A common novel splice variant ofSLC22A1(OCT1)is associated with impaired responses to imatinib in patients with chronic myeloid leukaemia

Chronic myeloid leukaemia: Biology and therapy

Chronic myeloid leukaemia (CML) is caused by BCR::ABL1. Tyrosine kinase-inhibitors (TKIs) are the initial therapy. Several organizations have reported milestones to evaluate response to initial TKI-therapy and suggest when a change of TKI should be considered. Achieving treatment-free remission (TFR) is increasingly recognized as the optimal therapy goal. Which TKI is the best initial therapy for which persons and what depth and duration of molecular remission is needed to achieve TFR are controversial. In this review we discuss these issues and suggest future research directions.

Parallelized Acquisition of Orbitrap and Astral Analyzers Enables High-Throughput Quantitative Analysis

The growing trend toward high-throughput proteomics demands rapid liquid chromatography-mass spectrometry (LC-MS) cycles that limit the available time to gather the large numbers of MS/MS fragmentation spectra required for identification. Orbitrap analyzers scale performance with acquisition time and necessarily sacrifice sensitivity and resolving power to deliver higher acquisition rates. We developed a new mass spectrometer that combines a mass-resolving quadrupole, the Orbitrap, and the novel Asymmetric Track Lossless (Astral) analyzer. The new hybrid instrument enables faster acquisition of high-resolution accurate mass (HRAM) MS/MS spectra compared with state-of-the-art mass spectrometers. Accordingly, new proteomics methods were developed that leverage the strengths of each HRAM analyzer, whereby the Orbitrap analyzer performs full scans with a high dynamic range and resolution, synchronized with the Astral analyzer's acquisition of fast and sensitive HRAM MS/MS scans. Substantial improvements are demonstrated over previous methods using current state-of-the-art mass spectrometers.

Effects of splicing-regulatory polymorphisms in ABCC2, ABCG2, and ABCB1 on methotrexate exposure in Chinese children with acute lymphoblastic leukemia

PURPOSE

Adenosine triphosphate (ATP)-binding cassette (ABC) transporters play an important role in the response to methotrexate (MTX). In this study, we investigated the frequency distribution of three splicing-regulatory polymorphisms in ABC transporters (ABCC2 rs2273697 G>A, ABCG2 rs2231142 G>T, and ABCB1 rs1128503 A>G) and their effects on MTX concentrations and the clinical outcome in a Chinese pediatric population with acute lymphoblastic leukemia (ALL).

METHODS

A fluorescence polarization immunoassay was used to measure the serum MTX concentrations in 24 h (C_24h) and 42 h (C_42h). The Sequenom MassARRAY system was used for single-nucleotide polymorphism (SNP) genotyping.

RESULTS

The study population had significantly lower frequencies of ABCC2 rs2273697 A, ABCG2 rs2231142 G, and ABCB1 rs1128503 G than African and European samples (P < 0.05). The dose-normalized MTX concentrations after 24 h and the proportion of C_42h > 0.5 µmol/L were significantly lower in patients with the ABCG2 rs2231142 GG genotype than in patients with the GT or TT genotype (P = 0.01 and 0.006, respectively). No significant effects on MTX pharmacokinetics were observed for ABCC2 rs2273697 and ABCB1 rs1128503 polymorphisms. Bioinformatics analysis suggested that the three SNPs overlapped with the putative binding sites of several splicing factors.

CONCLUSION

In conclusion, our study confirmed the ethnicity-based differences in the distribution of the three investigated SNPs. The ABCG2 rs2231142 polymorphism exerted a significant effect on the level of MTX exposure. These findings may help explain the variability in MTX responses and optimize MTX treatment in pediatric patients with ALL.

OCT-1 Expression in Patients with Chronic Myeloid Leukemia: A Comparative Analysis with Respect to Response to Imatinib Treatment

The introduction of tyrosine kinase inhibitors (TKI) has resulted in a significant improvement in the treatment of CML patients. However, some CML patients are resistant to imatinib therapy, the initial TKI therapy in the CML. Therefore, it is important to find prognostic markers for resistance. The OCT-1 gene involved in imatinib uptake is also suspected to cause imatinib resistance. The aim of this study was to investigate the role of OCT-1 in imatinib resistance by comparing OCT-1 expression levels in imatinib resistant and imatinib sensitive patients with chronic myeloid leukemia (CML). This study was conducted on 101 patients with CML [imatinib sensitive (n = 51) and imatinib resistant (n = 50)] who were treated with imatinib. Gene expression analysis was done using QRT-PCR. The relative expression levels of OCT-1 were calculated using 2(-ΔΔCT) method. OCT1 mRNA expression levels were 0.149 (0.011-2.532) and 0.119 (0.008-2.868) in imatinib-sensitive group and imatinib-resistant group, respectively. OCT-1 expression levels were not significantly different in the imatinib-sensitive group when compared to imatinib resistant group (p > 0.05). OCT-1 expression was also similar in BCR-ABL1 kinase domain mutation positive and negative cases (p > 0.05). The imatinib-resistant group had a higher rate of hydroxyurea or interferon-alpha treatment prior to imatinib therapy and a lower rate for first-line imatinib as the only treatment than the imatinib-sensitive group (p = 0.002 and p = 0.002, respectively). According to the results of our study, OCT-1 does not have a biomarker feature in the evaluation of imatinib response. In addition, the study should be performed in larger patient groups.

Pharmacogenetics of Drugs Used in the Treatment of Cancers

Pharmacogenomics is based on the understanding of the individual differences in drug use, the response to drug therapy (efficacy and toxicity), and the mechanisms underlying variable drug responses. The identification of DNA variants which markedly contribute to inter-individual variations in drug responses would improve the efficacy of treatments and decrease the rate of the adverse side effects of drugs. This review focuses only on the impact of polymorphisms within drug-metabolizing enzymes on drug responses. Anticancer drugs usually have a very narrow therapeutic index; therefore, it is very important to use appropriate doses in order to achieve the maximum benefits without putting the patient at risk of life-threatening toxicities. However, the adjustment of the appropriate dose is not so easy, due to the inheritance of specific polymorphisms in the genes encoding the target proteins and drug-metabolizing enzymes. This review presents just a few examples of such polymorphisms and their impact on the response to therapy.

Influence of Telomere Length on the Achievement of Deep Molecular Response With Imatinib in Chronic Myeloid Leukemia Patients

Tyrosine kinase inhibitors have dramatically changed the outcome of chronic myeloid leukemia (CML), and nowadays, one of the main treatment goals is the achievement of deep molecular responses (DMRs), which can eventually lead to therapy discontinuation approaches. Few biological factors at diagnosis have been associated with this level of response. Telomere length (TL) in peripheral blood cells of patients with CML has been related to disease stage, response to therapy and disease progression, but little is known about its role on DMR. In this study, we analyzed if age-adjusted TL (referred as “delta-TL”) at diagnosis of chronic phase (CP)-CML might correlate with the achievement of DMR under first-line imatinib treatment. TL from 96 CP-CML patients had been retrospectively analyzed at diagnosis by monochrome multiplex quantitative PCR. We observed that patients with longer age-adjusted telomeres at diagnosis had higher probabilities to achieve DMR with imatinib than those with shortened telomeres (P = 0.035 when delta-TL was studied as a continuous variable and P = 0.047 when categorized by the median). Moreover, patients carrying long telomeres also achieved major molecular response significantly earlier (P = 0.012). This study provides proof of concept that TL has a role in CML biology and when measured at diagnosis of CP-CML could help to identify patients likely to achieve DMR to first-line imatinib treatment.

Impact of alternative splicing on mechanisms of resistance to anticancer drugs

A shared characteristic of many tumors is the lack of response to anticancer drugs. Multiple mechanisms of pharmacoresistance (MPRs) are involved in permitting cancer cells to overcome the effect of these agents. Pharmacoresistance can be primary (intrinsic) or secondary (acquired), i.e., triggered or enhanced in response to the treatment. Moreover, MPRs usually result in the lack of sensitivity to several agents, which accounts for diverse multidrug-resistant (MDR) phenotypes. MPRs are based on the dynamic expression of more than one hundred genes, constituting the so-called resistome. Alternative splicing (AS) during pre-mRNA maturation results in changes affecting proteins involved in the resistome. The resulting splicing variants (SVs) reduce the efficacy of anticancer drugs by lowering the intracellular levels of active agents, altering molecular targets, enhancing both DNA repair ability and defensive mechanism of tumors, inducing changes in the balance between pro-survival and pro-apoptosis signals, modifying interactions with the tumor microenvironment, and favoring malignant phenotypic transitions. Reasons accounting for cancer-associated aberrant splicing include mutations that create or disrupt splicing sites or splicing enhancers or silencers, abnormal expression of splicing factors, and impaired signaling pathways affecting the activity of the splicing machinery. Here we have reviewed the impact of AS on MPR in cancer cells.

Effects of a Common Eight Base Pairs Duplication at the Exon 7-Intron 7 Junction on Splicing, Expression, and Function of OCT1

Organic cation transporter 1 (OCT1, SLC22A1) is localized in the sinusoidal membrane of human hepatocytes and mediates hepatic uptake of weakly basic or cationic drugs and endogenous compounds. Common amino acid substitutions in OCT1 were associated with altered pharmacokinetics and efficacy of drugs like sumatriptan and fenoterol. Recently, the common splice variant rs35854239 has also been suggested to affect OCT1 function. rs35854239 represents an 8 bp duplication of the donor splice site at the exon 7-intron 7 junction. Here we quantified the extent to which this duplication affects OCT1 splicing and, as a consequence, the expression and the function of OCT1. We used pyrosequencing and deep RNA-sequencing to quantify the effect of rs35854239 on splicing after minigene expression of this variant in HepG2 and Huh7 cells and directly in human liver samples. Further, we analyzed the effects of rs35854239 on OCT1 mRNA expression in total, localization and activity of the resulting OCT1 protein, and on the pharmacokinetics of sumatriptan and fenoterol. The 8 bp duplication caused alternative splicing in 38% (deep RNA-sequencing) to 52% (pyrosequencing) of the minigene transcripts when analyzed in HepG2 and Huh7 cells. The alternatively spliced transcript encodes for a truncated protein that after transient transfection in HEK293 cells was not localized in the plasma membrane and was not able to transport the OCT1 model substrate ASP⁺. In human liver, however, the alternatively spliced OCT1 transcript was detectable only at very low levels (0.3% in heterozygous and 0.6% in homozygous carriers of the 8 bp duplication, deep RNA-sequencing). The 8 bp duplication was associated with a significant reduction of OCT1 expression in the human liver, but explained only 9% of the general variability in OCT1 expression and was not associated with significant changes in the pharmacokinetics of sumatriptan and fenoterol. Therefore, the rs35854239 variant only partially changes splicing, causing moderate changes in OCT1 expression and may be of only limited therapeutic relevance.

Plasma imatinib levels and ABCB1 polymorphism influences early molecular response and failure-free survival in newly diagnosed chronic phase CML patients

Achieving early molecular response (EMR) has been shown to be associated with better event free survival in patients with chronic phase chronic myeloid leukemia (CP-CML) on Imatinib therapy. We prospectively evaluated the factors influencing the 2-year failure free survival (FFS) and EMR to imatinib therapy in these patients including day29 plasma Imatinib levels, genetic variants and the gene expression of target genes in imatinib transport and biotransformation. Patients with low and intermediate Sokal score had better 2-year FFS compared to those with high Sokal Score (p = 0.02). Patients carrying ABCB1-C1236T variants had high day29 plasma imatinib levels (P = 0.005), increased EMR at 3 months (P = 0.044) and a better 2 year FFS (P = 0.003) when compared to those with wild type genotype. This translates to patients with lower ABCB1 mRNA expression having a significantly higher intracellular imatinib levels (P = 0.029). Higher day29 plasma imatinib levels was found to be strongly associated with patients achieving EMR at 3 months (P = 0.022), MMR at 12 months (P = 0.041) which essentially resulted in better 2-year FFS (p = 0.05). Also, patients who achieved EMR at 3 months, 6 months and MMR at 12 months had better FFS when compared to those who did not. This study suggests the incorporation of these variables in to the imatinib dosing algorithm as predictive biomarkers of response to Imatinib therapy.

Comprehensive in-silico analysis of damage associated SNPs in hOCT1 affecting Imatinib response in chronic myeloid leukemia

Non-synonymous single nucleotide polymorphisms (nsSNPs) in hOCT1 (encoded by SLC22A1 gene) are expected to affect Imatinib uptake in chronic myeloid leukemia (CML). In this study, sequence homology-based genetic analysis of a set of 270 coding SNPs identified 18 nsSNPs to be putatively damaging/deleterious using eight different algorithms. Subsequently, based on conservation of amino acid residues, stability analysis, posttranscriptional modifications, and solvent accessibility analysis, the possible structural-functional relationship was established for high-confidence nsSNPs. Furthermore, based on the modeling results, some dissimilarities of mutant type amino acids from wild-type amino acids such as size, charge, interaction and hydrophobicity were revealed. Three highly deleterious mutations consisting of P283L, G401S and R402G in SLC22A1 gene that may alter the protein structure, function and stability were identified. These results provide a filtered data to explore the effect of uncharacterized nsSNP and find their association with Imatinib resistance in CML.

The human organic cation transporter OCT1 and its role as a target for drug responses

The human organic cation uptake transporter OCT1, encoded by the SLC22A1 gene, is highly expressed in the liver and reported to possess a broad substrate specificity. OCT1 operates by facilitated diffusion and allows the entry of nutrients into cells. Recent findings revealed that OCT1 can mediate the uptake of drugs for treating various diseases such as cancers. The levels of OCT1 expression correlate with the responses towards many drugs and functionally defective OCT1 lead to drug resistance. It has been recently proposed that OCT1 should be amongst the crucial drug targets used for pharmacogenomic analyses. Several single nucleotide polymorphisms exist and are distributed across the entire OCT1 gene. While there are differences in the OCT1 gene polymorphisms between populations, there are at least five variants that warrant consideration in any genetic screen. To date, and despite two decades of research into OCT1 functional role, it still remains uncertain what are the define substrates for this uptake transporter, although studies from mice revealed that one of the substrates is vitamin B1. It is also unclear how OCT1 recognizes a broad array of ligands and whether this involves specific modifications and interactions with other proteins. In this review, we highlight the current findings related to OCT1 with the aim of propelling further studies on this key uptake transporter.

Somatic variants in epigenetic modifiers can predict failure of response to imatinib but not to second-generation tyrosine kinase inhibitors

There are no validated molecular biomarkers to identify newly-diagnosed individuals with chronic-phase chronic myeloid leukemia likely to respond poorly to imatinib and who might benefit from first-line treatment with a more potent second-generation tyrosine kinase inhibitor. Our inability to predict these ‘high-risk’ individuals reflects the poorly understood heterogeneity of the disease. To investigate the potential of genetic variants in epigenetic modifiers as biomarkers at diagnosis, we used Ion Torrent next-generation sequencing of 71 candidate genes for predicting response to tyrosine kinase inhibitors and probability of disease progression. A total of 124 subjects with newly-diagnosed chronic-phase chronic myeloid leukemia began with imatinib (n=62) or second-generation tyrosine kinase inhibitors (n=62) and were classified as responders or non-responders based on the BCRABL1 transcript levels within the first year and the European LeukemiaNet criteria for failure. Somatic variants affecting 21 genes (e.g. ASXL1, IKZF1, DNMT3A, CREBBP) were detected in 30% of subjects, most of whom were non-responders (41% non-responders, 18% responders to imatinib, 38% non-responders, 25% responders to second-generation tyrosine kinase inhibitors). The presence of variants predicted the rate of achieving a major molecular response, event-free survival, progression-free survival and chronic myeloid leukemia-related survival in the imatinib but not the second-generation tyrosine kinase inhibitors cohort. Rare germline variants had no prognostic significance irrespective of treatment while some pre-leukemia variants suggest a multi-step development of chronic myeloid leukemia. Our data suggest that identification of somatic variants at diagnosis facilitates stratification into imatinib responders/non-responders, thereby allowing earlier use of second-generation tyrosine kinase inhibitors, which, in turn, may overcome the negative impact of such variants on disease progression.

Imatinib Uptake into Cells is Not Mediated by Organic Cation Transporters OCT1, OCT2, or OCT3, But is Influenced by Extracellular pH

BACKGROUND

Cancer cells undergo genetic and environmental changes that can alter cellular disposition of drugs, notably by alterations of transmembrane drug transporters expression. Whether the influx organic cation transporter 1 (OCT1) encoded by the gene SLC221A1 is implicated in the cellular uptake of imatinib is still controversial. Besides, imatinib ionization state may be modulated by the hypoxic acidic surrounding extracellular microenvironment.

OBJECTIVE

To determine the functional contribution of OCTs and extracellular pH on imatinib cellular disposition.

METHODS

We measured imatinib uptake in two different models of selective OCTs drug transporter expression (transfected Xenopus laevis oocytes and OCT-expressing HEK293 human cells), incubated at pH 7.4 and 6, using specific mass spectrometry analysis.

RESULTS

Imatinib cellular uptake occurred independently of OCT1- OCT2- or OCT3-mediated drug transport at pH 7.4. Uptake of the OCTs substrate tetraethylammonium in oocytes remained intact at pH 6, while the accumulation of imatinib in oocytes was 10-fold lower than at pH 7.4, irrespectively of OCTs expressions. In OCT1- and OCT2-HEK cells at pH 6, imatinib accumulation was reduced by 2- 3-fold regardless of OCTs expressions. Since 99.5% of imatinib at pH6 is under the cationic form, the reduced cellular accumulation of imatinib at such pH may be explained by the lower amount of uncharged imatinib remaining for passive diffusion across cellular membrane.

CONCLUSION

Imatinib is not a substrate of OCTs 1-3 while the environmental pH modulates cellular disposition of imatinib. The observation that a slightly acidic extracellular pH influences imatinib cellular accumulation is important, considering the low extracellular pH reported in the hematopoietic leukemia/ cancer cell microenvironment.

Impact of SLC22A1 and CYP3A5 genotypes on imatinib response in chronic myeloid leukemia: A systematic review and meta-analysis

Contrasting results have been reported on the role of rs628031 and rs683369 polymorphisms of SLC22A1 and rs776746 of CYP3A5 on imatinib treatment response in patients with chronic myeloid leukemia (CML). In the present study, we conducted a systematic review and meta-analysis of published studies to estimate the impact of the above-mentioned gene variants on major molecular response (MMR) or complete cytogenetic response (CCyR) in imatinib-treated CML patients. We performed a comprehensive search through PubMed, Web of Knowledge, and Cochrane databases up to September 2017. The pooled analyses showed association between carriers of SLC22A1 rs628031A allele (GA + AA vs GG, OR: 0.58, 95% CI: 0.38-0.88, P = 0.011) or rs683369G allele (CG + GG vs CC, OR: 0.64, 95% CI: 0.42-0.96, P = 0.032) and a lower MMR rate. The combined analyses also revealed a correlation between the dominant (GG + AG vs AA, OR: 2.43, 95%CI: 1.12-5.27, P = 0.024) or the allelic model (G vs A, OR: 1.72, 95% CI: 1.09-2.72, P = 0.020) of CYP3A5 rs776746 with higher CCyR rates. The subsequent sensitivity analysis confirmed the statistical significance of CYP3A5 rs776746 among Asian CML patients (dominant model OR: 3.90; 95%CI: 2.47-6.14, P < 0.001; allelic model OR: 2.08; 95% CI: 1.47-2.95, P < 0.001). In conclusion, the present meta-analysis supports the association of SLC22A1 and CYP3A5 genotypes with clinical imatinib response rates of CML patients, nevertheless further large studies, particularly in Caucasians, are still warranted to provide conclusive evidences.

Fine Mapping and Functional Analysis Reveal a Role of SLC22A1 in Acylcarnitine Transport

Genome-wide association studies have identified a signal at the SLC22A1 locus for serum acylcarnitines, intermediate metabolites of mitochondrial oxidation whose plasma levels associate with metabolic diseases. Here, we refined the association signal, performed conditional analyses, and examined the linkage structure to find coding variants of SLC22A1 that mediate independent association signals at the locus. We also employed allele-specific expression analysis to find potential regulatory variants of SLC22A1 and demonstrated the effect of one variant on the splicing of SLC22A1. SLC22A1 encodes a hepatic plasma membrane transporter whose role in acylcarnitine physiology has not been described. By targeted metabolomics and isotope tracing experiments in loss- and gain-of-function cell and mouse models of Slc22a1, we uncovered a role of SLC22A1 in the efflux of acylcarnitines from the liver to the circulation. We further validated the impacts of human variants on SLC22A1-mediated acylcarnitine efflux in vitro, explaining their association with serum acylcarnitine levels. Our findings provide the detailed molecular mechanisms of the GWAS association for serum acylcarnitines at the SLC22A1 locus by functionally validating the impact of SLC22A1 and its variants on acylcarnitine transport.

PTCH1 is a reliable marker for predicting imatinib response in chronic myeloid leukemia patients in chronic phase

Patched homolog 1 gene (PTCH1) expression and the ratio of PTCH1 to Smoothened (SMO) expression have been proposed as prognostic markers of the response of chronic myeloid leukemia (CML) patients to imatinib. We compared these measurements in a realistic cohort of 101 patients with CML in chronic phase (CP) using a simplified qPCR method, and confirmed the prognostic power of each in a competing risk analysis. Gene expression levels were measured in peripheral blood samples at diagnosis. The PTCH1/SMO ratio did not improve PTCH1 prognostic power (area under the receiver operating characteristic curve 0.71 vs. 0.72). In order to reduce the number of genes to be analyzed, PTCH1 was the selected measurement. High and low PTCH1 expression groups had significantly different cumulative incidences of imatinib failure (IF), which was defined as discontinuation of imatinib due to lack of efficacy (5% vs. 25% at 4 years, P = 0.013), probabilities of achieving a major molecular response (81% vs. 53% at first year, P = 0.02), and proportions of early molecular failure (14% vs. 43%, P = 0.015). Every progression to an advanced phase (n = 3) and CML-related death (n = 2) occurred in the low PTCH1 group (P<0.001 for both comparisons). PTCH1 was an independent prognostic factor for the prediction of IF. We also validated previously published thresholds for PTCH1 expression. Therefore, we confirmed that PTCH1 expression can predict the imatinib response in CML patients in CP by applying a more rigorous statistical analysis. Thus, PTCH1 expression is a promising molecular marker for predicting the imatinib response in CML patients in CP.

Impact of Membrane Drug Transporters on Resistance to Small-Molecule Tyrosine Kinase Inhibitors

Small-molecule inhibitors of tyrosine kinases (TKIs) are the mainstay of treatment for many malignancies and represent novel treatment options for other diseases such as idiopathic pulmonary fibrosis. Twenty-five TKIs are currently FDA-approved and >130 are being evaluated in clinical trials. Increasing evidence suggests that drug exposure of TKIs may significantly contribute to drug resistance, independently from somatic variation of TKI target genes. Membrane transport proteins may limit the amount of TKI reaching the target cells. This review highlights current knowledge on the basic and clinical pharmacology of membrane transporters involved in TKI disposition and their contribution to drug efficacy and adverse drug effects. In addition to non-genetic and epigenetic factors, genetic variants, particularly rare ones, in transporter genes are promising novel factors to explain interindividual variability in the response to TKI therapy.

Drug Transporters and Na+/H+ Exchange Regulatory Factor PSD-95/Drosophila Discs Large/ZO-1 Proteins

Drug transporters govern the absorption, distribution, and elimination of pharmacologically active compounds. Members of the solute carrier and ATP binding-cassette drug transporter family mediate cellular drug uptake and efflux processes, thereby coordinating the vectorial movement of drugs across epithelial barriers. To exert their physiologic and pharmacological function in polarized epithelia, drug transporters must be targeted and stabilized to appropriate regions of the cell membrane (i.e., apical versus basolateral). Despite the critical importance of drug transporter membrane targeting, the mechanisms that underlie these processes are largely unknown. Several clinically significant drug transporters possess a recognition sequence that binds to PSD-95/Drosophila discs large/ZO-1 (PDZ) proteins. PDZ proteins, such as the Na(+)/H(+) exchanger regulatory factor (NHERF) family, act to stabilize and organize membrane targeting of multiple transmembrane proteins, including many clinically relevant drug transporters. These PDZ proteins are normally abundant at apical membranes, where they tether membrane-delimited transporters. NHERF expression is particularly high at the apical membrane in polarized tissue such as intestinal, hepatic, and renal epithelia, tissues important to drug disposition. Several recent studies have highlighted NHERF proteins as determinants of drug transporter function secondary to their role in controlling membrane abundance and localization. Mounting evidence strongly suggests that NHERF proteins may have clinically significant roles in pharmacokinetics and pharmacodynamics of several pharmacologically active compounds and may affect drug action in cancer and chronic kidney disease. For these reasons, NHERF proteins represent a novel class of post-translational mediators of drug transport and novel targets for new drug development.

Role of SLC22A1 polymorphic variants in drug disposition, therapeutic responses, and drug-drug interactions

The SCL22A1 gene encodes the broad selectivity transporter hOCT1. hOCT1 is expressed in most epithelial barriers thereby contributing to drug pharmacokinetics. It is also expressed in different drug target cells, including immune system cells and others. Thus, this membrane protein might also contribute to drug pharmacodynamics. Up to 1000 hOCT1 polymorphisms have been identified so far, although only a small fraction of those have been mechanistically studied. A paradigm in the field of drug transporter pharmacogenetics is the impact of hOCT1 gene variability on metformin clinical parameters, affecting area under the concentration-time curve, Cmax and responsiveness. However, hOCT1 also mediates the translocation of a variety of drugs used as anticancer, antiviral, anti-inflammatory, antiemetic agents as well as drugs used in the treatment of neurological diseases among. This review focuses exclusively on those drugs for which some pharmacogenetic data are available, and aims at highlighting the need for further clinical research in this area.

Pharmacogenetics of BCR/ABL Inhibitors in Chronic Myeloid Leukemia

Chronic myeloid leukemia was the first haematological neoplasia that benefited from a targeted therapy with imatinib nearly 15 years ago. Since then, several studies have investigated the role of genes, their variants (i.e., polymorphisms) and their encoded proteins in the pharmacokinetics and pharmacodynamics of BCR-ABL1 tyrosine kinase activity inhibitors (TKIs). Transmembrane transporters seem to influence in a significant manner the disposition of TKIs, especially that of imatinib at both cellular and systemic levels. In particular, members of the ATP-binding cassette (ABC) family (namely ABCB1 and ABCG2) together with solute carrier (SLC) transporters (i.e., SLC22A1) are responsible for the differences in drug pharmacokinetics. In the case of the newer TKIs, such as nilotinib and dasatinib, the substrate affinity of these drugs for transporters is variable but lower than that measured for imatinib. In this scenario, the investigation of genetic variants as possible predictive markers has led to some discordant results. With the partial exception of imatinib, these discrepancies seem to limit the application of discovered biomarkers in the clinical settings. In order to overcome these issues, larger prospective confirmative trials are needed.

Genetic variations of hOCT1 gene and CYP3A4/A5 genes and their association with imatinib response in Chronic Myeloid Leukemia

There is an increasing body of evidence demonstrating that mechanisms independent of BCR/ABL gene also contribute to imatinib resistance in Chronic Myeloid Leukemia (CML). It has been extensively reported that polymorphisms of the genes associated with imatinib metabolization and imatinib influx/efflux play an important role in the disease resistance. We investigated the impact of 12 genetic variants of the two genes, CYP3A4/A5 and the human cation transporter 1 gene (hOCT1) on the clinical outcome, in a cohort of 106 newly diagnosed CML patients. In the patient cohort investigated, only 6 variant alleles could be detected. The others were not present and could not be investigated. Two polymorphisms, CYP3A5*3 (rs776746)and hOCT1 M408V (rs628031), were significantly associated with the Complete Cytogenetic Response (CCyR) at 6 months and Major Molecular Response (MMR) at 12 months. The presence of favourable alleles at M408V and M420del in combination was associated with a MMR at 12 months. Functional polymorphisms of the genes associated with imatinib influx and metabolization may play a role in predicting primary response to imatinib and treatment outcome.

OCT1 and imatinib transport in CML: is it clinically relevant?

Imatinib is a highly effective therapy for chronic phase-chronic myeloid leukaemia (CP-CML) patients; however, responses to frontline imatinib are variable. The human organic cation transporter 1 (OCT1; SLC22A1) has been reported to be the main influx transporter involved in imatinib uptake into CML cells. Furthermore, variation in the efficiency of imatinib influx via OCT1 has been demonstrated to result in the inter-patient variation observed in primary response to imatinib. Although studies have questioned the role of OCT1 in imatinib influx, these have been largely performed in non-clinical settings. Measuring both OCT1 mRNA levels and the functional activity of OCT1 in primary leukaemic cells has been demonstrated to predict molecular response and outcome in imatinib-treated CP-CML patients in several independent studies. Here, the role of OCT1 and OCT1 genetic variants in imatinib uptake and response prediction is summarised and data generated from model systems assessing the role of OCT1 in imatinib transport is discussed.

Metformin, cancer and glucose metabolism

Metformin is the first-line treatment for type 2 diabetes. Results from several clinical studies have indicated that type 2 diabetic patients treated with metformin might have a lower cancer risk. One of the primary metabolic changes observed in malignant cell transformation is an increased catabolic glucose metabolism. In this context, once it has entered the cell through organic cation transporters, metformin decreases mitochondrial respiration chain activity and ATP production that, in turn, activates AMP-activated protein kinase, which regulates energy homeostasis. In addition, metformin reduces cellular energy availability and glucose entrapment by inhibiting hexokinase-II, which catalyses the glucose phosphorylation reaction. In this review, we discuss recent findings on molecular mechanisms that sustain the anticancer effect of metformin through regulation of glucose metabolism. In particular, we have focused on the emerging action of metformin on glycolysis in normal and cancer cells, with a drug discovery perspective.