Drug resistance and methylation in myelodysplastic syndrome

Myelodysplastic syndrome is a clonal hematopoietic stem cell disorder that presents a poor survival for patients treated with standard therapies other than stem-cell transplantation. Multi-drug resistance (MDR) to simultaneous drugs used in chemotherapy is a major concern in the treatment of cancer and also in MDS. ATP-binding cassette (ABC) transporters are involved in the main mechanism that confers drug resistance to cells. Increased expression of drug resistance genes, such as MDR1, MRP1 and LRP, is involved with multi-drug resistance in MDS. The expression of these drug efflux transporters acts in synergy with other alterations, such as epigenetic events, increases in multidrug resistance in MDS. Methylation, the main epigenetic mechanism is widely explored in other hematological malignancies; however, in MDS, this mechanism is poorly investigated. Clinical trials evaluated or are under ongoing evaluation of drugs that abrogated ABC transporters action or reversed the abnormal methylation of some genes in MDS. In this report, we explore the data available in the field of drug resistance and methylation both in pediatric and adult MDS.     

The therapeutic potential of targeting ABC transporters to combat multi-drug resistance

Introduction: Most disseminated cancers remain fatal despite the availability of a variety of conventional and novel treatments including surgery, chemotherapy, radiotherapy, immunotherapy, and biologically targeted therapy. A major factor responsible for the failure of chemotherapy in the treatment of cancer is the development of multidrug resistance (MDR). The overexpression of various ABC transporters in cancer cells can efficiently remove the anticancer drug from the cell, thus causing the drug to lose its effect.

Areas covered: In this review, we summarised the ongoing research related to the mechanism, function, and regulation of ABC transporters. We integrated our current knowledge at different levels from molecular biology to clinical trials. We also discussed potential therapeutic strategies of targeting ABC transporters to reverse MDR in cancer cells.

Expert opinion: Involvement of various ABC transporters to cancer MDR lays the foundation for developing tailored therapies that can overcome MDR. An ideal MDR reversal agent should have broad-spectrum ABC-transporter inhibitory activity, be potent, have good pharmacokinetics, have no trans-stimulation effects, and have low or no toxicity. Alternatively, nanotechnology-based drug delivery systems containing both the cytotoxic drug and reversing agent may represent a useful approach to reversing MDR with minimal off-target toxicity.     

P450酶和转运体的DNA甲基化调控：提示药物反应的个体差异

细胞色素P450酶和转运体的基因多态性已被公认是导致临床上药物反应个体差异的重要原因,但个体间某些代谢酶、转运体的基因型和表型不一致的现象不能完全用基因多态性来解释。表观遗传药理学从表观遗传学的角度来研究遗传因素与药物治疗的关系,为药物反应的个体差异提供了新的解释。P450酶和转运体都受表观遗传因素控制。最常见表观遗传调控机制是DNA甲基化,它不会改变基因的遗传代码,而影响基因的表达。由于它对基因组序列的维护,DNA甲基化可用来解释某些基因多态性与表型的不一致现象。本综述总结了DNA甲基化表观遗传调控机制对P450酶和转运体的基因表达影响的最新进展。     

Epigenetic drug discovery for Alzheimer’s disease

Introduction: It is assumed that epigenetic modifications are reversible and could potentially be targeted by pharmacological and dietary interventions. Epigenetic drugs are gaining particular interest as potential candidates for the treatment of Alzheimer’s disease (AD).

Areas covered: This article covers relevant information from over 50 different epigenetic drugs including: DNA methyltransferase inhibitors; histone deacetylase inhibitors; histone acetyltransferase modulators; histone methyltransferase inhibitors; histone demethylase inhibitors; non-coding RNAs (microRNAs) and dietary regimes. The authors also review the pharmacoepigenomics and the pharmacogenomics of epigenetic drugs. The readers will gain insight into i) the classification of epigenetic drugs; ii) the mechanisms by which these drugs might be useful in AD; iii) the pharmacological properties of selected epigenetic drugs; iv) pharmacoepigenomics and the influence of epigenetic drugs on genes encoding CYP enzymes, transporters and nuclear receptors; and v) the genes associated with the pharmacogenomics of anti-dementia drugs.

Expert opinion: Epigenetic drugs reverse epigenetic changes in gene expression and might open future avenues in AD therapeutics. Unfortunately, clinical trials with this category of drugs are lacking in AD. The authors highlight the need for pharmacogenetic and pharmacoepigenetic studies to properly evaluate any efficacy and safety issues.     

Revisiting the ABCs of multidrug resistance in cancer chemotherapy

The adenosine tri-phosphate binding cassette (ABC) transporters are one of the largest transmembrane gene families in humans. The ABC transporters are present in a number of tissues, providing protection against xenobiotics and certain endogenous molecules. Unfortunately, their presence produces suboptimal chemotherapeutic outcomes in cancer patient tumor cells. It is well established that they actively efflux antineoplastic agents from cancer cells, producing the multidrug resistance (MDR) phenotype. The inadequate response to chemotherapy and subsequent poor prognosis in cancer patients can be in part the result of the clinical overexpression of ABC transporters. In fact, one of the targeted approaches for overcoming MDR in cancer cells is that directed towards blocking or inhibiting ABC transporters. Indeed, for almost three decades, research has been conducted to overcome MDR through pharmacological inhibition of ABC transporters with limited clinical success. Therefore, contemporary strategies to identify or to synthesize selective "resensitizers" of ABC transporters with limited nonspecific toxicity have been undertaken. Innovative approaches en route to understanding specific biochemical role of ABC transporters in MDR and tumorigenesis will prove essential to direct our knowledge towards more effective targeted therapies. This review briefly discusses the current knowledge regarding the clinical involvement of ABC transporters in MDR to antineoplastic drugs and highlights approaches undertaken so far to overcome ABC transporter-mediated MDR in cancer.     

Reversing agents for ATP-binding cassette (ABC) transporters: application in modulating multidrug resistance (MDR)

One of the main reasons for the failure in cancer chemotherapy is the existence of multidrug resistance (MDR) mechanisms. One form of MDR phenotype is contributed by a group of plasma membrane proteins that belong to a large superfamily of proteins called the ATP-binding cassette (ABC) transporters. There has been intense search for compounds, which can act at reversing MDR phenotype exhibited by ABC transporters such as P-glycoprotein (P-gp), multidrug-resistance protein (MRP) and breast cancer resistance protein (BCRP). Reversing agents can be designed to target MDR-associated ABC transporters at three levels - the protein, mRNA or DNA level. This review aims at describing, over-viewing and discussing currently known MDR reversing agents, which have been shown to act at either of the three levels against ABC transporters. Other potential agents and strategies, which can be used to reverse the MDR phenotype, are also discussed.     

Reversing epigenetic mechanisms of drug resistance in solid tumors using targeted microRNA delivery

ABSTRACT

Introduction: Tumor cells utilize many different mechanisms to desensitize themselves to the cytotoxic effects of drugs, but it has recently been recognized that alterations in epigenetic control of gene expression underly many of them. As master regulators of gene expression, microRNAs (miRNAs) present a promising therapeutic strategy for the reversal of epigenetic changes that lead to drug resistance phenotypes in tumor cells.

Areas covered: Effects of epigenetic changes on drug resistance in a variety of solid tumors are discussed. Specific miRNAs that are involved with the regulation of epigenetic machinery are highlighted. Further, we consider how delivery of miRNA or antagomirs may be utilized to resensitize drug-resistant tumor cells.

Expert opinion: Reversal of epigenetically controlled tumor drug resistance mechanisms via miRNA delivery presents a novel strategy for enhancing the efficacy of chemotherapeutics. Further, the ability to target delivery of miRNAs may provide the opportunity to go beyond reversal of resistance to hyper-sensitization of tumor cells to the cytotoxic effects of drugs. However, understanding of the role of miRNA in epigenetic regulation is still in its early stages and further research is critical for potential utility in improving therapeutic efficacy in cancer patients.     

Pharmacoepigenetics: an element of personalized therapy?

Introduction: Epigenetics is a rapidly growing field describing heritable alterations in gene expression that do not involve DNA sequence variations. Advances in epigenetics and epigenomics have influenced pharmacology, leading to the development of a new specialty, pharmacoepigenetics, the study of the epigenetic basis for the individual variation in drug response.

Areas covered: We present an overview of the major epigenetic mechanisms and their effects on the expression of drug metabolizing enzymes and drug transporters, as well as the epigenetic status of drug protein targets affecting therapy response. Recent advances in the development of pharmacoepigenetic biomarkers and epidrugs are also discussed.

Expert opinion: There is growing evidence that pharmacoepigenetics has the potential to become an important element of personalized medicine. Epigenetic modifications influence drug response, but they can also be modulated by drugs. Moreover, they can be monitored not only in the affected tissue, but also in body fluids. Nevertheless, there are very few examples of epigenetic biomarkers implemented in the clinical setting. Explanation of the interplay between genomic and epigenomic changes will contribute to the personalized medicine approach. Ultimately, both genetic biomarkers and epigenetic mechanisms should be taken into consideration in predicting drug response in the course of successful personalized therapy.     

Advancements in the delivery of epigenetic drugs

Introduction: Advancements in epigenetic treatments are not only coming from new drugs, but also from modifications or encapsulation of the existing drugs into different formulations leading to greater stability and enhanced delivery to the target site. The epigenome is highly regulated and complex; therefore, it is important that off-target effects of epigenetic drugs be minimized. The step from in vitro to in vivo treatment of these drugs often requires development of a method of effective delivery for clinical translation.

Areas covered: This review covers epigenetic mechanisms such as DNA methylation, chromatin remodeling and small-RNA-mediated gene regulation. There is a section in the review with examples of diseases where epigenetic alterations lead to impaired pathways, with an emphasis on cancer. Epigenetic drugs, their targets and clinical status are presented. Advantages of using a delivery method for epigenetic drugs as well as examples of current advancements and challenges are also discussed.

Expert opinion: Epigenetic drugs have the potential to be very effective therapy against a number of diseases, especially cancers and neurological disorders. As with many chemotherapeutics, undesired side effects need to be minimized. Finding a suitable delivery method means reducing side effects and achieving a higher therapeutic index. Each drug may require a unique delivery method exploiting the drug’s chemistry or other physical characteristic requiring interdisciplinary participation and would benefit from a better understanding of the mechanisms of action.     

The importance of drug transporters in human pluripotent stem cells and in early tissue differentiation

Introduction: Drug transporters are large transmembrane proteins which catalyse the movement of a wide variety of chemicals, including drugs as well as xeno- and endobiotics through cellular membranes. The major groups of these proteins include the ATP-binding cassette transporters which in eukaryotes work as ATP-fuelled drug ‘exporters’ and the Solute Carrier transporters, with various transport directions and mechanisms.

Areas covered: In this review, we discuss the key ATP-binding cassette and Solute Carrier drug transporters which have been reported to contribute to the function and/or protection of undifferentiated human stem cells and during tissue differentiation. We review the various techniques for studying transporter expression and function in stem cells, and the role of drug transporters in foetal and placental tissues is also discussed. We especially focus on the regulation of transporter expression by factors modulating cell differentiation properties and on the function of the transporters in adjustment to environmental challenges.

Expert opinion: The relatively new and as yet unexplored territory of transporters in stem cell biology may rapidly expand and bring important new information regarding the metabolic and epigenetic regulation of ‘stemness’ and the early differentiation properties. Drug transporters are clearly important protective and regulatory components in stem cells and differentiation.     

Independent regulation of apical and basolateral drug transporter expression and function in placental trophoblasts by cytokines, steroids, and growth factors.

Placental ATP binding cassette (ABC) transporters protect placental and fetal tissues by effluxing xenobiotics and endogenous metabolites. We have investigated the effects of cytokines and survival/growth factors, implicated in various placental pathologies, on ABC transporter expression and function in primary placental trophoblast cells. Treatment of primary term trophoblasts in vitro with tumor necrosis factor-alpha (TNF-alpha) or interleukin (IL)-1beta decreased mRNA and protein expression of apical transporters ABCB1/multidrug resistance gene product 1 (MDR1) and ABCG2/breast cancer resistance protein (BCRP) protein by 40 to 50% (P < 0.05). In contrast, IL-6 increased mRNA and protein expression of the basolateral transporter ABCB4/MDR3 (P < 0.05), whereas ABCC1/MRP1 expression was unaltered. Pretreatment of trophoblasts with TNF-alpha over 48 h resulted in significantly decreased BCRP efflux activity (increased mitoxantrone accumulation) with minimal changes in MDR1/3 activity. Epidermal growth factor (EGF) and insulin-like growth factor II, on the other hand, significantly increased BCRP expression at the mRNA and protein level (P < 0.05); EGF treatment also increased BCRP functional activity. Estradiol stimulated BCRP, MDR1, and MDR3 mRNA and protein expression by 40 to 60% and increased MDR1/3 functional activity (P < 0.05). Progesterone had modest positive effects on MRP1 mRNA and MDR1 protein expression (P < 0.05). In conclusion, this study shows that proinflammatory cytokines, sex steroids, and growth factors exert independent effects on expression of apical and basolateral placental ABC transporters in primary trophoblast. Such changes could alter placental drug disposition, increase fetal susceptibility to toxic xenobiotics, and impact on placental viability and function.     

DNA methyltransferase inhibitors in cancer: a chemical and therapeutic patent overview and selected clinical studies

Introduction: DNA methylation is an epigenetic modification that modulates gene expression without altering the DNA base sequence. It plays a crucial role in cancer by silencing tumor suppressor genes (TSG). The DNA methyltransferases (DNMT) are the enzymes that catalyze DNA methylation and they are interesting therapeutical targets since DNA methylation is reversible such that an aberrant hypermethylation of DNA can be reverted by inhibition of DNMTs. Today, two drugs are on the market for the treatment of myelodysplastic syndrome, azacitidine and decitabine.

Areas covered: Here, we present a review of the patents describing the chemistry and biological activities of novel DNMT inhibitors and discuss select clinical studies.

Expert opinion: DNMT inhibitors have shown efficacy in clinics. However, highly efficient and specific DNMT inhibitors have not yet been identified. Improving methods will certainly lead to the prediction of novel directly binding inhibitors in the future.     

Epigenetic modifications in multiple myeloma: recent advances on the role of DNA and histone methylation

Introduction: Multiple Myeloma (MM) is a clonal late B-cell disorder accounting for about 13% of hematological cancers and 1% of all neoplastic diseases. Recent studies on the molecular pathogenesis and biology of MM have highlighted a complex epigenomic landscape contributing to MM onset, prognosis and high individual variability.

Areas covered: We describe here the current knowledge on epigenetic events characterizing MM initiation and progression, focusing on the role of DNA and histone methylation and on the most promising epi-therapeutic approaches targeting the methylation pathway.

Expert opinion: Data published so far indicate that alterations of the epigenetic framework, which include aberrant global or gene/non-coding RNA specific methylation profiles, feature prominently in the pathobiology of MM. Indeed, the aberrant expression of components of the epigenetic machinery as well as the reversibility of the epigenetic marks make this pathway druggable, providing the basis for the design of epigenetic therapies against this still fatal malignancy.     

Advances in the molecular detection of ABC transporters involved in multidrug resistance in cancer

ATP-Binding Cassette (ABC) transporters are important mediators of multidrug resistance (MDR) in patients with cancer. Although their role in MDR has been extensively studied in vitro, their value in predicting response to chemotherapy has yet to be fully determined. Establishing a molecular diagnostic assay dedicated to the quantitation of ABC transporter genes is therefore critical to investigate their involvement in clinical MDR. In this article, we provide an overview of the methodologies that have been applied to analyze the mRNA expression levels of ABC transporters, by describing the technology, its pros and cons, and the experimental protocols that have been followed. We also discuss recent studies performed in our laboratory that assess the ability of the currently available high-throughput gene expression profiling platforms to discriminate between highly homologous genes. This work led to the conclusion that high-throughput TaqMan-based qRT-PCR platforms provide standardized clinical assays for the molecular detection of ABC transporters and other families of highly homologous MDR-linked genes encoding, for example, the uptake transporters (solute carriers-SLCs) and the phase I and II metabolism enzymes.     

Investigational ABC transporter inhibitors

INTRODUCTION: Multidrug resistance (MDR) is the main cause of failure in cancer therapy. One mechanism responsible for MDR is the active efflux of drugs by ATP-binding cassette (ABC) transporters. Several agents have been developed to block transporter-mediated drug efflux and some of these compounds have entered Phase II/III clinical testing. Evidence is also emerging of the role played by ABC transporters in cancer cell signalling that is likely to be important in disease progression and which is distinct from MDR. AREAS COVERED: This article reviews current literature to analyse the rationale for targeting ABC transporters in cancer. Preclinical and clinical results of ABC transporter inhibitors in early clinical trials, as single agents or in combination with other drugs, are described. The development of new strategies to target MDR and the emerging roles of ABC transporters in cancer signalling are discussed. EXPERT OPINION: The intense active search for safe and effective inhibitors of ABC transporters has led to some success in MDR reversal in preclinical studies. However, there has been little impact on clinical outcome. The discovery of novel, potent and nontoxic inhibitors as well as new treatment strategies is therefore needed.     

Emerging role of SETDB1 as a therapeutic target

Introduction: Epigenetic changes lead to aberrant gene expression in cancer. SETDB1, a histone lysine methyltransferase plays an important role in methylation and gene silencing. Aberrant histone methylation at H3K9 by SETDB1 promotes silencing of tumor suppressor genes and thus contributes to carcinogenesis. Recent studies indicate that SETDB1 is abnormally expressed in various human cancer conditions which contributed to enhanced tumor growth and metastasis. Hence, SETDB1 appears to be a promising epigenetic target for therapeutic intervention.

Areas covered: In this article, the structural features, localization and functions of SETDB1 are reviewed. Also, an overview of the role of SETDB1 in cancer and other disease mechanisms, the currently studied inhibitors for SETDB1 are mentioned.

Expert opinion: Silencing of tumor suppressor genes due to excessive trimethylation at H3K9 by amplified SETDB1 levels is found in various cancerous conditions. Since epigenetic changes are reversible, SETDB1 holds promise as an important therapeutic target for cancer. Therefore, a better understanding of the role of SETDB1 and its interaction with various proteins in cancer-related mechanisms along with therapeutic interventions specific for SETDB1 may improve targeted cancer therapy.