Discovery and Characterization of Potent Dual P-Glycoprotein and CYP3A4 Inhibitors: Design, Synthesis, Cryo-EM Analysis, and Biological Evaluations

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.

A comprehensive mechanistic investigation of factors affecting intestinal absorption and bioavailability of two PROTACs in rats

AIM

Proteolysis targeting chimeras (PROTACs) exhibit a unique and promising pharmacology. However, this comes with molecular properties exceeding the 'drug-like' rule of five chemical space, which often limits gastrointestinal absorption. This in vivo study aimed to investigate the contribution of luminal and plasma stability, intestinal effective permeability, P-glycoprotein (P-gp) efflux, and bile excretion, on the rat intestinal absorption and systemic exposure of two PROTACs, ARV-110 (812 Da, LogD7.4 4.8) and ARV-471 (724 Da, LogD7.4 4.6).

METHODS

Luminal stability and effective intestinal permeability were determined directly from luminal disappearance using single-pass intestinal perfusion, with and without a protease inhibitor, or a P-gp/Cytochrome P450 CYP3A inhibitor (ketoconazole) in rats. Plasma stability was tested by in vitro incubations. Intestinal absorption, systemic exposure, and biliary excretion were examined after intraduodenal and intravenous dosing with ketoconazole or the P-gp selective inhibitor (encequidar).

RESULTS AND DISCUSSION

Both PROTACs were degraded in the intestinal lumen and in plasma by peptidases. The intestinal effective permeability in rats was moderate for ARV-110 (0.62 × 10-4 cm/s) and low for ARV-471 (0.23 × 10-4 cm/s). P-gp inhibition increased the permeability 1.6- and 2.3-fold for ARV-110 and ARV-471, respectively. After intraduodenal dosing with the P-gp inhibitors a corresponding increase in systemic exposure was observed for both PROTACs. There was only a minor difference in the increased systemic exposure induced by the two inhibitors, suggesting that the mechanisms were primarily P-gp inhibition, rather than gut-wall and hepatic extraction. Biliary excretion was a minor pathway and did not affect the absorption and systemic exposure of the PROTACs to a large extent.

CONCLUSION

In the rat, ARV-110 and ARV-471 were enzymatically degraded in the intestinal lumen and in plasma, and their intestinal permeability and systemic exposure seem to be reduced due to P-gp efflux.

Insights into interactions between taxanes and P-glycoprotein using biophysical and in silico methods

Multidrug resistance mediated by P-glycoprotein (Pgp) is a significant obstacle to cancer chemotherapy. Taxane drugs, including paclitaxel, docetaxel, and cabazitaxel, are used to treat multiple types of cancer. All taxane drugs are Pgp substrates, but cabazitaxel is also a Pgp inhibitor, indicating potential differential interactions between Pgp and different taxanes. Here, we showed for the first time that cabazitaxel had a partial inhibitory effect on the ATPase activity at concentrations higher than 10 µM. We found the KD of paclitaxel, docetaxel, and cabazitaxel to Pgp are 0.85 µM, 40.59 µM, and 13.53 µM, respectively. Based on acrylamide quenching, paclitaxel induced Pgp into a wide inward-facing open conformation at a high concentration but a slightly occluded conformation at lower concentrations. Both docetaxel and cabazitaxel shifted Pgp towards occluded states, each drug resulting in a unique degree of occlusion. Furthermore, molecular docking and energy calculations revealed that cabazitaxel binds with the "access tunnel" and blocks the subsequent nucleotide-binding domain dimerization. Our results indicate that the preference of taxanes for different binding sites on Pgp leads to distinct transport mechanisms. These results provide valuable insight into the interaction between taxanes and Pgp, which will enhance future drug development.

A patent review of CYP3A4 inhibitors (2018 - present)

INTRODUCTION

Cytochrome P450 3A4 (CYP3A4), one of the most important xenobiotic-metabolizing enzymes, plays a central role in drug metabolism and acts as a key mediator in drug-drug interactions. CYP3A4 inhibitors can potentiate the in vivo therapeutic effects of CYP3A4-substrate drugs via enhancing their systematic exposure levels. Two CYP3A4 inhibitors (ritonavir and cobicistat) have already been approved for modulating the exposure levels of CYP3A4-substrate drugs.

AREAS COVERED

This review summarizes the newly patented CYP3A4 inhibitors in the period (2018-2024) by using the keywords 'CYP3A4' and 'inhibitor' in Espacenet database from academic institutions and industrial companies. The chemical structures and inhibition profiles of the patented CYP3A4 inhibitors, including the anti-CYP3A4 potency, inhibitory mechanisms, and other relevant information, are summarized and discussed.

EXPERT OPINION

Although diverse CYP3A4 inhibitors have been developed in the past few years, the development of more efficacious CYP3A4 inhibitors with favorable pharmacokinetic and safety profiles is still challenging. To maximize the benefit of CYP3A4 inhibitors, combination strategies should be used for the development of highly specific CYP3A4 inhibitors or degraders with efficacious anti-CYP3A4 effects and favorable pharmacokinetic profiles. Meanwhile, more efforts should be made to address the organ-targeting or tumor-targeting ability of CYP3A4 inhibitors for specific purposes.

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.

Functional and structural polypharmacology of indazole-based privileged ligands to tackle the undruggability of membrane transporters

Despite the significant roles of solute carrier (SLC) and ATP-binding cassette (ABC) transporters in human health and disease, most remain poorly characterized as intrinsic and/or xenobiotic ligands are unknown, rendering them as 'undruggable'. Polypharmacology, defined as the simultaneous engagement of multiple targets by a single ligand, offers a promising avenue for discovering novel lead compounds addressing these emerging pharmacological challenges - a major focus in contemporary medicinal chemistry. While common structural motifs among phylogenetically diverse proteins have been proposed to underlie polypharmacology through the concept of 'multitarget binding sites', a comprehensive analysis of these functional and structural aspects from a medicinal chemistry perspective has yet to be undertaken. In our study, we synthesized 65 distinct indazole derivatives and evaluated their activity across a broad biological assessment platform encompassing 17 specific and polyspecific SLC and ABC transporters. Notably, ten indazoles exhibited cross-target activity against challenging transporter targets associated with neurodegeneration (ABCA1), metabolic reprogramming (MCT4), and cancer multidrug resistance (ABCC10). Furthermore, molecular blind docking experiments and advanced binding site analyses revealed, for the first time, conserved binding motifs across monocarboxylate transporters (MCTs), organic anion transporting polypeptides (OATPs), organic cation transporters (OCTs), and ABC transporters, characterized by specific and recurring residues of tyrosine, phenylalanine, serine, and threonine. These findings highlight not only the potential of polypharmacology in drug discovery but also provide insights into the structural underpinnings of ligand binding across membrane transporters.

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.

An oxidation-reduction-triggered thiamine disulfide-based prodrug of 10-hydroxycamptothecin for selective tumor cell locking and therapeutic delivery

Chemotherapy, a primary method of cancer treatment, has been limited in clinical application due to its lack of specificity and tumor multidrug resistance, resulting in numerous undesired side effects. Herein, a small molecule conjugate, TDK-HCPT, was designed and synthesized, which could target tumor cells and prolong the retention of chemotherapy agents within tumor cells. Moreover, a similarly designed control system, TDK-Nap, has been developed as well to enable cancer cell imaging. Two design elements are incorporated into TDK-HCPT: the thiamine disulfide (TDS) and the thioketal subunit (tk). TDS can be reduced in the high glutathione (GSH) conditions within cancer cell to form thiazolium salt, and the resulting enhanced positive charge and lipophobicity make the system difficult to be pumped out of tumor cells, thereby effectively "locking" the chemotherapy drug HCPT inside the tumor cells. Additionally, the tk subunit serves as a ROS trigger, within the tumor cells, the "locked" HCPT were then released and activated by the high ROS conditions, optimizing its targeted potential. This allows TDK-HCPT to serve as a redox-liable molecular platform that targets cancer cells selectively which decreases cancer cell migration, retards tumor growth, and lowers tumorigenesis rates as evidenced by a combination of in vitro and in vivo studies. To the best of our knowledge, this is the first time a cancer cell "lock in" has been shown to prevent tumorigenesis in an animal model.

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.

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.

Investigating how HIV-1 antiretrovirals differentially behave as substrates and inhibitors of P-glycoprotein via molecular dynamics simulations

HIV-1 can rapidly infect the brain upon initial infection, establishing latent reservoirs that induce neuronal damage and/or death, resulting in HIV-Associated Neurocognitive Disorder. Though anti-HIV-1 antiretrovirals (ARVs) suppress viral load, the blood-brain barrier limits drug access to the brain, largely because of highly expressed efflux proteins like P-glycoprotein (P-gp). While no FDA-approved P-gp inhibitor currently exists, HIV-1 protease inhibitors show promise as partial P-gp inhibitors, potentially enhancing drug delivery to the brain. Herein, we employed docking and molecular dynamics simulations to elucidate key differences in P-gp's interactions with several antiretrovirals, including protease inhibitors, with known inhibitory or substrate-like behaviors towards P-gp. Our results led us to hypothesize new mechanistic details of small-molecule efflux by and inhibition of P-gp, where the "Lower Pocket" in P-gp's transmembrane domain serves as the primary initial site for small-molecule binding. Subsequently, this pocket merges with the more traditionally studied drug binding site-the "Upper Pocket"-thus funneling small-molecule drugs, such as ARVs, towards the Upper Pocket for efflux. Furthermore, our results reinforce the understanding that both binding energetics and changes in protein dynamics are crucial in discerning small molecules as non-substrates, substrates, or inhibitors of P-gp. Our findings indicate that interactions between P-gp and inhibitory ARVs induce bridging of transmembrane domain helices, impeding P-gp conformational changes and contributing to the inhibitory behavior of these ARVs. Overall, insights gained in this study could serve to guide the design of future P-gp-targeting therapeutics for a wide range of pathological conditions and diseases, including HIV-1.

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.

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.

Discovery of pyridoquinoxaline-based new P-gp inhibitors as coadjutant against Multi Drug Resistance in cancer

Multi-drug resistance (MDR) is a serious challenge in contemporary clinical practice and is mostly responsible for the failure of cancer medication therapies. Several experimental evidence links MDR to the overexpression of the drug efflux transporter P-gp, therefore, the discovery of novel P-glycoprotein inhibitors is required to treat or prevent MDR and to improve the absorption of chemotherapy drugs via the gastrointestinal system. In this work, we explored a series of novel pyridoquinoxaline-based derivatives designed from parental compounds, previously proved active in enhancing anticancer drugs in MDR nasopharyngeal carcinoma (KB). Among them, derivative 10d showed the most potent and selective inhibition of fluorescent dye efflux, if compared to reference compounds (MK-571, Novobiocin, Verapamil), and the highest MDR reversal activity when co-administered with the chemotherapeutic agents Vincristine and Etoposide, at non-cytotoxic concentrations. Molecular modelling predicted the two compound 10d binding mode in a ratio of 2:1 with the target protein. No cytotoxicity was observed in healthy microglia cells and off-target investigations showed the absence of CaV1.2 channel blockade. In summary, our findings indicated that 10d could potentially be a novel therapeutic coadjutant by inhibiting P-gp transport function in vitro, thereby reversing cancer multidrug resistance.

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.

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.

Design, synthesis, and ex vivo anti-drug resistant cervical cancer activity of novel molecularly targeted chalcone derivatives

Chemotherapy toxicity and tumor multidrug resistance remain the main reasons for clinical treatment failure in cervical cancer. In this study, 79 novel chalcone derivatives were designed and synthesized using the principle of active substructure splicing with the parent nucleus of licorice chalcone as the lead compound and VEGFR-2 and P-gp as the target of action and their potentials for anticervical cancer activity were preliminarily evaluated. The results showed that the IC50 values of candidate compound B20 against HeLa and HeLa/DDP cells were 3.66 ± 0.10 and 4.35 ± 0.21 μΜ, respectively, with a resistance index (RI) of 1.18, which was significantly higher than that of the positive drug cisplatin (IC50:13.60 ± 1.63, 100.03 ± 7.94 μΜ, RI:7.36). In addition, B20 showed significant inhibitory activity against VEGFR-2 kinase and P-gp-mediated rhodamine 123 efflux, as well as the ability to inhibit the phosphorylation of VEGFR-2 and downstream PI3K/AKT signaling pathway proteins, inducing apoptosis, blocking cells in the S-phase, and inhibiting invasive migration and tubule generation by HUVEC cells. Acceptable safety was demonstrated in acute toxicity tests when B20 was at 200 mg/kg. In the nude mouse HeLa/DDP cell xenograft tumor model, the inhibition rate of transplanted tumors was 39.2 % and 79.2 % when B20 was at 10 and 20 mg/kg, respectively. These results suggest that B20 is a potent VEGFR-2 and P-gp inhibitor with active potential for treating cisplatin-resistant cervical cancer.

Applicability of MDR1 Overexpressing Abcb1KO-MDCKII Cell Lines for Investigating In Vitro Species Differences and Brain Penetration Prediction

Implementing the 3R initiative to reduce animal experiments in brain penetration prediction for CNS-targeting drugs requires more predictive in vitro and in silico models. However, animal studies are still indispensable to obtaining brain concentration and determining the prediction performance of in vitro models. To reveal species differences and provide reliable data for IVIVE, in vitro models are required. Systems overexpressing MDR1 and BCRP are widely used to predict BBB penetration, highlighting the impact of the in vitro system on predictive performance. In this study, endogenous Abcb1 knock-out MDCKII cells overexpressing MDR1 of human, mouse, rat or cynomolgus monkey origin were used. Good correlations between ERs of 83 drugs determined in each cell line suggest limited species specificities. All cell lines differentiated CNS-penetrating compounds based on ERs with high efficiency and sensitivity. The correlation between in vivo and predicted Kp,uu,brain was the highest using total ER of human MDR1 and BCRP and optimized scaling factors. MDR1 interactors were tested on all MDR1 orthologs using digoxin and quinidine as substrates. We found several examples of inhibition dependent on either substrate or transporter abundance. In summary, this assay system has the potential for early-stage brain penetration screening. IC50 comparison between orthologs is complex; correlation with transporter abundance data is not necessarily proportional and requires the understanding of modes of transporter inhibition.

Discovery of new tricyclic spiroindole derivatives as potent P-glycoprotein inhibitors for reversing multidrug resistance enabled by a synthetic methodology-based library

The discovery of novel and highly effective P-gp inhibitors is considered to be an effective strategy for overcoming tumor drug resistance. In this paper, a phenotypic screening via a self-constructed synthetic methodology-based library identified a new class of tricyclic spiroindole derivatives with excellent tumor multidrug resistance reversal activity. A stereospecific compound OY-103-B with the best reversal activity was obtained based on a detailed structure-activity relationship study, metabolic stability optimization and chiral resolution. For the VCR-resistant Eca109 cell line (Eca109/VCR), co-administration of 5.0 μM OY-103-B resulted in a reversal fold of up to 727.2, superior to the typical third-generation P-gp inhibitor tariquidar. Moreover, the compound inhibited the proliferation of Eca109/VCR cells in a concentration-dependent manner in plate cloning and flow cytometry. Furthermore, fluorescence substrate accumulation assay and chemotherapeutic drug reversal activity tests demonstrated that OY-103-B reversed tumor drug resistance via P-gp inhibition. In conclusion, this study provides a novel skeleton that inspires the design of new P-gp inhibitors, laying the foundation for the treatment of drug-resistant tumors.

Cryo-EM structure of P-glycoprotein bound to triple elacridar inhibitor molecules

P-glycoprotein (P-gp) is an ATP-binding cassette transporter known for its roles in expelling xenobiotic compounds from cells and contributing to cellular drug resistance through multidrug efflux. This mechanism is particularly problematic in cancer cells, where it diminishes the therapeutic efficacy of anticancer drugs. P-gp inhibitors, such as elacridar, have been developed to circumvent the decrease in drug efficacy due to P-gp efflux. An earlier study reported the cryo-EM structure of human P-gp-Fab (MRK-16) complex bound by two elacridar molecules, at a resolution of 3.6 Å. In this study, we have obtained a higher resolution (2.5 Å) structure of the P-gp- Fab (UIC2) complex bound by three elacridar molecules. This finding, which exposes a larger space for compound-binding sites than previously acknowledged, has significant implications for the development of more selective inhibitors and enhances our understanding of the compound recognition mechanism of P-gp.

Model of P-Glycoprotein Ligand Binding and Validation with Efflux Substrate Matched Pairs

The blood-brain barrier (BBB) poses a significant obstacle in developing therapeutics for neurodegenerative diseases and central nervous system (CNS) disorders. P-glycoprotein (P-gp), a multidrug resistance protein, is a critical gatekeeper in the BBB and plays a role in cancer chemoresistance. This paper uses cryo-EM P-gp structures as starting points with an induced fit docking (IFD) model to evaluate 19 pairs of compounds with known P-gp efflux data. The study reveals significant differences in binding energy and sheds light on structural modifications' impact on efflux properties. In the cases examined, fluorine incorporation influences the efflux by altering the molecular conformation rather than proximal heteroatom basicity. Although there are limitations in addressing covalent interactions or when binding extends into the more flexible vestibule region of the protein, the results provide valuable insights and potential strategies to overcome P-gp efflux, contributing to the advancement of drug development for both CNS disorders and cancer therapies.

Membrane transporters in drug development and as determinants of precision medicine

The effect of membrane transporters on drug disposition, efficacy and safety is now well recognized. Since the initial publication from the International Transporter Consortium, significant progress has been made in understanding the roles and functions of transporters, as well as in the development of tools and models to assess and predict transporter-mediated activity, toxicity and drug-drug interactions (DDIs). Notable advances include an increased understanding of the effects of intrinsic and extrinsic factors on transporter activity, the application of physiologically based pharmacokinetic modelling in predicting transporter-mediated drug disposition, the identification of endogenous biomarkers to assess transporter-mediated DDIs and the determination of the cryogenic electron microscopy structures of SLC and ABC transporters. This article provides an overview of these key developments, highlighting unanswered questions, regulatory considerations and future directions.

Synthesis and evaluation of WK-X-34 derivatives as P-glycoprotein (P-gp/ABCB1) inhibitors for reversing multidrug resistance

The emergence of multidrug resistance (MDR) in malignant tumors is one of the leading threats encountered currently by many chemotherapeutic agents. A proposed strategy to overcome MDR is to disable the efflux function of P-glycoprotein (P-gp/ABCB1), a critical member of the ABC transporter family that significantly increases the efflux of various anticancer drugs from tumor cells. In this study, structural modification of a third-generation P-gp inhibitor WK-X-34 based on bioisosteric and fragment-growing strategies led to the discovery of the adamantane derivative PID-9, which exhibited the best MDR reversal activity (IC50 = 0.1338 μM, RF = 78.6) in this series, exceeding those of the reported P-gp inhibitors verapamil and WK-X-34. In addition, compared with WK-X-34, PID-9 showed decreased toxicity to cells. Furthermore, the mechanism studies revealed that the reversal activity of adamantane derivatives PID-5, PID-7, and PID-9 stemmed from the inhibition of P-gp efflux. These results indicated that compound PID-9 is the most effective P-gp inhibitor among them with low toxicity and high MDR reversal activity, which provided a fundamental structural reference for further discovery of novel, effective, and non-toxic P-gp inhibitors.

Consensus screening for a challenging target: the quest for P-glycoprotein inhibitors

ATP-binding cassette (ABC) transporters are a large family of proteins involved in membrane transport of a wide variety of substrates. Among them, ABCB1, also known as MDR-1 or P-glycoprotein (P-gp), is the most characterized. By exporting xenobiotics out of the cell, P-gp activity can affect the ADME properties of several drugs. Moreover, P-gp has been found to mediate multidrug resistance in cancer cells. Thus, the inhibition of P-gp activity may lead to increased absorption and/or intracellular accumulation of co-administered drugs, enhancing their effectiveness. Using the human-mouse chimeric cryoEM 3D structure of the P-gp in the inhibitor-bound intermediate form (PDBID: 6qee), approximately 200 000 commercially available natural compounds from the ZINC database were virtually screened. To build a model able to discriminate between substrate and inhibitors, two datasets of compounds with known activity, including P-gp inhibitors, substrates, and inactive molecules were also docked. The best docking pose of selected substrates and inhibitors were used to generate 3D common feature pharmacophoric models that were combined with the Autodock Vina binding energy values to prioritize compounds for visual inspection. With this consensus approach, 13 potential candidates were identified and then tested for their ability to inhibit P-gp, using zosuquidar, a third generation P-gp inhibitor, as a reference drug. Eight compounds were found to be active with 6 of them having an IC50 lower than 5 μM in a membrane-based ATPase activity assay. Moreover, the P-gp inhibitory activity was also confirmed by two different cell-based in vitro methods. Both retrospective and prospective results demonstrate the ability of the combined structure-based pharmacophore modeling and docking-based virtual screening approach to predict novel hit compounds with inhibitory activity toward P-gp. The resulting chemical scaffolds could serve as inspiration for the optimization of novel and more potent P-gp inhibitors.

Structural View of Cryo-Electron Microscopy-Determined ATP-Binding Cassette Transporters in Human Multidrug Resistance

ATP-binding cassette (ABC) transporters, acting as cellular "pumps," facilitate solute translocation through membranes via ATP hydrolysis. Their overexpression is closely tied to multidrug resistance (MDR), a major obstacle in chemotherapy and neurological disorder treatment, hampering drug accumulation and delivery. Extensive research has delved into the intricate interplay between ABC transporter structure, function, and potential inhibition for MDR reversal. Cryo-electron microscopy has been instrumental in unveiling structural details of various MDR-causing ABC transporters, encompassing ABCB1, ABCC1, and ABCG2, as well as the recently revealed ABCC3 and ABCC4 structures. The newly obtained structural insight has deepened our understanding of substrate and drug binding, translocation mechanisms, and inhibitor interactions. Given the growing body of structural information available for human MDR transporters and their associated mechanisms, we believe it is timely to compile a comprehensive review of these transporters and compare their functional mechanisms in the context of multidrug resistance. Therefore, this review primarily focuses on the structural aspects of clinically significant human ABC transporters linked to MDR, with the aim of providing valuable insights to enhance the effectiveness of MDR reversal strategies in clinical therapies.

Structure-ATPase Activity Relationship of Rhodamine Derivatives as Potent Inhibitors of P-Glycoprotein CmABCB1

Understanding the transport and inhibition mechanisms of substrates by P-glycoprotein (P-gp) is one of the important approaches in addressing multidrug resistance (MDR). In this study, we evaluated a variety of rhodamine derivatives as potential P-gp inhibitors targeting CmABCB1, a P-gp homologue, with a focus on their ATPase activity. Notably, a Q-rhodamine derivative with an o,o'-dimethoxybenzyl ester moiety (RhQ-DMB) demonstrated superior affinity and inhibitory activity, which was further confirmed by a drug susceptibility assay in yeast strains expressing CmABCB1. Results from a tryptophan fluorescence quenching experiment using a CmABCB1 mutant suggested that RhQ-DMB effectively enters and binds to the inner chamber of CmABCB1. These findings underscore the promising potential of RhQ-DMB as a tool for future studies aimed at elucidating the substrate-bound state of CmABCB1.

Molecular Modeling Studies to Probe the Binding Hypothesis of Novel Lead Compounds against Multidrug Resistance Protein ABCB1

The expression of drug efflux pump ABCB1/P-glycoprotein (P-gp), a transmembrane protein belonging to the ATP-binding cassette superfamily, is a leading cause of multidrug resistance (MDR). We previously curated a dataset of structurally diverse and selective inhibitors of ABCB1 to develop a pharmacophore model that was used to identify four novel compounds, which we showed to be potent and efficacious inhibitors of ABCB1. Here, we dock the inhibitors into a model structure of the human transporter and use molecular dynamics (MD) simulations to report the conformational dynamics of human ABCB1 induced by the binding of the inhibitors. The binding hypotheses are compared to the wider curated dataset and those previously reported in the literature. Protein-ligand interactions and MD simulations are in good agreement and, combined with LipE profiling, statistical and pharmacokinetic analyses, are indicative of potent and selective inhibition of ABCB1.

Structure-based discovery of novel P-glycoprotein inhibitors targeting the nucleotide binding domains

P-glycoprotein (P-gp), a membrane transport protein overexpressed in certain drug-resistant cancer cells, has been the target of numerous drug discovery projects aimed at overcoming drug resistance in cancer. Most characterized P-gp inhibitors bind at the large hydrophobic drug binding domain (DBD), but none have yet attained regulatory approval. In this study, we explored the potential of designing inhibitors that target the nucleotide binding domains (NBDs), by computationally screening a large library of 2.6 billion synthesizable molecules, using a combination of machine learning-guided molecular docking and molecular dynamics (MD). 14 of the computationally best-scoring molecules were subsequently tested for their ability to inhibit P-gp mediated calcein-AM efflux. In total, five diverse compounds exhibited inhibitory effects in the calcein-AM assay without displaying toxicity. The activity of these compounds was confirmed by their ability to decrease the verapamil-stimulated ATPase activity of P-gp in a subsequent assay. The discovery of these five novel P-gp inhibitors demonstrates the potential of in-silico screening in drug discovery and provides a new stepping point towards future potent P-gp inhibitors.

Design and Synthesis of Novel α-Methylchalcone Derivatives, Anti-Cervical Cancer Activity, and Reversal of Drug Resistance in HeLa/DDP Cells

In this study, a collection of newly developed α-methylchalcone derivatives were synthesized and assessed for their inhibitory potential against human cervical cancer cell lines (HeLa, SiHa, and C33A) as well as normal human cervical epithelial cells (H8). Notably, compound 3k exhibited substantial inhibitory effects on both HeLa and HeLa/DDP cells while demonstrating lower toxicity toward H8 cells. Furthermore, the compound 3k was found to induce apoptosis in both HeLa and HeLa/DDP cells while also inhibiting the G2/M phase, resulting in a decrease in the invasion and migration capabilities of these cells. When administered alongside cisplatin, 3k demonstrated a significant reduction in the resistance of HeLa/DDP cells to cisplatin, as evidenced by a decrease in the resistance index (RI) value from 7.90 to 2.10. Initial investigations into the underlying mechanism revealed that 3k did not impact the expression of P-gp but instead facilitated the accumulation of rhodamine 123 in HeLa/DDP cells. The results obtained from CADD docking analysis demonstrated that 3k exhibits stable binding to microtubule proteins and P-gp targets, forming hydrogen bonding interaction forces. Immunofluorescence analysis further revealed that 3k effectively decreased the fluorescence intensity of α and β microtubules in HeLa and HeLa/DDP cells, resulting in disruptions in cell morphology, reduction in cell numbers, nucleus coagulation, and cell rupture. Additionally, Western blot analysis indicated that 3k significantly reduced the levels of polymerized α and β microtubule proteins in both HeLa and HeLa/DDP cell lines while concurrently increasing the expression of dissociated α and β microtubule proteins. The aforementioned findings indicate a potential correlation between the inhibitory effects of 3k on HeLa and HeLa/DDP cells and its ability to inhibit tubulin and P-gp.

Advances in the structure, mechanism and targeting of chemoresistance-linked ABC transporters

Cancer cells frequently display intrinsic or acquired resistance to chemically diverse anticancer drugs, limiting therapeutic success. Among the main mechanisms of this multidrug resistance is the overexpression of ATP-binding cassette (ABC) transporters that mediate drug efflux, and, specifically, ABCB1, ABCG2 and ABCC1 are known to cause cancer chemoresistance. High-resolution structures, biophysical and in silico studies have led to tremendous progress in understanding the mechanism of drug transport by these ABC transporters, and several promising therapies, including irradiation-based immune and thermal therapies, and nanomedicine have been used to overcome ABC transporter-mediated cancer chemoresistance. In this Review, we highlight the progress achieved in the past 5 years on the three transporters, ABCB1, ABCG2 and ABCC1, that are known to be of clinical importance. We address the molecular basis of their broad substrate specificity gleaned from structural information and discuss novel approaches to block the function of ABC transporters. Furthermore, genetic modification of ABC transporters by CRISPR-Cas9 and approaches to re-engineer amino acid sequences to change the direction of transport from efflux to import are briefly discussed. We suggest that current information regarding the structure, mechanism and regulation of ABC transporters should be used in clinical trials to improve the efficiency of chemotherapeutics for patients with cancer.

An oral triple pill-based cocktail effectively controls acute myeloid leukemia with high translation

Acute myeloid leukemia (AML) is a deadly hematological malignancy characterized by oncogenic translational addiction that results in over-proliferation and apoptosis evasion of leukemia cells. Various chemo- and targeted therapies aim to reverse this hallmark, but most show only modest efficacy. Here we report a single oral pill containing a low-dose triple small molecule-based cocktail, a highly active anti-cancer therapy (HAACT) with unique mechanisms that can effectively control AML. The cocktail comprises oncogenic translation inhibitor HHT, drug efflux pump P-gpi ENC and anti-apoptotic protein Bcl-2i VEN. Mechanistically, the cocktail can potently kill both leukemia stem cells (LSC) and bulk leukemic cells via co-targeting oncogenic translation, apoptosis machinery, and drug efflux pump, resulting in deep and durable remissions of AML in diverse model systems. We also identified EphB4/Bcl-xL as the cocktail response biomarkers. Collectively, our studies provide proof that a single pill containing a triple combination cocktail might be a promising avenue for AML therapy.

Perspectives on drug repurposing to overcome cancer multidrug resistance mediated by ABCB1 and ABCG2

The overexpression of the human ATP-binding cassette (ABC) transporters in cancer cells is a common mechanism involved in developing multidrug resistance (MDR). Unfortunately, there are currently no approved drugs specifically designed to treat multidrug-resistant cancers, making MDR a significant obstacle to successful chemotherapy. Despite over two decades of research, developing transporter-specific inhibitors for clinical use has proven to be a challenging endeavor. As an alternative approach, drug repurposing has gained traction as a more practical method to discover clinically effective modulators of drug transporters. This involves exploring new indications for already-approved drugs, bypassing the lengthy process of developing novel synthetic inhibitors. In this context, we will discuss the mechanisms of ABC drug transporters ABCB1 and ABCG2, their roles in cancer MDR, and the inhibitors that have been evaluated for their potential to reverse MDR mediated by these drug transporters. Our focus will be on providing an up-to-date report on approved drugs tested for their inhibitory activities against these drug efflux pumps. Lastly, we will explore the challenges and prospects of repurposing already approved medications for clinical use to overcome chemoresistance in patients with high tumor expression of ABCB1 and/or ABCG2.

Architecture of the rings of 5-arylidenerhodanine derivatives versus P-gp inhibition

5-Arylidene derivatives of rhodanine show various biological activities. The new crystal structures of five derivatives investigated towards ABCB1 efflux pump modulation are reported, namely, 2-[5-([1,1'-biphenyl]-4-ylmethylidene]-4-oxo-2-thioxothiazolidin-3-yl)acetic acid dimethyl sulfoxide monosolvate, C18H13NO3S2·C2H6OS (1), 4-[5-([1,1'-biphenyl]-4-ylmethylidene]-4-oxo-2-thioxothiazolidin-3-yl)butanoic acid, C20H17NO3S2 (2), 5-[4-(benzyloxy)benzylidene]-2-thioxothiazolidin-4-one, C17H13NO2S2 (3), 4-{5-[4-(benzyloxy)benzylidene]-4-oxo-2-thioxothiazolidin-3-yl}butanoic acid, C21H19NO4S2 (4), and 5-[4-(diphenylamino)benzylidene]-2-thioxothiazolidin-4-one, C22H16N2OS2 (5). Compounds 1 and 3-5 crystallize in the triclinic space group P-1, while 2 crystallizes in the monoclinic space group P21/n, where the biphenyl moiety is observed in two positions (A and B). Two molecules are present in the asymmetric unit of 5 and, for the other four compounds, there is only one molecule; moreover, 1 crystallizes with one dimethyl sulfoxide molecule. The packing of the molecules containing a carboxyl group (1, 2 and 4) is determined by O-H...O hydrogen bonds, while in the other two compounds (3 and 5), the packing is determined by N-H...O hydrogen bonds. Additionally, induced-fit docking studies have been performed for the active compounds to investigate their putative binding mode inside the human glycoprotein P (P-gp) binding pocket.

Strategies to overcome cancer multidrug resistance (MDR) through targeting P-glycoprotein (ABCB1): An updated review

The emergence of multidrug resistance (MDR) in malignant tumors is one of the leading threats encountered currently in many chemotherapeutic agents. The overexpression of the ATP-binding cassette (ABC) transporters is involved in MDR. P-glycoprotein (P-gp)/ABCB1 is a member of the ABC transporter family that significantly increases the efflux of various anticancer drugs from tumor cells. Therefore, targeting P-gp with small molecule inhibitors is an effective therapeutic strategy to overcome MDR. Over the past four decades, diverse compounds with P-gp inhibitory activity have been identified to sensitize drug-resistant cells, but none of them has been proven clinically useful to date. Research efforts continue to discover an effective approach for circumventing MDR. This review has provided an overview of the most recent advances (last three years) in various strategies for circumventing MDR mediated by P-gp. It may be helpful for the scientists working in the field of drug discovery to further synthesize and discover new chemical entities/therapeutic modalities with less toxicity and more efficacies to overcome MDR in cancer chemotherapy.

Natural polymers as potential P-glycoprotein inhibitors: Pre-ADMET profile and computational analysis as a proof of concept to fight multidrug resistance in cancer

P-glycoprotein (P-gp) is known as the "multidrug resistance protein" because it contributes to tumor resistance to several different classes of anticancer drugs. The effectiveness of such polymers in treating cancer and delivering drugs has been shown in a wide range of in vitro and in vivo experiments. The primary objective of the present study was to investigate the inhibitory effects of several naturally occurring polymers on P-gp efflux, as it is known that P-gp inhibition can impede the elimination of medications. The objective of our study is to identify polymers that possess the potential to inhibit P-gp, a protein involved in drug resistance, with the aim of enhancing the effectiveness of anticancer drug formulations. The ADMET profile of all the selected polymers (Agarose, Alginate, Carrageenan, Cyclodextrin, Dextran, Hyaluronic acid, and Polysialic acid) has been studied, and binding affinities were investigated through a computational approach using the recently released crystal structure of P-gp with PDB ID: 7O9W. The advanced computational study was also done with the help of molecular dynamics simulation. The aim of the present study is to overcome MDR resulting from the activity of P-gp by using such polymers that can inhibit P-gp when used in formulations. The docking scores of native ligand, Agarose, Alginate, Carrageenan, Chitosan, Cyclodextrin, Dextran, Hyaluronic acid, and Polysialic acid were found to be -10.7, -8.5, -6.6, -8.7, -8.6, -24.5, -6.7, -8.3, and -7.9, respectively. It was observed that, Cyclodextrin possess multiple properties in drug delivery science and here also demonstrated excellent binding affinity. We propose that drug efflux-related MDR may be prevented by the use of Agarose, Carregeenan, Chitosan, Cyclodextrin, Hyaluronic acid, and/or Polysialic acid in the administration of anticancer drugs.

How Cryo-EM Has Expanded Our Understanding of Membrane Transporters

Over the past two decades, technological advances in membrane protein structural biology have provided insight into the molecular mechanisms that transporters use to move diverse substrates across the membrane. However, the plasticity of these proteins' ligand binding pockets, which allows them to bind a range of substrates, also poses a challenge for drug development. Here we highlight the structure, function, and transport mechanism of ATP-binding cassette/solute carrier transporters that are related to several diseases and multidrug resistance: ABCB1, ABCC1, ABCG2, SLC19A1, and SLC29A1. SIGNIFICANCE STATEMENT: ATP-binding cassette transporters and solute carriers play vital roles in clinical chemotherapeutic outcomes. This paper describes the current understanding of the structure of five pharmacologically relevant transporters and how they interact with their ligands.

The Alternating Access Mechanism in Mammalian Multidrug Resistance Transporters and Their Bacterial Homologs

Multidrug resistance (MDR) proteins belonging to the ATP-Binding Cassette (ABC) transporter group play a crucial role in the export of cytotoxic drugs across cell membranes. These proteins are particularly fascinating due to their ability to confer drug resistance, which subsequently leads to the failure of therapeutic interventions and hinders successful treatments. One key mechanism by which multidrug resistance (MDR) proteins carry out their transport function is through alternating access. This mechanism involves intricate conformational changes that enable the binding and transport of substrates across cellular membranes. In this extensive review, we provide an overview of ABC transporters, including their classifications and structural similarities. We focus specifically on well-known mammalian multidrug resistance proteins such as MRP1 and Pgp (MDR1), as well as bacterial counterparts such as Sav1866 and lipid flippase MsbA. By exploring the structural and functional features of these MDR proteins, we shed light on the roles of their nucleotide-binding domains (NBDs) and transmembrane domains (TMDs) in the transport process. Notably, while the structures of NBDs in prokaryotic ABC proteins, such as Sav1866, MsbA, and mammalian Pgp, are identical, MRP1 exhibits distinct characteristics in its NBDs. Our review also emphasizes the importance of two ATP molecules for the formation of an interface between the two binding sites of NBD domains across all these transporters. ATP hydrolysis occurs following substrate transport and is vital for recycling the transporters in subsequent cycles of substrate transportation. Specifically, among the studied transporters, only NBD2 in MRP1 possesses the ability to hydrolyze ATP, while both NBDs of Pgp, Sav1866, and MsbA are capable of carrying out this reaction. Furthermore, we highlight recent advancements in the study of MDR proteins and the alternating access mechanism. We discuss the experimental and computational approaches utilized to investigate the structure and dynamics of MDR proteins, providing valuable insights into their conformational changes and substrate transport. This review not only contributes to an enhanced understanding of multidrug resistance proteins but also holds immense potential for guiding future research and facilitating the development of effective strategies to overcome multidrug resistance, thus improving therapeutic interventions.

Simplified Derivatives of Tetrandrine as Potent and Specific P-gp Inhibitors to Reverse Multidrug Resistance in Cancer Chemotherapy

Targeted inhibition of a drug efflux transporter P-glycoprotein (P-gp) is an important strategy to reverse multidrug resistance in cancer chemotherapy. In this study, a rationally structural simplification to natural tetrandrine was performed based on molecular dynamics simulation and fragment growth, leading to an easily prepared, novel, and simplified compound OY-101 with high reversal activity and low cytotoxicity. Its excellent synergistic anti-cancer effect with vincristine (VCR) against drug-resistant cells Eca109/VCR was confirmed by reversal activity assay, flow cytometry, plate clone formation assay, and drug synergism analysis (IC50 = 9.9 nM, RF = 690). Further mechanism study confirmed that the OY-101 was a specific and efficient P-gp inhibitor. Importantly, OY-101 increased VCR sensitization in vivo without obvious toxicity. Overall, our findings may provide an alternative strategy for the design of novel specific P-gp inhibitor as an anti-tumor chemotherapy sensitizer.

Replacing the tropolonic methoxyl group of colchicine with methylamino increases tubulin binding affinity with improved therapeutic index and overcomes paclitaxel cross-resistance

AIMS

Microtubule inhibitors are widely used in first line cancer therapy, though drug resistance often develops and causes treatment failure. Colchicine binds to tubulins and inhibits tumor growth, but is not approved for cancer therapy due to systemic toxicity. In this study, we aim to improve the therapeutic index of colchicine through structural modification.

METHODS

The methoxyl group of the tropolonic ring in colchicine was replaced with amino groups. The cross-resistance of the derivatives with paclitaxel and vincristine was tested. Antitumor effects of target compounds were tested in vivo in A549 and paclitaxel-resistant A549/T xenografts. The interaction of target compounds with tubulins was measured using biological and chemical methods.

RESULTS

Methylamino replacement of the tropolonic methoxyl group of colchicine increases, while demethylation loses, selective tubulin binding affinity, G₂/M arrest and antiproliferation activity. Methylaminocolchicine is more potent than paclitaxel and vincristine to inhibit tumor growth in vitro and in vivo without showing cross-resistance to paclitaxel. Methylaminocolchicine binds to tubulins in unique patterns and inhibits P-gp with a stable pharmacokinetic profile.

CONCLUSION

Methylanimo replacement of the tropolonic methoxyl group of colchicine increases antitumor activity with improved therapeutic index. Methylaminocolchicine represents a new type of mitotic inhibitor with the ability of overcoming paclitaxel and vincristine resistance.

5-Arylidenerhodanines as P-gp Modulators: An Interesting Effect of the Carboxyl Group on ABCB1 Function in Multidrug-Resistant Cancer Cells

Multidrug resistance (MDR) is considered one of the major mechanisms responsible for the failure of numerous anticancer and antiviral chemotherapies. Various strategies to overcome the MDR phenomenon have been developed, and one of the most attractive research directions is focused on the inhibition of MDR transporters, membrane proteins that extrude cytotoxic drugs from living cells. Here, we report the results of our studies on a series newly synthesized of 5-arylidenerhodanines and their ability to inhibit the ABCB1 efflux pump in mouse T-lymphoma cancer cells. In the series, compounds possessing a triphenylamine moiety and the carboxyl group in their structure were of particular interest. These amphiphilic compounds showed over 17-fold stronger efflux pump inhibitory effects than verapamil. The cytotoxic and antiproliferative effects of target rhodanines on T-lymphoma cells were also investigated. A putative binding mode for 11, one of the most potent P-gp inhibitors tested here, was predicted by molecular docking studies and discussed with regard to the binding mode of verapamil.

Meta-Analysis of the Expansion in the Field of Structural Biology of ABC Transporters

ABC transporters are molecular machines which power the solute transport using ATP hydrolysis. The structural biology of ABC transporters has been exploding for the last few years, and this study explores timelines and trends for various attributes such as structural tools, resolution, fold, sources, and group leaders. This study also evidences the significance of mammalian expression systems, advancements in structural biology tools, and the developing interest of group leaders across the world in the remarkably progressing field. The field started in 2002 and bloomed in 2016, and COVID years were really productive to the field. Specifically, the study explores 337 structures of 58 unique ABC transporters deposited in the PDB database from which P-glycoprotein has the largest number of structures. Approximately, 62% of total structures are determined at the resolution of 3-4 Å and 53% of structures belong to fold IV type. With progressive advancements in the field, the field is shifting from prokaryotic to eukaryotic sources and X-ray crystallography to cryoelectron microscopy. In the nutshell, this study uniquely provides the detailed snapshot of the field of structural biology of ABC transporters with real-time data.

A commentary on the use of pharmacoenhancers in the pharmaceutical industry and the implication for DMPK drug discovery strategies

Paxlovid, a drug combining nirmatrelvir and ritonavir, was designed for the treatment of COVID-19 and its rapid development has led to emergency use approval by the FDA to reduce the impact of COVID-19 infection on patients.In order to overcome potentially suboptimal therapeutic exposures, nirmatrelvir is dosed in combination with ritonavir to boost the pharmacokinetics of the active product.Here we consider examples of drugs co-administered with pharmacoenhancers.Pharmacoenhancers have been adopted for multiple purposes such as ensuring therapeutic exposure of the active product, reducing formation of toxic metabolites, changing the route of administration, and increasing the cost-effectiveness of a therapy.We weigh the benefits and risks of this approach, examining the impact of technology developments on drug design and how enhanced integration between cross-discipline teams can improve the outcome of drug discovery.

An Oral 3D Printed PLGA-Tocopherol PEG Succinate Nanocomposite Hydrogel for High-Dose Methotrexate Delivery in Maintenance Chemotherapy

High-dose methotrexate (HDMTX) is one of the chemotherapeutic agents used to treat a variety of cancers in both adults and children. However, the toxicity associated with HDMTX has resulted in the spread of infections and treatment interruption. Further, poor bioavailability due to efflux pump activities mediated by P-glycoprotein has also been linked to poor therapeutic effects of methotrexate following oral administrations. D-α-Tocopheryl poly-ethylene glycol 1000 succinate (TPGS) is known to improve the bioavailability of poorly soluble drugs by inhibiting P-gp efflux activities, thus enhancing cellular uptake. Therefore, to achieve improved bioavailability for MTX, this study aimed to design and develop a novel drug delivery system employing TPGS and a biodegradable polymer, i.e., PLGA, to construct methotrexate-loaded nanoparticles fixated in alginate-gelatine 3D printable hydrogel ink to form a solid 3D printed tablet for oral delivery. The results indicated that high accuracy (>95%) of the 3D printed tablets was achieved using a 25 G needle. In vitro, drug release profiles were investigated at pH 1.2 and pH 7.4 to simulate the gastrointestinal environment. The in vitro release profile displayed a controlled and prolonged release of methotrexate over 24 h. The in silico modeling study displayed P-gp ATPase inhibition, suggesting enhanced MTX absorption from the gastrointestinal site. The 3D-printed hydrogel-based tablet has the potential to overcome the chemotherapeutic challenges that are experienced with conventional therapies.