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Most patients who die of cancer have disseminated disease that has become resistant to multiple therapeutic modalities. Ample evidence suggests that the expression of ATP- binding cassette (ABC) transporters, especially the multidrug resistance protein 1 (MDR1, also known as P- glycoprotein or P-gp), which is encoded by ABC subfamily B member 1 ( ABCB1 ), can confer resistance to cytotoxic and targeted chemotherapy. However, the development of MDR1 as a therapeutic target has been unsuccessful. At the time of its discovery, appropriate tools for the characterization and clinical development of MDR1 as a therapeutic target were lacking. Thirty years after the initial cloning and characterization of MDR1 and the implication of two additional ABC transporters, the multidrug resistance associated protein 1 (MRP1; encoded by ABCC1 )), and ABCG2, in multidrug resistance, interest in investigating these transporters as therapeutic targets has waned. However, with the emergence of new data and advanced techniques, we propose to re- evaluate whether these transporters play a clinical role in multidrug resistance. With this Opinion article, we present recent evidence indicating that it is time to revisit the investigation into the role of ABC transporters in efficient drug delivery in various cancer types and at the blood–brain barrier.
Curative cancer therapy remains a major challenge particularly in cancers displaying multidrug resistance (MDR). The MDR phenotype is characterized by cross-resistance to a wide array of anticancer drugs harboring distinct structures and mechanisms of action. The multiple factors involved in mediating MDR may include host factors, tumor factors as well as tumor-host interactions. Among the host factors are genetic variants and drug-drug interactions. The plethora of tumor factors involves decreased drug uptake primarily via impaired influx transporters, increased drug efflux predominantly due to the overexpression of MDR efflux transporters of the ATP-binding cassette superfamily or due to drug efflux mediated by extracellular vesicles (EVs) or drug-loaded lysosomes undergoing exocytosis, deregulation of cell death mechanisms (i.e. anti-apoptotic modalities), enhanced DNA damage repair, epigenetic alterations and/or deregulation of microRNAs. The intratumor heterogeneity and dynamics, along with cancer stem cell plasticity, are important tumor factors. Among the tumor-host interactions are the role of the tumor microenvironment, selective pressure of various stressor conditions and agents, acidic pH and the intracellular transfer of traits mediated by EVs. The involvement of these diverse factors in MDR, highlights the need for precision medicine and real-time personalized treatments of individual cancer patients. In this review, written by a group of researchers from COST Action STRATAGEM "New diagnostic and therapeutic tools against multidrug resistant tumors", we aim to bring together these multidisciplinary and interdisciplinary features of MDR cancers. Importantly, it is becoming increasingly clear that deciphering the molecular mechanisms underlying anticancer drug resistance, will pave the way towards the development of novel precision medicine treatment modalities that are able to surmount distinct and well-defined mechanisms of anticancer drug resistance.
The extrusion of anticancer drugs by members of the ATP-binding cassette (ABC) transporter family is one of the most widely recognized mechanisms of multidrug resistance, and can be considered a hijacking of their normal roles in the transport of xenobiotics, metabolites and signaling molecules across cell membranes. While roles in cancer multidrug resistance have been clearly demonstrated for P-glycoprotein (P-gp), Breast Cancer Resistance Protein (BCRP) and Multidrug Resistance Protein 1 (MRP1), direct evidence for a role in multidrug resistance in vivo is lacking for other family members. A less well understood but emerging theme is the drug efflux-independent contributions of ABC transporters to cancer biology, supported by a growing body of evidence that their loss or inhibition impacts on the malignant potential of cancer cells in vitro and in vivo. As with multidrug resistance, these contributions likely represent a hijacking of normal ABC transporter functions in the efflux of endogenous metabolites and signaling molecules, however they may expand the clinical relevance of ABC transporters beyond P-gp, BCRP and MRP1. This review summarizes established and emerging roles for ABC transporters in cancer, with a focus on neuroblastoma and ovarian cancer, and considers approaches to validate and better understand these roles.
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