Multidrug resistance in cancer chemotherapy

Resistance to chemotherapy is the single most important reason for treatment failure in cancer patients. Over the past 15 years, we have gained significant insight into one of the mechanisms responsible for this process: multidrug resistance (MDR). Far from being a phenomenon limited to the laboratory, multidrug resistance has been identified in a wide variety of newly diagnosed and recurrent human tumors. A number of compounds can block p-glycoprotein and overcome MDRin vitro andin vivo. Current strategies to block MDR are discussed in this review. Future research in this area will focus on the identification of more selective and potent MDR reversing agents and the development of entirely new approaches to overcoming multidrug resistance such as monoclonal antibodies, immunotoxins, and gene therapy.     

鲍曼不动杆菌抗生素多重耐药性: 耐药机制与感染治疗对策

多重以及泛耐药鲍曼不动杆菌导致的感染流行已成为全球关注的公共卫生问题.大多数细菌耐药基因与表型特征均充分反映在耐药鲍曼不动杆菌.鲍曼不动杆菌的外膜通透屏障与药物主动外排泵的协同作用使其呈现明显的天然多重耐药性,而其染色体可整合获取外源性由转座子/整合子可移动基因元件与多重耐药基因相联系的耐药岛区域或携带含有类似的整合子与多重耐药基因盒的质粒,这些基因结构导致该菌进化形成了对多类化学结构各异的临床常用抗生素的高度获得性耐药性,使其感染治疗的药物选择极其有限.尽管可考虑选用碳青霉烯类、含舒巴坦复合制剂、多黏菌素E或联合使用抗生素等治疗鲍曼不动杆菌感染,严重存在的多重耐药性等要求用药时更密切地观察药物的疗效,并参考药物敏感试验结果调整用药方案.虽然新的抗生素如替加环素等显示一定的体外抗鲍曼不动杆菌活性,但其临床疗效仍待继续研究.采取各种风险管理措施有效控制医院性细菌感染疾病包括合理使用各类抗生素是防止及减少鲍曼不动杆菌感染以及耐药性发生和传播的重要对策.     

Multidrug resistance in cancer: its mechanism and its modulation

One of the major problems related with the curative treatment of cancer patients is resistance against anticancer drugs. This resistance, which may occur from the beginning or is evident only later as an acquired phenomenon, is due to the action of drug transporters. These transmembrane proteins belong to the ATP-binding cassette (ABC) transporters which reduce bioavailability of drugs, but also determine the elimination of xenobiotics into bile, urine and feces. The present review summarizes recent knowledge in this area, highlighting the mechanism of action of these transporters, its clinical significance and its possible modulation. Novel approaches to overcome multidrug resistance include agents which inhibit or circumvent this efflux mechanism. For the latter category developments in nanomedicine may be of consequence. However, in spite of considerable progress in research regarding multidrug resistance, the phase of efficacious clinical use of this knowledge has not been reached yet.     

Targeting CSCs within the tumor microenvironment for cancer therapy: a potential role of mesenchymal stem cells

Introduction: Mesenchymal stem cells (MSCs) are one subgroup of adult stem cells and possess a proliferative potential and ability to differentiate into various ceells. Areas covered: Emerging evidence suggests that MSCs can reprogram toward cancer stem cells (CSCs), due to alterations of intrinsic and extrinsic microenvironments, leading to tumorigenesis. The CSC concept has fundamental clinical implications because of its involvement in cell migration/invasion, metastasis, and treatment resistance. Therefore, targeting CSCs provides a novel therapeutic strategy for cancer treatment. However, the origin of CSCs and its molecular connections are not fully understood. Emerging evidence suggests the existence of an inter-relationship between CSCs and epithelial-to-mesenchymal transition (EMT) phenotypic cells, in the context of inflammation and hypoxia, as well as the potential role of miRNAs. Expert opinion: We suggest that targeting CSC signatures along with EMT, inflammation, and hypoxia will provide a more effective therapeutic approach for the elimination of CSCs. To that end, curcumin especially its synthetic novel analog CDF have been shown to attenuate CSC characteristics along with the deregulation of multiple pathways and miRNAs, leading to the inhibition of human tumor growth in vivo, suggesting the potential role of CDF as an anti-tumor agent for the prevention/treatment of tumor progression.     

Targeting CSCs within the tumor microenvironment for cancer therapy: a potential role of mesenchymal stem cells

Introduction: Mesenchymal stem cells (MSCs) are one subgroup of adult stem cells and possess a proliferative potential and ability to differentiate into various ceells.

Areas covered: Emerging evidence suggests that MSCs can reprogram toward cancer stem cells (CSCs), due to alterations of intrinsic and extrinsic microenvironments, leading to tumorigenesis. The CSC concept has fundamental clinical implications because of its involvement in cell migration/invasion, metastasis, and treatment resistance. Therefore, targeting CSCs provides a novel therapeutic strategy for cancer treatment. However, the origin of CSCs and its molecular connections are not fully understood. Emerging evidence suggests the existence of an inter-relationship between CSCs and epithelial-to-mesenchymal transition (EMT) phenotypic cells, in the context of inflammation and hypoxia, as well as the potential role of miRNAs.

Expert opinion: We suggest that targeting CSC signatures along with EMT, inflammation, and hypoxia will provide a more effective therapeutic approach for the elimination of CSCs. To that end, curcumin especially its synthetic novel analog CDF have been shown to attenuate CSC characteristics along with the deregulation of multiple pathways and miRNAs, leading to the inhibition of human tumor growth in vivo, suggesting the potential role of CDF as an anti-tumor agent for the prevention/treatment of tumor progression.     

Plasmodium falciparum: Multidrug resistance

Malaria is the most lethal and debilitating disease caused by the protozoan parasite Plasmodium worldwide. The most severe forms of disease and the incidence rates of mortality are associated with P. falciparum infections. With the identification of disease source and symptoms, many chemical entities were developed naturally and synthetically for administration as a potential antimalarial drug. The major classes of approved antimalarial drugs that are governed as first‐line treatment in tropical and subtropical areas include quinolines, naphthoquinones, antifolates, 8‐aminoquinolines, and endoperoxides. However, the efficacy of antimalarial drugs has decreased due to ongoing multidrug resistance problem to current drugs. With increasing resistance to the current antimalarial artemisinin and its combination therapies, malaria prophylaxis has declined gradually. New‐generation antimalarial and novel drug target are required to check the incidence of malaria resistance. This review summarizes the emergence of multidrug resistance to known antimalarial and the development of new antimalarial to resolve drug resistance condition. Few essential proteins are also discussed that can be considered as novel drug target against malaria in future.     

Multidrug efflux pumps and their role in antibiotic and antiseptic resistance: a pharmacodynamic perspective

Introduction: Worrying levels of bacterial resistance have been reported worldwide involving the failure of many available antibiotic treatments. Multidrug resistance (MDR) in Gram-negative bacteria is often ascribed to the presence of multiple and different resistance mechanisms in the same strain. RND efflux pumps play a major role and are an attractive target to discover new antibacterial drugs.

Areas covered: This review discusses the prevalence of efflux pumps, their overexpression in clinical scenarios, their polyselectivity, their effect on the intracellular concentrations of various antibiotics associated with the alteration of the membrane permeability and their involvement in pathogenicity are discussed.

Expert opinion: Efflux pumps are new targets for the development of adjuvant in antibiotic treatments by of efflux pump inhibition. They may allow us to rejuvenate old antibiotics acting on their concentration inside the bacteria and thus potentiating their activity while blocking the release of virulence factors. It is a pharmacodynamic challenge to finalize new combined therapy.     

Multidrug resistance in epilepsy: a pharmacogenomic update

Multidrug resistance is one of the most serious problems in the treatment of epilepsy and is likely to have a complex genetic and environmental basis. Various experimental data support the hypothesis that overexpression of antiepileptic drug transporters may be important. However, key questions concerning their functionality remain unanswered. The first study reporting a positive association – between genetic variation in a putative antiepileptic drug transporter (P-glycoprotein, encoded by ABCB1) and multidrug resistant epilepsy was published in 2003. Since then, several other association genetics studies have sought to confirm this result, but, taken overall, do not support a major role for this polymorphism. Lessons learnt from the ABCB1 studies can help guide future association genetics studies, both for multidrug resistance in epilepsy, and for other epilepsy phenotypes.     

Multidrug resistance in pediatric oncology

Summary Cancer survival among children and adolescents has improved markedly due to evolution of multimodal treatment that incorporates combination chemotherapy, radiation therapy and/or surgery. However, 20–30% of children with malignancies will succumb to their disease or complications associated with their disease or treatment. A major limiting factor to improvement in survival among these patients is the occurrence of intrinsic and/or acquired resistance to our treatment interventions, chemotherapy and radiotherapy. Among these mechanisms, multidrug resistance, the focus of this review, is a well-documented phenomenon whose biochemistry, pharmacology and molecular biology has been extensively studied. A role for multidrug resistance in chemoresistance and therapeutic failure in childhood malignancies is suggested by the observation of clinical resistance to treatment regimes containing agents that are known substrates of multidrug resistance mechanisms. With the current results from studies in rhabdomyosarcoma, neuroblastoma, osteosarcoma, Ewing's sarcoma, leukemia and retinoblastoma, the role of multidrug resistance is still unclear. Earlier studies attempted to define a role for P-glycoprotein-mediated multidrug resistance; however, a limited number of reports suggest that the multidrug-associated resistance protein may play an active role in neuroblastoma. Further studies will be necessary using standardized and uniform approaches for the analyses of these mechanisms. Clinical trials directed toward reversal of multidrug resistance are premature since the exact role of P-glycoprotein is controversial in pediatric malignancies, the role of other mechanisms of multidrug resistance must be assessed and selective inhibitors of multidrug resistance have yet to be developed. Address for offprints: John F. Kuttesch, Jr., Division of Pediatrics, The University of Texas M.D., Anderson Cancer Center, 1515 Holcombe Blvd., Houston, TX 77030, USA     

Multidrug resistance in Mycobacterium tuberculosis

All but one of the four major mechanisms of resistance to antimicrobial agents-inactivation of the drug, altered cell wall permeability or drug efflux, drug titration due to target overproduction, and alteration of the target by mutation-appear to be employed by Mycobacterium tuberculosis in its resistance to components of short course chemotherapy regimens. To date no enzymes capable of inactivating any of the frontline drugs have been found. The most common resistance mechanism is alteration of the target leading to inadequate drug binding, or drug activation, as a result of mutations in chromosomal genes. This occurs in the case of the specific antituberculous drugs isoniazid, pyrazinamide and ethionamide as well as in resistance to the broad-spectrum antibiotics, rifampicin, streptomycin and the fluoroquinolones. Overproduction of the drug target also appears to lead to resistance to isoniazid and ethionamide whereas changes in permeability, or the activation of antibiotic-efflux systems, may contribute to the low-level resistance of the tubercle bacillus to streptomycin and fluoroquinolones.     

Multidrug resistance in epilepsy: a pharmacogenomic update

Multidrug resistance is one of the most serious problems in the treatment of epilepsy and is likely to have a complex genetic and environmental basis. Various experimental data support the hypothesis that overexpression of antiepileptic drug transporters may be important. However, key questions concerning their functionality remain unanswered. The first study reporting a positive association--between genetic variation in a putative antiepileptic drug transporter (P-glycoprotein, encoded by ABCB1) and multidrug resistant epilepsy was published in 2003. Since then, several other association genetics studies have sought to confirm this result, but, taken overall, do not support a major role for this polymorphism. Lessons learnt from the ABCB1 studies can help guide future association genetics studies, both for multidrug resistance in epilepsy, and for other epilepsy phenotypes.     

Multidrug resistance (MDR) in cancer: Mechanisms, reversal using modulators of MDR and the role of MDR modulators in influencing the pharmacokinetics of anticancer drugs

In recent years, there has been an increased understanding of P-glycoprotein (P-GP)-mediated pharmacokinetic interactions. In addition, its role in modifying the bioavailability of orally administered drugs via induction or inhibition has been also been demonstrated in various studies. This overview presents a background on some of the commonly documented mechanisms of multidrug resistance (MDR), reversal using modulators of MDR, followed by a discussion on the functional aspects of P-GP in the context of the pharmacokinetic interactions when multiple agents are coadministered. While adverse pharmacokinetic interactions have been documented with first and second generation MDR modulators, certain newer agents of the third generation class of compounds have been less susceptible in eliciting pharmacokinetic interactions. Although the review focuses on P-GP and the pharmacology of MDR reversal using MDR modulators, relevance of these drug transport proteins in the context of pharmacokinetic implications (drug absorption, distribution, clearance, and interactions) will also be discussed.     

Pancreatic cancer: the evolving role of systemic therapy

Pancreatic adenocarcinoma is the fourth leading cause of cancer mortality in the US. The outcome for patients with pancreatic cancer has not essentially altered over the past few decades. Several new drugs with activity against pancreatic cancer have recently been identified for use in palliative settings. Of these, gemcitabine is the most widely used agent against the disease, but its benefit is very modest. Pilot Phase II studies combining gemcitabine with 5-fluorouracil (5-FU), irinotecan, docetaxel or cisplatin show improved outcomes that need to be confirmed in randomised studies. Concurrent administration of gemcitabine and external beam radiation therapy (EBRT) for locally advanced pancreatic cancer is feasible and is currently undergoing efficacy evaluations. Current research in pancreatic cancer involves newer dosing schedules of gemcitabine, and combinations of gemcitabine with novel agents. Ultimately, better understanding of the molecular biology of pancreatic neoplasia will identify potential cellular targets for future development of new agents for pancreatic cancer.     

Multidrug resistance proteins (MRPs, ABCCs): importance for pathophysiology and drug therapy

The nine multidrug resistance proteins (MRPs) represent the major part of the 12 members of the MRP/CFTR subfamily belonging to the 48 human ATP-binding cassette (ABC) transporters. Cloning, functional characterization, and cellular localization of most MRP subfamily members have identified them as ATP-dependent efflux pumps with a broad substrate specificity for the transport of endogenous and xenobiotic anionic substances localized in cellular plasma membranes. Prototypic substrates include glutathione conjugates such as leukotriene C(4) for MRP1, MRP2, and MRP4, bilirubin glucuronosides for MRP2 and MRP3, and cyclic AMP and cyclic GMP for MRP4, MRP5, and MRP8. Reduced glutathione (GSH), present in living cells at millimolar concentrations, modifies the substrate specificities of several MRPs, as exemplified by the cotransport of vincristine with GSH by MRP1, or by the cotransport of GSH with bile acids or of GSH with leukotriene B(4) by MRP4.The role of MRP subfamily members in pathophysiology may be illustrated by the MRP-mediated release of proinflammatory and immunomodulatory mediators such as leukotrienes and prostanoids. Pathophysiological consequences of many genetic variants leading to a lack of functional MRP protein in the plasma membrane are observed in the hereditary MRP2 deficiency associated with conjugated hyperbilirubinemia in Dubin-Johnson syndrome, in pseudoxanthoma elasticum due to mutations in the MRP6 (ABCC6) gene, or in the type of human earwax and osmidrosis determined by single nucleotide polymorphisms in the MRP8 (ABCC8) gene. The hepatobiliary and renal elimination of many drugs and their metabolites is mediated by MRP2 in the hepatocyte canalicular membrane and by MRP4 as well as MRP2 in the luminal membrane of kidney proximal tubules. Therefore, inhibition of these efflux pumps affects pharmacokinetics, unless compensated by other ATP-dependent efflux pumps with overlapping substrate specificities.     

Multidrug resistance: retrospect and prospects in anti-cancer drug treatment

Conventional cancer chemotherapy is seriously limited by the multidrug resistance (MDR) commonly exhibited by tumour cells. One mechanism by which a living cell can achieve multiple resistances is via the active efflux of a broad range of anticancer drugs through the cellular membrane by MDR proteins. Such drugs are exported in both ATP-dependent and -independent manners, and can occur despite considerable concentration gradients. To the ATP-dependent group belongs the ATP-binding cassette (ABC) transporter family, which includes P-gp, MRP, BCRP, etc. Another protein related to MDR, though not belonging to the ABC transporter family, is lung resistance-related protein (LRP). All of these proteins are involved in diverse physiological processes, and are responsible for the uptake and efflux of a multitude of substances from cancer cells. Many inhibitors of MDR transporters have been identified over the years. Firstly, MDR drugs were not specifically developed for inhibiting MDR; in fact, they had other pharmacological properties, as well as a relatively low affinity for MDR transporters. They included compounds of diverse structure and function, such as verapamil and cyclosporine, and caused side effects. Secondly, the new drugs were more inhibitor-specific, in terms of MDR transport, and were designed to reduce such side effects (e.g., R-verapamil, dexniguldipine, etc.). Unfortunately, they displayed poor response in clinical studies. Recently, new compounds obtained from drug development programs conducted by the pharmaceutical industry are characterized by a high affinity to MDR transporters and are efficient at nanomolar concentrations. Some of these compounds (e.g., MS-209) are currently under clinical trials for specific forms of advanced cancers. We aim to provide an overview of the properties associated with those mammalian MDR transporters known to mediate significant transport of relevant drugs in cancer treatments. We also summarize recent advances concerning resistance to cancer drug therapies with respect to the function and overexpression of ABC and LRP multidrug transporters.     

Glioma stem cell maintenance: the role of the microenvironment

Glioblastomas are highly lethal cancers for which conventional therapies provide only palliation. The cellular heterogeneity of glioblastomas is manifest in genetic and epigenetic variation with both stochastic and hierarchical models informing cellular phenotypes. At the apex of the hierarchy is a self-renewing, tumorigenic, cancer stem cell (CSC). The significance of CSCs is underscored by their resistance to cytotoxic therapies, invasive potential, and promotion of angiogenesis. Thus, targeting CSCs may offer therapeutic benefit and sensitize tumors to conventional treatment, demanding elucidation of CSC regulation. Attention has been paid to intrinsic cellular systems in CSCs, but recognition of extrinsic factors is evolving. Glioma stem cells (GSCs) are enriched in functional niches--prominently the perivascular space and hypoxic regions. These niches provide instructive cues to maintain GSCs and induce cellular plasticity towards a stem-like phenotype. GSC-maintaining niches may therefore offer novel therapeutic targets but also signal additional complexity with perhaps different pools of GSCs governed by different molecular mechanisms that must be targeted for tumor control.