Reversal of P-glycoprotein mediated multidrug resistance by novel anthranilamide derivatives

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.

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.

Recent Advancements and Strategies for Overcoming the Blood-Brain Barrier Using Albumin-Based Drug Delivery Systems to Treat Brain Cancer, with a Focus on Glioblastoma

Glioblastoma multiforme (GBM) is a highly aggressive malignant tumor, and the most prevalent primary malignant tumor affecting the brain and central nervous system. Recent research indicates that the genetic profile of GBM makes it resistant to drugs and radiation. However, the main obstacle in treating GBM is transporting drugs through the blood-brain barrier (BBB). Albumin is a versatile biomaterial for the synthesis of nanoparticles. The efficiency of albumin-based delivery systems is determined by their ability to improve tumor targeting and accumulation. In this review, we will discuss the prevalence of human glioblastoma and the currently adopted treatment, as well as the structure and some essential functions of the BBB, to transport drugs through this barrier. We will also mention some aspects related to the blood-tumor brain barrier (BTBB) that lead to poor treatment efficacy. The properties and structure of serum albumin were highlighted, such as its role in targeting brain tumors, as well as the progress made until now regarding the techniques for obtaining albumin nanoparticles and their functionalization, in order to overcome the BBB and treat cancer, especially human glioblastoma. The albumin drug delivery nanosystems mentioned in this paper have improved properties and can overcome the BBB to target brain tumors.

Redox-responsive drug-inhibitor conjugate encapsulated in DSPE-PEG2k micelles for overcoming multidrug resistance to chemotherapy

Multidrug resistance (MDR) is a major cause of chemotherapy failure in cancer treatment. P-glycoprotein (P-gp) inhibitors are helpful for chemotherapy drugs to overcome tumor MDR effectively. With the traditional physical mixing of chemotherapy drugs and inhibitors, it is difficult to achieve satisfactory results due to the different pharmacokinetics and physicochemical properties between the two of them. Herein, we prepared a novel drug-inhibitor conjugate prodrug (PTX-ss-Zos) from a cytotoxin (PTX) and a third-generation P-gp inhibitor (Zos) linked with a redox-responsive disulfide. Then, PTX-ss-Zos was encapsulated in DSPE-PEG_2k micelles to form stable and uniform nanoparticles (PTX-ss-Zos@DSPE-PEG_2k NPs). PTX-ss-Zos@DSPE-PEG_2k NPs could be cleaved by the high-concentration GSH in cancer cells and release PTX and Zos simultaneously to inhibit MDR tumor growth synergistically without apparent systemic toxicity. The in vivo evaluation experiments exhibited that the tumor inhibition rates (TIR) of PTX-ss-Zos@DSPE-PEG_2k NPs were high up to 66.5% for HeLa/PTX tumor-bearing mice. This smart nanoplatform would bring new hope for cancer treatment in clinical trials.

The Tetrahydroisoquinoline Scaffold in ABC Transporter Inhibitors that Act as Multidrug Resistance (MDR) Reversers

BACKGROUND

The failure of anticancer chemotherapy is often due to the development of resistance to a variety of anticancer drugs. This phenomenon is called multidrug resistance (MDR) and is related to the overexpression of ABC transporters, such as P-glycoprotein, multidrug resistance- associated protein 1 and breast cancer resistance protein. Over the past few decades, several ABC protein modulators have been discovered and studied as a possible approach to evade MDR and increase the success of anticancer chemotherapy. Nevertheless, the co-administration of pump inhibitors with cytotoxic drugs, which are substrates of the transporters, does not appear to be associated with an improvement in the therapeutic efficacy of antitumor agents. However, more recently discovered MDR reversing agents, such as the two tetrahydroisoquinoline derivatives tariquidar and elacridar, are characterized by high affinity towards the ABC proteins and by reduced negative properties. Consequently, many analogs of these two derivatives have been synthesized, with the aim of optimizing their MDR reversal properties.

OBJECTIVE

This review aims to describe the MDR modulators carrying the tetraidroisoquinoline scaffold reported in the literature in the period 2009-2021, highlighting the structural characteristics that confer potency and/or selectivity towards the three ABC transport proteins.

RESULTS AND CONCLUSION

Many compounds have been synthesized in the last twelve years showing interesting properties, both in terms of potency and selectivity. Although clear structure-activity relationships can be drawn only by considering strictly related compounds, some of the compounds reviewed could be promising starting points for the design of new ABC protein inhibitors.

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

Targeted concurrent inhibition of intestinal drug efflux transporter P-glycoprotein (P-gp) and drug metabolizing enzyme cytochrome P450 3A4 (CYP3A4) is a promising approach to improve oral bioavailability of their common substrates such as docetaxel, while avoiding side effects arising from their pan inhibitions. Herein, we report the discovery and characterization of potent small molecule inhibitors of P-gp and CYP3A4 with encequidar (minimally absorbed P-gp inhibitor) as a starting point for optimization. To aid in the design of these dual inhibitors, we solved the high-resolution cryo-EM structure of encequidar bound to human P-gp. The structure guided us to prudently decorate the encequidar scaffold with CYP3A4 pharmacophores, leading to the identification of several analogues with dual potency against P-gp and CYP3A4. In vivo, dual P-gp and CYP3A4 inhibitor 3a improved the oral absorption of docetaxel by 3-fold as compared to vehicle, while 3a itself remained poorly absorbed.

Structure-Guided Design of a "Bump-and-Hole" Bromodomain-Based Degradation Tag

Chemical biology tools to modulate protein levels in cells are critical to decipher complex biology. Targeted protein degradation offers the potential for rapid and dose-dependent protein depletion through the use of protein fusion tags toward which protein degraders have been established. Here, we present a newly developed protein degradation tag BRD4_BD1L94V along with the corresponding cereblon (CRBN)-based heterobifunctional degrader based on a "bump-and-hole" approach. The resulting compound XY-06-007 shows a half-degradation concentration (DC_50, 6 h) of 10 nM against BRD4_BD1L94V with no degradation of off-targets, as assessed by whole proteome mass spectrometry, and demonstrates suitable pharmacokinetics for in vivo studies. We demonstrate that BRD4_BD1L94V can be combined with the dTAG approach to achieve simultaneous degrader-mediated depletion of their respective protein fusions. This orthogonal system complements currently available protein degradation tags and enables investigation into the consequences resulting from rapid degradation of previously undruggable disease codependencies.

Structural Basis of Drug Recognition by the Multidrug Transporter ABCG2

ABCG2 is an ATP-binding cassette (ABC) transporter whose function affects the pharmacokinetics of drugs and contributes to multidrug resistance of cancer cells. While its interaction with the endogenous substrate estrone-3-sulfate (E₁S) has been elucidated at a structural level, the recognition and recruitment of exogenous compounds is not understood at sufficiently high resolution. Here we present three cryo-EM structures of nanodisc-reconstituted, human ABCG2 bound to anticancer drugs tariquidar, topotecan and mitoxantrone. To enable structural insight at high resolution, we used Fab fragments of the ABCG2-specific monoclonal antibody 5D3, which binds to the external side of the transporter but does not interfere with drug-induced stimulation of ATPase activity. We observed that the binding pocket of ABCG2 can accommodate a single tariquidar molecule in a C-shaped conformation, similar to one of the two tariquidar molecules bound to ABCB1, where tariquidar acts as an inhibitor. We also found single copies of topotecan and mitoxantrone bound between key phenylalanine residues. Mutagenesis experiments confirmed the functional importance of two residues in the binding pocket, F439 and N436. Using 3D variability analyses, we found a correlation between substrate binding and reduced dynamics of the nucleotide binding domains (NBDs), suggesting a structural explanation for drug-induced ATPase stimulation. Our findings provide additional insight into how ABCG2 differentiates between inhibitors and substrates and may guide a rational design of new modulators and substrates.

P-Glycoprotein Inhibitors Differently Affect Toxoplasma gondii, Neospora caninum and Besnoitia besnoiti Proliferation in Bovine Primary Endothelial Cells

Apicomplexan parasites are obligatory intracellular protozoa. In the case of Toxoplasma gondii, Neospora caninum or Besnoitia besnoiti, to ensure proper tachyzoite production, they need nutrients and cell building blocks. However, apicomplexans are auxotrophic for cholesterol, which is required for membrane biosynthesis. P-glycoprotein (P-gp) is a transmembrane transporter involved in xenobiotic efflux. However, the physiological role of P-gp in cholesterol metabolism is unclear. Here, we analyzed its impact on parasite proliferation in T. gondii-, N. caninum- and B. besnoiti-infected primary endothelial cells by applying different generations of P-gp inhibitors. Host cell treatment with verapamil and valspodar significantly diminished tachyzoite production in all three parasite species, whereas tariquidar treatment affected proliferation only in B. besnoiti. 3D-holotomographic analyses illustrated impaired meront development driven by valspodar treatment being accompanied by swollen parasitophorous vacuoles in the case of T. gondii. Tachyzoite and host cell pre-treatment with valspodar affected infection rates in all parasites. Flow cytometric analyses revealed verapamil treatment to induce neutral lipid accumulation. The absence of a pronounced anti-parasitic impact of tariquidar, which represents here the most selective P-gp inhibitor, suggests that the observed effects of verapamil and valspodar are associated with mechanisms independent of P-gp. Out of the three species tested here, this compound affected only B. besnoiti proliferation and its effect was much milder as compared to verapamil and valspodar.

Discovery of Encequidar, First-in-Class Intestine Specific P-glycoprotein Inhibitor

Many chemotherapeutics, such as paclitaxel, are administered intravenously as they suffer from poor oral bioavailability, partly because of efflux mechanism of P-glycoprotein in the intestinal epithelium. To date, no drug has been approved by the U.S. Food and Drug Administration (FDA) that selectively blocks this efflux pump. We sought to identify a compound that selectively inhibits P-glycoprotein in the gastrointestinal mucosa with poor oral bioavailability, thus eliminating the issues such as bone marrow toxicity associated with systemic inhibition of P-glycoprotein. Here, we describe the discovery of highly potent, selective, and poorly orally bioavailable P-glycoprotein inhibitor 14 (encequidar). Clinically, encequidar was found to be well tolerated and minimally absorbed; and importantly, it enabled the oral delivery of paclitaxel.

Role of ABCB1 in mediating chemoresistance of triple-negative breast cancers

Triple-negative breast cancer (TNBC) is a group of breast cancers which neither express hormonal receptors nor human epidermal growth factor receptor. Hence, there is a lack of currently known targeted therapies and the only available line of systemic treatment option is chemotherapy or more recently immune therapy. However, in patients with relapsed disease after adjuvant or neoadjuvant therapy, resistance to chemotherapeutic agents has often developed, which results in poor treatment response. Multidrug resistance (MDR) has emerged as an important mechanism by which TNBCs mediate drug resistance and occurs primarily due to overexpression of ATP-binding cassette (ABC) transporter proteins such as P-glycoprotein (Pgp). Pgp overexpression had been linked to poor outcome, reduced survival rates and chemoresistance in patients. The aim of this mini-review is to provide a topical overview of the recent studies and to generate further interest in this critical research area, with the aim to develop an effective and safe approach for overcoming Pgp-mediated chemoresistance in TNBC.

ABCG: a new fold of ABC exporters and a whole new bag of riddles!

ATP-binding cassette (ABC) superfamily comprises membrane transporters that power the active transport of substrates across biological membranes. These proteins harness the energy of nucleotide binding and hydrolysis to fuel substrate translocation via an alternating-access mechanism. The primary structural blueprint is relatively conserved in all ABC transporters. A transport-competent ABC transporter is essentially made up of two nucleotide-binding domains (NBDs) and two transmembrane domains (TMDs). While the NBDs are conserved in their primary sequence and form at their interface two nucleotide-binding sites (NBSs) for ATP binding and hydrolysis, the TMDs are variable among different families and form the translocation channel. Transporters catalyzing the efflux of substrates from the cells are called exporters. In humans, they range from A to G subfamilies, with the B, C and G subfamilies being involved in chemoresistance. The recently elucidated structures of ABCG5/G8 followed by those of ABCG2 highlighted a novel structural fold that triggered extensive research. Notably, suppressor genetics in the orthologous yeast Pleiotropic Drug Resistance (PDR) subfamily proteins have pointed to a crosstalk between TMDs and NBDs modulating substrate export. Considering the structural information provided by their neighbors from the G subfamily, these studies provide mechanistic keys and posit a functional role for the non-hydrolytic NBS found in several ABC exporters. The present chapter provides an overview of structural and functional aspects of ABCG proteins with a special emphasis on the yeast PDR systems.

Water-soluble inhibitors of ABCG2 (BCRP) - A fragment-based and computational approach

A good balance between hydrophilicity and lipophilicity is a prerequisite for all bioactive compounds. If the hydrophilicity of a compound is low, its solubility in water will be meager. Many drug development failures have been attributed to poor aqueous solubility. ABCG2 inhibitors are especially prone to be insoluble since they have to address the extremely large and hydrophobic multidrug binding site in ABCG2. For instance, our previous, tariquidar-related ABCG2 inhibitor UR-MB108 (1) showed high potency (79 nM), but very low aqueous solubility (78 nM). To discover novel potent ABCG2 inhibitors with improved solubility we pursued a fragment-based approach. Substructures of 1 were optimized and the fragments 'enlarged' to obtain inhibitors, supported by molecular docking studies. Synthesis was achieved, i.a., via Sonogashira coupling, click chemistry and amide coupling. A kinetic solubility assay revealed that 1 and most novel inhibitors did not precipitate during the short time period of the applied biological assays. The solubility of the compounds in aqueous media at equilibrium was investigated in a thermodynamic solubility assay, where UR-Ant116 (40), UR-Ant121 (41), UR-Ant131 (48) and UR-Ant132 (49) excelled with solubilities between 1 μM and 1.5 μM - an up to 19-fold improvement compared to 1. Moreover, these novel N-phenyl-chromone-2-carboxamides inhibited ABCG2 in a Hoechst 33342 transport assay with potencies in the low three-digit nanomolar range, reversed MDR in cancer cells, were non-toxic and proved stable in blood plasma. All properties make them attractive candidates for in vitro assays requiring long-term incubation and in vivo studies, both needing sufficient solubility at equilibrium. 41 and 49 were highly ABCG2-selective, a precondition for developing PET tracers. The triple ABCB1/C1/G2 inhibitor 40 qualifies for potential therapeutic applications, given the concerted role of the three transporter subtypes at many tissue barriers, e.g. the BBB.

Cryo-EM structures reveal distinct mechanisms of inhibition of the human multidrug transporter ABCB1

ABCB1 detoxifies cells by exporting diverse xenobiotic compounds, thereby limiting drug disposition and contributing to multidrug resistance in cancer cells. Multiple small-molecule inhibitors and inhibitory antibodies have been developed for therapeutic applications, but the structural basis of their activity is insufficiently understood. We determined cryo-EM structures of nanodisc-reconstituted, human ABCB1 in complex with the Fab fragment of the inhibitory, monoclonal antibody MRK16 and bound to a substrate (the antitumor drug vincristine) or to the potent inhibitors elacridar, tariquidar, or zosuquidar. We found that inhibitors bound in pairs, with one molecule lodged in the central drug-binding pocket and a second extending into a phenylalanine-rich cavity that we termed the "access tunnel." This finding explains how inhibitors can act as substrates at low concentration, but interfere with the early steps of the peristaltic extrusion mechanism at higher concentration. Our structural data will also help the development of more potent and selective ABCB1 inhibitors.

An Overview of Privileged Scaffold: Quinolines and Isoquinolines in Medicinal Chemistry as Anticancer Agents

Cancer is one of the most difficult diseases and causes of death for many decades. Many pieces of research are continuously going on to get a solution for cancer. Quinoline and isoquinoline derivatives have shown their possibilities to work as an antitumor agent in anticancer treatment. The members of this privileged scaffold quinoline and isoquinoline have shown their controlling impacts on cancer treatment through various modes. In particular, this review suggests the current scenario of quinoline and isoquinoline derivatives as antitumor agents and refine the path of these derivatives to find and develop new drugs against an evil known as cancer.

Curcumin in cancer therapy: A novel adjunct for combination chemotherapy with paclitaxel and alleviation of its adverse effects

Dealing with cancer is of importance due to enhanced incidence rate of this life-threatening disorder. Chemotherapy is an ideal candidate in overcoming and eradication of cancer. To date, various chemotherapeutic agents have been applied in cancer therapy and paclitaxel (PTX) is one of them. PTX is a key member of taxane family with potential anti-tumor activity against different cancers. Notably, PTX has demonstrated excellent proficiency in elimination of cancer in clinical trials. This chemotherapeutic agent is isolated from Taxus brevifolia, and is a tricyclic diterpenoid. However, resistance of cancer cells into PTX chemotherapy has endangered its efficacy. Besides, administration of PTX is associated with a number of side effects such as neurotoxicity, hepatotoxicity, cardiotoxicity and so on, demanding novel strategies in obviating PTX issues. Curcumin is a pharmacological compound with diverse therapeutic effects including anti-tumor, anti-oxidant, anti-inflammatory, anti-diabetic and so on. In the current review, we demonstrate that curcumin, a naturally occurring nutraceutical compound is able to enhance anti-tumor activity of PTX against different cancers. Besides, curcumin administration reduces adverse effects of PTX due to its excellent pharmacological activities. These topics are discussed with an emphasis on molecular pathways to provide direction for further studies in revealing other signaling networks.

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

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

PD-L1 monoclonal antibody-decorated nanoliposomes loaded with Paclitaxel and P-gp transport inhibitor for the synergistic chemotherapy against multidrug resistant gastric cancers

Multidrug resistance (MDR) based on ATP-dependent efflux transporters (p-glycoprotein (p-gp)) remains a major obstacle in successful chemotherapy treatment. Herein, we have investigated the potential of PD-L1 mAb-conjugated nanoliposome to serve as a targeted delivery platform for the co-delivery of paclitaxel (PTX) and p-gp specific transport inhibitor (TQD, tariquidar) in drug-resistant gastric cancers. Two drugs, PTX and TQD, were co-loaded in a single vehicle in a precise ratio to enhance the prospect of combination chemotherapeutic effect. Cellular uptake study indicated that PD-PTLP had higher internalization efficiency in PD-L1 receptor overexpressing SGC7901/ADR cells than non-targeted PTLP. Highest synergy was observed at a weight fraction of 1/0.5 (PTX/TQD) and the combination of PTX and TQD resulted in obvious synergistic effect compared to that of individual drugs alone. Our in vitro results showed that TQD was effective in reversing the multidrug resistance in SGC7901/ADR cells. The IC50 value of PD-PTLP was 0.76 μg/ml compared to 6.58 μg/ml and 7.64 μg/ml for PTX and TQD, respectively. PD-TPLP triggered significantly higher levels of reactive oxygen species (ROS) and cell apoptosis compared to that of free PTX or TQD. Furthermore, the in vivo antitumor study showed that the combination chemotherapy of PD-PTLP displayed a significant inhibition of tumor burden of drug-resistant xenograft tumors with significantly higher terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL)-positive cells. Furthermore, free PTX resulted in significant increase in the levels of AST and ALT while PD-PTLP insignificantly different compared to that of control indicating the safety index. Overall, we believe that combination of anticancer drug with a p-gp inhibitor could provide a potential direction toward the treatment of drug-resistant gastric tumors.

Tariquidar-related triazoles as potent, selective and stable inhibitors of ABCG2 (BCRP)

Tariquidar derivatives have been described as potent and selective ABCG2 inhibitors. However, their susceptibility to hydrolysis limits their applicability. The current study comprises the synthesis and characterization of novel tariquidar-related inhibitors, obtained by bioisosteric replacement of the labile moieties in our previous tariquidar analog UR-ME22-1 (9). CuAAC ("click" reaction) gave convenient access to a triazole core as a substitute for the labile amide group and the labile ester moiety was replaced by different acyl groups in a Sugasawa reaction. A stability assay proved the enhancement of the stability in blood plasma. Compounds UR-MB108 (57) and UR-MB136 (59) inhibited ABCG2 in a Hoechst 33342 transport assay with an IC₅₀ value of about 80 nM and belong to the most potent ABCG2 inhibitors described so far. Compound 57 was highly selective, whereas its PEGylated analog 59 showed some potency at ABCB1. Both 57 and 59 produced an ABCG2 ATPase-depressing effect which is in agreement with our precedent cryo-EM study identifying 59 as an ATPase inhibitor that exerts its effect via locking the inward-facing conformation. Thermostabilization of ABCG2 by 57 and 59 can be taken as a hint to comparable binding to ABCG2. As reference substances, compounds 57 and 59 allow additional mechanistic studies on ABCG2 inhibition. Due to their stability in blood plasma, they are also applicable in vivo. The highly specific inhibitor 57 is suited for PET labeling, helping to further elucidate the (patho)physiological role of ABCG2, e.g. at the BBB.

Recent advances in the search of BCRP- and dual P-gp/BCRP-based multidrug resistance modulators

The development of multidrug resistance (MDR) is one of the major challenges to the success of chemotherapy treatment of cancer. This phenomenon is often associated with the overexpression of the ATP-binding cassette (ABC) transporters P-gp (P-glycoprotein, ABCB1), multidrug resistance-associated protein 1, ABCC1 and breast cancer resistance protein, ABCG2 (BCRP). These transporters are constitutively expressed in many tissues playing relevant protective roles by the regulation of the permeability of biological membranes, but they are also overexpressed in malignant tissues. P-gp is the first efflux transporter discovered to be involved in cancer drug resistance, and over the years, inhibitors of this pump have been disclosed to administer them in combination with chemotherapeutic agents. Three generations of inhibitors of P-gp have been examined in preclinical and clinical studies; however, these trials have largely failed to demonstrate that coadministration of pump inhibitors elicits an improvement in therapeutic efficacy of antitumor agents, although some of the latest compounds show better results. Therefore, new and innovative strategies, such as the fallback to natural products and the discover of dual activity ligands emerged as new perspectives. BCRP is the most recently ABC protein identified to be involved in multidrug resistance. It is overexpressed in several haematological and solid tumours together with P-gp, threatening the therapeutic effectiveness of different chemotherapeutic drugs. The chemistry of recently described BCRP inhibitors and dual P-gp/BCRP inhibitors, as well as their preliminary pharmacological evaluation are discussed, and the most recent advances concerning these kinds of MDR modulators are reviewed.

Multidrug Resistance Protein 1 Deficiency Promotes Doxorubicin-Induced Ovarian Toxicity in Female Mice

Multidrug resistance protein 1 (MDR1), a phase III drug transporter that exports substrates out of cells, has been discovered in both cancerous and normal tissues. The over expression of MDR1 in cancer cells contributes to multiple drug resistance, whereas the MDR1 in normal tissues protects them from chemical-induced toxicity. Currently, the role of MDR1 in the ovary has not been entirely understood. Our objective is to determine the function of MDR1 in protecting against chemotherapy-induced ovarian toxicity. Using both the in vivo transgenic mouse model and in vitro follicle culture model, we investigated the expression of MDR1 in the ovary, the effect of MDR1 deficiency on doxorubicin (DOX)-induced ovarian toxicity, and the ovarian steroid hormonal regulation of MDR1. Results showed that the MDR1 was expressed in the ovarian epithelial cells, stroma cells, theca cell layers, endothelial cells, and luteal cells. The lack of MDR1 did not affect female ovarian function and fertility; however, its deficiency significantly exacerbated the DOX-induced ovarian toxicity in both in vivo and in vitro models. The MDR1 showed significantly higher expression levels in the ovaries at estrus and metestrus stages than those at proestrus and diestrus stages. However, this dynamic expression pattern was not regulated by the ovarian steroid hormones of estrogen (E2) and progesterone (P4) but correlated to the number and status of corpus luteum. In conclusion, our study demonstrates that the lack of MDR1 promotes DOX-induced ovarian toxicity, suggesting the critical role of MDR1 in protecting female ovarian functions during chemotherapy.

Synthesis and evaluation of two new candidate high-affinity full agonist PET radioligands for imaging 5-HT1B receptors

INTRODUCTION

The serotonin 1B receptor subtype is of interest in the pathophysiology and treatment of depression, anxiety, and migraine. Over recent years 5-HT_1B receptor binding in human brain has been examined with PET using radioligands that are partial but not full agonists. To explore how the intrinsic activity of a PET radioligand may affect imaging performance, two high-affinity full 5-HT_1B receptor agonists (AZ11136118, 4; and AZ11895987, 5) were selected from a large compound library and radiolabeled for PET examination in non-human primates.

METHODS

[¹¹C]4 was obtained through Pd(0)-mediated insertion of [¹¹C]carbon monoxide between prepared iodoarene and homochiral amine precursors. [¹¹C]5 was obtained through N-¹¹C-methylation of N-desmethyl precursor 6 with [¹¹C]methyl triflate. [¹¹C]4 and [¹¹C]5 were studied with PET in rhesus or cynomolgus monkey. [¹¹C]4 was studied with PET in mice and rats to measure brain uptake and specific binding. Ex-vivo experiments in rats were performed to identify whether there were radiometabolites in brain. Physiochemical parameters for [¹¹C]4 (pKa, logD and conformational energetics) were evaluated.

RESULTS

Both [¹¹C]4 and [¹¹C]5 were successfully produced in high radiochemical purity and in adequate amounts for PET experiments. After intravenous injection of [¹¹C]4, brain radioactivity peaked at a low level (0.2 SUV). Pretreatment with tariquidar, an inhibitor of the brain P-gp efflux transporter, increased brain exposure four-fold whereas pretreatment with a high pharmacological dose of the 5-HT_1B antagonist, AR-A000002, had no effect on the binding. Ex-vivo experiments in rats showed no radiometabolites entering brain. [¹¹C]5 also failed to enter monkey brain under baseline conditions.

CONCLUSIONS

[¹¹C]4 and [¹¹C]5 show too low brain uptake and specific binding to be useful PET radioligands. Low brain uptake is partly ascribed to efflux transporter action as well as unfavorable conformations.

A Structural View on Medicinal Chemistry Strategies against Drug Resistance

The natural phenomenon of drug resistance is a widespread issue that hampers the performance of drugs in many major clinical indications. Antibacterial and antifungal drugs are affected, as well as compounds for the treatment of cancer, viral infections, or parasitic diseases. Despite the very diverse set of biological targets and organisms involved in the development of drug resistance, the underlying molecular mechanisms have been identified to understand the emergence of resistance and to overcome this detrimental process. Detailed structural information on the root causes for drug resistance is nowadays frequently available, so next-generation drugs can be designed that are anticipated to suffer less from resistance. This knowledge-based approach is essential for fighting the inevitable occurrence of drug resistance.

[O-methyl-11C]N-(4-(4-(3-Chloro-2-methoxyphenyl)-piperazin-1-yl)butyl)-1H-indole-2-carboxamide ([11C]BAK4-51) Is an Efflux Transporter Substrate and Ineffective for PET Imaging of Brain D₃ Receptors in Rodents and Monkey

Selective high-affinity antagonists for the dopamine D₃ receptor (D₃R) are sought for treating substance use disorders. Positron emission tomography (PET) with an effective D₃R radioligand could be a useful tool for the development of such therapeutics by elucidating pharmacological specificity and target engagement in vivo. Currently, a D₃R-selective radioligand does not exist. The D₃R ligand, N-(4-(4-(3-chloro-2-methoxyphenyl)piperazin-1-yl)butyl)-1H-indole-2-carboxamide (BAK4-51, 1), has attractive properties for PET radioligand development, including full antagonist activity, very high D₃R affinity, D₃R selectivity, and moderate lipophilicity. We labeled 1 with the positron-emitter carbon-11 (t_1/2 = 20.4 min) in the methoxy group for evaluation as a radioligand in animals with PET. However, [¹¹C]1 was found to be an avid substrate for brain efflux transporters and lacked D₃R-specific signal in rodent and monkey brain in vivo.

Novel Potent ABCB1 Modulator, Phenethylisoquinoline Alkaloid, Reverses Multidrug Resistance in Cancer Cell

ATP-binding cassette (ABC) transporters, which are concerned with the efflux of anticancer drugs from cancer cells, have a pivotal role in multidrug resistance (MDR). In particular, ABCB1 is a well-known ABC transporter that develops MDR in many cancer cells. Some ABCB1 modulators can reverse ABCB1-mediated MDR; however, no modulators with clinical efficacy have been approved. The aim of this study was to identify novel ABCB1 modulators by using high-throughput screening. Of the 5861 compounds stored at Tohoku University, 13 compounds were selected after the primary screening via a fluorescent plate reader-based calcein acetoxymethylester (AM) efflux assay. These 13 compounds were validated in a flow cytometry-based calcein AM efflux assay. Two isoquinoline derivatives were identified as novel ABCB1 inhibitors, one of which was a phenethylisoquinoline alkaloid, (±)-7-benzyloxy-1-(3-benzyloxy-4-methoxyphenethyl)-1,2,3,4-tetrahydro-6-methoxy-2-methylisoquinoline oxalate. The compound, a phenethylisoquinoline alkaloid, was subsequently evaluated in the cytotoxicity assay and shown to significantly enhance the reversal of ABCB1-mediated MDR. In addition, the compound activated the ABCB1-mediated ATP hydrolysis and inhibited the photolabeling of ABCB1 with [¹²⁵I]-iodoarylazidoprazosin. Furthermore, the compound also reversed the resistance to paclitaxel without increasing the toxicity in the ABCB1-overexpressing KB-V1 cell xenograft model. Overall, we concluded that the newly identified phenethylisoquinoline alkaloid reversed ABCB1-mediated MDR through direct interaction with the substrate-binding site of ABCB1. These findings may contribute to the development of more potent and less toxic ABCB1 modulators, which could overcome ABCB1-mediated MDR.

Chemically Induced Degradation of Anaplastic Lymphoma Kinase (ALK)

We present the development of the first small molecule degraders that can induce anaplastic lymphoma kinase (ALK) degradation, including in non-small-cell lung cancer (NSCLC), anaplastic large-cell lymphoma (ALCL), and neuroblastoma (NB) cell lines. These degraders were developed through conjugation of known pyrimidine-based ALK inhibitors, TAE684 or LDK378, and the cereblon ligand pomalidomide. We demonstrate that in some cell types degrader potency is compromised by expression of drug transporter ABCB1. In addition, proteomic profiling demonstrated that these compounds also promote the degradation of additional kinases including PTK2 (FAK), Aurora A, FER, and RPS6KA1 (RSK1).

Structural basis of small-molecule inhibition of human multidrug transporter ABCG2

ABCG2 is an ATP-binding cassette (ABC) transporter that protects tissues against xenobiotics, affects the pharmacokinetics of drugs and contributes to multidrug resistance. Although many inhibitors and modulators of ABCG2 have been developed, understanding their structure-activity relationship requires high-resolution structural insight. Here, we present cryo-EM structures of human ABCG2 bound to synthetic derivatives of the fumitremorgin C-related inhibitor Ko143 or the multidrug resistance modulator tariquidar. Both compounds are bound to the central, inward-facing cavity of ABCG2, blocking access for substrates and preventing conformational changes required for ATP hydrolysis. The high resolutions allowed for de novo building of the entire transporter and also revealed tightly bound phospholipids and cholesterol interacting with the lipid-exposed surface of the transmembrane domains (TMDs). Extensive chemical modifications of the Ko143 scaffold combined with in vitro functional analyses revealed the details of ABCG2 interactions with this compound family and provide a basis for the design of novel inhibitors and modulators.

Preparation and characterization of surface-modified PLGA-polymeric nanoparticles used to target treatment of intestinal cancer

Docetaxel (DTX), a cytotoxic taxane, is a poor water-soluble drug and exhibits less oral bioavailability. Current research investigates the effective transport, for DTX-loaded chitosan (CS)-coated-poly-lactide-co-glycolide (PLGA)-nanoparticles (NPs) (DTX-CS-PLGA-NPs) and DTX-PLGA-NPs as well as a novel third-generation P-gp inhibitor i.e. GF120918 (Elacridar), across intestinal epithelium with its successive uptake by the tumour cells in an in vitro model. The prepared NPs showed a spherical shape particle size i.e. <123.96 nm with polydispersity index (PDI) of <0.290 whereas for CS-coated NPs, the zeta potential was converted from negative to positive value along with a small modification in particle size distribution. The entrapment efficiency observed for DTX-CS-PLGA-NPs was 74.77%, whereas the in vitro release profile revealed an initial rapid DTX release followed by a sustained release pattern. For apparent permeability, DTX-CS-PLGA-NPs and DTX-PLGA-NPs along with GF120918 showed a five-fold (p < .01) and 2.2-fold enhancement, respectively, as observed in rat ileum permeation study. Similarly, for pharmacokinetic (PK) studies, higher oral bioavailability was observed from DTX-CS-PLGA-NPs (5.11-folds) and DTX-PLGA-NPs (3.29-folds) as compared with DTX-suspension (DTX-S). Cell uptake studies on A549 cells as performed for DTX-CS-PLGA-NPs and DTX-PLGA-NPs loaded with rhodamine 123 dye, exhibited enhanced uptake as compared with plain dye solution. The enhanced uptake for DTX-CS-PLGA-NPs and DTX-PLGA-NPs formulations in the presence of GF120918 was confirmed further with the help of confocal laser scanning microscopic images (CLSM). The potential of the third-generation novel P-gp inhibitor (GF120918) investigated for the effective delivery of DTX as well as investigation of permeability and uptake studies whereby a strong potential of GF120918 for effective oral delivery was established.

Osimertinib (AZD9291), a Mutant-Selective EGFR Inhibitor, Reverses ABCB1-Mediated Drug Resistance in Cancer Cells

In recent years, tyrosine kinase inhibitors (TKIs) have been shown capable of inhibiting the ATP-binding cassette (ABC) transporter-mediated multidrug resistance (MDR). In this study, we determine whether osimertinib, a novel selective, irreversible EGFR (epidermal growth factor receptor) TKI, could reverse ABC transporter-mediated MDR. The results showed that, at non-toxic concentrations, osimertinib significantly sensitized both ABCB1-transfected and drug-selected cell lines to substrate anticancer drugs colchicine, paclitaxel, and vincristine. Osimertinib significantly increased the accumulation of [³H]-paclitaxel in ABCB1 overexpressing cells by blocking the efflux function of ABCB1 transporter. In contrast, no significant alteration in the expression levels and localization pattern of ABCB1 was observed when ABCB1 overexpressing cells were exposed to 0.3 µM osimertinib for 72 h. In addition, ATPase assay showed osimertinib stimulated ABCB1 ATPase activity. Molecular docking and molecular dynamic simulations showed osimertinib has strong and stable interactions at the transmembrane domain of human homology ABCB1. Taken together, our findings suggest that osimertinib, a clinically-approved third-generation EGFR TKI, can reverse ABCB1-mediated MDR, which supports the combination therapy with osimertinib and ABCB1 substrates may potentially be a novel therapeutic stategy in ABCB1-positive drug resistant cancers.

Reversal of Chemoresistance in Ovarian Cancer by Co-Delivery of a P-Glycoprotein Inhibitor and Paclitaxel in a Liposomal Platform

The overexpression of permeability-glycoprotein (P-gp), an ABC transporter involved in the cellular exclusion of chemotherapeutic drugs, is a major factor in paclitaxel-resistant ovarian cancer. However, in clinical trials, co-administration of P-gp inhibitors and anticancer drugs has not resulted in the efficient reversal of drug resistance. To improve administration, we encapsulated the third-generation P-gp inhibitor tariquidar (XR-9576, XR), alone or in combination with paclitaxel (PCT) in liposomes (LP). After optimization, the liposomes demonstrated favorable physicochemical properties and the ability to reverse chemoresistance in experiments using chemosensitive/chemoresistant ovarian cancer cell line pairs. Analyzing publicly available datasets, we found that overexpression of P-gp in ovarian cancer is associated with a shorter progression-free and overall survival. In vitro, LP(XR) significantly increased the cellular retention of rhodamine 123, a P-gp substrate. LP(XR,PCT) synergistically inhibited cell viability, blocked proliferation, and caused G₂-M arrest in paclitaxel-resistant SKOV3-TR and HeyA8-MDR cell lines overexpressing P-gp. Holographic imaging cytometry revealed that LP(XR,PCT) treatment of SKOV3-TR cells induced almost complete mitotic arrest, whereas laser scanning cytometry showed that the treatment induced apoptosis. In proof-of-concept preclinical studies, LP(XR,PCT), when compared with LP(PCT), significantly reduced tumor weight (43.2% vs. 16.9%, P = 0.0007) and number of metastases (44.4% vs. 2.8%, P = 0.012) in mice bearing orthotopic HeyA8-MDR ovarian tumors. In the xenografts, LP(XR,PCT) efficiently induced apoptosis and impaired proliferation. Our findings suggest that co-delivery of a P-gp inhibitor and paclitaxel using a liposomal platform can sensitize paclitaxel-resistant ovarian cancer cells to paclitaxel. LP(XR,PCT) should be considered for clinical testing in patients with P-gp-overexpressing tumors. Mol Cancer Ther; 15(10); 2282-93. ©2016 AACR.

Synthesis and Investigation of Tetrahydro-β-carboline Derivatives as Inhibitors of the Breast Cancer Resistance Protein (ABCG2)

The breast cancer resistance protein (ABCG2) transports chemotherapeutic drugs out of cells, which makes it a major player in mediating multidrug resistance (MDR) of cancer cells. To overcome this mechanism, inhibitors of ABCG2 can be used. Only a few potent and selective ABCG2 inhibitors have been discovered, i.e., fumitremorgin C (FTC), Ko143, and the alkaloid harmine, which contain a tetrahydro-β-carboline or β-carboline backbone, respectively. However, toxicity and or instability prevent their use in vivo. Therefore, there is a need for further potent inhibitors. We synthesized and pharmacologically investigated 37 tetrahydro-β-carboline derivatives. The inhibitory activity of two compounds (51, 52) is comparable to that of Ko143, and they are selective for ABCG2 over ABCB1. Furthermore, they are able to reverse the ABCG2-mediated resistance toward SN-38 and inhibit the ATPase activity. The cytotoxicity data show that their inhibitory effect is substantially higher than their toxicity.

Synthesis and Biological Evaluation of 4-Anilino-quinazolines and -quinolines as Inhibitors of Breast Cancer Resistance Protein (ABCG2)

Chemotherapeutic treatment of cancer often fails due to overexpression of the ATP-binding cassette (ABC) transport proteins, like ABCG2, triggering active efflux of various structurally unrelated drugs. This so-called multidrug resistance (MDR) may be reversed by selective, potent, and nontoxic inhibitors of ABCG2. As only a few potent inhibitors are known, new compounds based on a 4-substituted-2-phenylquinazoline scaffold were investigated. Substitution with hydroxy, cyano, nitro, acetamido, and fluoro led to high inhibitory activities toward ABCG2. The ability to reverse MDR of the most active compounds was confirmed in a MTT efficacy assay. Moreover, a negligibly low intrinsic cytotoxicity was found resulting in a high therapeutic ratio. Investigations of the inhibitory activity toward ABCB1 and ABCC1 yielded a high selectivity toward ABCG2 for the quinazoline compounds. Quinoline-based analogues showed lower inhibitory activity and selectivity. The study yielded a variety of promising compounds, some with superior properties compared to those of the standard inhibitor Ko143.

Flavonoid derivatives as selective ABCC1 modulators: Synthesis and functional characterization

A series of chromones, bearing substituted amino groups or N-substituted carboxamide moieties in position 2, was synthesized and characterized in cellular assays for modulation of the ABC transporters ABCC1 (MDCKII-MRP1 cells), ABCB1 (Kb-V1 cells) and ABCG2 (MCF-7/Topo cells). The most potent ABCC1 modulators identified among these flavonoid-type compounds were comparable to the reference compound reversan regarding potency, but superior in terms of selectivity concerning ABCB1 and ABCG2 (2-[4-(Benzo[c][1,2,5]oxadiazol-5-ylmethyl)piperazin-1-yl]-5,7-dimethoxy-4H-chromen-4-one (51): ABCC1, IC50 11.3 μM; inactive at ABCB1 and ABCG2). Compound 51 was as effective as reversan in reverting ABCC1-mediated resistance to cytostatics in MDCKII-MRP1 cells and proved to be stable in mouse plasma and cell culture medium. Modulators, such as compound 51, are of potential value as pharmacological tools for the investigation of the (patho)physiological role of ABCC1.

Therapeutic strategies to improve drug delivery across the blood-brain barrier

Resection of brain tumors is followed by chemotherapy and radiation to ablate remaining malignant cell populations. Targeting these populations stands to reduce tumor recurrence and offer the promise of more complete therapy. Thus, improving access to the tumor, while leaving normal brain tissue unscathed, is a critical pursuit. A central challenge in this endeavor lies in the limited delivery of therapeutics to the tumor itself. The blood-brain barrier (BBB) is responsible for much of this difficulty but also provides an essential separation from systemic circulation. Due to the BBB's physical and chemical constraints, many current therapies, from cytotoxic drugs to antibody-based proteins, cannot gain access to the tumor. This review describes the characteristics of the BBB and associated changes wrought by the presence of a tumor. Current strategies for enhancing the delivery of therapies across the BBB to the tumor will be discussed, with a distinction made between strategies that seek to disrupt the BBB and those that aim to circumvent it.

Reversal of P-gp and BCRP-mediated MDR by tariquidar derivatives

With an aim to generate non-toxic, specific and highly potent multidrug resistance (MDR) modulators, a novel series of anthranilic acid amide-substituted tariquidar derivatives were synthesized. The new compounds were evaluated for their cytotoxicity toward normal human colon fibroblasts (CCD18-Co), human gastric epithelial cell line (HFE) and primary rat liver cells, and for their ability to inhibit P-gp/BCRP-mediated drug efflux and reversal of P-gp and BCRP-mediated MDR in parental and drug-resistant cancer cell lines (LCC6 MDR1, MCF-7 FLV1000, R-HepG2, SW620-Ad300). While tariquidar is highly toxic to normal cells, the new derivatives exhibited much lower or negligible cytotoxicity. Some of the new tariquidar derivatives inhibited both P-gp and BCRP-mediated drug efflux whereas a few of them bearing a sulfonamide functional group (1, 5, and 16) are specific to P-gp. The new compounds were also found to potentiate the anticancer activity of the transporter substrate anticancer drugs in the corresponding transporter-overexpressing cell lines. The extent of resistance reversal was found to be consistent with the transporter inhibitory effect of the new derivatives. To further understand the mechanism of P-gp and BCRP inhibition, the tariquidar derivatives were found to interact with the transporters using an antibody-based UIC2 or 5D3 shift assay. Moreover, the transporters-inhibiting derivatives were found to modulate the ATPase activities of the two MDR transporters. Our data thus advocate further development of the new compounds for the circumvention of MDR.

HM30181 Derivatives as Novel Potent and Selective Inhibitors of the Breast Cancer Resistance Protein (BCRP/ABCG2)

The breast cancer resistance protein (BCRP, ABCG2) belongs to the superfamily of ATP binding-cassette (ABC) proteins. In addition to other physiological functions, it transports potentially cell-damaging compounds out of the cell using the energy from ATP hydrolysis. Certain tumors overexpressing BCRP were found to become resistant against various anticancer drugs. In previous work, we found that tariquidar analogues lacking the tetrahydroisoquinoline moiety selectively inhibit BCRP. In the present study, we synthesized 21 derivatives of the third-generation P-gp inhibitor HM30181, which is structurally related to tariquidar. The compounds were tested for their inhibitory activities against BCRP and screened against P-glycoprotein (P-gp, ABCB1) and multidrug resistance protein 1 (MRP1, ABCC1) to confirm the selectivity toward BCRP. The most potent compounds are selective toward BCRP and 2-fold more potent than the reference Ko143. Qualitative structure-activity relationship (SAR) analysis revealed that the presence of a methoxy group in the ortho or para position of at least one phenyl ring is beneficial for inhibitory activity. Furthermore, the cytotoxicity and multidrug resistance (MDR)-reversal ability of selected compounds were investigated. It was shown that they have a low cytotoxicity and the ability to reverse the BCRP-mediated SN-38 resistance.

Blood brain barrier: a challenge for effectual therapy of brain tumors

Brain tumors are one of the most formidable diseases of mankind. They have only a fair to poor prognosis and high relapse rate. One of the major causes of extreme difficulty in brain tumor treatment is the presence of blood brain barrier (BBB). BBB comprises different molecular components and transport systems, which in turn create efflux machinery or hindrance for the entry of several drugs in brain. Thus, along with the conventional techniques, successful modification of drug delivery and novel therapeutic strategies are needed to overcome this obstacle for treatment of brain tumors. In this review, we have elucidated some critical insights into the composition and function of BBB and along with it we have discussed the effective methods for delivery of drugs to the brain and therapeutic strategies overcoming the barrier.

Inhibition or knockdown of ABC transporters enhances susceptibility of adult and juvenile schistosomes to Praziquantel

Parasitic flatworms of the genus Schistosoma cause schistosomiasis, a neglected tropical disease that affects hundreds of millions. Treatment of schistosomiasis depends almost entirely on the drug praziquantel (PZQ). Though essential to treating and controlling schistosomiasis, a major limitation of PZQ is that it is not active against immature mammalian-stage schistosomes. Furthermore, there are reports of field isolates with heritable reductions in PZQ susceptibility, and researchers have selected for PZQ-resistant schistosomes in the laboratory. P-glycoprotein (Pgp; ABCB1) and other ATP binding cassette (ABC) transporters remove a wide variety of toxins and xenobiotics from cells, and have been implicated in multidrug resistance (MDR). Changes in ABC transporter structure or expression levels are also associated with reduced drug susceptibility in parasitic helminths, including schistosomes. Here, we show that the activity of PZQ against schistosome adults and juveniles ex vivo is potentiated by co-administration of either the highly potent Pgp inhibitor tariquidar or combinations of inhibitors targeting multiple ABC multidrug transporters. Adult worms exposed to sublethal PZQ concentrations remain active, but co-administration of ABC transporter inhibitors results in complete loss of motility and disruption of the tegument. Notably, juvenile schistosomes (3–4 weeks post infection), normally refractory to 2 µM PZQ, become paralyzed when transporter inhibitors are added in combination with the PZQ. Experiments using the fluorescent PZQ derivative (R)-PZQ-BODIPY are consistent with the transporter inhibitors increasing effective intraworm concentrations of PZQ. Adult worms in which expression of ABC transporters has been suppressed by RNA interference show increased responsiveness to PZQ and increased retention of (R)-PZQ-BODIPY consistent with an important role for these proteins in setting levels of PZQ susceptibility. These results indicate that parasite ABC multidrug transporters might serve as important targets for enhancing the action of PZQ. They also suggest a potentially novel and readily-available strategy for overcoming reduced PZQ susceptibility of schistosomes.

Schistosomes are parasitic flatworms that cause schistosomiasis, a tropical disease affecting hundreds of millions worldwide. Praziquantel (PZQ) is the current drug of choice against schistosomiasis, and, indeed, is the only approved antischistosomal treatment available in most parts of the world. Though effective overall, PZQ has limitations, including its lack of activity against immature schistosomes. Furthermore, reported cure rates in the field are often below optimal levels, and there is increasing evidence that schistosomes can become resistant to the drug. ABC transporters such as P-glycoprotein are efflux transporters that mediate detoxification of cells via removal of toxins and xenobiotics, including drugs. They underlie multidrug resistance in mammalian cells, and are also associated with drug resistance in parasitic worms, including schistosomes. Here, we show that compounds that inhibit these efflux transporters potentiate the activity of PZQ against schistosomes, including normally PZQ-insensitive juvenile worms. Similarly, suppressing expression of these transporters also increases adult worm responsiveness to PZQ. Our experiments may provide insights into the role of these drug transporters in PZQ action, and could also translate into new therapeutic strategies for augmenting treatment of schistosome infections and overcoming drug resistance.

Interactions of the multidrug resistance modulators tariquidar and elacridar and their analogues with P-glycoprotein

Tariquidar and elacridar are among the most potent inhibitors of the multidrug resistance transporter P-glycoprotein (P-gp), but how they interact with the protein is yet unknown. In this work, we describe a possible way in which these inhibitors interact with P-gp. We rely on structure-activity relationship analysis of a small group of tariquidar and elacridar analogues that was purposefully selected, designed, and tested. Structural modifications of the compounds relate to the presence or absence of functional groups in the tariquidar and elacridar scaffolds. The activity of the compounds was evaluated by their effects on the accumulation of P-gp substrates rhodamine 123 and Hoechst 33342 in resistant tumor cells. The data allow estimation of the ability of the compounds to interact with the experimentally proposed R- and H-sites to which rhodamine 123 and Hoechst 33342 bind, respectively. Using an inward-facing homology model of human P-gp based on the crystallographic structure of mouse P-gp, we demonstrate that these binding sites may overlap with the binding sites of the QZ59 ligands co-crystallized with mouse P-gp. Based on this SAR analysis, and using flexible alignment and docking, we propose possible binding modes for tariquidar and elacridar. Our results suggest the possibility for the studied compounds to bind to sites that coincide or overlap with the binding sites of rhodamine 123 and Hoechst 33342. These results contribute to further understanding of structure-function relationships of P-gp and can help in the design of selective and potent P-gp inhibitors with potential clinical use.