Considerations in the design and development of transport inhibitors as adjuncts to drug therapy

Exploring the potential of P-glycoprotein inhibitors in the targeted delivery of anti-cancer drugs: A comprehensive review

Due to the high prevalence of cancer, progress in the management of cancer is the need of the hour. Most cancer patients develop chemotherapeutic drug resistance, and many remain insidious due to overexpression of Multidrug Resistance Protein 1 (MDR1), also known as Permeability-glycoprotein (P-gp) or ABCB1 transporter (ATP-binding cassette subfamily B member 1). P-gp, a transmembrane protein that protects vital organs from outside chemicals, expels medications from malignant cells. The blood-brain barrier (BBB), gastrointestinal tract (GIT), kidneys, liver, pancreas, and cancer cells overexpress P-gp on their apical surfaces, making treatment inefficient and resistant. Compounds that compete with anticancer medicines for transportation or directly inhibit P-gp may overcome biological barriers. Developing nanotechnology-based formulations may help overcome P-gp-mediated efflux and improve bioavailability and cell chemotherapeutic agent accumulation. Nanocarriers transport pharmaceuticals via receptor-mediated endocytosis, unlike passive diffusion, which bypasses ABCB1. Anticancer drugs and P-gp inhibitors in nanocarriers may synergistically increase drug accumulation and chemotherapeutic agent toxicity. The projection of desirable binding and effect may be procured initially by molecular docking of the inhibitor with P-gp, enabling the reduction of preliminary trials in formulation development. Here, P-gp-mediated efflux and several possible outcomes to overcome the problems associated with currently prevalent cancer treatments are highlighted.

The evidence for repurposing anti-epileptic drugs to target cancer

Antiepileptic drugs are versatile drugs with the potential to be used in functional drug formulations with drug repurposing approaches. In the present review, we investigated the anticancer properties of antiepileptic drugs and interlinked cancer and epileptic pathways. Our focus was primarily on those drugs that have entered clinical trials with positive results and those that provided good results in preclinical studies. Many contributing factors make cancer therapy fail, like drug resistance, tumor heterogeneity, and cost; exploring all alternatives for efficient treatment is important. It is crucial to find new drug targets to find out new antitumor molecules from the already clinically validated and approved drugs utilizing drug repurposing methods. The advancements in genomics, proteomics, and other computational approaches speed up drug repurposing. This review summarizes the potential of antiepileptic drugs in different cancers and tumor progression in the brain. Valproic acid, oxcarbazepine, lacosamide, lamotrigine, and levetiracetam are the drugs that showed potential beneficial outcomes against different cancers. Antiepileptic drugs might be a good option for adjuvant cancer therapy, but there is a need to investigate further their efficacy in cancer therapy clinical trials.

SuperNatural inhibitors to reverse multidrug resistance emerged by ABCB1 transporter: Database mining, lipid-mediated molecular dynamics, and pharmacokinetics study

An effective approach to reverse multidrug resistance (MDR) is P-glycoprotein (P-gp, ABCB1) transport inhibition. To identify such molecular regulators, the SuperNatural II database, which comprises > 326,000 compounds, was virtually screened for ABCB1 transporter inhibitors. The Lipinski rule was utilized to initially screen the SuperNatural II database, identifying 128,126 compounds. Those natural compounds were docked against the ABCB1 transporter, and those with docking scores less than zosuquidar (ZQU) inhibitor were subjected to molecular dynamics (MD) simulations. Based on MM-GBA binding energy (ΔGbinding) estimations, UMHSN00009999 and UMHSN00097206 demonstrated ΔGbinding values of -68.3 and -64.1 kcal/mol, respectively, compared to ZQU with a ΔGbinding value of -49.8 kcal/mol. For an investigation of stability, structural and energetic analyses for UMHSN00009999- and UMHSN00097206-ABCB1 complexes were performed and proved the high steadiness of these complexes throughout 100 ns MD simulations. Pharmacokinetic properties of the identified compounds were also predicted. To mimic the physiological conditions, MD simulations in POPC membrane surroundings were applied to the UMHSN00009999- and UMHSN00097206-ABCB1 complexes. These results demonstrated that UMHSN00009999 and UMHSN00097206 are promising ABCB1 inhibitors for reversing MDR in cancer and warrant additional in-vitro/in-vivo studies.

Computational binding affinity and molecular dynamic characterization of annonaceous acetogenins at nucleotide binding domain (NBD) of multi-drug resistance ATP-binding cassette sub-family B member 1 (ABCB1)

Multi drug resistance (MDR) in tumor might be caused leading to the overexpression of transporters, such as ATP-binding cassette sub-family B member 1 (ABCB1). A combination of non-toxic and potent ABC inhibitors along with conventional anti-cancer drugs is needed to reverse MDR in tumors. A variety of phytochemicals have been previously shown to reverse MDR. Annonaceous acetogenins (AAs) with C₃₅/C₃₇ long-chain fatty acids were reported for their anti-tumor activity, however, their effect on reversing MDR is not yet investigated. We aimed to investigate some selective AAs against the B1 subtype of ABC transporter using computational studies. Various modules of Maestro software were utilized for our in-silico analysis. Few well-characterized AAs were screened for their drug-likeness properties and tested for binding affinity at ATP and drug binding sites of ABCB1 through molecular docking. The stability of the ligand-protein complex (lowest docking score) was then determined by a molecular dynamic (MD) simulation study. Out of 24 AAs, Annonacin A (-8.10 kcal/mol) and Annohexocin (-10.49 kcal/mol) docked with a greater binding affinity at the ATP binding site than the first-generation inhibitor of ABCB1 (Verapamil: -3.86 kcal/mol). MD simulation of Annonacin A: ABCB1 complex for 100 ns also indicated that Annonacin A would stably bind to the ATP binding site. We report that Annonacin A binds at a greater affinity with ABCB1 and might act as a potential drug lead to reverse MDR in tumor cells. Communicated by Ramaswamy H. Sarma.

Nanocracker capable of simultaneously reversing both P-glycoprotein and tumor microenvironment

Here, we describe a multidrug-resistant nanocracker (MDRC) that can treat multi-drug resistant (MDR) cancer by recognizing the acidic microenvironment and inhibiting two mechanisms of MDR such as P-glycoprotein (P-gp) and vacuolar-type ATPase (V-ATPase). MDRC is a liposome formulation co-loading pantoprazole (PZ) and paclitaxel (PTX). PZ acts as a chemosensitizer that enhances the MDR cancer treatment effect of PTX by disrupting the pH gradient and inhibiting P-gp. MDRC-encapsulated PZ and PTX have different release rates, with PZ released within 12 h and PTX sustained release for 48 h in the plasma. MDRC could increase cell uptake by inhibiting the P-gp overexpressed MCF-7/mdr cells and UV-2237M cells, which are human breast MDR cancer cells and murine fibrosarcoma cells, respectively. MDRC can also increase the cytotoxic efficacy of PTX by increasing intracellular pH. MDRC has a 10.5-fold reduced IC₅₀ value in the P-gp overexpressed human breast adenocarcinoma and a 6.3- to 9.5-fold reduced IC₅₀ value in the P-gp non-expressed human breast adenocarcinoma compared to the mixture of PZ and PTX, respectively. Intravenous injection of MDRC did not cause weight loss, liver dysfunction, or major organ toxicity. MDRC exhibited 80% complete remission of murine fibrosarcoma. The excellent therapeutic effect of MDRC on MDR tumors was accompanied by an increase in dendritic cell maturation and cytotoxic T cells. In other words, MDRC has the potential to terminate MDR therapy through the complete remission of MDR tumors.

Novel application of rhein and its prodrug diacerein for reversing cancer-related multidrug resistance through the dual inhibition of P-glycoprotein efflux and STAT3-mediated P-glycoprotein expression

Multidrug resistance (MDR) is a multifactorial issue in cancer treatment. Drug efflux transporters, particularly P-glycoprotein (P-gp), are major contributors to such resistance. In the present study, we evaluated the P-gp-inhibiting and MDR-reversing effects of two compounds, namely rhein, an anthraquinone, and diacerein, the acetylated prodrug of rhein. ABCB1/Flp-In-293 was used as a model for investigating the related molecular mechanisms, and the multi-drug-resistant cancer cell line KB/VIN was used as a platform for evaluating the reversal of MDR0. The results indicated that at a concentration of 2.5 μM, both diacerein and rhein significantly inhibited P-gp efflux function. They also downregulated P-gp expression by interacting with the signal transducer and activator of transcription 3. Further investigation of the inhibitory mechanism of these compounds revealed that both stimulated P-gp ATPase activity dose dependently and engaged in the noncompetitive inhibition of rhodamine 123 efflux. Furthermore, rhein was revealed to be a potent reverser of MDR in cancer, and the combination of 30 μM rhein and 1000 nM vincristine exerted a strong synergistic effect, achieving a high combination index (CI) of 0.092. Diacerein demonstrated potential applications as a selective cytotoxic agent against multi-drug-resistant cancer cells at a concentration of > 18.92 μM and as a mild MDR reverser at doses of < 10 μM. In conclusion, diacerein and rhein are potential candidates for P-gp inhibition and MDR reversal in cancer cells.

Pharmaceutical Formulations with P-Glycoprotein Inhibitory Effect as Promising Approaches for Enhancing Oral Drug Absorption and Bioavailability

P-glycoprotein (P-gp) is crucial in the active transport of various substrates with diverse structures out of cells, resulting in poor intestinal permeation and limited bioavailability following oral administration. P-gp inhibitors, including small molecule drugs, natural constituents, and pharmaceutically inert excipients, have been exploited to overcome P-gp efflux and enhance the oral absorption and bioavailability of many P-gp substrates. The co-administration of small molecule P-gp inhibitors with P-gp substrates can result in drug–drug interactions and increased side effects due to the pharmacological activity of these molecules. On the other hand, pharmaceutically inert excipients, including polymers, surfactants, and lipid-based excipients, are safe, pharmaceutically acceptable, and are not absorbed from the gut. Notably, they can be incorporated in pharmaceutical formulations to enhance drug solubility, absorption, and bioavailability due to the formulation itself and the P-gp inhibitory effects of the excipients. Different formulations with inherent P-gp inhibitory activity have been developed. These include micelles, emulsions, liposomes, solid lipid nanoparticles, polymeric nanoparticles, microspheres, dendrimers, and solid dispersions. They can bypass P-gp by different mechanisms related to their properties. In this review, we briefly introduce P-gp and P-gp inhibitors, and we extensively summarize the current development of oral drug delivery systems that can bypass and inhibit P-gp to improve the oral absorption and bioavailability of P-gp substrates. Since many drugs are limited by P-gp-mediated efflux, this review is helpful for designing suitable formulations of P-gp substrates to enhance their oral absorption and bioavailability.

Drug Resistance in Metastatic Breast Cancer: Tumor Targeted Nanomedicine to the Rescue

Breast cancer, specifically metastatic breast, is a leading cause of morbidity and mortality in women. This is mainly due to relapse and reoccurrence of tumor. The primary reason for cancer relapse is the development of multidrug resistance (MDR) hampering the treatment and prognosis. MDR can occur due to a multitude of molecular events, including increased expression of efflux transporters such as P-gp, BCRP, or MRP1; epithelial to mesenchymal transition; and resistance development in breast cancer stem cells. Excessive dose dumping in chemotherapy can cause intrinsic anti-cancer MDR to appear prior to chemotherapy and after the treatment. Hence, novel targeted nanomedicines encapsulating chemotherapeutics and gene therapy products may assist to overcome cancer drug resistance. Targeted nanomedicines offer innovative strategies to overcome the limitations of conventional chemotherapy while permitting enhanced selectivity to cancer cells. Targeted nanotheranostics permit targeted drug release, precise breast cancer diagnosis, and importantly, the ability to overcome MDR. The article discusses various nanomedicines designed to selectively target breast cancer, triple negative breast cancer, and breast cancer stem cells. In addition, the review discusses recent approaches, including combination nanoparticles (NPs), theranostic NPs, and stimuli sensitive or “smart” NPs. Recent innovations in microRNA NPs and personalized medicine NPs are also discussed. Future perspective research for complex targeted and multi-stage responsive nanomedicines for metastatic breast cancer is discussed.

Development of a Novel Quinoline Derivative as a P-Glycoprotein Inhibitor to Reverse Multidrug Resistance in Cancer Cells

Multidrug resistance (MDR) is one of conventional cancer chemotherapy’s limitations. Our group previously synthesized a series of quinoline-based compounds in an attempt to identify novel anticancer agents. With a molecular docking analysis, the novel compound 160a was predicted to target p-glycoprotein, an MDR candidate. The purpose of this study is to evaluate 160a’s MDR reversal effect and investigate the underlying mechanism at the molecular level. To investigate 160a’s inhibitory effect, we used a series of parental cancer cell lines (A549, LCC6, KYSE150, and MCF-7), the corresponding doxorubicin-resistant cell lines, an MTS cytotoxicity assay, an intracellular doxorubicin accumulation test, and multidrug resistance assays. The Compusyn program confirmed, with a combination index (CI) value greater than 1, that 160a combined with doxorubicin exerts a synergistic effect. Intracellular doxorubicin accumulation and transported calcein acetoxymethyl (AM) (a substrate for p-glycoprotein) were both increased when cancer cells with MDR were treated with compound 160a. We also showed that compound 160a’s MDR reversal effect can persist for at least 1 h. Taken together, these results suggest that the quinoline compound 160a possesses high potential to reverse MDR by inhibiting p-glycoprotein-mediated drug efflux in cancer cells with MDR.

Bioavailability Enhancement of Poorly Soluble Drugs: The Holy Grail in Pharma Industry

Background:

Bioavailability, one of the prime pharmacokinetic properties of a drug, is defined as the fraction of an administered dose of unchanged drug that reaches the systemic circulation and is used to describe the systemic availability of a drug. Bioavailability assessment is imperative in order to demonstrate whether the drug attains the desirable systemic exposure for effective therapy. In recent years, bioavailability has become the subject of importance in drug discovery and development studies.

Methods:

A systematic literature review in the field of bioavailability and the approaches towards its enhancement have been comprehensively done, purely focusing upon recent papers. The data mining was performed using databases like PubMed, Science Direct and general Google searches and the collected data was exhaustively studied and summarized in a generalized manner.

Results:

The main prospect of this review was to generate a comprehensive one-stop summary of the numerous available approaches and their pharmaceutical applications in improving the stability concerns, physicochemical and mechanical properties of the poorly water-soluble drugs which directly or indirectly augment their bioavailability.

Conclusion:

The use of novel methods, including but not limited to, nano-based formulations, bio-enhancers, solid dispersions, lipid-and polymer-based formulations which provide a wide range of applications not only increases the solubility and permeability of the poorly bioavailable drugs but also improves their stability, and targeting efficacy. Although, these methods have drastically changed the pharmaceutical industry demand for the newer potential methods with better outcomes in the field of pharmaceutical science to formulate various dosage forms with adequate systemic availability and improved patient compliance, further research is required.

Discovery of novel multidrug resistance protein 4 (MRP4) inhibitors as active agents reducing resistance to anticancer drug 6-Mercaptopurine (6-MP) by structure and ligand-based virtual screening

Multidrug resistance protein 4 (MRP4/ABCC4) is an ATP-binding cassette (ABC) transporter. It is associated with multidrug resistance (MDR), which is becoming a growing challenge to the treatment of cancer and infections. In the context of several types of cancer in which MRP4 is overexpressed, MRP4 inhibition manifests striking effects against cancer progression and drug resistance. In this study, we combined ligand-based and structure-based drug design strategy, by searching the SPECS chemical library to find compounds that are most likely to bind to MRP4. Clustering analysis based on a two-dimensional fingerprint was performed to help with visual selection of potential compounds. Cell viability assays with potential inhibitors and the anticancer drug 6-MP were carried out to identify their bioactivity. As a result, 39 compounds were tested and seven of them reached inhibition above 55% with 6-MP. Then compound Cpd23 was discovered to improve HEK293/MRP4 cell sensibility to 6-MP dramatically, and low concentration Cpd23 (5 μM) achieved the equivalent effect of 50 μM MK571. The accumulation of 6-MP was determined by validated high-performance liquid chromatography methods, and pretreatment of the HEK293/MRP4 cells with 50 μM MK571 or Cpd23 resulted in significantly increased accumulation of 6-MP by approximately 1.5 times. This compound was first reported with a novel scaffold compared with previously known MRP4 inhibitors, which is a hopeful molecular tool that can be used for overcoming multidrug resistance research.

Overcoming P-Glycoprotein-Mediated Multidrug Resistance in Colorectal Cancer: Potential Reversal Agents among Herbal Medicines

OBJECTIVES

Multidrug resistance (MDR) is the major reason for the failure of chemotherapy in colorectal cancer (CRC), and the primary determinant of MDR in CRC patients is active drug efflux owing to overexpression of P-glycoprotein (P-gp) in cancer tissues. Despite research efforts to overcome P-gp-mediated drug efflux, the high toxicity of P-gp inhibitors has been a major obstacle for the clinical use of these agents. The aim of this study was to review the literature for potential P-gp reversal agents among traditional herbal medicines, which offer the advantages of safety and potential synergetic effects in CRC chemotherapy.

METHODS

We searched ten databases including 3 English databases, 1 Chinese medical database, and 6 Korean medical databases up to July 2018 and included in vivo and in vitro studies evaluating the effects of herbal medicines as P-gp reversal agents in CRC.

RESULTS

A total of 28 potentially related studies were identified and 16 articles were included. Involving 3 studies about Salvia miltiorrhiza and 2 studies about Curcuma longa, finally we found 14 kinds of traditional herbal medicines-Salvia miltiorrhiza, Curcuma longa, Sinomenium acutum, Stephania tetrandra, Bufo gargarizans, Coptis japonica, Piper nigrum and Piper longum, Hedyotis diffusa, Schisandra chinensis, Glycyrrhiza glabra, Glycyrrhiza inflate, Daphne genkwa, Stemona tuberosa Lour, and Andrographis paniculata-as showing efficacy as P-gp inhibitors in anticancer drug-resistant CRC cells in vitro and in vivo.

CONCLUSIONS

This brief account provides insight into the relationship between P-gp and CRC. Further studies on herbal medicines with demonstrated effects against P-gp overexpression will aid in improving the efficacy of chemotherapy in CRC.

Expression of multidrug resistance protein P-glycoprotein in correlation with markers of hypoxia (HIF-1α, EPO, EPO-R) in invasive breast cancer with metastasis to lymph nodes

INTRODUCTION

Overexpression of the mdr-1 gene is the earliest discovered mechanism of multidrug resistance, which is associated with P-glycoprotein (P-gp) - a cell membrane protein responsible for the efflux of drugs of various structures out of cancer cells. Although the expression of P-glycoprotein has been demonstrated in many cancer types, its relation to markers of hypoxia such as HIF-1α, EPO-R or EPO in invasive breast cancer is not well established. The aim of this research was to analyze the co-expression of P-glycoprotein and the markers of tissue hypoxia HIF-1α, EPO, and EPO-R by immunohistochemistry in invasive breast cancer classified according to the presence of steroid receptors and the HER2 receptors.

MATERIAL AND METHODS

Tissue samples were collected from 58 patients with the diagnosis of invasive breast cancer with lymph node metastases. The expression of P-gp, HIF-1α, EPO-R and EPO was determined by immunohistochemistry.

RESULTS

Of all the invasive breast cancers with lymph node metastases, 15.5% expressed P-gp in cell membrane and tumor blood vessels. In our research, there was a significant positive correlation between HER2-positive tumors that did not express steroid receptors (ER-/PR-/HER2+), and P-gp expression (p = 0.049, r = 0.105). Moreover, there was a significant positive correlation between EPO expression and P-gp (p < 0.001, r = 0.474), and between HIF-1α expression and P-gp (p = 0.00475, r = 0.371).

CONCLUSIONS

We found that HIF-1α and EPO expression is significantly associated with P-gp expression in invasive breast cancer with lymph node metastases. An important result of our study is the demonstration of a correlation between P-gp expression and patients with HER2-positive breast tumors that do not express steroid receptors.

Blood-Brain Barrier Driven Pharmacoresistance in Amyotrophic Lateral Sclerosis and Challenges for Effective Drug Therapies

The blood-brain barrier (BBB) is essential for proper neuronal function, homeostasis, and protection of the central nervous system (CNS) microenvironment from blood-borne pathogens and neurotoxins. The BBB is also an impediment for CNS penetration of drugs. In some neurologic conditions, such as epilepsy and brain tumors, overexpression of P-glycoprotein, an efflux transporter whose physiological function is to expel catabolites and xenobiotics from the CNS into the blood stream, has been reported. Recent studies reported that overexpression of P-glycoprotein and increase in its activity at the BBB drives a progressive resistance to CNS penetration and persistence of riluzole, the only drug approved thus far for treatment of amyotrophic lateral sclerosis (ALS), rapidly progressive and mostly fatal neurologic disease. This review will discuss the impact of transporter-mediated pharmacoresistance for ALS drug therapy and the potential therapeutic strategies to improve the outcome of ALS clinical trials and efficacy of current and future drug treatments.

Stimulus-Responsive Short Peptide Nanogels for Controlled Intracellular Drug Release and for Overcoming Tumor Resistance

Multidrug resistance (MDR) poses a major burden to cancer treatment. As one important factor contributing to MDR, overexpression of P-glycoprotein (P-gp) results in a reduced intracellular drug accumulation. Hence, the ability to effectively block the efflux protein and to accumulate the therapeutics in cancer cells is of great significance in clinical practice. In this work, we successfully developed a smart stimulus-responsive short peptide-assembled system, termed as PD/VER nanogels, which synergistically combined the acid-activatable antitumor prodrug doxorubicin (Dox) with the P-gp inhibitor verapamil (VER) for reversing MDR. Systematic studies demonstrated that such an inhibitor-encapsulated nanogel could effectively enhance the accumulation of Dox in resistant cancer cells, thereby revealing significantly higher antitumor activity compared to free Dox molecules. This work showed that the assembly of bioactive agents with a synergistic effect into nano-drugs could provide a useful strategy to overcome cancer drug resistance.

Molecular Modelling, Dynamics, and Docking of Membrane Proteins

Computational tools and techniques are now most popular and promising to progress the research at rapid rate. Molecular modelling studies contribute their maximum role in wide variety of disciplines especially in proteomics and drug discovery strategies. Molecular dynamics and molecular docking algorithms are now became an essential part in daily research activities of every laboratory throughout the world. These strategies are now well established and standardised to study any specific protein of interest and drug molecule. But still there exist considerable drawbacks in a special concern with membrane proteins as the presently available tools and methods cannot be applied directly to them. Modelling, dynamics and docking studies of membrane proteins need a special care and attention as several challenges are to be crossed with an intensive care to produce a reliable result. This chapter is aimed to discuss such challenges and solutions to handle membrane proteins.

Substrate Specificity of Aglaia loheri Active Isolate towards P-glycoprotein in Multidrug-Resistant Cancer Cells

Multidrug resistance (MDR) is a major contributory factor in the failure of chemotherapy. Concrete interpretation of P-glycoprotein (P-gp) substrate specificity, whether a substance is a substrate or an inhibitor, represents an important feature of a compound's pharmaceutical profiling in drug design and development. In this work, the P-gp substrate specificity of Maldi 531.2[M+H]+, a phenol ester from Aglaia loheri Blanco leaves was investigated. This study focuses on the effect of Maldi 531.2[M+H]+ on P-gp ATPase activity, which was examined by measuring the amount of inorganic phosphates (Pi) released as a result of ATP hydrolysis. To test the effects of Maldi 531.2[M+H]+ on MDR activity, an attempt to combine Maldi 531.2[M+H]+ with a potent P-gp substrate such as verapamil was performed. As a result of this combination treatment, two distinct patterns of interaction with P-gp activity were determined by a calcein-acetoxymethyl ester (AM) assay. Depending on the concentratgion, both stimulation and inhibition of MDR activity were observed at certain drug concentrations suggesting biphasic reactions, which can be understood as cooperative stimulation and competitive inhibition, respectively. Verapamil is a strong substrate to P-gp. Substrate specificity of Maldi 531.2[M+H]+ may be less than the substrate specificity of verapamil, but it acts additively together with low concentrations of verapamil in stimulating ATPase activity. On the one hand, verapamil and Maldi 531.2[M+H]+ exerted cooperative stimulation on P-gp. On the other hand, Maldi 531.2[M+H]+ acts as competitive inhibitor at higher concentrations.

Turning the gun on cancer: Utilizing lysosomal P-glycoprotein as a new strategy to overcome multi-drug resistance

Oxidative stress plays a role in the development of drug resistance in cancer cells. Cancer cells must constantly and rapidly adapt to changes in the tumor microenvironment, due to alterations in the availability of nutrients, such as glucose, oxygen and key transition metals (e.g., iron and copper). This nutrient flux is typically a consequence of rapid growth, poor vascularization and necrosis. It has been demonstrated that stress factors, such as hypoxia and glucose deprivation up-regulate master transcription factors, namely hypoxia inducible factor-1α (HIF-1α), which transcriptionally regulate the multi-drug resistance (MDR), transmembrane drug efflux transporter, P-glycoprotein (Pgp). Interestingly, in addition to the established role of plasma membrane Pgp in MDR, a new paradigm of intracellular resistance has emerged that is premised on the ability of lysosomal Pgp to transport cytotoxic agents into this organelle. This mechanism is enabled by the topological inversion of Pgp via endocytosis resulting in the transporter actively pumping agents into the lysosome. In this way, classical Pgp substrates, such as doxorubicin (DOX), can be actively transported into this organelle. Within the lysosome, DOX becomes protonated upon acidification of the lysosomal lumen, causing its accumulation. This mechanism efficiently traps DOX, preventing its cytotoxic interaction with nuclear DNA. This review discusses these effects and highlights a novel mechanism by which redox-active and protonatable Pgp substrates can utilize lysosomal Pgp to gain access to this compartment, resulting in catastrophic lysosomal membrane permeabilization and cell death. Hence, a key MDR mechanism that utilizes Pgp (the "gun") to sequester protonatable drug substrates safely within lysosomes can be "turned on" MDR cancer cells to destroy them from within.

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.

Chemical characterization, antioxidant and cytotoxic activities of the methanolic extract of Hymenocrater longiflorus grown in Iraq

Hymenocrater longiflorus was collected from northern Iraq, and the chemical composition and antioxidant and cytotoxic activities of this plant were investigated. Ten compounds detected by HPLC-ESI/MS were identified as flavonoids and phenolic acids. The free radical scavenging activity of the 70% methanol extract was evaluated using the 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay. The antioxidant activities of the extract may be attributed to its polyphenolic composition. The cytotoxicity of the plant extract against the osteosarcoma (U2OS) cell line was assessed with the 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT) assay. The extract significantly reduced the viability of cells in a concentration- and time-dependent manner. Cells were arrested during the S-phase of the cell cycle, and DNA damage was revealed by antibodies against histone H2AX. The apoptotic features of cell shrinkage and decrease in cell size were also observed. Western blot analysis revealed cleavage of poly (ADP-ribose)-polymerase 1 (PARP-1), in addition to increases in the proteins p53, p21, and γ-H2AX. Collectively, our findings demonstrate that the H. longiflorus extract is highly cytotoxic to U2OS cells, most likely due to its polyphenolic composition.

The reversal of antineoplastic drug resistance in cancer cells by β-elemene

Multidrug resistance (MDR), defined as the resistance of cancer cells to compounds with diverse structures and mechanisms of actions, significantly limits the efficacy of antitumor drugs. A major mechanism that mediates MDR in cancer is the overexpression of adenosine triphosphate (ATP)-binding cassette transporters. These transporters bind to their respective substrates and catalyze their efflux from cancer cells, thereby lowering the intracellular concentrations of the substrates and thus attenuating or even abolishing their efficacy. In addition, cancer cells can become resistant to drugs via mechanisms that attenuate apoptosis and cell cycle arrest such as alterations in the p53, check point kinase, nuclear factor kappa B, and the p38 mitogen-activated protein kinase pathway. In this review, we discuss the mechanisms by which β-elemene, a compound extracted from Rhizoma zedoariae that has clinical antitumor efficacy, overcomes drug resistance in cancer.

Pathways and progress in improving drug delivery through the intestinal mucosa and blood-brain barriers

One of the major hurdles in developing therapeutic agents is the difficulty in delivering drugs through the intestinal mucosa and blood-brain barriers (BBB). The goal here is to describe the general structures of the biological barriers and the strategies to enhance drug delivery across these barriers. Prodrug methods used to improve drug penetration via the transcellular pathway have been successfully developed, and some prodrugs have been used to treat patients. The use of transporters to improve absorption of some drugs (e.g., antiviral agents) has also been successful in treating patients. Other methods, including blocking the efflux pumps to improve transcellular delivery, and modulation of cell-cell adhesion in the intercellular junctions to improve paracellular delivery across biological barriers, are still in the investigational stage.

The pharmacology of novel oral anticoagulants

Anticoagulation for the prevention of stroke is an important aspect of the management of atrial fibrillation. Novel anticoagulants including oral factor Xa inhibitors rivaroxaban and apixaban and the direct thrombin inhibitor dabigatran have emerged as important therapeutic treatment options for prevention of stroke in non-valvular atrial fibrillation. These agents offer practical advantages over traditional vitamin K antagonists, however an understanding of their individual pharmacokinetic and other agent-specific differences is essential for identifying appropriate candidates for therapy, and for selecting the appropriate agent that will be effective and safe. Here, we review the pharmacokinetic process of oral medication use, summarize the newer anticoagulants, their pharmacology, individual pharmacokinetic features, and explore possible explanations for the differences in bleeding outcomes observed in the clinical trials.

Selenorhodamine photosensitizers for photodynamic therapy of P-glycoprotein-expressing cancer cells

We examined a series of selenorhodamines with amide and thioamide functionality at the 5-position of a 9-(2-thienyl) substituent on the selenorhodamine core for their potential as photosensitizers for photodynamic therapy (PDT) in P-glycoprotein (P-gp) expressing cells. These compounds were examined for their photophysical properties (absorption, fluorescence, and ability to generate singlet oxygen), for their uptake into Colo-26 cells in the absence or presence of verapamil, for their dark and phototoxicity toward Colo-26 cells, for their rates of transport in monolayers of multidrug-resistant, P-gp-overexpressing MDCKII-MDR1 cells, and for their colocalization with mitochondrial specific agents in Colo-26 cells. Thioamide derivatives 16b and 18b were more effective photosensitizers than amide derivatives 15b and 17b. Selenorhodamine thioamides 16b and 18b were useful in a combination therapy to treat Colo-26 cells in vitro: a synergistic therapeutic effect was observed when Colo-26 cells were exposed to PDT and treatment with the cancer drug doxorubicin.

Polyoxypregnane steroids from the stems of Marsdenia tenacissima

A new polyoxypregnane aglycone, tenacigenin D (1), and seven new C21 steroid glycosides, tenacissimosides D-J (2-8), were isolated from the stems of Marsdenia tenacissima. Their structures were determined by interpretation of their 1D and 2D NMR and other spectroscopic data, as well as by comparison with published values for related known compounds. Compound 1 was found to circumvent P-glycoprotein (P-gp)-mediated multidrug resistance through an inhibitory effect on P-gp with a similar potency to verapamil. In addition, compound 1 potentiated the activity of erlotinib and gefitinib in epidermal growth factor receptor tyrosine kinase inhibitor (EGFR TKI)-resistant non-small-cell lung cancer cells.

Nanodrug delivery in reversing multidrug resistance in cancer cells

Different mechanisms in cancer cells become resistant to one or more chemotherapeutics is known as multidrug resistance (MDR) which hinders chemotherapy efficacy. Potential factors for MDR includes enhanced drug detoxification, decreased drug uptake, increased intracellular nucleophiles levels, enhanced repair of drug induced DNA damage, overexpression of drug transporter such as P-glycoprotein(P-gp), multidrug resistance-associated proteins (MRP1, MRP2), and breast cancer resistance protein (BCRP). Currently nanoassemblies such as polymeric/solid lipid/inorganic/metal nanoparticles, quantum dots, dendrimers, liposomes, micelles has emerged as an innovative, effective, and promising platforms for treatment of drug resistant cancer cells. Nanocarriers have potential to improve drug therapeutic index, ability for multifunctionality, divert ABC-transporter mediated drug efflux mechanism and selective targeting to tumor cells, cancer stem cells, tumor initiating cells, or cancer microenvironment. Selective nanocarrier targeting to tumor overcomes dose-limiting side effects, lack of selectivity, tissue toxicity, limited drug access to tumor tissues, high drug doses, and emergence of multiple drug resistance with conventional or combination chemotherapy. Current review highlights various nanodrug delivery systems to overcome mechanism of MDR by neutralizing, evading, or exploiting the drug efflux pumps and those independent of drug efflux pump mechanism by silencing Bcl-2 and HIF1α gene expressions by siRNA and miRNA, modulating ceramide levels and targeting NF-κB. “Theragnostics” combining a cytotoxic agent, targeting moiety, chemosensitizing agent, and diagnostic imaging aid are highlighted as effective and innovative systems for tumor localization and overcoming MDR. Physical approaches such as combination of drug with thermal/ultrasound/photodynamic therapies to overcome MDR are focused. The review focuses on newer drug delivery systems developed to overcome MDR in cancer cell.

β-Elemene, a compound derived from Rhizoma zedoariae, reverses multidrug resistance mediated by the ABCB1 transporter

In the present in vitro study, we examined the effect of the compound β-elemene on the response of KB-C2 cells overexpressing the ABCB1 transporter to specific antineoplastic compounds. The MTT assay was used to determine the effects of β-elemene in combination with other anticancer drugs on ABCB1-overexpressing cancer cell lines. Furthermore, we used [3H]-paclitaxel accumulation, efflux assay, immunofluorescence experiments, western blot assays and docking analysis to ascertain the mechanism of action of β-elemene. The incubation of KB-C2 cells overexpressing ABCB1 transporter with β-elemene (100 µM) significantly augmented the antineoplastic efficacy of colchicine, vinblastine and paclitaxel when compared to KB-C2 cells incubated with these drugs alone. In HEK293 cells overexpressing the ABCB1 transporter, β-elemene significantly increased the cytotoxicity of paclitaxel. In addition, 100 µM of β-elemene significantly increased the accumulation of [3H]-paclitaxel and this was due to a decrease in [3H]-paclitaxel efflux when compared to controls. The incubation of KB-C2 cells with β-elemene (100 µM) for 72 h did not significantly alter the expression of ABCB1 protein levels. Immunofluorescence experiments indicated that β-elemene did not significantly alter the subcellular localization of the ABCB1 transporter. Docking analysis indicated that β-elemene binds to the drug-binding site of ABCB1 transporter. Finally, β-elemene at 100 µM partially (~50%) increased the sensitivity of the BCRP-overexpressing cell line, NCI-H460/MX20, to mitoxantrone, but β-elemene did not significantly alter the resistance of MRP1-transfected HEK293/MRP1 cells to vincristine. Overall, our in vitro findings indicated that β-elemene potentiates the cytotoxic effects of various antineoplastic drugs in cell lines overexpressing the ABCB1 transporter and that this is due to the inhibition of the efflux component of the ABCB1 transporter.

Overview of P-glycoprotein inhibitors: a rational outlook

P-glycoprotein (P-gp), a transmembrane permeability glycoprotein, is a member of ATP binding cassette (ABC) super family that functions specifically as a carrier mediated primary active efflux transporter. It is widely distributed throughout the body and has a diverse range of substrates. Several vital therapeutic agents are substrates to P-gp and their bioavailability is lowered or a resistance is induced because of the protein efflux. Hence P-gp inhibitors were explored for overcoming multidrug resistance and poor bioavailability problems of the therapeutic P-gp substrates. The sensitivity of drug moieties to P-gp and vice versa can be established by various experimental models in silico, in vitro and in vivo. Ever since the discovery of P-gp, the research plethora identified several chemical structures as P-gp inhibitors. The aim of this review was to emphasize on the discovery and development of newer, inert, non-toxic, and more efficient, specifically targeting P-gp inhibitors, like those among the natural herb extracts, pharmaceutical excipients and formulations, and other rational drug moieties. The applications of cellular and molecular biology knowledge, in silico designed structural databases, molecular modeling studies and quantitative structure-activity relationship (QSAR) analyses in the development of novel rational P-gp inhibitors have also been mentioned.

OSI-930 analogues as novel reversal agents for ABCG2-mediated multidrug resistance

OSI-930, a dual c-Kit and KDR tyrosine kinase inhibitor, is reported to have undergone a Phase I dose escalation study in patients with advanced solid tumors. A series of fifteen pyridyl and phenyl analogues of OSI-930 were designed and synthesized. Extensive screening of these compounds led to the discovery that nitropyridyl and ortho-nitrophenyl analogues, VKJP1 and VKJP3, were effective in reversing ABC subfamily G member 2 (ABCG2) transporter-mediated multidrug resistance (MDR). VKJP1 and VKJP3 significantly sensitized ABCG2-expressing cells to established substrates of ABCG2 including mitoxantrone, SN-38, and doxorubicin in a concentration-dependent manner, but not to the non-ABCG2 substrate cisplatin. However, they were unable to reverse ABCB1- or ABCC1-mediated MDR indicating their selectivity for ABCG2. Western blotting analysis was performed to evaluate ABCG2 expression and it was found that neither VKJP1 nor VKJP3 significantly altered ABCG2 protein expression for up to 72 h. [(3)H]-mitoxantrone accumulation study demonstrated that VKJP1 and VKJP3 increased the intracellular accumulation of [(3)H]-mitoxantrone, a substrate of ABCG2. VKJP1 and VKJP3 also remarkably inhibited the transport of [(3)H]-methotrexate by ABCG2 membrane vesicles. Importantly, both VKJP1 and VKJP3 were efficacious in stimulating the activity of ATPase of ABCG2 and inhibited the photoaffinity labeling of this transporter by its substrate [(125)I]-iodoarylazidoprazosin. The results suggested that VKJP1 and VKJP3, specifically inhibit the function of ABCG2 through direct interaction with its substrate binding site(s). Thus VKJP1 and VKJP3 represent a new class of drugs for reducing MDR in ABCG2 over-expressing tumors.

P-glycoprotein silencing with siRNA delivered by DOPE-modified PEI overcomes doxorubicin resistance in breast cancer cells

AIMS

Multidrug resistance (MDR) mediated by overexpression of drug efflux transporters such as P-glycoprotein (P-gp), is a major problem, limiting successful chemotherapy of breast cancer. The use of siRNA to inhibit P-gp expression in MDR tumors is an attractive strategy to improve the effectiveness of anticancer drugs.

METHOD

We have synthesized a novel conjugate between a phospholipid (dioleoylphosphatidylethanolamine) and polyethylenimine (PEI) for siRNA delivery, for the purpose of silencing P-gp to overcome doxorubicin resistance in MCF-7 human breast cancer cells.

RESULTS

The dioleoylphosphatidylethanolamine-PEI conjugate enhanced the transfection efficacy of low-molecular-weight PEI, which was otherwise totally ineffective. In addition, the polyethylene glycol/lipid coating of the new complexes gave rise to small micelle-like nanoparticles with improved biocompatibility properties. Both coated and noncoated formulations delivered P-gp-specific siRNA to MDR cells.

DISCUSSION

The combination of doxorubicin and P-gp silencing formulations led to a twofold increase of doxorubicin uptake and a significant improvement of the therapeutic effect of doxorubicin in resistant cells.

Calcein assay: a high-throughput method to assess P-gp inhibition

Transporter mediated drug-drug interactions (tDDI) mediated by ABCB1 have been shown to be clinically relevant. Hence, the assessment of the ABCB1 tDDI potential early in the drug development process has gained interest. We have evaluated the Calcein assay as a means of assessing the ABCB1 tDDI that is amenable to high throughout and compared it with the monolayer efflux assay. We found the Calcein assay, when performed in K562MDR cells using the protocol originally published more sensitive than digoxin transport inhibition in MDCKII-MDR1 cells. Application of the Calcein assay to cell lines containing different amounts of ABCB1, yielded IC(50) values that varied 10-100-fold. The differences observed for IC(50) values for the same compounds were in the following rank order: IC(50, MDCKII-MDR1) >IC(50, K562MDR)>IC(50, hCMEC/D3). Higher IC(50) values were obtained in cells with higher ABCB1 expression. The Calcein assay is a high-throughput alternative to digoxin transport inhibition as it appears to have a comparable selectivity but higher sensitivity than previously published digoxin transport inhibition in MDCKII-MDR1 cells. In addition, it can be performed in a barrier-specific manner highlighting the dependence of ABCB1 IC(50) values on different ABCB1 expression levels.

Proteasome inhibitor-I enhances tunicamycin-induced chemosensitization of prostate cancer cells through regulation of NF-κB and CHOP expression

Although endoplasmic reticulum (ER) stress induction by some anticancer drugs can lead to apoptotic death of cancer cells, combination therapy with other chemicals would be much more efficient. It has been reported that proteasome inhibitors could induce cancer cell death through ER-stress. Our study, however, showed a differential mechanism of proteasome inhibitor-I (Pro-I)-induced cell death. Pro-I significantly enhanced apoptotic death of PC3 prostate cancer cells pretreated with tunicamycin (TM) while other signaling inhibitors against p38, mitogen activated kinase (MEK) and phosphatidyl-inositol 3-kinase (PI3K) did not, as evidenced by cell proliferation and cell cycle analyses. NF-κB inhibition by Pro-I, without direct effect on ER-stress, was found to be responsible for the TM-induced chemosensitization of PC3 cells. Moreover, TM-induced/enhancer-binding protein (C/EBP) homologous protein (CHOP) expression was enhanced by Pro-I without change in GRP78 expression. CHOP knockdown by siRNA also showed a significant decrease in Pro-I chemosensitization. All these data suggest that although TM could induce both NF-κB activation and CHOP expression through ER-stress, both NF-κB inhibition and increased CHOP level by Pro-I are required for enhanced chemosensitization of PC3 prostate cancer cells. Thus, our study might contribute to the identification of anticancer targets against prostate cancer cells.

Marine sponge-derived sipholane triterpenoids reverse P-glycoprotein (ABCB1)-mediated multidrug resistance in cancer cells

Previously, we reported sipholenol A, a sipholane triterpenoid from the Red Sea sponge Callyspongia siphonella, as a potent reversal of multidrug resistance (MDR) in cancer cells that overexpressed P-glycoprotein (P-gp). Through extensive screening of several related sipholane triterpenoids that have been isolated from the same sponge, we identified sipholenone E, sipholenol L and siphonellinol D as potent reversals of MDR in cancer cells. These compounds enhanced the cytotoxicity of several P-gp substrate anticancer drugs, including colchicine, vinblastine and paclitaxel, and significantly reversed the MDR-phenotype in P-gp-overexpressing MDR cancer cells KB-C2 in a dose-dependent manner. Moreover, these three sipholanes had no effect on the response to cytotoxic agents in cells lacking P-gp expression or expressing MRP1 (ABCC1) or MRP7 (ABCC10) or breast cancer resistance protein (BCRP/ABCG2). All three sipholanes (IC(50) >50 μM) were not toxic to all the cell lines that were used. [(3)H]-Paclitaxel accumulation and efflux studies demonstrated that all three triterpenoids time-dependently increased the intracellular accumulation of [(3)H]-paclitaxel by directly inhibiting P-gp-mediated drug efflux. Sipholanes also inhibited calcein-AM transport from P-gp-overexpressing cells. The Western blot analysis revealed that these three triterpenoids did not alter the expression of P-gp. However, they stimulated P-gp ATPase activity in a concentration-dependent manner and inhibited the photolabeling of this transporter with its transport substrate [(125)I]-iodoarylazidoprazosin. In silico molecular docking aided the virtual identification of ligand binding sites of these compounds. In conclusion, sipholane triterpenoids efficiently inhibit the function of P-gp through direct interactions and may represent potential reversal agents for the treatment of MDR.

Influence of blood-brain barrier efflux pumps on the distribution of vincristine in brain and brain tumors

Vincristine (VCR) is efficacious in some but not all brain cancers and an established substrate of Pgp and Mrp1. However, the extent to which such transporters affect the VCR penetration through the blood-brain barrier (BBB) is poorly understood. To evaluate the role of Pgp and Mrp1 in VCR CNS distribution, VCR concentrations were analyzed under steady-state conditions in normal brain, brain tumor, and bone marrow in wild-type (WT), Mrp1 ko (mrp1-/-), Pgp ko (mdr1a-/-:mdr1b-/-), and TKO (mdr1a-/-:mdr1b-/-:mrp1-/-) mice. VCR normal brain partition coefficients (i.e. tissue/plasma VCR concentrations) in TKO mice were greater than those in WT mice at both targeted 10 and 50 ng/mL plasma VCR concentrations, and ranged from 1.3- to 3.6-fold. VCR brain tumor partition coefficients in Mrp1 mice were greater than WT mice at both doses, being 1.5- and 2.4-fold higher at low and high doses, respectively. TKO mice also showed elevated VCR brain tumor penetration with a brain tumor partition coefficient of 1.9-fold greater than that in WT mice at the high-dose level. The bone marrow partition coefficient in Mrp1 ko mice was 1.65-fold greater than that in WT mice. Within strain comparisons revealed that VCR brain tumor concentrations were significantly greater than normal brain in all strains, ranging from 9- to 40-fold. These findings indicate that disruption of the BBB caused the largest enhancement in VCR tumor concentrations, yet the absence of Mrp1 on the brain tumor vasculature could enhance the penetration compared with that in normal brain.

Sex-related differences in the gastrointestinal disposition of ivermectin in the rat: P-glycoprotein involvement and itraconazole modulation

Ivermectin (IVM), a macrocyclic lactone used as antiparasite agent, has been reported as a P-glycoprotein (P-gp) substrate. The participation of P-gp in the IVM excretion process has been previously demonstrated. Sex-related differences in the kinetic behaviour of some macrocyclic lactone compounds have been observed. The aim of this work was to characterize in-vivo the comparative gastrointestinal disposition of IVM in male and female rats. The sex-related influence on the itraconazole (ITZ) modulation of P-gp-mediated IVM intestinal transport was also assessed. Sixty Wistar rats (30 male, 30 female) received IVM alone or co-administered with ITZ. Rats were killed between 6 and 72h after treatment and blood, gastrointestinal tissues and lumen contents were collected. IVM concentrations were determined by high performance liquid chromatography. Substantial sex-related differences in the IVM disposition kinetics were observed. Higher IVM systemic availability was observed in female rats. The ITZ-mediated modulation of the IVM disposition kinetics had a differential impact between male and female rats. Co-administration with ITZ resulted in a marked increase in the IVM concentrations in the wall tissue from different portions of the gastrointestinal tract of male rats. The presence of ITZ induced drastic sex-related changes on the P-gp-mediated IVM gastrointestinal disposition.