Structural insight into substrate and inhibitor discrimination by human P-glycoprotein

Design, synthesis, and biological activity study of 6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline derivatives against multidrug resistance in Eca109/VCR cells

The advent of multidrug resistance (MDR) in tumors markedly diminishes the effectiveness of anticancer therapies. P-glycoprotein (P-gp) plays a crucial role in tumor MDR by mediating the efflux of drugs and cytotoxic agents. Presently, small molecule agents targeting P-gp are among the promising therapeutic approaches to counteract MDR. In previous research, our team identified a novel class of P-gp inhibitors featuring a 6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline scaffold. To further delineate the structure-activity relationship, this study conducted an extensive structural optimization, synthesizing 42 novel compounds. Evaluation on the drug-resistant cell line Eca109/VCR indicated that the majority of these compounds exhibited remarkable MDR-reversing activity. Notably, the optimized compound 41 demonstrated an outstanding ability to reverse MDR, with a reversal fold of up to 467.7, surpassing the efficacy of the standard third-generation P-gp inhibitor TQ, as evidenced by plate cloning assay and flow cytometry analysis. Subsequent mechanism validation experiments-including western blotting, chemosensitization tests, and fluorescent substrate accumulation assays-complemented by molecular docking studies, confirmed that compound 41 exerts its MDR-reversing effects through P-gp inhibition. This research offers new perspectives for the development of drug sensitizers targeting resistant tumors based on the tetrahydroisoquinoline scaffold.

Potential P-glycoprotein (P-gp) inhibitors from SuperDRUG2 database toward reversing multidrug resistance in cancer treatment: Database mining, molecular dynamics, and binding energy estimations

P-glycoprotein (P-gp) transporter is included in the failure of various carcinoma chemotherapeutics because of the multidrug resistance (MDR) phenomenon, in which the chemotherapeutic drugs are eliminated from target cells. Consequently, inhibiting P-gp transporter function is a prospective strategy for cancer treatment. In the current study, the SuperDRUG2 database containing >4600 pharmaceutical compounds was virtually screened toward the P-gp transporter utilizing the docking predictions. For inhibitors with a docking score lower than -10.5 kcal/mol, molecular dynamics (MD) simulations were performed, accompanied by binding energy evaluations using the MM-GBSA approach. In accordance with the MM-GBSA//100 ns MD, angiotensin amide (SD003508), terlipressin (SD002603), argipressin (SD002535), and lanreotide (SD001365) exhibited potential binding affinities against the P-gp transporter with ΔGbinding < -120.0 kcal/mol. The outstanding consistency of the investigated inhibitors inside the P-gp binding pocket was shown by the post-dynamics analyses. Additionally, MD simulations of the inhibitor-P-gp complexes in a POPC membrane environment were conducted to mimic the physiological conditions. These results demonstrated that angiotensin amide, terlipressin, argipressin, and lanreotide are promising P-gp inhibitors and deserve additional in-vitro/in-vivo studies.

Design, synthesis, and biological evaluation of C-12 modified ocotillol-type derivatives as novel P-glycoprotein modulators for overcoming multidrug resistance

Ocotillol-type ginsenoside derivatives exhibit significant potential as modulators of P-glycoprotein (Pgp). To date, structural investigations of Ocotillol-type saponins have predominantly focused on modifications at the C-3 position of the A-ring, with limited exploration of the C-12 position on the C-ring. In this study, we designed and synthesized a series of C-12 modified ocotillol-type derivatives and assessed their efficacy in reversing multidrug resistance (MDR) in KBV cells. Most of the newly synthesized derivatives exhibited minimal cytotoxicity and potent MDR reversal capabilities. Notably, compound 9e emerged as the most effective agent in reversing tumor MDR in vitro, showing more than twice the potency of verapamil. Furthermore, 9e displayed high selectivity for Pgp, being 40- and 20-fold more effective than verapamil in inhibiting Rh123 efflux and enhancing doxorubicin sensitivity, respectively. Molecular docking analysis revealed that 9e possesses a unique T-shaped configuration that occupies the access channel of Pgp, obstructing the peristaltic extrusion mechanism of TM12 and TM9, thereby inhibiting the efflux function of Pgp. Overall, 9e represents a promising lead compound for the development of novel Pgp modulators to overcome MDR in cancer therapy.

Discovery and biological evaluation of hederagenin derivatives as non-substrate inhibitors of P-glycoprotein-mediated multidrug resistance

Multidrug Resistance (MDR) is an essential cause of failure of tumor chemotherapy, and P-glycoprotein (P-gp) overexpression is one of the major causes of MDR in tumor cells. Hederagenin (HRG) derivatives showed significant inhibitory effects in P-gp-mediated tumor MDR. Herein, we designed and synthesized 30 HRG derivatives and evaluated these compounds' tumor MDR reversal ability. For the first time, we identified a potential P-gp non-substrate inhibitor of the HRG derivatives 15, which binds to non-substrate active sites in transmembrane structural domains (TMDs) with high binding affinity. Subsequent assays confirmed that 15 exerted significant tumor MDR reversal activity by binding to P-gp and inhibiting P-gp function rather than affecting its expression. It could not be pumped out of the cell by P-gp. In addition, 15 inhibited Rhodamine123 efflux, rendered the KBV cells sensitive to paclitaxel (Ptx), blocked the cells in the G2/M phase, and induced apoptosis. Notably, 15 increased Ptx sensitivity in vivo, significantly inhibited the growth of KBV cell-derived xenograft tumors in nude mice, with a tumor suppression rate as high as 63.71 %.

Structures and mechanisms of the ABC transporter ABCB1 from Arabidopsis thaliana

The Arabidopsis thaliana auxin transporter ABCB1 plays a fundamental role in the regulation of plant growth and development. While its homolog ABCB19 was previously shown to transport brassinosteroids (BR), another class of essential hormones, the ability of ABCB1 to mediate BR transport has remained unexplored. In this study we show that ABCB1 also transports brassinosteroids with an in vitro brassinolide (BL) transport assay. Using single-particle cryo-electron microscopy, we determined ABCB1 structures in multiple inward-facing conformations in the apo state, ANP-bound state, BL-bound state, and the both BL- and ANP-bound state. BL binds to the large cavity of two transmembrane domains, inducing a slight conformational change. Additionally, we obtained the structure of ABCB1 in an outward-facing conformation. By comparing these different conformations, we elucidated the possible mechanism of hormone transport by ABCB1. These high-resolution structures help us to understand the structural basis for hormone recognition and transport mechanisms of ABCB1.

Mapping the Role of P-gp in Multidrug Resistance: Insights from Recent Structural Studies

P-glycoprotein (P-gp/ABCB1), a key ATP-binding cassette (ABC) transporter, plays a central role in multidrug resistance (MDR), one of the leading causes of chemotherapy failure in cancer treatment. P-gp actively pumps chemotherapeutic agents out of cancer cells, reducing intracellular drug concentration and compromising therapeutic efficacy. Recent advancements in structural biology, particularly cryogenic electron microscopy (cryo-EM), have revealed detailed conformational states of P-gp, providing unprecedented insights into its transport mechanisms. In parallel, studies have identified various P-gp mutants in cancer patients, many of which are linked to altered drug efflux activity and resistance phenotypes. This review systematically examines recent structural studies of P-gp, correlates known patient-derived mutations to their functional consequences, and explores their impact on MDR. We propose plausible mechanisms by which these mutations affect P-gp's activity based on structural evidence and discuss their implications for chemotherapy resistance. Additionally, we review current approaches for P-gp inhibition, a critical strategy to restore drug sensitivity in resistant cancers, and outline future research directions to combat P-gp-mediated MDR.

Plant steroids on the move: mechanisms of brassinosteroid export

Brassinosteroids (BRs) are essential plant steroidal hormones that regulate growth and development. The recent discoveries of ATP-binding cassette subfamily B (ABCB) members, ABCB19 and ABCB1, as BR transporters highlight the significance of active export to the apoplast in maintaining BR homeostasis and enabling effective signaling. This review focuses on the latest progress in understanding ABCB-mediated BR transport, with particular attention to the structural and functional characterization of arabidopsis ABCB19 and ABCB1. These findings reveal both conserved and distinct features in substrate recognition and transport mechanisms, providing valuable insights into their roles in hormonal regulation. Additionally, the evolutionary conservation of ABC transporters in mediating steroid-based signaling across biological kingdoms underscores their fundamental biological significance.

Cryo-EM of human P-glycoprotein reveals an intermediate occluded conformation during active drug transport

P-glycoprotein (Pgp) is an important human multidrug transporter that contributes to pharmacokinetics and multidrug resistance. Despite decades of study, the conformation transition cycle of Pgp undergoing active drug transport is not defined, thus the precise relevance of all available Pgp structures to uninterrupted multidrug transport remains unclear. Here, we use cryo-EM of membrane-embedded human Pgp under continuous turnover conditions to analyze the conformational ensembles of Pgp transporting distinct substrates. These results delineate multiple conformations including inward-facing and closed conformations, highlighting the occluded conformation as a critical intermediate state between transporter closure and substrate release. A combination of structural, functional, and computational studies reveals the transmembrane helices 4 and 10 undergoing drastic rearrangement to coordinate substrate binding, occlusion, and release, and identifies a peripheral site involved in substrate capture and Pgp inhibition. Together, our results provide a set of snapshots of Pgp undergoing continuous drug transport, unveiling the intricate interplay between transporter dynamics and drug movement, and shed light on the mechanism of polyspecificity.

Conversion of Human Multidrug Transporter P-glycoprotein (ABCB1) from Drug Efflux to Uptake Pump: Evidence for a Switch Region Modulating the Direction of Substrate Transport

The multidrug transporter P-glycoprotein (P-gp), is pivotal in exporting various chemically dissimilar amphipathic compounds including anti-cancer drugs, thus causing multidrug resistance during cancer treatment. P-gp is composed of two transmembrane domains (TMDs), each containing six homologous transmembrane helices (TMHs). Among these helices, TMH 6 and 12 align oppositely, lining a drug-binding pocket in the transmembrane region which acts as a pathway for drug efflux. Previously, we demonstrated that specific mutations within TMH 6 and 12 resulted in loss of substrate efflux and altered the transport direction from efflux to uptake for some substrates. This suggested the presence of a regulatory switch that governs the direction of transport. In this study, we sought to elucidate the mechanism of switch region modulation of the uptake function by engineering several mutants via substituting specific residues in TMH 6 and 12. We discovered that the alanine substitution of four residues (V974, L975, V977, and F978) within the upper region of TMH 12, along with three residues (V334, F336, and F343) within TMH 6, was sufficient to convert P-gp from an efflux to an uptake pump. Additional mutagenesis of the residues in the middle region of TMH 12 revealed that the uptake function, like efflux, is reversible. Further studies, including molecular dynamics simulations, revealed that the switch region appears to act during the substrate translocation step. We propose that the switch region in TMH 6 and 12, which modulates the direction of transport by P-gp, provides a novel approach to selectively target P-gp-expressing cancer cells.

Spiroindoline quinazolinedione derivatives as inhibitors of P-glycoprotein: potential agents for overcoming multidrug resistance in cancer therapy

Multidrug resistance (MDR) presents a major challenge for effectiveness of chemotherapy. This study investigates the effectiveness of spiroindoline quinazolinediones in reversing MDR mediated by P-glycoprotein (P-gp) overexpression in cancer cells. A series of synthesized hybrid spiro[indoline-3,2'-quinazoline]-2,4'(3'H)-dione derivatives (compounds 5a-5l) were analyzed for their ability to enhance rhodamine 123 (Rhd123) accumulation in the MES-SA/DX5 cell line using flow cytometry. The MTT assay was also employed to evaluate the compounds' effectiveness in reversing drug resistance. Additionally, docking studies and molecular dynamics simulations were conducted to investigate the interaction of these compounds with the P-gp transporter. The Rhd123 accumulation assay in MDR cancer cells revealed that most compounds, in particular 5f, 5g, 5h, 5i, 5j, 5k, and 5l, exhibited significant potential as P-gp inhibitors. Among the tested derivatives, compounds 5g and 5l demonstrated the best effects, and increased Rhd123 accumulation up to 12.9 times compared to untreated cells. Additionally, compounds 5f through 5 l bearing methylbenzyl (5f), benzyl (5g), pentyl (5 ), p-bromobenzyl (5i), p-chlorobenzyl (5j), dichlorobenzyl (5k), and tert-butylbenzyl (5l) substituents on the isatin ring effectively restored sensitivity to doxorubicin at their non-toxic concentrations in resistant MES-SA/DX5 cells. Among these, compound 5l at 5 μM exhibited the highest inhibitory potential, and lowered doxorubicin's IC50 value 10.1 times compared to control. Moreover, in silico investigation identified the potential interactions of test compounds with critical residues of P-gp involved in its efflux function. Our study suggests that the synthesized spiroindoline quinazolinediones may have high potentials as agents capable of reversing MDR in cancer cells.

In vitro assessment of ATP-binding cassette transporters and their functional genetic polymorphisms on fluoroquinolone accumulation in human embryonic kidney 293 recombinant cell lines

Fluoroquinolone tissue distribution and cellular accumulation are hindered by efflux transporters, including ATP-binding cassette subfamily B member 1 (ABCB1), ATP-binding cassette subfamily G member 2 (ABCG2), and ATP-binding cassette subfamily C member 4 (ABCC4). Genetic polymorphisms (single-nucleotide polymorphisms) can impact transporter activity, leading to interindividual variability in the systemic and cellular pharmacokinetics of their substrates. This study assesses the impact of these transporters on moxifloxacin and ciprofloxacin (CIP) cellular accumulation in vitro, and the effect of common single-nucleotide polymorphisms in ABCB1 [c.1199G>A (rs2229109); common haplotype c.1236C>T (rs1128503), c.2677G>T/A (rs2032582), and c.3435C>T (rs1045642)] and ABCG2 [c.421C>A (rs2231142)]. Recombinant human embryonic kidney (HEK) cell lines overexpressing wild-type or variant transporters were generated via stable plasmid transfection. The impact of transporter overexpression on fluoroquinolone cell disposition was assessed through accumulation experiments in the presence of specific inhibitors to establish the link between transporter expression and differential accumulation. Results indicated that ABCB1 overexpression reduced moxifloxacin cellular concentration by 30% but inconsistently with that of CIP and that zosuquidar or elacridar reversed these effects. ABCG2 had no impact. ABCC4 markedly reduced CIP accumulation by 25%, even at the basal level, an effect reversed by MK517. Contrarily to the wild-type and the c.1199A carriers, ABCB1 CGT and TTT variants did not reduce antibiotic accumulation. In conclusion, moxifloxacin and CIP are substrates of the wild-type and 1199G>A ABCB1, while CGT and TTT haplotypes had a marginal impact on fluoroquinolone transport by ABCB1. CIP is a preferential ABCC4 substrate. Because of the large body distribution of these transporters, our findings may help rationalize their role and the impact of their polymorphisms in fluoroquinolone disposition in tissues and cells. SIGNIFICANCE STATEMENT: This study demonstrates that moxifloxacin and ciprofloxacin are substrates of ABCB1, with ciprofloxacin also transported by ABCC4. Specific ABCB1 polymorphisms (CGT and TTT haplotypes) reduce the ABCB1 transport capacity toward fluoroquinolones. These findings highlight the importance of considering ABCB1 and ABCC4 inducers or inhibitors, which may affect fluoroquinolone disposition in tissues and cells, as well as ABCB1 polymorphisms that could explain interindividual variability in pharmacokinetic profiles.

Membrane-dependent dynamics and dual translocation mechanisms of ABCB4: Insights from molecular dynamics simulations

ABCB4 is an ATP-binding cassette transporter expressed at the canalicular membrane of hepatocytes and responsible for translocating phosphatidylcholine into bile. Despite the recent cryo-EM structures of ABCB4, knowledge about the molecular mechanism of phosphatidylcholine transport remains fragmented. In this study, we used all-atom molecular dynamics simulations to investigate ABCB4 dynamics during its transport cycle, leveraging both symmetric and asymmetric membrane models. Our results demonstrate that membrane composition influences the local conformational dynamics of ABCB4, revealing distinct lipid-binding patterns across different conformers, particularly for cholesterol. We explored the two potential mechanisms for phosphatidylcholine translocation: the canonical ATP-driven alternating access model and the "credit-card swipe" model. Critical residues were identified for phosphatidylcholine binding and transport pathway modulation, supporting the canonical mechanism while also indicating a possible additional pathway. The conformer-specific roles of kinking in transmembrane helices (TMH4 and TMH10) were highlighted as key events in substrate translocation. Overall, ABCB4 may utilize a cooperative transport mechanism, integrating elements of both models to facilitate efficient phosphatidylcholine motion across the membrane. This study provides new insights into the relationship between membrane environment and ABCB4 function, contributing to our understanding of its role in bile physiology and susceptibility to genetic and xenobiotic influences.

Organic anion transporting polypeptides: Pharmacology, toxicology, structure, and transport mechanisms

Organic anion transporting polypeptides (OATPs) are membrane proteins that mediate the uptake of a wide range of substrates across the plasma membrane of various cells and tissues. They are classified into 6 subfamilies, OATP1 through OATP6. Humans contain 12 OATPs encoded by 11 solute carrier of organic anion transporting polypeptide (SLCO) genes: OATP1A2, OATP1B1, OATP1B3, the splice variant OATP1B3-1B7, OATP1C1, OATP2A1, OATP2B1, OATP3A1, OATP4A1, OATP4C1, OATP5A1, and OATP6A1. Most of these proteins are expressed in epithelial cells, where they mediate the uptake of structurally unrelated organic anions, cations, and even neutral compounds into the cytoplasm. The best-characterized members are OATP1B1 and OATP1B3, which have an important role in drug metabolism by mediating drug uptake into the liver and are involved in drug-drug interactions. In this review, we aimed to (1) provide a historical perspective on the identification of OATPs and their nomenclature and discuss their phylogenic relationships and molecular characteristics; (2) review the current knowledge of the broad substrate specificity and their role in drug disposition and drug-drug interactions, with a special emphasis on human hepatic OATPs; (3) summarize the different experimental systems that are used to study the function of OATPs and discuss their advantages and disadvantages; (4) review the available experimental 3-dimensional structures and examine how they can help elucidate the transport mechanisms of OATPs; and (5) finally, summarize the current knowledge of the regulation of OATP expression, discuss clinically important single-nucleotide polymorphisms, and outline challenges of physiologically based pharmacokinetic modeling and in vitro to in vivo extrapolation. SIGNIFICANCE STATEMENT: Organic anion transporting polypeptides (OATPs) are a family of 12 uptake transporters in the solute carrier superfamily. Several members, particularly the liver-expressed OATP1B1 and OATP1B3, are important drug transporters. They mediate the uptake of several endobiotics and xenobiotics, including statins and numerous other drugs, into hepatocytes, and their inhibition by other drugs or reduced expression due to single-nucleotide polymorphisms can lead to adverse drug effects. Their recently solved 3-dimensional structures should help to elucidate their transport mechanisms and broad substrate specificities.

Reversal potentials of Tween 20 in ABC transporter-mediated multidrug-resistant cancer and treatment-resistant depression through interacting with both drug-binding and ATP-binding areas on MDR proteins

Drug efflux transporters, especially those belonging to the ATP-binding cassette (ABC) transporter superfamily, play a crucial role in various drug resistance issues, including multidrug resistance (MDR) in cancer and treatment-resistant depression (TRD) in individuals with major depressive disorder. Key transporters in this context include P-glycoprotein (P-gp), multidrug resistance protein 1 (MRP1), and breast cancer resistance protein (BCRP). This study aimed to investigate the modulatory effects of polyoxyethylene (20) sorbitan monolaurate (Tween 20) on these efflux transporters in vitro and to evaluate its potential for overcoming drug resistance in two models: an in vitro cancer MDR model and an in vivo TRD model. The findings indicated that 0.001% Tween 20 significantly inhibited the efflux actions of all three transporters. Additionally, 0.005% Tween 20 effectively reversed resistance to paclitaxel, vincristine, doxorubicin, and mitoxantrone in various cancer MDR cell lines. In the in vivo depression-like behaviour model, 0.01% Tween 20 markedly enhanced the antidepressant and anxiolytic effects of fluoxetine. Given its strong inhibitory effects on P-gp, MRP1, and BCRP, along with its capacity to reverse drug resistance both in vitro and in vivo, Tween 20 is a compelling candidate for tackling transporter-mediated drug resistance.

Lipid-mediated mechanism of drug extrusion by a heterodimeric ABC exporter

Multidrug transport by ATP binding cassette (ABC) exporters entails a mechanism to modulate drug affinity across the transport cycle. Here, we combine cryo-EM and molecular dynamics (MD) simulations to illuminate how lipid competition modulates substrate affinity to drive its translocation by ABC exporters. We determined cryo-EM structures of the ABC transporter BmrCD in drug-loaded inward-facing (IF) and outward-facing (OF) conformations in lipid nanodiscs to reveal the structural basis of alternating access, details of drug-transporter interactions, and the scale of drug movement between the two conformations. Remarkably, the structures uncovered lipid molecules bound in or near the transporter vestibule along with the drugs. MD trajectories from the IF structure show that these lipids stimulate drug disorder and translocation towards the vestibule apex. Similarly, bound lipids enter the OF vestibule and weaken drug-transporter interactions facilitating drug release. Our results complete a near-atomic model of BmrCD's conformational cycle and advance a general mechanism of lipid-driven drug transport by ABC exporters.

Molecular mechanism of type ib MET inhibitors and their potential for CNS tumors

The emergence of targeted therapies for MET exon 14 (METex14) skipping mutations has significantly changed the treatment landscape for NSCLC and other solid tumors. The skipping of METex14 results in activating the MET-HGF pathway and promoting tumor cell proliferation, migration, and preventing apoptosis. Type Ib MET inhibitors, designed to selectively target the "DFG-in" conformation of MET, characteristically bind to the ATP-binding pocket of MET in a U-shaped conformation, extending into the solvent-accessible region and interact strongly with residue Y1230 through π-π interactions, have shown remarkable efficacy in treating METex14-altered NSCLC, including cases with brain metastases (BMs). Notably, vebreltinib and capmatinib have demonstrated superior blood-brain barrier (BBB) permeability in both computational and experimental models, highlighting their potential for treating the central nervous system (CNS) metastases. P-glycoprotein (P-gp) is highly expressed in the BBB, which limits the brain uptake of many highly lipophilic drugs. Despite challenges posed by P-gp mediated efflux, vebreltinib has emerged as a promising candidate for CNS treatment due to its favorable pharmacokinetic profile and minimal susceptibility to P-gp efflux. This study underscores the importance of molecular dynamics simulations in predicting drug efficacy and BBB penetration, providing valuable insights for the development of CNS-targeted metastases therapies.

Squeeze pumping of lipids and insecticides by ABCH transporter

ATP-binding cassette (ABC) transporter subfamily H is only identified in arthropods and zebrafish. It transports lipids and is related to insecticide resistance. However, the precise mechanisms of its functions remain elusive. Here, we report cryoelectron microscopy (cryo-EM) structures of an ABCH from Tribolium castaneum, a worldwide pest of stored grains, in complex with an HEK293 cell-ceramide lipid, a fluorescent-labeled ceramide, a carbamate insecticide, and a maltose detergent inhibitor. We revealed a narrow, long, and arched substrate-binding tunnel in the transmembrane domains of the transporter dimer with two arginine-gated cytoplasmic entries for the binding and transport of lipids or insecticides. A pair of glutamines above the tunnel acts as a gate for directing substrate to be extruded via a vent-like hydrophilic exit to the extracellular side of the membrane upon ATP binding. Our structures and biochemical data provide mechanistic understanding of lipid transport, insecticide detoxification, and the inhibition of transporter activity by branched maltose detergents.

Rhodamine6G and Hœchst33342 narrow BmrA conformational spectrum for a more efficient use of ATP

Multidrug ABC transporters harness the energy of ATP binding and hydrolysis to translocate substrates out of the cell and detoxify them. While this involves a well-accepted alternating access mechanism, molecular details of this interplay are still elusive. Rhodamine6G binding on a catalytic inactive mutant of the homodimeric multidrug ABC transporter BmrA triggers a cooperative binding of ATP on the two identical nucleotide-binding-sites, otherwise michaelian. Here, we investigate this asymmetric behavior via a structural-enzymology approach, solving cryoEM structures of BmrA at defined ATP ratios, highlighting the plasticity of BmrA as it undergoes the transition from inward to outward facing conformations. Analysis of continuous heterogeneity within cryoEM data and structural dynamics, reveals that Rhodamine6G narrows the conformational spectrum explored by the nucleotide-binding domains. We observe the same behavior for the other drug Hœchst33342. Following on these findings, the effect of drug-binding showed an ATPase stimulation and a maximal transport activity of the wild-type protein at the concentration-range where the cooperative transition occurs. Altogether, these findings provide a description of the influence of drug binding on the ATP-binding sites through a change in conformational dynamics.

Vodobatinib overcomes cancer multidrug resistance by attenuating the drug efflux function of ABCB1 and ABCG2

Multidrug resistance (MDR) remains a significant obstacle in cancer treatment, primarily attributable to the overexpression of ATP-binding cassette (ABC) transporters such as ABCB1 and ABCG2 within cancer cells. These transporters actively diminish the effectiveness of cytotoxic drugs by facilitating ATP hydrolysis-dependent drug efflux, thereby reducing intracellular drug accumulation. Given the absence of approved treatments for multidrug-resistant cancers and the established benefits of combining tyrosine kinase inhibitors (TKIs) with conventional anticancer drugs, we investigate the potential of vodobatinib, a potent c-Abl TKI presently in clinical trials, to restore sensitivity to chemotherapeutic agents in multidrug-resistant cancer cells overexpressing ABCB1 and ABCG2. Results indicate that vodobatinib, administered at sub-toxic concentrations, effectively restores the sensitivity of multidrug-resistant cancer cells to cytotoxic drugs in a concentration-dependent manner. Moreover, vodobatinib enhances drug-induced apoptosis in these cells by inhibiting the drug-efflux function of ABCB1 and ABCG2, while maintaining their expression levels. Moreover, we found that while vodobatinib enhances the ATPase activity of ABCB1 and ABCG2, the overexpression of these transporters does not induce resistance to vodobatinib. These results strongly suggest that increased levels of ABCB1 or ABCG2 are unlikely to play a significant role in the development of resistance to vodobatinib in cancer patients. Overall, our findings unveil an additional pharmacological facet of vodobatinib against ABCB1 and ABCG2 activity, suggesting its potential incorporation into combination therapy for a specific subset of patients with tumors characterized by high ABCB1 or ABCG2 levels. Further investigation is warranted to fully elucidate the clinical implications of this therapeutic approach.

PROTAC-attractive site as a new target for suppressing P-glycoprotein activity

P-glycoprotein (P-gp) plays an important role in the rapid release of various small molecule substances from the cell. In turn, inhibition of this efflux transporter is an attractive strategy for both overcoming chemoresistance and facilitating oral absorption of drugs or CNS drug delivery. In this work, we adopt an approach typical for PROteolysis Targeting Chimera (PROTAC), which is based on the artificial drawing together of the target protein to E3 ubiquitin ligase, to P-gp. Forced ubiquitinylation of a transmembrane protein will provoke its removal from the cell membrane and promote its subsequent degradation. Within this concept, we investigated the possibility of P-gp ubiquitinylation by a number of PROTAC-specific E3 ligases using several approaches. We also identified the most promising site for the development of P-gp ligands. By screening a diversified library of compounds, we not only identified a number of scaffolds suitable for the construction of specific ligands, but also proposed dorsomorphin as a convenient platform for creating the constituent of a bifunctional chimera. We show that dorsomorphin both has the structural characteristics necessary to develop a PROTAC-like molecule and exhibits P-gp inhibitory activity. In conclusion, the proposed approach is universal and can be applied to other transmembrane proteins associated with the pathogenesis of certain diseases.

Successful strategies for expression and purification of ABC transporters

ATP-binding cassette (ABC) transporters are proteins responsible for active transport of various compounds, from small ions to macromolecules, across membranes. Proteins from this superfamily also pump drugs out of the cell resulting in multidrug resistance. Based on the cellular functions of ABC-transporters they are commonly associated with diseases like cancer and cystic fibrosis. To understand the molecular mechanism of this critical family of integral membrane proteins, structural characterization is a powerful tool which in turn requires successful recombinant production of stable and functional protein in good yields. In this review we have used high resolution structures of ABC transporters as a measure of successful protein production and summarized strategies for prokaryotic and eukaryotic proteins, respectively. In general, Escherichia coli is the most frequently used host for production of prokaryotic ABC transporters while human embryonic kidney 293 (HEK293) cells are the preferred host system for eukaryotic proteins. Independent of origin, at least two-steps of purification were required after solubilization in the most used detergent DDM. The purification tag was frequently cleaved off before structural characterization using cryogenic electron microscopy, or crystallization and X-ray analysis for prokaryotic proteins.

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.

The C-terminal α-helix is crucial for the activity of the bacterial ABC transporter BmrA

ABC transporters are membrane integral proteins that consist of a transmembrane domain and nucleotide-binding domain (NBD). Two monomers (half-transporters) of the Bacillus subtilis ABC transporter Bacillus multidrug-resistance ATP (BmrA) dimerize to build a functional full-transporter. As all ABC exporters, BmrA uses the free energy of ATP hydrolysis to transport substrate molecules across the cell membrane. For substrate transport, a BmrA dimer undergoes major conformational changes. ATP binding drives dimerization of the NBDs followed by the hydrolysis of the nucleotides. Conserved structural elements within the NBD and transmembrane domain are crucial for dimerization and the activity of BmrA. In the BmrA structure, an α-helix is present at the C-terminus, which can be subdivided in two smaller helices. As shown here, the very C-terminal helix (fragment) is not crucial for the BmrA activity. In fact, based on Cys-scanning mutagenesis, this region is highly flexible. In contrast, a BmrA variant lacking the entire C-terminal α-helix, showed no ATPase and transport activity. Via Ala-scanning, we identified residues in the N-terminal fragment of the helix that are crucial for the BmrA activity, most likely via establishing contacts to structural elements involved in ATP recognition, binding, and/or hydrolysis.

Structural insights into brassinosteroid export mediated by the Arabidopsis ABC transporter ABCB1

Brassinosteroids (BRs) are steroidal phytohormones indispensable for plant growth, development, and responses to environmental stresses. The export of bioactive BRs to the apoplast is essential for BR signaling initiation, which requires binding of a BR molecule to the extracellular domains of the plasma membrane-localized receptor complex. We have previously shown that the Arabidopsis thaliana ATP-binding cassette (ABC) transporter ABCB19 functions as a BR exporter and, together with its close homolog ABCB1, positively regulates BR signaling. Here, we demonstrate that ABCB1 is another BR transporter. The ATP hydrolysis activity of ABCB1 can be stimulated by bioactive BRs, and its transport activity was confirmed in proteoliposomes and protoplasts. Structures of ABCB1 were determined in substrate-unbound (apo), brassinolide (BL)-bound, and ATP plus BL-bound states. In the BL-bound structure, BL is bound to the hydrophobic cavity formed by the transmembrane domain and triggers local conformational changes. Together, our data provide additional insights into ABC transporter-mediated BR export.

Structural basis for the transport and regulation mechanism of the multidrug resistance-associated protein 2

Multidrug resistance-associated protein 2 (MRP2) is an ATP-powered exporter important for maintaining liver homeostasis and a potential contributor to chemotherapeutic resistance. Using cryogenic electron microscopy (cryo-EM), we determine the structures of human MRP2 in three conformational states: an autoinhibited state, a substrate-bound pre-translocation state, and an ATP-bound post-translocation state. In the autoinhibited state, the cytosolic regulatory (R) domain plugs into the transmembrane substrate-binding site and extends into the cytosol to form a composite ATP-binding site at the surface of nucleotide-binding domain 2. Substrate displaces the R domain, permitting conformational changes necessary for transport. These observations suggest that the R domain functions as a selectivity gauge, where only at sufficiently high concentrations can the substrate effectively initiate transport. Comparative structural analyzes of MRP2 bound to various substrates, as determined in this study and others, reveal how MRP2 recognizes a diverse array of compounds, supporting its role in multidrug resistance.

c-MET tyrosine kinase inhibitors reverse drug resistance mediated by the ATP-binding cassette transporter B1 (ABCB1) in cancer cells

This study investigated the potential of MET kinase inhibitors, cabozantinib, crizotinib, and PHA665752, in reversing multidrug resistance (MDR) mediated by ABCB1 in cancer cells. The accumulation of the fluorescent probe, Rhodamine 123, was assessed using flow cytometry and fluorescence microscopy in MDR MES-SA/DX5 and parental cells. The growth inhibitory activity of MET inhibitors as monotherapies and in combination with chemotherapeutic drugs was evaluated by MTT assay. CalcuSyn software was used to analyze the combination index (CI) as an index of drug-drug interaction in combination treatments. Results showed that at concentrations of 5, and 25 μM, c-MET inhibitors significantly increased Rhodamine 123 accumulation in MDR cells, with ratios up to 17.8 compared to control cells, while exhibiting no effect in parental cells. Additionally, the combination of c-MET inhibitors with the chemotherapeutic agent doxorubicin synergistically enhanced cytotoxicity in MDR cells, as evidenced by combination index (CI) values of 0.54 ± 0.08, 0.69 ± 0.1, and 0.85 ± 0.07 for cabozantinib, crizotinib, and PHA665752, respectively. While all three c-MET inhibitors stimulated ABCB1 ATPase activity in different manners at certain concentrations, PHA-665752 suppressed it at high concentration. In silico analysis also suggested that the transmembrane domains (TMD) of ABCB1 transporters could be considered potential target for these agents. Our results suggest that c-MET inhibitors can serve as promising MDR reversal agents in ABCB1-medicated drug-resistant cancer cells.

Mobocertinib antagonizes multidrug resistance in ABCB1- and ABCG2-overexpressing cancer cells: In vitro and in vivo studies

Overexpression of ATP-binding cassette (ABC) transporters, particularly ABCB1 and ABCG2, strongly correlates with multidrug resistance (MDR), rendering cancer chemotherapy ineffective. Exploration and identification of novel inhibitors targeting ABCB1 and ABCG2 are necessary to overcome the related MDR. Mobocertinib is an approved EGFR/HER2 inhibitor for non-small cell lung cancer (NSCLC) with EGFR exon 20 insertion mutations. This study demonstrates that mobocertinib can potentially reverse ABCB1- and ABCG2-mediated MDR. Our findings indicate a strong interaction between mobocertinib and these two proteins, supported by its high binding affinity with ABCB1 and ABCG2 models. Through inhibiting the drug efflux function of ABCB1 and ABCG2, mobocertinib facilitates substrate drugs accumulation, thereby re-sensitizing substrate drugs in drug-resistant cancer cells. Additionally, mobocertinib inhibited the ATPase activity of ABCB1 and ABCG2 without changing the expression levels or subcellular localization. In the tumor-bearing mouse model, mobocertinib boosted the antitumor effect of paclitaxel and topotecan, resulting in tumor regression. In summary, our study uncovers a novel potential for repurposing mobocertinib as a dual inhibitor of ABCB1 and ABCG2, and suggests the combination of mobocertinib with substrate drugs as a strategy to counteract MDR.

Lathyrane diterpenoids with multidrug resistance reversal activity from the tubers of Euphorbia antiquorum

Nine previously unreported lathyrane diterpenoids named euphorantesters A-I, along with 16 known analogues, have been separated from the tubers of Euphorbia antiquorum. Their structures were established by means of spectroscopic analyses, time-dependent density functional theory based electronic circular dichroism calculation and single crystal X-ray crystallography. Their reversal ability against P-glycoprotein-mediated multidrug resistance (MDR) in MCF-7/ADR cell line was then evaluated, and 15 ones exhibited moderate MDR reversal activity with reversal fold falling in the range of 1.12-13.15. The most active euphorantester B could effectively increase the sensitivity of MCF-7/ADR cell to adriamycin comparably to the reference drug verapamil.

Mechanistic insights into P-glycoprotein ligand transport and inhibition revealed by enhanced molecular dynamics simulations

P-glycoprotein (P-gp) plays a crucial role in cellular detoxification and drug efflux processes, transitioning between inward-facing (IF) open, occluded, and outward-facing (OF) states to facilitate substrate transport. Its role is critical in cancer therapy, where P-gp contributes to the multidrug resistance phenotype. In our study, classical and enhanced molecular dynamics (MD) simulations were conducted to dissect the structural and functional features of the P-gp conformational states. Our advanced MD simulations, including kinetically excited targeted MD (ketMD) and adiabatic biasing MD (ABMD), provided deeper insights into state transition and translocation mechanisms. Our findings suggest that the unkinking of TM4 and TM10 helices is a prerequisite for correctly achieving the outward conformation. Simulations of the IF-occluded conformations, characterized by kinked TM4 and TM10 helices, consistently demonstrated altered communication between the transmembrane domains (TMDs) and nucleotide binding domain 2 (NBD2), suggesting the implication of this interface in inhibiting P-gp's efflux function. A particular emphasis was placed on the unstructured linker segment connecting the NBD1 to TMD2 and its role in the transporter's dynamics. With the linker present, we specifically noticed a potential entrance of cholesterol (CHOL) through the TM4-TM6 portal, shedding light on crucial residues involved in accommodating CHOL. We therefore suggest that this entry mechanism could be employed for some P-gp substrates or inhibitors. Our results provide critical data for understanding P-gp functioning and developing new P-gp inhibitors for establishing more effective strategies against multidrug resistance.

ATP-Binding Cassette and Solute Carrier Transporters: Understanding Their Mechanisms and Drug Modulation Through Structural and Modeling Approaches

The ATP-binding cassette (ABC) and solute carrier (SLC) transporters play pivotal roles in cellular transport mechanisms, influencing a wide range of physiological processes and impacting various medical conditions. Recent advancements in structural biology and computational modeling have provided significant insights into their function and regulation. This review provides an overview of the current knowledge of human ABC and SLC transporters, emphasizing their structural and functional relationships, transport mechanisms, and the contribution of computational approaches to their understanding. Current challenges and promising future research and methodological directions are also discussed.

Development of Natural-Product-Inspired ABCB1 Inhibitors Through Regioselective Tryptophan C3-Benzylation

The emergence of drug resistance in cancer cells eventually causing relapse is a serious threat that demands new advances. Upregulation of the ATP-dependent binding cassette (ABC) transporters, such as ABCB1, significantly contributes to the emergence of drug resistance in cancer. Despite more than 30 years of therapeutic discovery, and several generations of inhibitors against P-gp, the search for effective agents that minimize toxicity to human cells, while maintaining efflux pump inhibition is still underway. Leads derived from natural product scaffolds are well-known to be effective in various therapeutic approaches. Inspired by the biosynthetic pathway to Nocardioazine A, a marine alkaloid known to inhibit the P-gp efflux pump in cancer cells, we devised a regioselective pathway to create structurally unique indole-C3-benzyl cyclo-L-Trp-L-Trp diketopiperazines (DKPs). Using bat cells as a model to derive effective ABCB1 inhibitors for targeting human P-gp efflux pumps, we have recently identified exo-C3-N-Dbn-Trp2 (13) as a lead ABCB1 inhibitor. This C3-benzylated lead inhibited ABCB1 better than Verapamil.[21] Additionally, C3-N-Dbn-Trp2 restored chemotherapy sensitivity in drug-resistant human cancer cells and had no adverse effect on cell proliferation in cell cultures. For a clearer structure-activity relationship, we developed a broader screen to test C3-functionalized pyrroloindolines as ABCB1 inhibitors and observed that C3-benzylation is outperforming respective isoprenylated derivatives. Results arising from the molecular docking studies indicate that the interactions at the access tunnel between ABCB1 and the inhibitor result in a powerful predictor for the efficacy of the inhibitor. Based on fluorescence-based assays, we conclude that the most efficacious inhibitor is the p-cyano-derived exo-C3-N-Dbn-Trp2 (33 a), closely followed by the p-nitro substituted analogue. By combining assay results with molecular docking studies, we further correlate that the predictions based on the inhibitor interactions at the access tunnel provide clues about the design of improved ABCB1 inhibitors. As it has been well documented that ABCB1 itself is powerfully engaged in multi-drug resistance, this work lays the foundation for the design of a new class of inhibitors based on the endogenous amino acid-derived cyclo-L-Trp-L-Trp DKP scaffold.

Highly Potent and Intestine Specific P-Glycoprotein Inhibitor to Enable Oral Delivery of Taxol

Taxol is widely used in cancer chemotherapy; however, the oral absorption of Taxol remains a formidable challenge. Since the intestinal p-glycoprotein (P-gp) mediated drug efflux is one of the primary causes, the development of P-gp inhibitor is emerging as a promising strategy to realize Taxol's oral delivery. Because P-gp exists in many tissues, the non-selective P-gp inhibitors would lead to toxicity. Correspondingly, a potent and intestine specific P-gp inhibitor would be an ideal solution to boost the oral absorption of Taxol and avoid exogenous toxicity. Herein, we would like to report a highly potent and intestine specific P-gp inhibitor to enable oral delivery of Taxol in high efficiency. Through a multicomponent reaction and post-modification, various benzofuran-fused-piperidine derivatives were achieved and the biological evaluation identified 16 c with potent P-gp inhibitory activity. Notably, 16 c was intestine specific and showed almost none absorption (F=0.82 %), but possessing higher efficacy than Encequidar to improve the oral absorption of Taxol. In MDA-MB-231 xenograft model, the oral administration of Taxol and 16 c showed high therapeutic efficiency and low toxicity, thus providing a valuable chemotherapy strategy.

Unraveling the secrets: Evolution of resistance mediated by membrane proteins

Membrane protein-mediated resistance is a multidisciplinary challenge that spans fields such as medicine, agriculture, and environmental science. Understanding its complexity and devising innovative strategies are crucial for treating diseases like cancer and managing resistant pests in agriculture. This paper explores the dual nature of resistance mechanisms across different organisms: On one hand, animals, bacteria, fungi, plants, and insects exhibit convergent evolution, leading to the development of similar resistance mechanisms. On the other hand, influenced by diverse environmental pressures and structural differences among organisms, they also demonstrate divergent resistance characteristics. Membrane protein-mediated resistance mechanisms are prevalent across animals, bacteria, fungi, plants, and insects, reflecting their shared survival strategies evolved through convergent evolution to address similar survival challenges. However, variations in ecological environments and biological characteristics result in differing responses to resistance. Therefore, examining these differences not only enhances our understanding of adaptive resistance mechanisms but also provides crucial theoretical support and insights for addressing drug resistance and advancing pharmaceutical development.

Epertinib counteracts multidrug resistance in cancer cells by antagonizing the drug efflux function of ABCB1 and ABCG2

A significant hurdle in cancer treatment arises from multidrug resistance (MDR), often due to overexpression of ATP-binding cassette (ABC) transporters like ABCB1 and/or ABCG2 in cancer cells. These transporters actively diminish the efficacy of cytotoxic drugs by facilitating ATP hydrolysis-dependent drug efflux and reducing intracellular drug accumulation in cancer cells. Addressing multidrug-resistant cancers poses a significant challenge due to the lack of approved treatments, prompting the exploration of alternative avenues like drug repurposing (also referred to as drug repositioning) of molecularly targeted agents to reverse MDR-mediated by ABCB1 and/or ABCG2 in multidrug-resistant cancer cells. Epertinib, a potent inhibitor of EGFR and HER2 currently in clinical trials for solid tumors, was investigated for its potential to resensitize ABCB1- and ABCG2-overexpressing multidrug-resistant cancer cells to chemotherapeutic agents. Our findings reveal that at sub-toxic, submicromolar concentrations, epertinib restores the sensitivity of multidrug-resistant cancer cells to cytotoxic drugs in a concentration-dependent manner. The results demonstrate that epertinib enhances drug-induced apoptosis in these cancer cells by impeding the drug-efflux function of ABCB1 and ABCG2 without altering their expression. ATPase activity and molecular docking were employed to reveal potential interaction sites between epertinib and the drug-binding pockets of ABCB1 and ABCG2. In summary, our study demonstrates an additional pharmacological capability of epertinib against the activity of ABCB1 and ABCG2. These findings suggest that incorporating epertinib into combination therapy could be advantageous for a specific patient subset with tumors exhibiting high levels of ABCB1 or ABCG2, warranting further exploration.

Zosuquidar Promotes Antitumor Immunity by Inducing Autophagic Degradation of PD-L1

The intracellular distribution and transportation process are essential for maintaining PD-L1 (programmed death-ligand 1) expression, and intervening in this cellular process may provide promising therapeutic strategies. Here, through a cell-based high content screening, it is found that the ABCB1 (ATP binding cassette subfamily B member 1) modulator zosuquidar dramatically suppresses PD-L1 expression by triggering its autophagic degradation. Mechanistically, ABCB1 interacts with PD-L1 and impairs COP II-mediated PD-L1 transport from ER (endoplasmic reticulum) to Golgi apparatus. The treatment of zosuquidar enhances ABCB1-PD-L1 interaction and leads the ER retention of PD-L1, which is subsequently degraded in the SQSTM1-dependent selective autophagy pathway. In CT26 mouse model and a humanized xenograft mouse model, zosuquidar significantly suppresses tumor growth and accompanies by increased infiltration of cytotoxic T cells. In summary, this study indicates that ABCB1 serves as a negative regulator of PD-L1, and zosuquidar may act as a potential immunotherapy agent by triggering PD-L1 degradation in the early secretory pathway.

P-glycoprotein (P-gp)-driven cancer drug resistance: biological profile, non-coding RNAs, drugs and nanomodulators

Drug resistance has compromised the efficacy of chemotherapy. The dysregulation of drug transporters including P-glycoprotein (P-gp) can mediate drug resistance through drug efflux. In this review, we highlight the role of P-gp in cancer drug resistance and the related molecular pathways, including phosphoinositide 3-kinase (PI3K)-Akt, phosphatase and tensin homolog (PTEN) and nuclear factor-κB (NF-κB), along with non-coding RNAs (ncRNAs). Extracellular vesicles secreted by the cells can transport ncRNAs and other proteins to change P-gp activity in cancer drug resistance. P-gp requires ATP to function, and the induction of mitochondrial dysfunction or inhibition of glutamine metabolism can impair P-gp function, thus increasing chemosensitivity. Phytochemicals, small molecules and nanoparticles have been introduced as P-gp inhibitors to increase drug sensitivity in human cancers.

A BPTF Inhibitor That Interferes with the Multidrug Resistance Pump to Sensitize Murine Triple-Negative Breast Cancer Cells to Chemotherapy

Triple-negative breast cancer (TNBC) is associated with a generally poor prognosis due to its highly aggressive and metastatic nature, lack of targetable receptors, as well as the frequent development of resistance to chemotherapy. We previously reported that AU1, a small molecule developed as an inhibitor of BPTF (bromodomain PHD finger-containing transcription factor), was capable of sensitizing preclinical models of TNBC to chemotherapy in part via the promotion of autophagy. In studies reported here, we identify an additional property of this compound, specifically that sensitization is associated with the inhibition of the P-glycoprotein (P-gp) efflux pump. In silico molecular docking studies indicate that AU1 binds to active regions of the efflux pump in a manner consistent with the inhibition of the pump function. This work identifies a novel chemical structure that can influence multidrug efflux, an established mechanism of drug resistance in TNBC, that has not yet been successfully addressed by clinical efforts.

P-glycoproteins in anthelmintic safety, efficacy, and resistance

P-glycoprotein (PGP) is a pivotal transmembrane transporter governing the cellular flux of diverse substances shielding mammals from toxics. It can thwart the effectiveness of medicines such as ivermectin (IVM) and other macrocyclic lactone (ML) anthelmintics, undermining therapeutic efforts. We analyze the role of PGPs in limiting the toxicity of these drugs in hosts, and their potential contribution to anthelmintic resistance in nematodes. Targeting nematode PGPs to increase drug sensitivity to MLs seems interesting, but is hampered by the lack of selective inhibitors. The nuclear hormone receptor (NHR)-8 should be seriously considered as a target because it upregulates multiple PGPs involved in anthelmintic resistance and it is specific to nematodes. This would advance our understanding of host-pathogen dynamics and foster innovative therapeutic strategies.

Computationally accelerated identification of P-glycoprotein inhibitors

Overexpression of the polyspecific efflux transporter, P-glycoprotein (P-gp, MDR1, ABCB1 ), is a major mechanism by which cancer cells acquire multidrug resistance (MDR), the resistance to diverse chemotherapeutic drugs. Inhibiting drug transport by P-gp can resensitize cancer cells to chemotherapy, but there are no P-gp inhibitors available to patients. Clinically unsuccessful P-gp inhibitors tend to bind at the pump's transmembrane drug binding domains and are often P-gp transport substrates, resulting in lowered intracellular concentration of the drug and altered pharmacokinetics. In prior work, we used computationally accelerated drug discovery to identify novel P-gp inhibitors that target the pump's cytoplasmic nucleotide binding domains. Our first-draft study provided conclusive evidence that the nucleotide binding domains of P-gp are viable targets for drug discovery. Here we develop an enhanced, computationally accelerated drug discovery pipeline that expands upon our prior work by iteratively screening compounds against multiple conformations of P-gp with molecular docking. Targeted molecular dynamics simulations with our homology model of human P-gp were used to generate docking receptors in conformations mimicking a putative drug transport cycle. We offset the increased computational complexity using custom Tanimoto chemical datasets, which maximize the chemical diversity of ligands screened by docking. Using our expanded, virtual-assisted pipeline, we identified nine novel P-gp inhibitors that reverse MDR in two types of P-gp overexpressing human cancer cell lines, reflecting a 13.4% hit rate. Of these inhibitors, all were non-toxic to non-cancerous human cells, and six were not likely to be transport substrates of P-gp. Our novel P-gp inhibitors are chemically diverse and are good candidates for lead optimization. Our results demonstrate that the nucleotide binding domains of P-gp are an underappreciated target in the effort to reverse P-gp-mediated multidrug resistance in cancer.

Progress in Research of Nanotherapeutics for Overcoming Multidrug Resistance in Cancer

Chemotherapy has been widely applied in oncotherapy. However, the development of multidrug resistance (MDR) has diminished the effectiveness of anticancer drugs against tumor cells. Such resistance often results in tumor recurrence, metastasis, and patient death. Fortunately, nanoparticle-based drug delivery systems provide a promising strategy by codelivery of multiple drugs and MDR reversal agents and the skillful, flexible, smart modification of drug targets. Such systems have demonstrated the ability to bypass the ABC transporter biological efflux mechanisms due to drug resistance. Hence, how to deliver drugs and exert potential antitumor effects have been successfully explored, applied, and developed. Furthermore, to overcome multidrug resistance, nanoparticle-based systems have been developed due to their good therapeutic effect, low side effects, and high tumor metastasis inhibition. In view of this, we systematically discuss the molecular mechanisms and therapeutic strategies of MDR from nanotherapeutics. Finally, we summarize intriguing ideas and future trends for further research in overcoming MDR.

Mutational analysis reveals the importance of residues of the access tunnel inhibitor site to human P-glycoprotein (ABCB1)-mediated transport

Human P-glycoprotein (P-gp) utilizes energy from ATP hydrolysis for the efflux of chemically dissimilar amphipathic small molecules and plays an important role in the development of resistance to chemotherapeutic agents in most cancers. Efforts to overcome drug resistance have focused on inhibiting P-gp-mediated drug efflux. Understanding the features distinguishing P-gp inhibitors from substrates is critical. Cryo-electron microscopy has revealed distinct binding patterns, emphasizing the role of the L-site or access tunnel in inhibition. We substituted 5-9 residues of the L-site with alanine to investigate whether the binding of a second inhibitor molecule to the L-site is required for inhibiting drug efflux. We reveal, for the first time, that mutations in the L-site affect the drug efflux activity of P-gp, despite their distance from the substrate-binding pocket (SBP). Surprisingly, after the mutations were introduced, inhibitors such as tariquidar and zosuquidar still inhibited drug efflux by mutant P-gps. Communication between the transmembrane helices (TMHs) and nucleotide-binding domains (NBDs) was evaluated using the ATPase assay, revealing distinct modulation patterns by inhibitors for the mutants, with zosuquidar exhibiting substrate-like stimulation of ATPase. Furthermore, L-site mutations abolished ATP-dependent thermal stabilization. In silico molecular docking studies corroborated the altered inhibitor binding due to mutations in the L-site residues, shedding light on their critical role in substrate transport and inhibitor interactions with P-gp. These findings suggest that inhibitors bind either to the SBP alone, and/or to alternate site(s) when the L-site is disabled by mutagenesis.

Dynamics design of a non-natural transcription factor responding to androst-4-ene-3,17-dione

The production of androst-4-ene-3,17-dione (AD) by the steroidal microbial cell factory requires transcription factors (TFs) to participate in metabolic regulation. However, microbial cell factory lacks effective TFs that can respond to AD in its metabolic pathway. Additionally, finding and obtaining natural TFs that specifically respond to AD is a complex and onerous task. In this study, we devised an artificial TF that responds to AD, termed AdT, based on structure-guided molecular dynamics (MD) simulation. According to MD analysis of the conformational changes of AdT after binding to AD, an LBD in which the N- and C-termini exhibited convergence tendencies was used as a microswitch to guide the assembly of a DNA-binding domain lexA, a linker (GGGGS)2, and a transcription activation domain B42 into an artificial TF. As a proof of design, a AD biosensor was designed and constructed in yeast on the basis of the ligand-binding domain (LBD) of hormone receptor. In addition, the transcription factor activity of AdT was increased by 1.44-fold for its variant F320Y. Overall, we created non-natural TF elements for AD microbial cell factory, and expected that the design TF strategy will be applied to running in parallel to the signaling machinery of the host cell.

Lansoprazole (LPZ) reverses multidrug resistance (MDR) in cancer through impeding ATP-binding cassette (ABC) transporter-mediated chemotherapeutic drug efflux and lysosomal sequestration

AIMS

Lansoprazole is one of the many proton pump inhibitors (PPIs) that acts more strongly with ABCB1 and ABCG2. The present study is to investigate the potential of lansoprazole on reversal of ABCB1/G2-mediated MDR in cancer, in vitro and in vivo.

METHODS

Reversal studies and combination evaluation were conducted to determine the synergistic anti-MDR effects on lansoprazole. Lysosomal staining was used to determination of lansoprazole on ABCB1-mediated lysosomal sequestration. Substrate accumulation and efflux assays, ATPase activity, and molecular docking were conducted to evaluate lansoprazole on ABCB1/G2 functions. Western blot and immunofluorescence were used to detect lansoprazole on ABCB1/G2 expression and subcellular localization. MDR nude mice models were established to evaluate the effects of lansoprazole on MDR in vivo.

RESULTS

Lansoprazole attenuated ABCB1/G2-mediated MDR and exhibited synergistic effects with substrate drugs in MDR cells. In vivo experiments demonstrated that lansoprazole attenuated ABCB1/G2-mediated MDR and exhibited synergistic effects that augmented the sensitivity of substrate anticancer drugs in ABCB1/G2-mediated settings without obvious toxicity. Lansoprazole impeded lysosomal sequestration mediated by ABCB1, leading to a substantial increase in intracellular accumulation of substrate drugs. The effects of lansoprazole were not attributable to downregulation or alterations in subcellular localization of ABCB1/G2. Lansoprazole promoted the ATPase activity of ABCB1/G2 and competitively bound to the substrate-binding region of ABCB1/G2.

CONCLUSIONS

These findings present novel therapeutic avenues whereby the combination of lansoprazole and chemotherapeutic agents mitigates MDR mediated by ABCB1/G2 overexpression.

Coupling the role of lipids to the conformational dynamics of the ABC transporter P-glycoprotein

The ATP-binding cassette transporter P-glycoprotein (P-gp) is a multidrug efflux pump that is overexpressed in a variety of cancers and associated with the drug-resistance phenomenon. P-gp structures were previously determined in detergent and in nanodiscs, in which different transmembrane helix conformations were found, "straight" and "kinked," respectively, indicating a possible role of the lipid environment on the P-gp structural ensemble. Here, we investigate the dynamic conformational ensembles and protein-lipid interactions of two human P-gp inward-open conformers, straight and kinked, employing all-atom molecular dynamics (MD) simulations in asymmetric multicomponent lipid bilayers that mimic the highly specialized hepatocyte membrane in which P-gp is expressed. The two conformers are found to differ in terms of the accessibility of the substrate cavity. The MD simulations show how cholesterol and different lipid species wedge, snorkel, and partially enter into the cavity of the straight P-gp conformer solved in detergent. However, access to the cavity of the kinked P-gp conformer solved in nanodiscs is restricted. Furthermore, the volume and dynamic fluctuations of the substrate cavity largely differ between the two P-gp conformers and are modulated by the presence (or absence) of cholesterol in the membrane and/or of ATP. From the mechanistic perspective, the findings indicate that the straight conformer likely precedes the kinked conformer in the functional working cycle of P-gp, with the latter conformation representing a post substrate-bound state. The inaccessibility of the main transmembrane cavity in the kinked conformer might be crucial in preventing substrate disengagement and transport withdrawal. Remarkably, in our unbiased MD simulations, one transmembrane helix (TM10) of the straight conformer underwent a spontaneous transition to a kinked conformation, underlining the relevance of both conformations in a native phospholipid environment and revealing structural descriptors defining the transition between the two P-gp conformers.

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.

Sitagliptin synergizes 5-fluorouracil efficacy in colon cancer cells through MDR1-mediated flux impairment and down regulation of NFκB2 and p-AKT survival proteins

5-fluorouracil (5-FU) is an inexpensive treatment for colon cancer; however, its efficacy is limited by chemoresistance. This study investigates the combination therapy approach of 5-FU with Sitagliptin (Sita), a diabetic drug with potential cancer-modulating effects. The combination was evaluated in vitro and in silico, focusing on the effects of Sita and 5-FU on colon cancer cells. The results showed that the addition of Sita significantly decreased the IC50 of 5-FU compared to 5-Fu monotherapy. The study also found that Sita and 5-FU interact synergistically, with a combination index below 1. Sita successfully lowered the 5-FU dosage reduction index, decreasing the expression of MDR1 mRNA and p-AKT and NFκB2 subunits p100/p52 protein. Molecular docking analyses confirmed Sita's antagonistic action on MDR1 and thymidylate synthase proteins. The study concludes that sitagliptin can target MDR1, increase apoptosis, and significantly reduce the expression of p-AKT and NFκB2 cell-survival proteins. These effects sensitize colon cancer cells to 5-FU. Repurposing sitagliptin may enhance the anticancer effects of 5-FU at lower dosages.

Effect of substituents at the C3´, C3´N, C10 and C2-meta-benzoate positions of taxane derivatives on their activity against resistant cancer cells

We tested the effect of substituents at the (1) C3´, C3´N, (2) C10, and (3) C2-meta-benzoate positions of taxane derivatives on their activity against sensitive versus counterpart paclitaxel-resistant breast (MCF-7) and ovarian (SK-OV-3) cancer cells. We found that (1) non-aromatic groups at both C3´ and C3´N positions, when compared with phenyl groups at the same positions of a taxane derivative, significantly reduced the resistance of ABCB1 expressing MCF-7/PacR and SK-OV-3/PacR cancer cells. This is, at least in the case of the SB-T-1216 series, accompanied by an ineffective decrease of intracellular levels in MCF-7/PacR cells. The low binding affinity of SB-T-1216 in the ABCB1 binding cavity can elucidate these effects. (2) Cyclopropanecarbonyl group at the C10 position, when compared with the H atom, seems to increase the potency and capability of the derivative in overcoming paclitaxel resistance in both models. (3) Derivatives with fluorine and methyl substituents at the C2-meta-benzoate position were variously potent against sensitive and resistant cancer cells. All C2 derivatives were less capable of overcoming acquired resistance to paclitaxel in vitro than non-substituted analogs. Notably, fluorine derivatives SB-T-121205 and 121,206 were more potent against sensitive and resistant SK-OV-3 cells, and derivatives SB-T-121405 and 121,406 were more potent against sensitive and resistant MCF-7 cells. (4) The various structure-activity relationships of SB-T derivatives observed in two cell line models known to express ABCB1 favor their complex interaction not based solely on ABCB1.

Low-dose doxorubicin loaded extracellular vesicles combined Fas/FasL pathway-mediated chemo-sensitization and immunotherapy against tumor

The clinical application of doxorubicin (DOX) is mainly restricted by its serious side effects, poor drug delivery efficiency, and limited immunogenic death (ICD) effect. To improve DOX-based chemotherapy and ameliorate its adverse effects, we utilized 3LL cell-derived extracellular vesicles to encapsulate DOX and sodium nitroprusside (SNP) to obtain DOX/SNP@CM, which could effectively target the tumor site by harnessing the inherent homologous targeting property of tumor cell membranes. DOX performed its role on chemotherapy, and SNP successfully respond to the intracellular GSH to continuously generate nitric oxide (NO). The in situ-produced NO upregulated the Fas expression on the tumor cell surface, thereby sensitizing the Fas/FasL pathway-mediated tumor cell apoptosis of DOX. Furthermore, NO also boosted the intratumoral infiltration of cytotoxic T cells by promoted ICD effect towards tumor cells. Importantly, the anti-tumor immunity tightly cooperated with Fas/FasL mediated tumor cell apoptosis by NO-mediated manipulation on Fas/FasL interaction, collectively making DOX/SNP@CM exert significant tumor growth inhibition with low-dose DOX. Remarkably, DOX and SNP both are widely used clinical medicines, ensuring DOX/SNP@CM a potential opportunity for future practical applications.

Exploring bat-inspired cyclic tryptophan diketopiperazines as ABCB1 Inhibitors

Chemotherapy-induced drug resistance remains a major cause of cancer recurrence and patient mortality. ATP binding cassette subfamily B member 1 (ABCB1) transporter overexpression in tumors contributes to resistance, yet current ABCB1 inhibitors have been unsuccessful in clinical trials. To address this challenge, we propose a new strategy using tryptophan as a lead molecule for developing ABCB1 inhibitors. Our idea stems from our studies on bat cells, as bats have low cancer incidences and high ABCB1 expression. We hypothesized that potential ABCB1 substrates in bats could act as competitive inhibitors in humans. By molecular simulations of ABCB1-substrate interactions, we generated a benzylated Cyclo-tryptophan (C3N-Dbn-Trp2) that inhibits ABCB1 activity with efficacy comparable to or better than the classical inhibitor, verapamil. C3N-Dbn-Trp2 restored chemotherapy sensitivity in drug-resistant human cancer cells with no adverse effect on cell proliferation. Our unique approach presents a promising lead toward developing effective ABCB1 inhibitors to treat drug-resistant cancers.

Design, synthesis, molecular modeling, in vitro and in vivo biological evaluation of potent anthranilamide derivatives as dual P-glycoprotein and CYP3A4 inhibitors

Paclitaxel (PTX) is considered the blockbuster chemotherapy treatment for cancer. Paclitaxel's (PTX) oral administration has proven to be extremely difficult, mostly because of its susceptibility to intestinal P-glycoprotein (P-gp) and cytochrome P450 (CYP3A4). The concurrent local inhibition of intestinal P-gp and CYP3A4 is a promising approach to improve the oral bioavailability of paclitaxel while avoiding potential unfavorable side effects of their systemic inhibition. Herein, we report the rational design and evaluation of novel dual potent inhibitors of P-gp and CYP3A4 using an anthranilamide derivative tariquidar as a starting point for their structural optimizations. Compound 14f, bearing N-imidazolylbenzyl side chain, was found to have potent and selective P-gp (EC50 = 28 nM) and CYP3A4 (IC50 = 223 nM) inhibitory activities with low absorption potential (Papp (A-to-B) <0.06). In vivo, inhibitor 14f improved the oral absorption of paclitaxel by 6 times in mice and by 30 times in rats as compared to vehicle, while 14f itself remained poorly absorbed. Compound 14f, possessing dual P-gp and CYP3A4 inhibitory activities, offered additional enhancement in paclitaxel oral absorption compared to tariquidar in mice. Evaluating the CYP effect of 14f on oral absorption of paclitaxel requires considering the variations in CYP expression between animal species. This study provides further medicinal chemistry advice on strategies for resolving concerns with the oral administration of chemotherapeutic agents.