Inhibition of the multidrug resistance P-glycoprotein: time for a change of strategy?

Molecular Mimics of Classic P-Glycoprotein Inhibitors as Multidrug Resistance Suppressors and Their Synergistic Effect on Paclitaxel

P-glycoprotein (Pgp) is a membrane bound efflux pump spread in a variety of tumor cells and considered as a main component of multidrug resistance (MDR) to chemotherapies. In this work, three groups of compounds (imidazolone, oxazolone and vinyl dipeptide derivatives) were synthesized aiming to develop a molecular framework that effectively suppresses MDR. When tested for their influence on Pgp activity, four compounds coded Cur1-01, Cur1-12V, Curox-1 and Curox-3 significantly decreased remaining ATP concentration indicating Pgp substrate site blocking. On the other hand, Cur-3 and Cur-10 significantly increased remaining ATP concentration, which is indicative of Pgp ATPase inhibition. The cytotoxicity of synthesized compounds was examined against Pgp expressing/highly resistant colorectal cancer cell lines (LS-174T). Compounds Cur-1 and Cur-3 showed considerable cytotoxicity with IC50 values of 7.6 and 8.9 μM, respectively. Equitoxic combination (at IC50 concentrations) of PTX and Cur-3 greatly diminished resistant cell clone from 45.7% to 2.5%, albeit with some drop in potency from IC50 of 7.9 nM to IC50 of 23.8 nM. On the other hand, combination of PTX and the non-cytotoxic Cur1-12V (10 μM) significantly decreased the IC50 of PTX to 3.8 nM as well as the resistant fraction to 16.2%. The combination test was confirmed using the same protocol but on another resistant CRC cell line (HCT-116) as we obtained similar results. Both Cur-3 and Cur1-12V (10 μM) significantly increased the cellular entrapment of Pgp probe (doxorubicin) elevating its intracellular concentration from 1.9 pmole/cell to 3.0 and 2.9 pmole/cell, respectively.

Chemotherapy curable malignancies and cancer stem cells: a biological review and hypothesis

Background

Cytotoxic chemotherapy brings routine cures to only a small select group of metastatic malignancies comprising gestational trophoblast tumours, germ cell tumours, acute leukemia, Hodgkin’s disease, high grade lymphomas and some of the rare childhood malignancies.

We have previously postulated that the extreme sensitivity to chemotherapy for these malignancies is linked to the on-going high levels of apoptotic sensitivity that is naturally linked with the unique genetic events of nuclear fusion, meiosis, VDJ recombination, somatic hypermutation, and gastrulation that have occurred within the cells of origin of these malignancies.

In this review we will examine the cancer stem cell/cancer cell relationship of each of the chemotherapy curable malignancies and how this relationship impacts on the resultant biology and pro-apoptotic sensitivity of the varying cancer cell types.

Discussion

In contrast to the common epithelial cancers, in each of the chemotherapy curable malignancies there are no conventional hierarchical cancer stem cells. However cells with cancer stem like qualities can arise stochastically from within the general tumour cell population. These stochastic stem cells acquire a degree of resistance to DNA damaging agents but also retain much of the key characteristics of the cancer cells from which they develop. We would argue that the balance between the acquired resistance of the stochastic cancer stem cell and the inherent chemotherapy sensitivity of parent tumour cell determines the overall chemotherapy curability of each diagnosis.

Summary

The cancer stem cells in the chemotherapy curable malignancies appear to have two key biological differences from those of the more common chemotherapy incurable malignancies. The first difference is that the conventional hierarchical pattern of cancer stem cells is absent in each of the chemotherapy curable malignancies.

The other key difference, we suggest, is that the stochastic stem cells in the chemotherapy curable malignancies take on a significant aspect of the biological characteristics of their parent cancer cells. This action includes for the chemotherapy curable malignancies the heightened pro-apoptotic sensitivity linked to their respective associated unique genetic events.

For the chemotherapy curable malignancies the combination of the relationship of their cancer stem cells combined with the extreme inherent sensitivity to induction of apoptosis from DNA damaging agents plays a key role in determining their overall curability with chemotherapy.

Machine learning-, rule- and pharmacophore-based classification on the inhibition of P-glycoprotein and NorA

The efflux pumps P-glycoprotein (P-gp) in humans and NorA in Staphylococcus aureus are of great interest for medicinal chemists because of their important roles in multidrug resistance (MDR). The high polyspecificity as well as the unavailability of high-resolution X-ray crystal structures of these transmembrane proteins lead us to combining ligand-based approaches, which in the case of this study were machine learning, perceptual mapping and pharmacophore modelling. For P-gp inhibitory activity, individual models were developed using different machine learning algorithms and subsequently combined into an ensemble model which showed a good discrimination between inhibitors and noninhibitors (acctrain-diverse = 84%; accinternal-test = 92% and accexternal-test = 100%). For ligand promiscuity between P-gp and NorA, perceptual maps and pharmacophore models were generated for the detection of rules and features. Based on these in silico tools, hit compounds for reversing MDR were discovered from the in-house and DrugBank databases through virtual screening in an attempt to restore drug sensitivity in cancer cells and bacteria.

Self-assembling nanoparticles encapsulating zoledronic acid revert multidrug resistance in cancer cells

The overexpression of ATP binding cassette (ABC) transporters makes tumor cells simultaneously resistant to several cytotoxic drugs. Impairing the energy metabolism of multidrug resistant (MDR) cells is a promising chemosensitizing strategy, but many metabolic modifiers are too toxic in vivo. We previously observed that the aminobisphosphonate zoledronic acid inhibits the activity of hypoxia inducible factor-1α (HIF-1α), a master regulator of cancer cell metabolism. Free zoledronic acid, however, reaches low intratumor concentration. We synthesized nanoparticle formulations of the aminobisphosphonate that allow a higher intratumor delivery of the drug. We investigated whether they are effective metabolic modifiers and chemosensitizing agents against human MDR cancer cells in vitro and in vivo.

At not toxic dosage, nanoparticles carrying zoledronic acid chemosensitized MDR cells to a broad spectrum of cytotoxic drugs, independently of the type of ABC transporters expressed. The nanoparticles inhibited the isoprenoid synthesis and the Ras/ERK1/2-driven activation of HIF-1α, decreased the transcription and activity of glycolytic enzymes, the glucose flux through the glycolysis and tricarboxylic acid cycle, the electron flux through the mitochondrial respiratory chain, the synthesis of ATP. So doing, they lowered the ATP-dependent activity of ABC transporters, increasing the chemotherapy efficacy in vitro and in vivo. These effects were more pronounced in MDR cells than in chemosensitive ones and were due to the inhibition of farnesyl pyrophosphate synthase (FPPS), as demonstrated in FPPS-silenced tumors.

Our work proposes nanoparticle formulations of zoledronic acid as the first not toxic metabolic modifiers, effective against MDR tumors.

Imaging of Metabolic Status in 3D Cultures with an Improved AMPK FRET Biosensor for FLIM

We describe an approach to non-invasively map spatiotemporal biochemical and physiological changes in 3D cell culture using Forster Resonance Energy Transfer (FRET) biosensors expressed in tumour spheroids. In particular, we present an improved Adenosine Monophosphate (AMP) Activated Protein Kinase (AMPK) FRET biosensor, mTurquoise2 AMPK Activity Reporter (T2AMPKAR), for fluorescence lifetime imaging (FLIM) readouts that we have evaluated in 2D and 3D cultures. Our results in 2D cell culture indicate that replacing the FRET donor, enhanced Cyan Fluorescent Protein (ECFP), in the original FRET biosensor, AMPK activity reporter (AMPKAR), with mTurquoise2 (mTq2FP), increases the dynamic range of the response to activation of AMPK, as demonstrated using the direct AMPK activator, 991. We demonstrated 3D FLIM of this T2AMPKAR FRET biosensor expressed in tumour spheroids using two-photon excitation.

Jatrophane diterpenes and cancer multidrug resistance - ABCB1 efflux modulation and selective cell death induction

BACKGROUND

Modulation of P-glycoprotein (ABCB1) and evaluation of the collateral sensitivity effect are among the most promising approaches to overcome multidrug resistance (MDR) in cancer. In a previous study, two rare 12,17-cyclojatrophanes (1-2) and other novel jatrophanes (3-4), isolated from Euphorbia welwitschii, were screened for collateral sensitivity effect. Herein, the isolation of another jatrophane (5) is presented, being the broader goal of this work to investigate the role of euphowelwitschines A (1) and B (2), welwitschene (3), epoxywelwitschene (4) and esulatin M (5) as ABCB1 modulators and/or collateral sensitivity agents.

METHODS

Compounds 1-5 were evaluated for ABCB1 modulation ability through combination of transport and chemosensitivity assays, using a mouse T-lymphoma MDR1-transfected cell model. Moreover, the nature of interaction of compound 4 with ABCB1 was studied, using an ATPase assay. The MDR-selective antiproliferative activity of compound 5 was evaluated against gastric (EPG85-257) and pancreatic (EPP85-181) human cancer cells and their drug-selected counterparts (EPG85-257RDB, EPG85-257RNOV, EPP85-181RDB, EPP85-181RNOV). The drug induced cell death was investigated for compounds 4 and 5, using the annexin V/PI staining and the active caspase-3 assay.

RESULTS

The jatrophanes 1-5 were able to modulate the efflux activity of ABCB1, and at 2µM, 3-5 maintained the strong modulator profile. Structure activity results indicated that high conformational flexibility of the twelve-membered ring of compounds 3-5 favored ABCB1 modulation, in contrast to the tetracyclic scaffold of compounds 1 and 2. The effects of epoxywelwitschene (4) on the ATPase activity of ABCB1 showed it to interact with the transporter and to be able to reduce the transport of a second subtrate. Drug combination experiments also corroborated the anti-MDR potential of these diterpenes due to their synergistic interaction with doxorubicin (combination index <0.7). Esulatin M (5) showed a strong MDR-selective antiproliferative activity against EPG85-257RDB and EPP85-181RDB cells, with IC50 of 1.8 and 4.8 µM, respectively. Compounds 4 and 5 induced apoptosis via caspase-3 activation. A significant discrimination was observed between the resistant cell lines and parental cells.

CONCLUSIONS

This study strengthens the role of jatrophane diterpenes as lead candidates for the development of MDR reversal agents, higlighting the action of compounds 4 and 5.

Enhancing Activity of Anticancer Drugs in Multidrug Resistant Tumors by Modulating P-Glycoprotein through Dietary Nutraceuticals

Multidrug resistance is a principal mechanism by which tumors become resistant to structurally and functionally unrelated anticancer drugs. Resistance to chemotherapy has been correlated with overexpression of p-glycoprotein (p-gp), a member of the ATP-binding cassette (ABC) superfamily of membrane transporters. P-gp mediates resistance to a broad-spectrum of anticancer drugs including doxorubicin, taxol, and vinca alkaloids by actively expelling the drugs from cells. Use of specific inhibitors/blocker of p-gp in combination with clinically important anticancer drugs has emerged as a new paradigm for overcoming multidrug resistance. The aim of this paper is to review p-gp regulation by dietary nutraceuticals and to correlate this dietary nutraceutical induced-modulation of p-gp with activity of anticancer drugs.

Nuclear-targeting TAT-PEG-Asp8-doxorubicin polymeric nanoassembly to overcome drug-resistant colon cancer

Aim:

Drug efflux-associated multidrug resistance (MDR) is a main obstacle to effective cancer chemotherapy. Large molecule drugs are not the substrates of P-glycoprotein, and can circumvent drug efflux and be retained inside cells. In this article we report a polymer-drug conjugate nanoparticulate system that can overcome MDR based on size-related exclusion effect.

Methods:

Doxorubicin was coupled with the triblock polymeric material cell-penetrating TAT-PEG-poly(aspartic acid). The amphiphilic macromolecules (termed TAT-PEG-Asp₈-Dox) could self-assemble into nanoparticles (NPs) in water. The antitumor activity was evaluated in drug-resistant human colon cancer HCT8/ADR cells in vitro and in nude mice bearing HCT8/ADR tumor.

Results:

The self-assembling TAT-PEG-Asp₈-Dox NPs were approximately 150 nm with a narrow particle size distribution, which not only increased the cellular uptake efficiency, but also bypassed P-glycoprotein-mediated drug efflux and improved the intracellular drug retention, thus yielding an enhanced efficacy for killing drug-resistant HCT8/ADR colon cancer cells in vitro. Importantly, the TAT-PEG-Asp₈-Dox NPs enhanced the intranuclear disposition of drugs for grater inhibition of DNA/RNA biosynthesis. In nude mice bearing xenografted HCT8/ADR colon cancers, intravenous or peritumoral injection of TAT-PEG-Asp₈-Dox NPs for 22 d effectively inhibited tumor growth.

Conclusion:

TAT-PEG-Asp₈-Dox NPs can increase cellular drug uptake and intranuclear drug delivery and retain effective drug accumulation inside the cells, thus exhibiting enhanced anticancer activity toward the drug-resistant human colon cancer HCT8/ADR cells.

Mitochondria-Targeted Doxorubicin: A New Therapeutic Strategy against Doxorubicin-Resistant Osteosarcoma

Doxorubicin is one of the leading drugs for osteosarcoma standard chemotherapy. A total of 40% to 45% of high-grade osteosarcoma patients are unresponsive, or only partially responsive, to doxorubicin (Dox), due to the overexpression of the drug efflux transporter ABCB1/P-glycoprotein (Pgp). The aim of this work is to improve Dox-based regimens in resistant osteosarcomas. We used a chemically modified mitochondria-targeted Dox (mtDox) against Pgp-overexpressing osteosarcomas with increased resistance to Dox. Unlike Dox, mtDox accumulated at significant levels intracellularly, exerted cytotoxic activity, and induced necrotic and immunogenic cell death in Dox-resistant/Pgp-overexpressing cells, fully reproducing the activities exerted by anthracyclines in drug-sensitive tumors. mtDox reduced tumor growth and cell proliferation, increased apoptosis, primed tumor cells for recognition by the host immune system, and was less cardiotoxic than Dox in preclinical models of drug-resistant osteosarcoma. The increase in Dox resistance was paralleled by a progressive upregulation of mitochondrial metabolism. By widely modulating the expression of mitochondria-related genes, mtDox decreased mitochondrial biogenesis, the import of proteins and metabolites within mitochondria, mitochondrial metabolism, and the synthesis of ATP. These events were paralleled by increased reactive oxygen species production, mitochondrial depolarization, and mitochondria-dependent apoptosis in resistant osteosarcoma cells, where Dox was completely ineffective. We propose mtDox as a new effective agent with a safer toxicity profile compared with Dox that may be effective for the treatment of Dox-resistant/Pgp-positive osteosarcoma patients, who strongly need alternative and innovative treatment strategies. Mol Cancer Ther; 15(11); 2640-52. ©2016 AACR.

Computational predictive models for P-glycoprotein inhibition of in-house chalcone derivatives and drug-bank compounds

The human P-glycoprotein (P-gp) efflux pump is of great interest for medicinal chemists because of its important role in multidrug resistance (MDR). Because of the high polyspecificity as well as the unavailability of high-resolution X-ray crystal structures of this transmembrane protein, ligand-based, and structure-based approaches which were machine learning, homology modeling, and molecular docking were combined for this study. In ligand-based approach, individual two-dimensional quantitative structure-activity relationship models were developed using different machine learning algorithms and subsequently combined into the Ensemble model which showed good performance on both the diverse training set and the validation sets. The applicability domain and the prediction quality of the developed models were also judged using the state-of-the-art methods and tools. In our structure-based approach, the P-gp structure and its binding region were predicted for a docking study to determine possible interactions between the ligands and the receptor. Based on these in silico tools, hit compounds for reversing MDR were discovered from the in-house and DrugBank databases through virtual screening using prediction models and molecular docking in an attempt to restore cancer cell sensitivity to cytotoxic drugs.

In vitro and in silico analysis of the vascular effects of asymmetrical N,N-bis(alkanol)amine aryl esters, novel multidrug resistance-reverting agents

Asymmetrical N,N-bis(alkanol)amine aryl esters (FRA77, GDE6, and GDE19) are potent multidrug resistance (MDR) reversers. Their structures loosely remind that of the Ca(2+) antagonist verapamil. Therefore, the aim of this study was to investigate their vascular activity in vitro. Their effects on the mechanical activity of fresh and cultured rat aorta rings on Cav1.2 channel current (I Ca1.2) of A7r5 cells and their cytotoxicity on A7r5 and EA.hy926 cells were analyzed. Docking at the rat α1C subunit of the Cav1.2 channel was simulated in silico. Compounds tested were cytotoxic at concentrations >1 μM (FRA77, GDE6, GDE19) and >10 μM (verapamil) in EA.hy926 cells, or >10 μM (FRA77, GDE6, GDE19) and at 100 μM (verapamil) in A7r5 cells. In fresh rings, the three compounds partly antagonized phenylephrine and 60 mM K(+) (K60)-induced contraction at concentrations ≥1 and ≥3 μM, respectively. On the contrary, verapamil fully relaxed rings pre-contracted with both agents. In cultured rings, 10 μM GDE6, GDE19, FRA77, and verapamil significantly reduced the contractile response to both phenylephrine and K60. Similarly to verapamil, the three compounds docked at the α1C subunit, interacting with the same amino acids residues. FRA77, GDE6, and GDE19 inhibited I Ca1.2 with IC50 values 1 order of magnitude higher than that of verapamil. FRA77-, GDE6-, and GDE19-induced vascular effects occurred at concentrations that are at least 1 order of magnitude higher than those effectively reverting MDR. Though an unambiguous divergence between MDR reverting and vascular activity is of overwhelming importance, these findings consistently contribute to the design and synthesis of novel and potent chemosensitizers.

Nano-based strategies to overcome p-glycoprotein-mediated drug resistance

INTRODUCTION

The discussion about cancer treatment has a long history. Chemotherapy, one of the promising approaches in cancer therapy, is limited in the clinic as plenty of factors evolve and prevent appropriate therapeutic response to drugs. Multi-drug resistance (MDR), which is mostly P-glycoprotein-mediated, is described as the most well-known impediment in this contribution. It extrudes several agents out of cells, arising MDR and decreasing the bioavailability of drugs. Hence, cancer cells become insensitive to chemotherapy.

AREAS COVERED

Many agents have been developed to reverse MDR, but it is difficult to deliver them into cancer sites and cancer cells. The emerging nano-based drug delivery systems have been more effective to overcome P-glycoprotein-mediated MDR by increasing the intracellular delivery of these agents. Here, we represent systems including siRNA-targeted inhibition of P-gp, monoclonal antibodies, natural extracts, conventional inhibitors, hard nanoparticles and soft nanoparticles as delivery systems in addition to a novel approach applying cell penetrating peptides.

EXPERT OPINION

Overcoming cancer drug resistance using innovative nanotechnology is being increasingly used and developed. Among resistance mechanisms, drug efflux transporter inhibitors and MDR gene expression silencing are among the those being investigated. In the near future, it seems some of these nanomedical approaches might become the mainstay of effective treatment of important human conditions like cancer.

Effective co-delivery of doxorubicin and curcumin using a glycyrrhetinic acid-modified chitosan-cystamine-poly(ε-caprolactone) copolymer micelle for combination cancer chemotherapy

A glycyrrhetinic acid-modified chitosan-cystamine-poly(ε-caprolactone) copolymer (PCL-SS-CTS-GA) micelle was developed for the co-delivery of doxorubicin (DOX) and curcumin (CCM) to hepatoma cells. Glycyrrhetinic acid (GA) was used as a targeting unit to ensure specific delivery. Co-encapsulation of DOX and CCM was facilitated by the incorporation of poly(ε-caprolactone) (PCL) groups. The highest drug loading content was 19.8% and 8.9% (w/w) for DOX and CCM, respectively. The PCL-SS-CTS-GA micelle presented a spherical or ellipsoidal geometry with a mean diameter of approximately 110nm. The surface charge of the micelle changed from negative to positive, when the pH value of the solution decreased from 7.4 to 6.8. Meanwhile, it also exhibited a character of redox-responsive drug release and GA/pH-mediated endocytosis in vitro. In simulated body fluid with 10mM glutathione, the release rate in 12h was 80.6% and 67.2% for DOX and CCM, respectively. The cell uptake of micelles was significantly higher at pH 6.8 than pH 7.4. The combined administration of DOX and CCM was facilitated by PCL-SS-CTS-GA micelle. Results showed that there was strong synergic effect between the two drugs. The PCL-SS-CTS-GA micelle might turn into a promising and effective carrier for improved combination chemotherapy.

Guizhi Fuling Wan, a Traditional Chinese Herbal Formula, Sensitizes Cisplatin-Resistant Human Ovarian Cancer Cells through Inactivation of the PI3K/AKT/mTOR Pathway

The aim of the study was to explore the possible mechanisms that Guizhi Fuling Wan (GFW) enhances the sensitivity of the SKOV3/DDP ovarian cancer cells and the resistant xenograft tumours to cisplatin. Rat medicated sera containing GFW were prepared by administering GFW to rats, and the primary bioactive constituents of the sera were gallic acid, paeonol, and paeoniflorin analysed by HPLC/QqQ MS. Cell counting kit-8 analysis was shown that coincubation of the sera with cisplatin/paclitaxel enhanced significantly the cytotoxic effect of cisplatin or paclitaxel in SKOV3/DDP cells. The presence of the rat medicated sera containing GFW resulted in an increase in rhodamine 123 accumulation by flow cytometric assays and a decrease in the protein levels of P-gp, phosphorylation of AKT at Ser473, and mTOR in a dose-dependent manner in SKOV3/DDP cells by western blot analysis, but the sera had no effect on the protein levels of PI3K p110α and total AKT. The low dose of GFW enhanced the anticancer efficacy of cisplatin and paclitaxel treatment in resistant SKOV3/DDP xenograft tumours. GFW could sensitize cisplatin-resistant SKOV3/DDP cells by inhibiting the protein level and function of P-gp, which may be medicated through inactivation of the PI3K/AKT/mTOR pathway.

Pharmacokinetic Properties of Anticancer Agents for the Treatment of Central Nervous System Tumors: Update of the Literature

Despite significant improvement in outcomes for patients with hematologic malignancies and solid tumors over the past 10 years, patients with primary or metastatic brain tumors continue to have a poor prognosis. A primary reason for this is the inability of many chemotherapeutic drugs to penetrate into the brain and brain tumors at concentrations high enough to exert an antitumor effect because of unique barriers and efflux transporters. Several studies have been published recently examining the central nervous system pharmacokinetics of various anticancer drugs in patients with primary and metastatic brain tumors. To summarize recent advances in the field, this review critically presents studies published within the last 9 years examining brain and cerebrospinal fluid penetration of clinically available anticancer agents for patients with central nervous system tumors.

Targeting tumor microenvironment with PEG-based amphiphilic nanoparticles to overcome chemoresistance

Multidrug resistance is one of the biggest obstacles in the treatment of cancer. Recent research studies highlight that tumor microenvironment plays a predominant role in tumor cell proliferation, metastasis, and drug resistance. Hence, targeting the tumor microenvironment provides a novel strategy for the evolution of cancer nanomedicine. The blooming knowledge about the tumor microenvironment merging with the design of PEG-based amphiphilic nanoparticles can provide an effective and promising platform to address the multidrug resistant tumor cells. This review describes the characteristic features of tumor microenvironment and their targeting mechanisms with the aid of PEG-based amphiphilic nanoparticles for the development of newer drug delivery systems to overcome multidrug resistance in cancer cells.

FROM THE CLINICAL EDITOR

Cancer is a leading cause of death worldwide. Many cancers develop multidrug resistance towards chemotherapeutic agents with time and strategies are urgently needed to combat against this. In this review article, the authors discuss the current capabilities of using nanomedicine to target the tumor microenvironments, which would provide new insight to the development of novel delivery systems for the future.

Enhanced antitumoral activity of doxorubicin against lung cancer cells using biodegradable poly(butylcyanoacrylate) nanoparticles

Doxorubicin (Dox) is widely used for the combined chemotherapy of solid tumors. However, the use of these drug associations in lung cancer has low antitumor efficacy. To improve its efficacious delivery and activity in lung adenocarcinoma cells, we developed a biodegradable and noncytotoxic nanoplatform based on biodegradable poly(butylcyanoacrylate) (PBCA). The reproducible formulation method was based on an anionic polymerization process of the PBCA monomer, with the antitumor drug being entrapped within the nanoparticle (NP) matrix during its formation. Improved drug-entrapment efficiencies and sustained (biphasic) drug-release properties were made possible by taking advantage of the synthesis conditions (drug, monomer, and surfactant-agent concentrations). Dox-loaded NPs significantly enhanced cellular uptake of the drug in the A549 and LL/2 lung cancer cell lines, leading to a significant improvement of the drug's antitumoral activity. In vivo studies demonstrated that Dox-loaded NPs clearly reduced tumor volumes and increased mouse-survival rates compared to the free drug. These results demonstrated that PBCA NPs may be used to optimize the antitumor activity of Dox, thus exhibiting a potential application in chemotherapy against lung adenocarcinoma.

MRP1 and its role in anticancer drug resistance

The phenomenon of multidrug resistance (MDR) in cancer is associated with the overexpression of the ATP-binding cassette (ABC) transporter proteins, including multidrug resistance-associated protein 1 (MRP1) and P-glycoprotein. MRP1 plays an active role in protecting cells by its ability to efflux a vast array of drugs to sub-lethal levels. There has been much effort in elucidating the mechanisms of action, structure and substrates and substrate binding sites of MRP1 in the last decade. In this review, we detail our current understanding of MRP1, its clinical relevance and highlight the current environment in the search for MRP1 inhibitors. We also look at the capacity for the rapid intercellular transfer of MRP1 phenotype from spontaneously shed membrane vesicles known as microparticles and discuss the clinical and therapeutic significance of this in the context of cancer MDR.

P-glycoprotein mediated drug interactions in animals and humans with cancer

Drug–drug interactions can cause unanticipated patient morbidity and mortality. The consequences of drug–drug interactions can be especially severe when anticancer drugs are involved because of their narrow therapeutic index. Veterinary clinicians have traditionally been taught that drug–drug interactions result from alterations in drug metabolism, renal excretion or protein binding. More recently, drug–drug interactions resulting from inhibition of P‐glycoprotein‐mediated drug transport have been identified in both human and veterinary patients. Many drugs commonly used in veterinary patients are capable of inhibiting P‐glycoprotein function and thereby causing an interaction that results in severe chemotherapeutic drug toxicity. The intent of this review is to describe the mechanism and clinical implications of drug–drug interactions involving P‐glycoprotein and anticancer drugs. Equipped with this information, veterinarians can prevent serious drug–drug interactions by selecting alternate drugs or adjusting the dose of interacting drugs.

Reversal of multidrug resistance by co-delivery of paclitaxel and lonidamine using a TPGS and hyaluronic acid dual-functionalized liposome for cancer treatment

Multidrug resistance (MDR) remains the primary issue in cancer therapy, which is characterized by the overexpressed P-glycoprotein (P-gp)-included efflux pump or the upregulated anti-apoptotic proteins. In this study, a D-alpha-tocopheryl poly (ethylene glycol 1000) succinate (TPGS) and hyaluronic acid (HA) dual-functionalized cationic liposome containing a synthetic cationic lipid, 1,5-dioctadecyl-N-histidyl-L-glutamate (HG2C18) was developed for co-delivery of a small-molecule chemotherapeutic drug, paclitaxel (PTX) with a chemosensitizing agent, lonidamine (LND) to treat the MDR cancer. It was demonstrated that the HG2C18 lipid contributes to the endo-lysosomal escape of the liposome following internalization for efficient intracellular delivery. The TPGS component was confirmed able to elevate the intracellular accumulation of PTX by inhibiting the P-gp efflux, and to facilitate the mitochondrial-targeting of the liposome. The intracellularly released LND suppressed the intracellular ATP production by interfering with the mitochondrial function for enhanced P-gp inhibition, and additionally, sensitized the MDR breast cancer (MCF-7/MDR) cells to PTX for promoted induction of apoptosis through a synergistic effect. Functionalized with the outer HA shell, the liposome preferentially accumulated at the tumor site and showed a superior antitumor efficacy in the xenograft MCF-7/MDR tumor mice models. These findings suggest that this dual-functional liposome for co-delivery of a cytotoxic drug and an MDR modulator provides a promising strategy for reversal of MDR in cancer treatment.

Genome-Wide Identification and Characterization of Novel Factors Conferring Resistance to Topoisomerase II Poisons in Cancer

The topoisomerase II poisons doxorubicin and etoposide constitute longstanding cornerstones of chemotherapy. Despite their extensive clinical use, many patients do not respond to these drugs. Using a genome-wide gene knockout approach, we identified Keap1, the SWI/SNF complex, and C9orf82 (CAAP1) as independent factors capable of driving drug resistance through diverse molecular mechanisms, all converging on the DNA double-strand break (DSB) and repair pathway. Loss of Keap1 or the SWI/SNF complex inhibits generation of DSB by attenuating expression and activity of topoisomerase IIα, respectively, whereas deletion of C9orf82 augments subsequent DSB repair. Their corresponding genes, frequently mutated or deleted in human tumors, may impact drug sensitivity, as exemplified by triple-negative breast cancer patients with diminished SWI/SNF core member expression who exhibit reduced responsiveness to chemotherapy regimens containing doxorubicin. Collectively, our work identifies genes that may predict the response of cancer patients to the broadly used topoisomerase II poisons and defines alternative pathways that could be therapeutically exploited in treatment-resistant patients.

Anticancer efficacy and toxicokinetics of a novel paclitaxel-clofazimine nanoparticulate co-formulation

Contemporary chemotherapy is limited by disseminated, resistant cancer. Targeting nanoparticulate drug delivery systems that encapsulate synergistic drug combinations are a rational means to increase the therapeutic index of chemotherapeutics. A lipopolymeric micelle co-encapsulating an in vitro optimized, synergistic fixed-ratio combination of paclitaxel (PTX) and clofazimine (B663) has been developed and called Riminocelles™. The present pre-clinical study investigated the acute toxicity, systemic exposure, repeat dose toxicity and efficacy of Riminocelles in parallel to Taxol® at an equivalent PTX dose of 10 mg/kg. Daily and weekly dosing schedules were evaluated against Pgp-expressing human colon adenocarcinoma (HCT-15) xenografts implanted subcutaneously in athymic mice. Riminocelles produced statistically significant (p <  .05) tumor growth delays of 3.2 and 2.7 days for the respective schedules in contrast to Taxol delaying growth by 0.5 and 0.6 days. Using the control tumor doubling time of 4.2 days, tumor-cell-kill values of 0.23 for Riminocelles and 0.04 for Taxol following daily schedules were calculated. A significant weight loss of 5.7% after 14 days (p < 0.05) relative to the control group (n = 8) was observed for the daily Taxol group whereas Riminocelles did not incur significant weight loss neither were blood markers of toxicity elevated after acute administration (n = 3). The safety and efficacy of Riminocelles is statistically superior to Taxol. However, passive tumor targeting was not achieved and the tumor burden progressed quickly. Prior to further animal studies, the in vivo thermodynamic instability of the simple lipopolymeric micellular delivery system requires improvement so as to maintain and selectively deliver the fixed-ratio drug combination.

Repurposing Drugs in Oncology (ReDO)-itraconazole as an anti-cancer agent

Itraconazole, a common triazole anti-fungal drug in widespread clinical use, has evidence of clinical activity that is of interest in oncology. There is evidence that at the clinically relevant doses, itraconazole has potent anti-angiogenic activity, and that it can inhibit the Hedgehog signalling pathway and may also induce autophagic growth arrest. The evidence for these anticancer effects, in vitro, in vivo, and clinical are summarised, and the putative mechanisms of their action outlined. Clinical trials have shown that patients with prostate, lung, and basal cell carcinoma have benefited from treatment with itraconazole, and there are additional reports of activity in leukaemia, ovarian, breast, and pancreatic cancers. Given the evidence presented, a case is made that itraconazole warrants further clinical investigation as an anti- cancer agent. Additionally, based on the properties summarised previously, it is proposed that itraconazole may synergise with a range of other drugs to enhance the anti-cancer effect, and some of these possible combinations are presented in the supplementary materials accompanying this paper.

Thyroid hormone and P-glycoprotein in tumor cells

P-glycoprotein (P-gp; multidrug resistance pump 1, MDR1; ABCB1) is a plasma membrane efflux pump that when activated in cancer cells exports chemotherapeutic agents. Transcription of the P-gp gene (MDR1) and activity of the P-gp protein are known to be affected by thyroid hormone. A cell surface receptor for thyroid hormone on integrin αvβ3 also binds tetraiodothyroacetic acid (tetrac), a derivative of L-thyroxine (T₄) that blocks nongenomic actions of T₄ and of 3,5,3′-triiodo-L-thyronine (T₃) at αvβ3. Covalently bound to a nanoparticle, tetrac as nanotetrac acts at the integrin to increase intracellular residence time of chemotherapeutic agents such as doxorubicin and etoposide that are substrates of P-gp. This action chemosensitizes cancer cells. In this review, we examine possible molecular mechanisms for the inhibitory effect of nanotetrac on P-gp activity. Mechanisms for consideration include cancer cell acidification via action of tetrac/nanotetrac on the Na⁺/H⁺ exchanger (NHE1) and hormone analogue effects on calmodulin-dependent processes and on interactions of P-gp with epidermal growth factor (EGF) and osteopontin (OPN), apparently via αvβ3. Intracellular acidification and decreased H⁺ efflux induced by tetrac/nanotetrac via NHE1 is the most attractive explanation for the actions on P-gp and consequent increase in cancer cell retention of chemotherapeutic agent-ligands of MDR1 protein.

Demethoxycurcumin modulates human P-glycoprotein function via uncompetitive inhibition of ATPase hydrolysis activity

Curcuminoids are major components of Curcuma longa L., which is widely used as spice in food. This study aimed at identifying whether curcumin, demethoxycurcumin, and bisdemethoxycurcumin could modulate efflux function of human P-glycoprotein and be used as chemosensitizers in cancer treatments. Without altering P-glycoprotein expression levels and conformation, the purified curcuminoids significantly inhibited P-glycoprotein efflux function. In rhodamine 123 efflux and calcein-AM accumulation assays, demethoxycurcumin demonstrated the highest inhibition potency (inhibitory IC50 = 1.56 ± 0.13 μM) among the purified curcuminoids, as well as in the fold of reversal assays. Demethoxycurcumin inhibited P-glycoprotein-mediated ATP hydrolysis under concentrations of <1 μM and efficiently inhibited 200 μM verapamil-stimulated ATPase activity, indicating a high affinity of demethoxycurcumin for P-glycoprotein. These results suggested that demethoxycurcumin may be a potential additive natural product in combination with chemotherapeutic agents in drug-resistant cancers.

The role of transporters in toxicity and disease

The significance of transporters in the disposition, metabolism, and elimination of drugs is well recognized. One gap in our knowledge is a comprehensive understanding of how drug transporters change functionality (their amount and activity) in response to disease and how disease and its inevitable pathology change transporter expression. In this issue of Drug Metabolism and Disposition a series of review and primary research articles are presented to highlight the importance of transporters in toxicity and disease. Because of the central role of the liver in drug metabolism, many of the articles in this theme issue focus on transporters in the liver and how pathology or alterations in physiology affects transporter expression. The contributing authors have also considered the role of transporters in drug interactions as well as drug-induced liver injury. Noninvasive approaches to assessing transporter function in vivo are also described. Several articles highlight important issues in oncology where toxicity must be balanced against efficacy. In total, this theme issue will provide a stepping-stone to future studies that will establish a more comprehensive understanding of transporters in disease.