Circumvention of Methotrexate Resistance in Childhood Leukemia Subtypes by Rationally Designed Antifolates

Cellular methotrexate (MTX) resistance may cause treatment failure in childhood common/preB-acute lymphoblastic leukemia (c/preB-ALL), T-lineage ALL (T-ALL), and acute myeloid leukemia (AML). The ex vivo potency of several antifolates (MTX, trimetrexate [TMQ], GW1843U89, multitargeted antifolate [MTA], Raltitrexed, and ZD9331) was studied via in situ inhibition of thymidylate synthase (TS). After short-term exposure, relapsed c/preB-ALL (rALL, n = 21), T-ALL (n = 22), and AML (n = 22) were 3-fold, 10-fold, and 6-fold less sensitive to MTX (P ≤ .01) compared with initial c/preB-ALL (n = 43). This difference in resistance was not observed for TMQ. Also for GW1843U89 and MTA, no resistance was observed in rALL and AML compared with c/preB-ALL. T-ALL compared with c/preB-ALL tended to be less resistant to GW1843U89 (3-fold) and MTA (6-fold) than to MTX (10-fold) (P= .06). Raltitrexed was more active against c/preB-ALL compared with the other subtypes. After 21 hours continuous incubation, T-ALL and AML samples were equally sensitive as c/preB-ALL to MTX, but rALL was 3-fold resistant to MTX compared with initial c/preB-ALL (P = .003). The resistance of rALL was circumvented by TMQ (1-fold; P = .03) and GW1843U89 (1.4-fold; P= .004). Novel antifolates, except MTA, displayed a more potent TS inhibition than MTX during continuous exposure. These results suggest that MTX resistance in AML and T-ALL can be circumvented by continuous exposure, and that novel antifolates should be explored further for MTX-resistant T-ALL, rALL, and AML cells.

Circumvention of Methotrexate Resistance in Childhood Leukemia Subtypes by Rationally Designed Antifolates

Cellular methotrexate (MTX) resistance may cause treatment failure in childhood common/preB-acute lymphoblastic leukemia (c/preB-ALL), T-lineage ALL (T-ALL), and acute myeloid leukemia (AML). The ex vivo potency of several antifolates (MTX, trimetrexate [TMQ], GW1843U89, multitargeted antifolate [MTA], Raltitrexed, and ZD9331) was studied via in situ inhibition of thymidylate synthase (TS). After short-term exposure, relapsed c/preB-ALL (rALL, n = 21), T-ALL (n = 22), and AML (n = 22) were 3-fold, 10-fold, and 6-fold less sensitive to MTX (P ≤ .01) compared with initial c/preB-ALL (n = 43). This difference in resistance was not observed for TMQ. Also for GW1843U89 and MTA, no resistance was observed in rALL and AML compared with c/preB-ALL. T-ALL compared with c/preB-ALL tended to be less resistant to GW1843U89 (3-fold) and MTA (6-fold) than to MTX (10-fold) (P= .06). Raltitrexed was more active against c/preB-ALL compared with the other subtypes. After 21 hours continuous incubation, T-ALL and AML samples were equally sensitive as c/preB-ALL to MTX, but rALL was 3-fold resistant to MTX compared with initial c/preB-ALL (P = .003). The resistance of rALL was circumvented by TMQ (1-fold; P = .03) and GW1843U89 (1.4-fold; P= .004). Novel antifolates, except MTA, displayed a more potent TS inhibition than MTX during continuous exposure. These results suggest that MTX resistance in AML and T-ALL can be circumvented by continuous exposure, and that novel antifolates should be explored further for MTX-resistant T-ALL, rALL, and AML cells.

Phase II study of 2-chlorodeoxyadenosine in combination with chlorambucil in previously untreated B-cell chronic lymphocytic leukemia

The objective was to determine the safety and efficacy of adding a maximally tolerated dose of 2-chlorodeoxyadenosine (2-CdA) to standard chlorambucil (CLB) therapy in previously untreated B-cell chronic lymphocytic leukemia (CLL). Thirty patients with CLL (median age, 64 years) received two courses of 2-CdA given intravenously (2 mg/m2 daily for 7 days) added to biweekly administration of CLB at 30 mg/m2 given orally. The diagnosis of CLL, treatment indications, and response criteria were according to the National Cancer Institute established guidelines. Sixteen patients (53%) had advanced-stage disease, and four (13%) had trisomy 12 abnormality. The overall remission rate was 80%, including 20% complete remission (CR), 30% nodular partial remission (nPR), and 30% partial remission (PR). Minimal residual disease was detected phenotypically in two of five patients with CR and in eight of nine with nPR. Overall, CR, nPR, and PR rates were not influenced significantly by the presence of cytogenetic abnormalities or advanced clinical stage. With a median follow-up of 33 months, 58% of patients who had a response had relapse. Median time to progression in all 30 patients was 30 months, and time to progression and progression-free survival were not significantly different for the different response groups, clinical stages, or cytogenetic groups. Severe neutropenia and thrombocytopenia occurred in 33% and 7% of patients, respectively. Only two patients had documented bacterial infections, and four had herpetic infections. Concurrent combination chemotherapy with abbreviated doses of 2-CdA and standard-dose CLB is feasible and safe in previously untreated CLL. Antitumor activity may be superior to that of CLB alone given in conventional doses. Whether a different schedule of combining these two agents would result in improved outcome is being investigated.

Molecular and cellular correlates of methotrexate response in childhood acute lymphoblastic leukemia

The improved outlook for children diagnosed today with acute lymphoblastic leukemia (ALL) over that 40 years ago is remarkable. With modern therapies and supportive care, complete remissions are achieved in up to 95% of patients and long-term disease-free survival rates approach 80%. Methotrexate is a key component in ALL consolidation and maintenance therapies and is administered intrathecally in the prophylaxis and treatment of central nervous system leukemia. Recent reports have significantly extended the results of preclinical studies of methotrexate response and resistance to patients with ALL. The application of new and sensitive molecular biology techniques makes it possible to study specific chromosomal and genetic alterations [t(12;21), hyperdiploidy, deletions or methylation of p15INK4B and p16INK4A] which potentially contribute to methotrexate response and resistance in childhood ALL. Studies of the relationships between genetic alterations and ALL progression, methotrexate pharmacology, and long term event-free-survivals may lead to the better identification of subgroups of patients who exhibit unique levels of sensitivity or resistance to chemotherapy including methotrexate. Further, by characterizing the roles of translocation-generated fusion genes (TEL-AML 1) and tumor suppressor genes (p15INK4B and p16INK4A) in treatment response, it may be possible to identify new and selective targets and/or treatment strategies for both children and adults with ALL who are refractory to current therapies.

Differential Methotrexate Resistance in Childhood T- Versus Common/PreB-Acute Lymphoblastic Leukemia Can Be Measured by an In Situ Thymidylate Synthase Inhibition Assay, But Not by the MTT Assay

Methotrexate (MTX) is not cytotoxic to patient-derived acute lymphoblastic leukemia (ALL) cells in total-cell-kill assays, such as the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, putatively due to the rescue effects of hypoxanthine and thymidine released from dying cells. This was mimicked by a diminished methotrexate (MTX) cytotoxicity for the cell lines HL60 and U937 in the presence of hypoxanthine, thymidine, or lysed ALL cells. However, enzymatic depletion or inhibition of nucleoside membrane transport did not result in MTX dose-dependent cytotoxicity in patient samples. Alternatively, a thymidylate synthase inhibition assay (TSIA), based on inhibition of the TS-catalyzed conversion of 3H-dUMP to dTMP and 3H2O, correlated with the MTT assay for antifolate sensitivity in four human leukemia cell lines with different modes of MTX resistance. For 86 ALL patient samples, TSI50 values after 21 hours exposure to MTX were not different between T- and c/preB-ALL (P = .46). After 3 hours incubation with MTX followed by an 18-hour drug-free period, T-ALL samples were 3.4-fold more resistant to MTX compared with c/preB-ALL samples (P = .001) reflecting the clinical differences in MTX sensitivity. TSI50 values correlated with MTX accumulation (r = −.58, P &lt; .001). In conclusion, the TSIA, but not the MTT assay, can measure dose-response curves for MTX in patient-derived ALL cells and showed relative MTX resistance in T-ALL compared with c/preB-ALL.

Differential Methotrexate Resistance in Childhood T- Versus Common/PreB-Acute Lymphoblastic Leukemia Can Be Measured by an In Situ Thymidylate Synthase Inhibition Assay, But Not by the MTT Assay

Methotrexate (MTX) is not cytotoxic to patient-derived acute lymphoblastic leukemia (ALL) cells in total-cell-kill assays, such as the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, putatively due to the rescue effects of hypoxanthine and thymidine released from dying cells. This was mimicked by a diminished methotrexate (MTX) cytotoxicity for the cell lines HL60 and U937 in the presence of hypoxanthine, thymidine, or lysed ALL cells. However, enzymatic depletion or inhibition of nucleoside membrane transport did not result in MTX dose-dependent cytotoxicity in patient samples. Alternatively, a thymidylate synthase inhibition assay (TSIA), based on inhibition of the TS-catalyzed conversion of 3H-dUMP to dTMP and 3H2O, correlated with the MTT assay for antifolate sensitivity in four human leukemia cell lines with different modes of MTX resistance. For 86 ALL patient samples, TSI50 values after 21 hours exposure to MTX were not different between T- and c/preB-ALL (P = .46). After 3 hours incubation with MTX followed by an 18-hour drug-free period, T-ALL samples were 3.4-fold more resistant to MTX compared with c/preB-ALL samples (P = .001) reflecting the clinical differences in MTX sensitivity. TSI50 values correlated with MTX accumulation (r = −.58, P < .001). In conclusion, the TSIA, but not the MTT assay, can measure dose-response curves for MTX in patient-derived ALL cells and showed relative MTX resistance in T-ALL compared with c/preB-ALL.