Drug-protein hydrogen bonds govern the inhibition of the ATP hydrolysis of the multidrug transporter P-glycoprotein

Structural insights into binding-site access and ligand recognition by human ABCB1

ABCB1 is a broad-spectrum efflux pump central to cellular drug handling and multidrug resistance in humans. However, how it is able to recognize and transport a wide range of diverse substrates remains poorly understood. Here we present cryo-EM structures of lipid-embedded human ABCB1 in conformationally distinct apo-, substrate-bound, inhibitor-bound, and nucleotide-trapped states at 3.4-3.9 Å resolution, in the absence of stabilizing antibodies or mutations. The substrate-binding site is located within one half of the molecule and, in the apo state, is obstructed by the transmembrane helix (TM) 4. Substrate and inhibitor binding are distinguished by major TM rearrangements and their ligand binding chemistry, with TM4 playing a central role in all conformational transitions. Furthermore, our data identify secondary structure-breaking residues that impart localized TM flexibility and asymmetry between the two transmembrane domains. The resulting structural changes and lipid interactions that are induced by substrate and inhibitor binding can predict substrate-binding profiles and may direct ABCB1 inhibitor design.

USP14 increases the sensitivity of retinoblastoma to cisplatin by mediating the ferroptosis

The aim of this study is to explore the function of USP14 on the sensitivity of retinoblastoma (RB) to cisplatin (DDP) and the underlying mechanism. USP14 was knockdown in Y79 cells by transfecting three siRNAs (si-USP14-1, si-USP14-2, and si-USP14-3), with si-USP14 NC as the negative control. si-USP14-3 was selected by results of Western blotting. The CCK-8 assay was used to detect the IC50 of Y79 cells and the growth curve. The cell cycle, cell apoptosis, and ROS level were measured by flow cytometry. The expression level of P-GP, ERCC1, survivin, GPX4, FTH1, ACSL4, NOX1, COX2, and FASN was determined by the Western blotting assay. CO-IP assay was utilized to evaluate the interaction between USP14 and FASN. The IC50 of DDP in Y79 cells and Y79/DDP cells was 7.83 µM and 24.67 µM, respectively. Compared to control and si-USP14 NC groups, increased apoptotic rate and ROS level, and arrested cell cycle in S phase were observed in USP14-knockdown Y79 cells. Compared to control and si-USP14 NC groups, increased apoptotic rate and arrested cell cycle in G0/G1 phase were observed in USP14-knockdown Y79/DDP cells. Compared to control, increased ROS level was observed in USP14-knockdown Y79/DDP cells. Compared to the si-USP14 NC groups, extremely downregulated P-GP, ERCC1, survivin, GPX4, FTH1, NOX1, COX2, and FASN were observed in USP14-knockdown Y79 cells or Y79/DDP cells, accompanied by the elevated expression of ACSL4. The interaction between USP14 and FASN was identified according to the result of CO-IP assay. By silencing USP14 in Y79 and Y79/DDP cells, levels of resistance-related proteins (P-GP, ERCC1, and survivin), ferroptosis-related proteins (FTH1 and GPX4), and lipid metabolism-related proteins (NOX1, COX2, and FASN) were dramatically reduced, accompanied by enhanced ROS level, increased apoptosis, and restrained DNA content, indicating that USP14 might suppress the DDP resistance in RB by mediating ferroptosis, which is an important target for treating RB.

Structural insight into binding site access and ligand recognition by human ABCB1

ABCB1 is a broad-spectrum efflux pump central to cellular drug handling and multidrug resistance in humans. However, its mechanisms of poly-specific substrate recognition and transport remain poorly resolved. Here we present cryo-EM structures of lipid embedded human ABCB1 in its apo, substrate-bound, inhibitor-bound, and nucleotide-trapped states at 3.4-3.9 Å resolution without using stabilizing antibodies or mutations and each revealing a distinct conformation. The substrate binding site is located within one half of the molecule and, in the apo state, is obstructed by transmembrane helix (TM) 4. Substrate and inhibitor binding are distinguished by major differences in TM arrangement and ligand binding chemistry, with TM4 playing a central role in all conformational transitions. Our data offer fundamental new insights into the role structural asymmetry, secondary structure breaks, and lipid interactions play in ABCB1 function and have far-reaching implications for ABCB1 inhibitor design and predicting its substrate binding profiles.

Formononetin Defeats Multidrug-Resistant Cancers by Induction of Oxidative Stress and Suppression of P-Glycoprotein

Multidrug resistance (MDR) remains the most difficult problem facing conventional chemotherapy for cancers. Astragalus membranaceus is a historically traditional Chinese medicine. One of its bioactive components, formononetin, exhibits antitumor effects on various cancers. However, the effects of formononetin on MDR cancers have not been evaluated. Therefore, we investigated the defense's effects of formononetin on MDR. We used rhodamine 123 and doxorubicin efflux assays to analyze the inhibition kinetics of P-glycoprotein (P-gp) mediated-efflux. Cell viability was detected by sulforhodamine B assay, and the synergistic effects of formononetin combined with chemotherapeutic agents were further calculated using CompuSyn software. Molecular docking was performed with iGEMDOCK. We discovered that formononetin considerably induced oxidative stress and the disruption of mitochondrial membrane potential in MDR cancer cells. Furthermore, formononetin inhibits the P-gp efflux function by ATPase stimulation and the uncompetitive inhibition of P-gp-mediated effluxes of rhodamine 123 and doxorubicin. The molecular docking model indicates that formononetin may bind to P-gp by strong hydrogen bonds at Arginine (Arg) 489 and Glutamine (Gln) 912. Formononetin exhibits significant synergistic effects with vincristine and doxorubicin toward MDR cancer cells, and it synergistically suppressed tumor growth in vivo with paclitaxel. These results suggest that formononetin should be seen as a potential candidate for the adjuvant therapy of MDR cancers.

A Novel 3-meta-Pyridine-1,2,4-oxadiazole Derivative of Glycyrrhetinic Acid as a Safe and Promising Candidate for Overcoming P-Glycoprotein-Mediated Multidrug Resistance in Tumor Cells

Given the pharmacophore properties of the nitrogen-containing moiety in the molecular structure of P-glycoprotein (P-gp) inhibitors, we report the evaluation of the P-gp inhibitory and MDR reversal activities of 2g, a 3-meta-pyridin-1,2,4-oxadiazole derivative of 18βH-glycyrrhetinic acid. Through molecular docking, we have shown that 2g has the potential to directly interact with the transmembrane domain of P-gp with a low free binding energy (-10.2 kcal/mol). Using KB-8-5 human cervical carcinoma cells and RLS40 murine lymphosarcoma cells, both of which exhibit a multidrug-resistant (MDR) phenotype mediated by P-gp activation, we have shown that 2g, at nontoxic concentrations, effectively increased the intracellular accumulation of fluorescent P-gp substrates (rhodamine 123 or doxorubicin (DOX)), leading to a marked sensitization of the model cells to the cytotoxic effect of DOX. Considering the comparable activity of 2g with verapamil, a known P-gp inhibitor, 2g can be considered as a promising candidate for the development of agents capable of overcoming P-gp-mediated MDR in tumor cells.

Study Models of Drug-Drug Interactions Involving P-Glycoprotein: The Potential Benefit of P-Glycoprotein Modulation at the Kidney and Intestinal Levels

P-glycoprotein (P-gp) is a crucial membrane transporter situated on the cell's apical surface, being responsible for eliminating xenobiotics and endobiotics. P-gp modulators are compounds that can directly or indirectly affect this protein, leading to changes in its expression and function. These modulators can act as inhibitors, inducers, or activators, potentially causing drug-drug interactions (DDIs). This comprehensive review explores diverse models and techniques used to assess drug-induced P-gp modulation. We cover several approaches, including in silico, in vitro, ex vivo, and in vivo methods, with their respective strengths and limitations. Additionally, we explore the therapeutic implications of DDIs involving P-gp, with a special focus on the renal and intestinal elimination of P-gp substrates. This involves enhancing the removal of toxic substances from proximal tubular epithelial cells into the urine or increasing the transport of compounds from enterocytes into the intestinal lumen, thereby facilitating their excretion in the feces. A better understanding of these interactions, and of the distinct techniques applied for their study, will be of utmost importance for optimizing drug therapy, consequently minimizing drug-induced adverse and toxic effects.

Second-site suppressor mutations reveal connection between the drug-binding pocket and nucleotide-binding domain 1 of human P-glycoprotein (ABCB1)

Human P-glycoprotein (P-gp) or ABCB1 is overexpressed in many cancers and has been implicated in altering the bioavailability of chemotherapeutic drugs due to their efflux, resulting in the development of chemoresistance. To elucidate the mechanistic aspects and structure-function relationships of P-gp, we previously utilized a tyrosine (Y)-enriched P-gp mutant (15Y) and demonstrated that at least 15 conserved residues in the drug-binding pocket of P-gp are responsible for optimal substrate interaction and transport. To further understand the role of these 15 residues, two new mutants were generated, namely 6Y with the substitution of six residues (F72, F303, I306, F314, F336 and L339) with Y in transmembrane domain (TMD) 1 and 9Y with nine substitutions (F732, F759, F770, F938, F942, M949, L975, F983 and F994) in TMD2. Although both the mutants were expressed at normal levels at the cell surface, the 6Y mutant failed to transport all the tested substrates except Bodipy-verapamil, whereas the 9Y mutant effluxed all tested substrates in a manner very similar to that of the wild-type protein. Further mutational analysis revealed that two second-site mutations, one in intracellular helix (ICH) 4 (F916Y) and one in the Q loop of nucleotide-binding domain (NBD) 1 (F480Y) restored the transport function of 6Y. Additional biochemical data and comparative molecular dynamics simulations of the 6Y and 6Y+F916Y mutant indicate that the Q-loop of NBD1 of P-gp communicates with the substrate-binding sites in the transmembrane region through ICH4. This is the first evidence for the existence of second-site suppressors in human P-gp that allow recovery of the loss of transport function caused by primary mutations. Further study of such mutations could facilitate mapping of the communication pathway between the substrate-binding pocket and the NBDs of P-gp and possibly other ABC drug transporters.

Synergistic Inhibitory Effect of Quercetin and Cyanidin-3O-Sophoroside on ABCB1

The human ABCB1 (P-glycoprotein, Pgp) protein is an active exporter expressed in the plasma membrane of cells forming biological barriers. In accordance with its broad substrate spectrum and tissue expression pattern, it affects the pharmacokinetics of numerous chemotherapeutic drugs and it is involved in unwanted drug-drug interactions leading to side effects or toxicities. When expressed in tumor tissues, it contributes to the development of chemotherapy resistance in malignancies. Therefore, the understanding of the molecular details of the ligand-ABCB1 interactions is of crucial importance. In a previous study, we found that quercetin (QUR) hampers both the transport and ATPase activity of ABCB1, while cyandin-3O-sophroside (C3S) stimulates the ATPase activity and causes only a weak inhibition of substrate transport. In the current study, when QUR and C3S were applied together, both a stronger ATPase inhibition and a robust decrease in substrate transport were observed, supporting their synergistic ABCB1 inhibitory effect. Similar to cyclosporine A, a potent ABCB1 inhibitor, co-treatment with QUR and C3S shifted the conformational equilibrium to the "inward-facing" conformer of ABCB1, as it was detected by the conformation-selective UIC2 mAb. To gain deeper insight into the molecular details of ligand-ABCB1 interactions, molecular docking experiments and MD simulations were also carried out. Our in silico studies support that QUR and C3S can bind simultaneously to ABCB1. The most favourable ligand-ABCB1 interaction is obtained when C3S binds to the central substrate binding site and QUR occupies the "access tunnel". Our results also highlight that the strong ABCB1 inhibitory effect of the combined treatment with QUR and C3S may be exploited in chemotherapy protocols for the treatment of multidrug-resistant tumors or for improving drug delivery through pharmacological barriers.

In Silico and In Vitro Identification of P-Glycoprotein Inhibitors from a Library of 375 Phytochemicals

Cancer therapy with clinically established anticancer drugs is frequently hampered by the development of drug resistance of tumors and severe side effects in normal organs and tissues. The demand for powerful, but less toxic, drugs is high. Phytochemicals represent an important reservoir for drug development and frequently exert less toxicity than synthetic drugs. Bioinformatics can accelerate and simplify the highly complex, time-consuming, and expensive drug development process. Here, we analyzed 375 phytochemicals using virtual screenings, molecular docking, and in silico toxicity predictions. Based on these in silico studies, six candidate compounds were further investigated in vitro. Resazurin assays were performed to determine the growth-inhibitory effects towards wild-type CCRF-CEM leukemia cells and their multidrug-resistant, P-glycoprotein (P-gp)-overexpressing subline, CEM/ADR5000. Flow cytometry was used to measure the potential to measure P-gp-mediated doxorubicin transport. Bidwillon A, neobavaisoflavone, coptisine, and z-guggulsterone all showed growth-inhibitory effects and moderate P-gp inhibition, whereas miltirone and chamazulene strongly inhibited tumor cell growth and strongly increased intracellular doxorubicin uptake. Bidwillon A and miltirone were selected for molecular docking to wildtype and mutated P-gp forms in closed and open conformations. The P-gp homology models harbored clinically relevant mutations, i.e., six single missense mutations (F336Y, A718C, Q725A, F728A, M949C, Y953C), three double mutations (Y310A-F728A; F343C-V982C; Y953A-F978A), or one quadruple mutation (Y307C-F728A-Y953A-F978A). The mutants did not show major differences in binding energies compared to wildtypes. Closed P-gp forms generally showed higher binding affinities than open ones. Closed conformations might stabilize the binding, thereby leading to higher binding affinities, while open conformations may favor the release of compounds into the extracellular space. In conclusion, this study described the capability of selected phytochemicals to overcome multidrug resistance.

On-target inhibition of Cryptosporidium parvum by nitazoxanide (NTZ) and paclitaxel (PTX) validated using a novel MDR1-transgenic host cell model and algorithms to quantify the effect on the parasite target

Cryptosporidium parvum is a globally distributed zoonotic protozoan parasite that causes moderate to severe, sometime deadly, watery diarrhea in humans and animals, for which fully effective treatments are yet unavailable. In studying the mechanism of action of drugs against intracellular pathogens, it is important to validate whether the observed anti-infective activity is attributed to the drug action on the pathogen or host target. For the epicellular parasite Cryptosporidium, we have previously developed a concept that the host cells with significantly increased drug tolerance by transient overexpression of the multidrug resistance protein-1 (MDR1) could be utilized to evaluate whether and how much the observed anti-cryptosporidial activity of an inhibitor was attributed to the inhibitor’s action on the parasite target. However, the transient transfection model was only applicable to evaluating native MDR1 substrates. Here we report an advanced model using stable MDR1-transgenic HCT-8 cells that allows rapid development of novel resistance to non-MDR1 substrates by multiple rounds of drug selection. Using the new model, we successfully validated that nitazoxanide, a non-MDR1 substrate and the only FDA-approved drug to treat human cryptosporidiosis, killed C. parvum by fully (100%) acting on the parasite target. We also confirmed that paclitaxel acted fully on the parasite target, while several other inhibitors including mitoxantrone, doxorubicin, vincristine and ivermectin acted partially on the parasite targets. Additionally, we developed mathematical models to quantify the proportional contribution of the on-parasite-target effect to the observed anti-cryptosporidial activity and to evaluate the relationships between several in vitro parameters, including antiparasitic efficacy (ECi), cytotoxicity (TCi), selectivity index (SI) and Hill slope (h). Owning to the promiscuity of the MDR1 efflux pump, the MDR1-transgenic host cell model could be applied to assess the on-parasite-target effects of newly identified hits/leads, either substrates or non-substrates of MDR1, against Cryptosporidium or other epicellular pathogens.

Analysis of P-Glycoprotein Transport Cycle Reveals a New Way to Identify Efflux Inhibitors

P-glycoprotein (P-gp) is found to be of considerable interest for the design of drugs capable of treating chemoresistant tumors. This transporter is an interesting target for which an efficient approach has not yet been developed in terms of computer simulation. In this work, we use a combination of docking, molecular dynamics, and metadynamics to fully explore the states that occur during the capture of a ligand and subsequent efflux by P-gp. The proposed approach allowed us to substantiate a number of experimentally established facts, as well as to develop a new criterion for identifying potential P-gp inhibitors.

Vinca alkaloid binding to P-glycoprotein occurs in a processive manner

A mechanistic understanding of how P-glycoprotein (Pgp) is able to bind and transport its astonishing range of substrates remains elusive. Pharmacological data demonstrated the presence of at least four distinct binding sites, but their locations have not been fully elucidated. The combination of biochemical and structural data suggests that initial binding may occur in the central cavity or at the lipid-protein interface. Our objective was to define the binding sites for two transported substrates of Pgp; the anticancer drug vinblastine and the fluorescent probe rhodamine 123. A series of mutations was generated in positions proximal to previously defined drug-interacting residues on Pgp. The protein was purified and reconstituted into styrene-maleic acid lipid particles (SMALPs) to measure the apparent drug binding constant or into liposomes for assessment of drug-stimulated ATP hydrolysis. The biochemical data were reconciled with structural models of Pgp using molecular docking. The data indicated that the binding of rhodamine 123 occurred predominantly within the central cavity of Pgp. In contrast, the significantly more hydrophobic vinblastine bound to both the lipid-protein interface and within the central cavity. The data suggest that the initial interaction of vinca alkaloids with Pgp occurs at the lipid interface followed by internalisation into the central cavity, which also provides the transport conduit. This model is supported by recent structural observations with Pgp and early biophysical and cross-linking approaches. Moreover, the proposed model illustrates that the broad substrate profile for Pgp is underpinned by a combination of multiple initial interaction sites and an accommodating transport conduit.

Overcoming multidrug resistance through targeting ABC transporters: lessons for drug discovery

INTRODUCTION

The argument around cancer therapy is an old one. Using chemotherapeutic drugs, as one of the most effective strategies in treatment of malignancies, is restricted by various issues that progress during therapy and avoid achieving clinical endpoints. Multidrug resistance (MDR), frequently mediated by ATP-binding cassette (ABC) transporters, is one of the most recognized obstacles in the success of pharmacological anticancer approaches. These transporters efflux diverse drugs to extracellular environment, causing MDR and responsiveness of tumor cells to chemotherapy diminishes.

AREAS COVERED

Several strategies have been used to overcome MDR phenomenon. Succession in this field requires complete knowledge about features and mechanism of ABC transporters. In this review, conventional synthetic and natural inhibitors are discussed first and then novel approaches including RNA, monoclonal antibodies, nanobiotechnology, and structural modification techniques are represented.

EXPERT OPINION

With increasing frequency of MDR in cancer cells, it is essential to develop new drugs to inhibit MDR. Using knowledge acquired about ABC transporter's structure, rational design of inhibitors is possible. Also, some herbal products have shown to be potential lead compounds in drug discovery for reversal of MDR.

Novel Epoxides of Soloxolone Methyl: An Effect of the Formation of Oxirane Ring and Stereoisomerism on Cytotoxic Profile, Anti-Metastatic and Anti-Inflammatory Activities In Vitro and In Vivo

It is known that epoxide-bearing compounds display pronounced pharmacological activities, and the epoxidation of natural metabolites can be a promising strategy to improve their bioactivity. Here, we report the design, synthesis and evaluation of biological properties of αO-SM and βO-SM, novel epoxides of soloxolone methyl (SM), a cyanoenone-bearing derivative of 18βH-glycyrrhetinic acid. We demonstrated that the replacement of a double-bound within the cyanoenone pharmacophore group of SM with α- and β-epoxide moieties did not abrogate the high antitumor and anti-inflammatory potentials of the triterpenoid. It was found that novel SM epoxides induced the death of tumor cells at low micromolar concentrations (IC₅₀^(24h) = 0.7–4.1 µM) via the induction of mitochondrial-mediated apoptosis, reinforced intracellular accumulation of doxorubicin in B16 melanoma cells, probably by direct interaction with key drug efflux pumps (P-glycoprotein, MRP1, MXR1), and the suppressed pro-metastatic phenotype of B16 cells, effectively inhibiting their metastasis in a murine model. Moreover, αO-SM and βO-SM hampered macrophage functionality in vitro (motility, NO production) and significantly suppressed carrageenan-induced peritonitis in vivo. Furthermore, the effect of the stereoisomerism of SM epoxides on the mentioned bioactivities and toxic profiles of these compounds in vivo were evaluated. Considering the comparable antitumor and anti-inflammatory effects of SM epoxides with SM and reference drugs (dacarbazine, dexamethasone), αO-SM and βO-SM can be considered novel promising antitumor and anti-inflammatory drug candidates.

Epifriedelanol enhances adriamycin-induced cytotoxicity towards K562/ADM cells by down regulating of P-gp and MRP2

1. Multidrug resistance (MDR) is a critical issue during chemotherapy of cancers. Epifriedelanol (Epi) is the effective compounds from the Root Bark of Ulmus davidiana. This study aims to investigate the effect of Epi on MDR and its potential mechanism in the adriamycin (Adr)-resistant K562/ADM cells.2. The effect of Epi on MDR, P-glycoprotein (P-gp) and multidrug resistance-associated proteins (MRPs) were investigated in the adriamycin (Adr)-resistant K562/ADM cells. In addition, the alterations of nuclear receptor pregnane X receptor (PXR) and constitutive androstane receptor (CAR) mRNA expression levels in K562/ADM cells after Epi treatment were also examined.3. Epi significantly enhanced Adr-induced cytotoxicity towards K562/ADM cells. Combination of Epi and Adr can significantly reduce the 50% inhibitory concentration (IC₅₀) of K562/ADM cells to Adr. The reversal fold was 1.83 and 3.64 after treated with Epi at 10 and 20 μM, respectively. The intracellular accumulation of Adr was significant increased after exposure to Epi at 5-20 μM compared with the control group. Furthermore, Epi treatment significantly decreased the mRNA and protein expression of P-gp and MRP2 in K562/ADM cells.4. The present study demonstrated that Epi could enhance Adr-induced cytotoxicity towards K562/ADM cells accompanied by the down-regulation of P-gp and MRP2.

JAK2 Inhibitor, Fedratinib, Inhibits P-gp Activity and Co-Treatment Induces Cytotoxicity in Antimitotic Drug-Treated P-gp Overexpressing Resistant KBV20C Cancer Cells

P-glycoprotein (P-gp) overexpression is one of the major mechanisms of multidrug resistance (MDR). Previously, co-treatment with Janus kinase 2 (JAK2) inhibitors sensitized P-gp-overexpressing drug-resistant cancer cells. In this study, we assessed the cytotoxic effects of JAK2 inhibitor, fedratinib, on drug-resistant KBV20C cancer cells. We found that co-treatment with fedratinib at low doses induced cytotoxicity in KBV20C cells treated with vincristine (VIC). However, fedratinib-induced cytotoxicity was little effect on VIC-treated sensitive KB parent cells, suggesting that these effects are specific to resistant cancer cells. Fluorescence-activated cell sorting (FACS), Western blotting, and annexin V analyses were used to further investigate fedratinib’s mechanism of action in VIC-treated KBV20C cells. We found that fedratinib reduced cell viability, increased G2 arrest, and upregulated apoptosis when used as a co-treatment with VIC. G2 phase arrest and apoptosis in VIC–fedratinib-co-treated cells resulted from the upregulation of p21 and the DNA damaging marker pH2AX. Compared with dimethyl sulfoxide (DMSO)-treated cells, fedratinib-treated KBV20C cells showed two-fold higher P-gp-inhibitory activity, indicating that VIC–fedratinib sensitization is dependent on the activity of fedratinib. Similar to VIC, fedratinib co-treatment with other antimitotic drugs (i.e., eribulin, vinorelbine, and vinblastine) showed increased cytotoxicity in KBV20C cells. Furthermore, VIC–fedratinib had similar cytotoxic effects to co-treatment with other JAK2 inhibitors (i.e., VIC–CEP-33779 or VIC–NVP-BSK805) at the same dose; similar cytotoxic mechanisms (i.e., early apoptosis) were observed between treatments, suggesting that co-treatment with JAK2 inhibitors is generally cytotoxic to P-gp-overexpressing resistant cancer cells. Given that fedratinib is FDA-approved, our findings support its application in the co-treatment of P-gp-overexpressing cancer patients showing MDR.

Long Range Communication between the Drug-Binding Sites and Nucleotide Binding Domains of the Efflux Transporter ABCB1

The ABC efflux pump P-glycoprotein (P-gp) transports a wide variety of drugs and is inhibited by others. Some drugs stimulate ATP hydrolysis at the nucleotide binding domains (NBDs) and are transported, others uncouple ATP hydrolysis and transport, and others inhibit ATP hydrolysis. The molecular basis for the different behavior of these drugs is not well understood despite the availability of several structural models of P-gp complexes with ligands bound. Hypothetically, ligands differentially alter the conformational dynamics of peptide segments that mediate the coupling between the drug binding sites and the NBDs. Here, we explore by hydrogen-deuterium exchange mass spectrometry the dynamic consequences of a classic substrate and inhibitor, vinblastine and zosuquidar, binding to mouse P-gp (mdr1a) in lipid nanodiscs. The dynamics of P-gp in nucleotide-free, pre-hydrolysis, and post-hydrolysis states in the presence of each drug reveal distinct mechanisms of ATPase stimulation and implications for transport. For both drugs, there are common regions affected in a similar manner, suggesting that particular networks are the key to stimulating ATP hydrolysis. However, drug binding effects diverge in the post-hydrolysis state, particularly in the intracellular helices (ICHs 3 and 4) and neighboring transmembrane helices. The local dynamics and conformational equilibria in this region are critical for the coupling of drug binding and ATP hydrolysis and are differentially modulated in the catalytic cycle.

Characterization and tissue localization of zebrafish homologs of the human ABCB1 multidrug transporter

Capillary endothelial cells of the human blood–brain barrier (BBB) express high levels of P-glycoprotein (P-gp, encoded by ABCB1) and ABCG2 (encoded by ABCG2). However, little information is available regarding ATP-binding cassette transporters expressed at the zebrafish BBB, which has emerged as a potential model system. We report the characterization and tissue localization of two genes that are similar to ABCB1, zebrafish abcb4 and abcb5. When stably expressed in HEK293 cells, both Abcb4 and Abcb5 conferred resistance to P-gp substrates; however, Abcb5 poorly transported doxorubicin and mitoxantrone compared to zebrafish Abcb4. Additionally, Abcb5 did not transport the fluorescent P-gp probes BODIPY-ethylenediamine or LDS 751, while they were transported by Abcb4. High-throughput screening of 90 human P-gp substrates confirmed that Abcb4 has an overlapping substrate specificity profile with P-gp. In the brain vasculature, RNAscope probes for abcb4 colocalized with staining by the P-gp antibody C219, while abcb5 was not detected. The abcb4 probe also colocalized with claudin-5 in brain endothelial cells. Abcb4 and Abcb5 had different tissue localizations in multiple zebrafish tissues, potentially indicating different functions. The data suggest that zebrafish Abcb4 functionally phenocopies P-gp and that the zebrafish may serve as a model to study the role of P-gp at the BBB.

Protein functional dynamics from the rigorous global analysis of DEER data: Conditions, components, and conformations

The potential of spin labeling to reveal the dynamic dimension of macromolecules has been recognized since the dawn of the methodology in the 1960s. However, it was the development of pulsed electron paramagnetic resonance spectroscopy to detect dipolar coupling between spin labels and the availability of turnkey instrumentation in the 21st century that realized the full promise of spin labeling. Double electron-electron resonance (DEER) spectroscopy has seen widespread applications to channels, transporters, and receptors. In these studies, distance distributions between pairs of spin labels obtained under different biochemical conditions report the conformational states of macromolecules, illuminating the key movements underlying biological function. These experimental studies have spurred the development of methods for the rigorous analysis of DEER spectroscopic data along with methods for integrating these distributions into structural models. In this tutorial, we describe a model-based approach to obtaining a minimum set of components of the distance distribution that correspond to functionally relevant protein conformations with a set of fractional amplitudes that define the equilibrium between these conformations. Importantly, we review and elaborate on the error analysis reflecting the uncertainty in the various parameters, a critical step in rigorous structural interpretation of the spectroscopic data.

Transporter-interfering chemicals inhibit P-glycoprotein of yellowfin tuna (Thunnus albacares)

Marine pollutants bioaccumulate at high trophic levels of marine food webs and are transferred to humans through consumption of apex species. Yellowfin tuna (Thunnus albacares) are marine predators, and one of largest commercial fisheries in the world. Previous studies have shown that yellowfin tuna can accumulate high levels of persistent organic pollutants, including Transporter Interfering Chemicals (TICs), which are chemicals shown to bind to mammalian xenobiotic transporters and interfere with their function. Here, we examined the extent to which these same compounds might interfere with the activity of the yellowfin tuna (Thunnus albacares) ortholog of this transporter. To accomplish this goal we identified, expressed, and functionally assayed tuna ABCB1. The results demonstrated a common mode of vertebrate ABCB1 interaction with TICs that predicts effects across these species, based on high conservation of specific interacting residues. Importantly several TICs showed potent inhibition of Ta-ABCB1, such as the organochlorine pesticides Endrin (EC₅₀ = 1.2 ± 0.2 μM) and Mirex (EC₅₀ = 2.3 ± 0.9 μM). However, unlike the effects observed on mouse ABCB1, low concentrations of the organochlorine pesticide TICs p,p'-DDT and its metabolite p,p'-DDD co-stimulated verapamil-induced Ta-ABCB1 ATPase activity possibly suggesting a low transport activity for these ligands in tuna. These results provide a mechanistic basis for understanding the potential vulnerability of tuna to these ubquitous pollutants.

Jatrophane diterpenoids as multidrug resistance modulators from Euphorbia sororia

Eight new jatrophane diterpenoids, Euphosorophane F-M (1-8), as well as fourteen known jatrophane diterpenoids (9-22) were separated and purified from the fructus of Euphorbia sororia, and the chemical structures were determined based on extensive spectroscopic analysis, 1D, 2D NMR and HRESIMS data included. Their absolute configurations of compounds 1, 2, 9, and 22 were elucidated by X-ray crystallographic analysis. These jatrophane diterpenoids showed lower cytotoxicity and compounds 3, 4, 11, 12, 13, 14, and 20 revealed promising multidrug resistance (MDR) reversal ability as modulators compared to verapamil (VRP) by MTT assay. The structure-activity relationship (SAR) exhibited that the absence of keto-carbonyl at C-9 and C-14 was essential to MDR reversal activity and the acyloxies substitution at C-5, C-7, C-8, and C-14 also made the activity difference. Euphosorophane I (4) particularly unfold greater potency (EC₅₀ = 1.82 μM) in reversing P-gp-mediated resistance to doxorubicin (DOX). As shown by fluorescence microscopy, 4 promoted intracellular accumulation of rhodamine 123 (Rh123) and DOX in a dose-dependentmanner than VRP. Flow cytometry indicated that 4 inhibitedP-glycoprotein (P-gp) -dependentRh123 efflux in drug-resistant MCF-7/ADR cells. 4 stimulated P-gp-ATPase activity in a concentration-dependent way and inhibited DOX transport activity. Western blot and real-time qPCR results further illustrated that 4 exhibited superior MDR reversal effect in MCF-7/ADR cells attributed to the activation of ATPase rather than the upregulation of P-gp expression and mRNA levels. In addition, 4 bond to the drug-binding site of P-gp predicted by the molecular docking analysis. Collectively, these results indicated that 4 efficiently reversed P-gp-mediated MDR via inhibiting the ABCB1 drug efflux function. 4 with the advantage of low toxicity and efficient could be used as an adjuvanttherapy drug for breast cancer.

Lysine 268 adjacent to transmembrane helix 5 of hamster P-glycoprotein is the major photobinding site of iodomycin in CHO B30 cells

P‐glycoprotein (Pgp) detoxifies cells by exporting hundreds of chemically dissimilar hydrophobic and amphipathic compounds and is implicated in multidrug resistance (MDR) in the treatment of cancers. Photoaffinity labeling of plasma membrane vesicles of MDR CHO B30 cells with the anthracycline [¹²⁵I]‐iodomycin, subsequent sequential cleavage with BNPS‐skatol and endoproteinase Lys‐C, and the Edman sequencing of the purified photoaffinity‐labeled peptide identified the lysine residue at position 268 in the hamster Pgp primary sequence as the major photobinding site of iodomycin in CHO B30 cells. Lysine 268 is located adjacent to the cytosolic terminus of transmembrane 5. According to thermodynamic and kinetic analyses, this location should present the equilibrium binding site of ATP‐free Pgp for daunomycin and iodomycin in B30 cells.

Disruption of small molecule transporter systems by Transporter-Interfering Chemicals (TICs)

Small molecule transporters (SMTs) in the ABC and SLC families are important players in disposition of diverse endo- and xenobiotics. Interactions of environmental chemicals with these transporters were first postulated in the 1990s, and since validated in numerous in vitro and in vivo scenarios. Recent results on the co-crystal structure of ABCB1 with the flame-retardant BDE-100 demonstrate that a diverse range of man-made and natural toxic molecules, hereafter termed transporter-interfering chemicals (TICs), can directly bind to SMTs and interfere with their function. TIC-binding modes mimic those of substrates, inhibitors, modulators, inducers, and possibly stimulants through direct and allosteric mechanisms. Similarly, the effects could directly or indirectly agonize, antagonize or perhaps even prime the SMT system to alter transport function. Importantly, TICs are distinguished from drugs and pharmaceuticals that interact with transporters in that exposure is unintended and inherently variant. Here, we review the molecular mechanisms of environmental chemical interaction with SMTs, the methodological considerations for their evaluation, and the future directions for TIC discovery.

P-Glycoprotein: One Mechanism, Many Tasks and the Consequences for Pharmacotherapy of Cancers

P-glycoprotein or multidrug resistance protein (MDR1) is an adenosine triphosphate (ATP) binding cassette transporter (ABCB1) intensely investigated because it is an obstacle to successful pharmacotherapy of cancers. P-glycoprotein prevents cellular uptake of a large number of structurally and functionally diverse compounds, including most cancer therapeutics and in this way causes multidrug resistance (MDR). To overcome MDR, and thus improve cancer treatment, an understanding of P-glycoprotein inhibition at the molecular level is required. With this goal in mind, we propose rules that predict whether a compound is a modulator, substrate, inhibitor, or inducer of P-glycoprotein. This new set of rules is derived from a quantitative analysis of the drug binding and transport properties of P-glycoprotein. We further discuss the role of P-glycoprotein in immune surveillance and cell metabolism. Finally, the predictive power of the proposed rules is demonstrated with a set of FDA approved drugs which have been repurposed for cancer therapy.

Sapitinib Reverses Anticancer Drug Resistance in Colon Cancer Cells Overexpressing the ABCB1 Transporter

The efficacy of anti-cancer drugs in patients can be attenuated by the development of multi-drug resistance (MDR) due to ATP-binding cassette (ABC) transporters overexpression. In this in vitro study, we determined the reversal efficacy of the epidermal growth factor receptor (EFGR) inhibitor, saptinib, in SW620 and SW720/Ad300 colon cancer cells and HEK293/ABCB1 cells which overexpress the ABCB1 transporter. Sapitinib significantly increased the efficacy of paclitaxel and doxorubicin in ABCB1 overexpressing cells without altering the expression or the subcellular location of the ABCB1 transporter. Sapitinib significantly increased the accumulation of [³H]-paclitaxel in SW620/AD300 cells probably by stimulating ATPase activity which could competitively inhibit the uptake of [³H]-paclitaxel. Furthermore, sapitinib inhibited the growth of resistant multicellular tumor spheroids (MCTS). The docking study indicated that sapitinib interacted with the efflux site of ABCB1 transporter by π-π interaction and two hydrogen bonds. In conclusion, our study suggests that sapitinib surmounts MDR mediated by ABCB1 transporter in cancer cells.

Inhibitory effect of Ubenimex combined with fluorouracil on multiple drug resistance and P-glycoprotein expression level in non-small lung cancer

Tumour drug resistance is one of the most urgent issues faced by anti‐tumour therapies. P‐glycoprotein (P‐gp) has been reported to be correlated with drug resistance. In this study, we aimed to study the synergistic effect of fluorouracil (5FU) and Ubenimex (UBE) on drug resistance in lung cancer. In this study, the tumour inhibitory role of 5FU and UBE was assessed in nude mice bearing A549 or A549/ADR. Real‐time polymerase chain reaction, Western blot and immunohistochemical were performed to analyse the mRNA and protein expression of P‐gp. TUNEL assay was used to evaluate the apoptosis of A549/ADR cells under 5FU and UBE treatment. MTT assay was performed to calculate the IC50 value of 5FU and UBE in A549 or A549/ADR. Combined administration of 5FU and UBE significantly inhibited the tumour growth of multidrug‐resistant cell lines A549/ADR in nude mice by down‐regulating the mRNA and protein expression of P‐gp. The apoptosis of A549/ADR was remarkably elevated in nude mice treated with 5FU and UBE. The IC50 value of 5FU and UBE was dramatically declined in A549/ADR cells compared with that of 5FU or UBE alone. Combined treatment of 5FU and UBE remarkably enhanced the apoptosis of A549/ADR cells by enhancing the intracellular accumulation of the drugs. The results of this study demonstrated that UBE combined with fluorouracil attenuated multiple drug resistance and inhibited the expression of P‐gp in lung cancer.

Antimicrobial Peptide Reverses ABCB1-Mediated Chemotherapeutic Drug Resistance

Multidrug resistance (MDR) of tumor cells to chemotherapeutic agents is the main reason for the failure of cancer chemotherapy. Overexpression of ABCB1 transporter that actively pumps various drugs out of the cells has been considered a major contributing factor for MDR. Over the past decade, many antimicrobial peptides with antitumor activity have been identified or synthesized, and some antitumor peptides have entered the clinical practice. In this study, we report that peptide HX-12C has the effect of reversing ABCB1-mediated chemotherapy resistance. In ABCB1-overexpressing cells, nontoxic dose of peptide HX-12C inhibited drug resistance and increased the effective intracellular concentration of paclitaxel and other ABCB1 substrate drugs. The mechanism study showed that peptide HX-12C stimulated ABCB1 ATPase activity without changing the expression level and localization patterns of ABCB1. Molecular docking predicted the binding modes between peptide HX-12C and ABCB1. Overall, we found that peptide HX-12C reverses ABCB1-mediated MDR through interacting with ABCB1 and blocking its function without affecting the transporter's expression and cellular localization. Our findings suggest that this antimicrobial peptide may be used as a novel prospective cancer therapeutic strategy in combination with conventional anticancer agents.

ATP-dependent thermostabilization of human P-glycoprotein (ABCB1) is blocked by modulators

P-glycoprotein (P-gp), an ATP-binding cassette transporter associated with multidrug resistance in cancer cells, is capable of effluxing a number of xenobiotics as well as anticancer drugs. The transport of molecules through the transmembrane (TM) region of P-gp involves orchestrated conformational changes between inward-open and inward-closed forms, the details of which are still being worked out. Here, we assessed how the binding of transport substrates or modulators in the TM region and the binding of ATP to the nucleotide-binding domains (NBDs) affect the thermostability of P-gp in a membrane environment. P-gp stability after exposure at high temperatures (37-80°C) was assessed by measuring ATPase activity and loss of monomeric P-gp. Our results show that P-gp is significantly thermostabilized (>22°C higher IT50) by the binding of ATP under non-hydrolyzing conditions (in the absence of Mg2+). By using an ATP-binding-deficient mutant (Y401A) and a hydrolysis-deficient mutant (E556Q/E1201Q), we show that thermostabilization of P-gp requires binding of ATP to both NBDs and their dimerization. Additionally, we found that transport substrates do not affect the thermal stability of P-gp either in the absence or presence of ATP; in contrast, inhibitors of P-gp including tariquidar and zosuquidar prevent ATP-dependent thermostabilization in a concentration-dependent manner, by stabilizing the inward-open conformation. Altogether, our data suggest that modulators, which bind in the TM regions, inhibit ATP hydrolysis and drug transport by preventing the ATP-dependent dimerization of the NBDs of P-gp.

Low-Dose Crizotinib, a Tyrosine Kinase Inhibitor, Highly and Specifically Sensitizes P-Glycoprotein-Overexpressing Chemoresistant Cancer Cells Through Induction of Late Apoptosis in vivo and in vitro

We investigated possible conditions or drugs that could target P-glycoprotein (P-gp)-overexpressing drug-resistant KBV20C cancer cells. Specifically, we focused on identifying a single treatment with a relatively low half maximal inhibitory concentration (IC₅₀). Our approach utilized repurposing drugs, which are already used in clinical practice. We evaluated 13 TKIs (gefitinib, imatinib, erlotinib, nilotinib, pazopanib, masatinib, sunitinib, sorafenib, regorafenib, lapatinib, vandetanib, cediranib, and crizotinib) for their sensitizing effects on P-gp-overexpressing drug-resistant KBV20C cells. We found that crizotinib had a much greater sensitization effect than the other tested drugs at relatively low doses. In a detailed quantitative analysis using both lower doses and time-duration treatments, we demonstrated that crizotinib, which increased the levels of apoptosis and G2 arrest, was the best TKI to induce sensitization in P-gp-overexpressing KBV20C cells. Upon comparing resistant KBV20C cells and sensitive KB parent cells, crizotinib was found to markedly sensitize drug-resistant KBV20C cancer cells compared with other TKIs. This suggests that crizotinib is a resistant cancer cell-sensitizing drug that induces apoptosis. In mice bearing xenografted P-gp-overexpressing KBV20C cells, we confirmed that crizotinib significantly reduced tumor growth and weight, without apparent side effects. In addition, although lapatinib and crizotinib have a high P-gp inhibitory activity, we found that co-treatment with crizotinib and vincristine (VIC) did not have much of a sensitization effect on KBV20C cells, whereas lapatinib had a high sensitization effect on VIC-treated KBV20C cells. This suggests that crizotinib is a single-treatment specific drug for resistant cancer cells. These findings provide valuable information regarding the sensitization of drug-resistant cells and indicate that low-dose crizotinib monotherapy may be used in patients with specific P-gp-overexpressing chemoresistant cancer.

Active transport of rhodamine 123 by the human multidrug transporter P-glycoprotein involves two independent outer gates

Human P-glycoprotein (P-gp) is a multispecific drug-efflux transporter, which plays an important role in drug resistance and drug disposition. Recent cryo-electron microscopy structures confirmed its rotationally symmetric architecture, which allows dual interaction with ATP and substrates. We here report the existence of two distinct, symmetry-related outer gates. Experiments were aided by availability of the X-ray structure of a homodimeric eukaryotic homolog of P-gp from red alga (CmABCB1), which defined the role of an apical tyrosine residue (Y358) in outer gate formation. We mutated analogous tyrosine residues in each half of the human full-length transporter (Y310, Y953) to alanine. These mutants were introduced in engineered transporters which bind rhodamine 123 in one of two symmetry-related binding modes only. Outer gate dysfunction was detected by a loss of active transport characteristics, while these mutants retained the ability for outward downhill transport. Our data demonstrate that symmetric tyrosine residues Y310 and Y953 are involved in formation of two distinct symmetry-related outer gates, which operate contingent on the rhodamine 123 binding mode. Hence, the rotationally symmetric architecture of P-gp, which determines duality in ATP binding and rhodamine 123 interaction, also forms the basis for the existence of two independently operating outer gates.

Considerations for Determining Direct Versus Indirect Functional Effects of Solubilizing Excipients on Drug Transporters for Enhancing Bioavailability

Excipients used in drug formulations at clinically safe levels have been considered to be pharmacologically inert; however, numerous studies have suggested that many solubilizing agents may modulate drug transporter activities and intestinal absorption. Here, the reported interactions between various solubilizing excipients and drug transporters are evaluated to consider various potential underlying mechanisms. This forms the basis for debate in the field in regard to whether or not the effects are based on "direct" interactions or "indirect" consequences arising from the role of the excipients. For example, an increase in apparent drug solubility can give rise to saturation of transporters according to Michaelis-Menten kinetics. This is also drawing the attention of regulatory agencies as they seek to understand the role of formulation additives. The continued application of excipients as a tool in solubility enhancement is crucial in the drug development process, creating a need for additional data to verify the proposed mechanism behind these changes. A literature review is provided here with some guidance on other factors that should be considered to delineate the effects that arise from direct physiological interactions or indirect effects. The results of such studies may aid the rational design of bioavailability-enhancing formulations.

Replacing the eleven native tryptophans by directed evolution produces an active P-glycoprotein with site-specific, non-conservative substitutions

P-glycoprotein (Pgp) pumps an array of hydrophobic compounds out of cells, and has major roles in drug pharmacokinetics and cancer multidrug resistance. Yet, polyspecific drug binding and ATP hydrolysis-driven drug export in Pgp are poorly understood. Fluorescence spectroscopy using tryptophans (Trp) inserted at strategic positions is an important tool to study ligand binding. In Pgp, this method will require removal of 11 endogenous Trps, including highly conserved Trps that may be important for function, protein-lipid interactions, and/or protein stability. Here, we developed a directed evolutionary approach to first replace all eight transmembrane Trps and select for transport-active mutants in Saccharomyces cerevisiae. Surprisingly, many Trp positions contained non-conservative substitutions that supported in vivo activity, and were preferred over aromatic amino acids. The most active construct, W(3Cyto), served for directed evolution of the three cytoplasmic Trps, where two positions revealed strong functional bias towards tyrosine. W(3Cyto) and Trp-less Pgp retained wild-type-like protein expression, localization and transport function, and purified proteins retained drug stimulation of ATP hydrolysis and drug binding affinities. The data indicate preferred Trp substitutions specific to the local context, often dictated by protein structural requirements and/or membrane lipid interactions, and these new insights will offer guidance for membrane protein engineering.

Medicinal chemistry strategies to discover P-glycoprotein inhibitors: An update

The presence of multidrug resistance (MDR) in malignant tumors is one of the primary causes of treatment failure in cancer chemotherapy. The overexpression of the ATP binding cassette (ABC) transporter, P-glycoprotein (P-gp), which significantly increases the efflux of certain anticancer drugs from tumor cells, produces MDR. Therefore, inhibition of P-gp may represent a viable therapeutic strategy to overcome cancer MDR. Over the past 4 decades, many compounds with P-gp inhibitory efficacy (referred to as first- and second-generation P-gp inhibitors) have been identified or synthesized. However, these compounds were not successful in clinical trials due to a lack of efficacy and/or untoward toxicity. Subsequently, third- and fourth-generation P-gp inhibitors were developed but dedicated clinical trials did not indicate a significant therapeutic effect. In recent years, an extraordinary array of highly potent, selective, and low-toxicity P-gp inhibitors have been reported. Herein, we provide a comprehensive review of the synthetic and natural products that have specific inhibitory activity on P-gp drug efflux as well as promising chemosensitizing efficacy in MDR cancer cells. The present review focuses primarily on the structural features, design strategies, and structure-activity relationships (SAR) of these compounds.

Propafenone analogue with additional H-bond acceptor group shows increased inhibitory activity on P-glycoprotein

P-glycoprotein (P-gp) is an ATP-dependent efflux pump that has a marked impact on the absorption, distribution, and excretion of therapeutic drugs. As P-gp inhibition can result in drug-drug interactions and altered drug bioavailability, identifying molecular properties that are linked to inhibition is of great interest in drug development. In this study, we combined chemical synthesis, in vitro testing, quantitative structure-activity relationship analysis, and docking studies to investigate the role of hydrogen bond (H-bond) donor/acceptor properties in transporter-ligand interaction. In a previous work, it has been shown that propafenone analogs with a 4-hydroxy-4-piperidine moiety exhibit a generally 10-fold higher P-gp inhibitory activity than expected based on their lipophilicity. Here, we specifically expanded the data set by introducing substituents at position 4 of the 4-phenylpiperidine moiety to assess the importance of H-bond donor/acceptor features in this region. The results suggest that indeed an H-bond acceptor, such as hydroxy and methoxy, increases the affinity by forming a H-bond with Tyr310.

Mechanism of allosteric modulation of P-glycoprotein by transport substrates and inhibitors

The ATP-binding cassette subfamily B member 1 (ABCB1) multidrug transporter P-glycoprotein plays a central role in clearance of xenobiotics in humans and is implicated in cancer resistance to chemotherapy. We used double electron electron resonance spectroscopy to uncover the basis of stimulation of P-glycoprotein adenosine 5'-triphosphate (ATP) hydrolysis by multiple substrates and illuminate how substrates and inhibitors differentially affect its transport function. Our results reveal that substrate-induced acceleration of ATP hydrolysis correlates with stabilization of a high-energy, post-ATP hydrolysis state characterized by structurally asymmetric nucleotide-binding sites. By contrast, this state is destabilized in the substrate-free cycle and by high-affinity inhibitors in favor of structurally symmetric nucleotide binding sites. Together with previous data, our findings lead to a general model of substrate and inhibitor coupling to P-glycoprotein.

Evidence for the Interaction of A3 Adenosine Receptor Agonists at the Drug-Binding Site(s) of Human P-glycoprotein (ABCB1)

P-glycoprotein (P-gp) is a multidrug transporter that is expressed on the luminal surface of epithelial cells in the kidney, intestine, bile-canalicular membrane in the liver, blood-brain barrier, and adrenal gland. This transporter uses energy of ATP hydrolysis to efflux from cells a variety of structurally dissimilar hydrophobic and amphipathic compounds, including anticancer drugs. In this regard, understanding the interaction with P-gp of drug entities in development is important and highly recommended in current US Food and Drug Administration guidelines. Here we tested the P-gp interaction of some A₃ adenosine receptor agonists that are being developed for the treatment of chronic diseases, including rheumatoid arthritis, psoriasis, chronic pain, and hepatocellular carcinoma. Biochemical assays of the ATPase activity of P-gp and by photolabeling P-gp with its transport substrate [¹²⁵I]-iodoarylazidoprazosin led to the identification of rigidified (N)-methanocarba nucleosides (i.e., compound 3 as a stimulator and compound 8 as a partial inhibitor of P-gp ATPase activity). Compound 8 significantly inhibited boron-dipyrromethene (BODIPY)-verapamil transport mediated by human P-gp (IC₅₀ 2.4 ± 0.6 µM); however, the BODIPY-conjugated derivative of 8 (compound 24) was not transported by P-gp. In silico docking of compounds 3 and 8 was performed using the recently solved atomic structure of paclitaxel (Taxol)-bound human P-gp. Molecular modeling studies revealed that both compounds 3 and 8 bind in the same region of the drug-binding pocket as Taxol. Thus, this study indicates that nucleoside derivatives can exhibit varied modulatory effects on P-gp activity, depending on structural functionalization. SIGNIFICANCE STATEMENT: Certain A₃ adenosine receptor agonists are being developed for the treatment of chronic diseases. The goal of this study was to test the interaction of these agonists with the human multidrug resistance-linked transporter P-glycoprotein (P-gp). ATPase and photolabeling assays demonstrated that compounds with rigidified (N)-methanocarba nucleosides inhibit the activity of P-gp; however, a fluorescent derivative of one of the compounds was not transported by P-gp. Furthermore, molecular docking studies revealed that the binding site for these compounds overlaps with the site for paclitaxel in the drug-binding pocket. These results suggest that nucleoside derivatives, depending on structural functionalization, can modulate the function of P-gp.

Tepotinib reverses ABCB1-mediated multidrug resistance in cancer cells

Overexpression of ABCB1 transporters plays a crucial role in mediating multidrug resistance (MDR). Therefore, it is important to inhibit ABCB1 activity in order to maintain an effective intracellular level of chemotherapeutic drugs. Tepotinib is a MET tyrosine kinase inhibitor with potential anticancer effect and it is currently in clinical trials. In this study, we investigated whether tepotinib could antagonize ABC transporters-mediated MDR. Our results suggest that tepotinib significantly reversed ABCB1-mediated MDR but not ABCG2- or ABCC1-mediated MDR. Mechanistic studies show that tepotinib significantly reversed ABCB1-mediated MDR by attenuating the efflux activity of ABCB1 transporter. The ATPase assay showed that tepotinib inhibited the ATPase activity of ABCB1 in a concentration-dependent manner. Furthermore, treatment with tepotinib did not change protein expression or subcellular localization of ABCB1. Docking analysis indicated that tepotinib interacted with the drug-binding site of the ABCB1 transporter. Our study provides a potential chemotherapeutic strategy of co-administrating tepotinib with other conventional chemotherapeutic agents to overcome MDR and improve therapeutic effect.

Establishment and Use of Human Mouth Epidermal Carcinoma (KB) Cells Overexpressing P-Glycoprotein To Characterize Structure Requirements for Flavonoids Transported by the Efflux Transporter

This study was aimed to determine the mechanism for flavonoid poor absorption related to P-glycoprotein (P-gp). The cellular uptake (CU) of 40 flavonoids was investigated in P-gp overexpressing KB/multidrug-resistant (MDR) cells. A total of 9 flavonoids, including 5,7,3',4'-tetramethoxyflavone, with a significant ( p < 0.05) CU_KBE (2.90 ± 0.146 μmol/g) higher than CU_KBP (1.57 ± 0.129 μmol/g) were identified as P-gp substrates. Besides, 8 substrates, including tangeretin, showed a significant ( p < 0.05) CU_KB (9.72 ± 1.09 μmol/g) higher than its CU_KBP (7.36 ± 0.692 μmol/g). A total of 7 of 17 flavonoid substrates stimulated the P-gp efflux of rhodamine 123, and most substrates increased P-gp expression in KB/MDR cells. Docking analyses showed a good correlation ( R = 0.764; p < 0.01) between efflux fold and S_scoring of flavonoids to the P-gp model, indicating consistency between in silico and in vitro results. A structure-affinity relationship exhibited that 3-OH, 5-OH, 3'-OCH₃, and 4'-OCH₃ are crucial for flavonoids binding to P-gp. These results provide valuable information for finding a solution to improve the absorption of flavonoids.

Tenulin and isotenulin inhibit P-glycoprotein function and overcome multidrug resistance in cancer cells

BACKGROUND

Multidrug resistance (MDR) in cancer is one of the main obstacles in treatment with chemotherapy. Drug efflux through P-glycoprotein is the major mechanism involved in MDR. A potential strategy to provide the best possible clinical outcomes is to develop P-glycoprotein (P-gp) inhibitors from natural products.

PURPOSE

The present study investigated the effects of the natural sesquiterpene lactone tenulin and its derivative isotenulin on human P-gp; the mechanisms of kinetic interactions were also explored.

METHODS

The human P-gp (ABCB1/Flp-In™-293) stable expression cells were established by using the Flp-In™ system. The effects of tenulin and isotenulin on cell viability were evaluated by SRB assays in established cell lines, sensitive cancer cell line (HeLaS3), and resistant cancer cell line (KB-vin). The transporter inhibition ability was evaluated by calcein-AM uptake assays. The P-gp inhibition kinetics of tenulin and isotenulin were evaluated by rhodamine123 and doxorubicin efflux assays. The ATPase activity was evaluated with the Pgp-Glo™ Assay System.

RESULTS

Tenulin and isotenulin significantly inhibited the P-gp efflux function by stimulating P-gp ATPase activity. Tenulin and isotenulin interacted with the effluxes of rhodamine 123 and doxorubicin through a competitive and noncompetitive mechanism, respectively. The combinations of tenulin and isotenulin with chemotherapeutic drugs significantly resensitized MDR cancer cells.

CONCLUSION

These results suggested that tenulin and isotenulin are potential candidates to be developed for synergistic treatment of MDR cancers.

Different structures of berberine and five other protoberberine alkaloids that affect P-glycoprotein-mediated efflux capacity

Berberine, berberrubine, thalifendine, demethyleneberberine, jatrorrhizine, and columbamine are six natural protoberberine alkaloid (PA) compounds that display extensive pharmacological properties and share the same protoberberine molecular skeleton with only slight substitution differences. The oral delivery of most PAs is hindered by their poor bioavailability, which is largely caused by P-glycoprotein (P-gp)-mediated drug efflux. Meanwhile, P-gp undergoes large-scale conformational changes (from an inward-facing to an outward-facing state) when transporting substrates, and these changes might strongly affect the P-gp-binding specificity. To confirm whether these six compounds are substrates of P-gp, to investigate the differences in efflux capacity caused by their trivial structural differences and to reveal the key to increasing their binding affinity to P-gp, we conducted a series of in vivo, in vitro, and in silico assays. Here, we first confirmed that all six compounds were substrates of P-gp by comparing the drug concentrations in wild-type and P-gp-knockout mice in vivo. The efflux capacity (net efflux) ranked as berberrubine > berberine > columbamine ~ jatrorrhizine > thalifendine > demethyleneberberine based on in vitro transport studies in Caco-2 monolayers. Using molecular dynamics simulation and molecular docking techniques, we determined the transport pathways of the six compounds and their binding affinities to P-gp. The results suggested that at the early binding stage, different hydrophobic and electrostatic interactions collectively differentiate the binding affinities of the compounds to P-gp, whereas electrostatic interactions are the main determinant at the late release stage. In addition to hydrophobic interactions, hydrogen bonds play an important role in discriminating the binding affinities.

Structural and functional aspects of P-glycoprotein and its inhibitors

P-glycoprotein (P-gp) is a member of ATP-binding cassette (ABC) superfamily which extrudes chemotherapeutic agents out of the cell. Suppression of this efflux activity has been the subject of numerous attempts to develop P-gp inhibitors. The aim of this review is to present up-to-date information on the structural and functional aspects of P-gp and its known inhibitors. The data presented also provide some information on drug discovery approaches for candidate P-gp inhibitors. Nucleotide-binding domains (NBDs) and drug-binding domains (DBDs) have been extensively studied to gain more information about P-gp inhibition and it looks that the ATPase activity of this pump has been the most attractive target for designing inhibitors. Hydrophobic and π-π (aromatic) interactions between P-gp binding domains and inhibitors are dominant intermolecular forces that have been reported in many studies using different methods. Many synthetic and natural products have been found to possess inhibitory or modulatory effects on drug transporter proteins. Log P value is an important factor in studying these inhibitors and has a crucial role on absorption, distribution, metabolism, and excretion (ADME) properties of candidate P-gp inhibitors.

Evidence for the critical role of transmembrane helices 1 and 7 in substrate transport by human P-glycoprotein (ABCB1)

P-glycoprotein (P-gp) is an ABC transporter that exports many amphipathic or hydrophobic compounds, including chemically and functionally dissimilar anticancer drugs, from cells. To understand the role of transmembrane helices (TMH) 1 and 7 in drug-binding and transport, we selected six residues from both TMH1 (V53, I59, I60, L65, M68 and F72) and TMH7 (V713, I719, I720, Q725, F728 and F732); and substituted them with alanine by gene synthesis to generate a variant termed "TMH1,7 mutant P-gp". The expression and function of TMH1,7 mutant P-gp with twelve mutations was characterized using the BacMam baculovirus-HeLa cell expression system. The expression and conformation of TMH1,7 mutant P-gp was not altered by the introduction of the twelve mutations, as confirmed by using the human P-gp-specific antibodies UIC2, MRK16 and 4E3. We tested 25 fluorescently-labeled substrates and found that only three substrates, NBD-cyclosporine A, Rhod-2-AM and X-Rhod-1-AM were transported by the TMH1,7 mutant. The basal ATPase activity of TMH1,7 mutant P-gp was lower (40-50%) compared to wild-type (WT) P-gp, despite similar level of expression. Although most of the substrates modulate ATPase activity of P-gp, the activity of TMH1,7 mutant transporter was not significantly modulated by any of the tested substrates. Docking of selected substrates in homology models showed comparable docking scores for the TMH1,7 mutant and WT P-gp, although the binding conformations were different. Both the ATPase assay and in silico docking analyses suggest that the interactions with residues in the drug-binding pocket are altered as a consequence of the mutations. We demonstrate that it is possible to generate a variant of P-gp with a loss of broad substrate specificity and propose that TMH1 and TMH7 play a critical role in the drug efflux function of this multidrug transporter.

Combination of dihydromyricetin and ondansetron strengthens antiproliferative efficiency of adriamycin in K562/ADR through downregulation of SORCIN: A new strategy of inhibiting P-glycoprotein

Though the advancement of chemotherapy drugs alleviates the progress of cancer, long-term therapy with anticancer agents gradually leads to acquired multidrug resistance (MDR), which limits the survival outcomes in patients. It was shown that dihydromyricetin (DMY) could partly reverse MDR by suppressing P-glycoprotein (P-gp) and soluble resistance-related calcium-binding protein (SORCIN) independently. To reverse MDR more effectively, a new strategy was raised, that is, circumventing MDR by the coadministration of DMY and ondansetron (OND), a common antiemetic drug, during cancer chemotherapy. Meanwhile, the interior relation between P-gp and SORCIN was also revealed. The combination of DMY and OND strongly enhanced antiproliferative efficiency of adriamycin (ADR) because of the increasing accumulation of ADR in K562/ADR-resistant cell line. DMY could downregulate the expression of SORCIN and P-gp via the ERK/Akt pathways, whereas OND could not. In addition, it was proved that SORCIN suppressed ERK and Akt to inhibit P-gp by the silence of SORCIN, however, not vice versa. Finally, the combination of DMY, OND, and ADR led to G2/M cell cycle arrest and apoptosis via resuming P53 function and restraining relevant proteins expression. These fundamental findings provided a promising approach for further treatment of MDR.

Mapping discontinuous epitopes for MRK-16, UIC2 and 4E3 antibodies to extracellular loops 1 and 4 of human P-glycoprotein

P-glycoprotein (P-gp), an ATP-dependent efflux pump, is associated with the development of multidrug resistance in cancer cells. Antibody-mediated blockade of human P-gp activity has been shown to overcome drug resistance by re-sensitizing resistant cancer cells to anticancer drugs. Despite the potential clinical application of this finding, the epitopes of the three human P-gp-specific monoclonal antibodies MRK-16, UIC2 and 4E3, which bind to the extracellular loops (ECLs) have not yet been mapped. By generating human-mouse P-gp chimeras, we mapped the epitopes of these antibodies to ECLs 1 and 4. We then identified key amino acids in these regions by replacing mouse residues with homologous human P-gp residues to recover binding of antibodies to the mouse P-gp. We found that changing a total of ten residues, five each in ECL1 and ECL4, was sufficient to recover binding of both MRK-16 and 4E3 antibodies, suggesting a common epitope. However, recovery of the conformation-sensitive UIC2 epitope required replacement of thirteen residues in ECL1 and the same five residues replaced in the ECL4 for MRK-16 and 4E3 binding. These results demonstrate that discontinuous epitopes for MRK-16, UIC2 and 4E3 are located in the same regions of ECL1 and 4 of the multidrug transporter.

Retina Compatible Interactions and Effective Modulation of Blood Ocular Barrier P-gp Activity by Third-Generation Inhibitors Improve the Ocular Penetration of Loperamide

Effective drug delivery to the deeper ocular tissues remains an unresolved conundrum mainly due to the expression of multidrug resistance efflux proteins, besides tight junction proteins, in the blood ocular barriers (BOBs). Hence, the purpose of the current research was to investigate the ability of the third-generation efflux protein inhibitors, elacridar (EQ), and tariquidar (TQ), to diminish P-glycoprotein (P-gp) mediated efflux transport of loperamide (LOP), a P-gp substrate, across the BOB in Sprague Dawley rats. Initially, Western blot analysis confirmed the expression of P-gp in the iris-ciliary bodies and the retina choroid in the wild type rats. Next, the ocular distribution of LOP, in the presence and absence of EQ/TQ (at 2 doses), was evaluated. The significantly higher aqueous humor/plasma (D_AH) and vitreous humor (VH)/plasma (D_VH) distribution ratios of LOP in the rats pretreated with EQ or TQ demonstrated effective inhibition of P-gp activity in the BOB. Interestingly, the modulation of P-gp activity by EQ/TQ was more pronounced at the lower dose. The normal functioning and architecture of the retina, as indicated by electroretinography studies, confirmed the cytocompatibility of LOP and EQ/TQ interactions at the doses tested.

An NMR strategy to detect conformational differences in a protein complexed with highly analogous inhibitors in solution

This manuscript presents an NMR strategy to investigate conformational differences in protein-inhibitor complexes, when the inhibitors tightly bind to a protein at sub-nanomolar dissociation constants and are highly analogous to each other. Using HIV-1 protease (PR), we previously evaluated amide chemical shift differences, ΔCSPs, of PR bound to darunavir (DRV) compared to PR bound to several DRV analogue inhibitors, to investigate subtle but significant long-distance conformation changes caused by the inhibitor's chemical moiety variation [Khan, S. N., Persons, J. D. Paulsen, J. L., Guerrero, M., Schiffer, C. A., Kurt-Yilmaz, N., and Ishima, R., Biochemistry, (2018), 57, 1652-1662]. However, ΔCSPs are not ideal for investigating subtle PR-inhibitor interface differences because intrinsic differences in the electron shielding of the inhibitors affect protein ΔCSPs. NMR relaxation is also not suitable as it is not sensitive enough to detect small conformational differences in rigid regions among similar PR-inhibitor complexes. Thus, to gain insight into conformational differences at the inhibitor-protein interface, we recorded ¹⁵N-half filtered NOESY spectra of PR bound to two highly analogous inhibitors and assessed NOEs between PR amide protons and inhibitor protons, between PR amide protons and hydroxyl side chains, and between PR amide protons and water protons. We also verified the PR amide-water NOEs using 2D water-NOE/ROE experiments. Differences in water-amide proton NOE peaks, possibly due to amide-protein hydrogen bonds, were observed between subunit A and subunit B, and between the DRV-bound form and an analogous inhibitor-bound form, which may contribute to remote conformational changes.