Reversal of P-gp and BCRP-mediated MDR by tariquidar derivatives. Eur J Med Chem. 2015;101:560-572. [PMID: 26197160 DOI: 10.1016/j.ejmech.2015.06.049]

Novel pyxinol amide derivatives bearing an aliphatic heterocycle as P-glycoprotein modulators for overcoming multidrug resistance

P-glycoprotein (Pgp) modulators are promising agents for overcoming multidrug resistance (MDR) in cancer chemotherapy. In this study, via structural optimization of our lead compound S54 (nonsubstrate allosteric inhibitor of Pgp), 29 novel pyxinol amide derivatives bearing an aliphatic heterocycle were designed, synthesized, and screened for MDR reversal activity in KBV cells. Unlike S54, these active derivatives were shown to transport substrates of Pgp. The most potent derivative 4c exhibited promising MDR reversal activity (IC50 of paclitaxel = 8.80 ± 0.56 nM, reversal fold = 211.8), which was slightly better than that of third-generation Pgp modulator tariquidar (IC50 of paclitaxel = 9.02 ± 0.35 nM, reversal fold = 206.6). Moreover, the cytotoxicity of this derivative was 8-fold lower than that of tariquidar in human normal HK-2 cells. Furthermore, 4c blocked the efflux function of Pgp and displayed high selectivity for Pgp but had no effect on its expression and distribution. Molecular docking revealed that 4c bound preferentially to the drug-binding domain of Pgp. Overall, 4c is a promising lead compound for developing Pgp modulators.

Lycorine and homolycorine derivatives for chemo-sensitizing resistant human ovarian adenocarcinoma cells

BACKGROUND

Multidrug resistance is the major obstacle to cancer chemotherapy. Modulation of P-glycoprotein and drug combination approaches have been considered important strategies to overcome drug resistance.

PURPOSE

Aiming at generating a small library of Amaryllidaceae-type alkaloids to overcome drug resistance, two major alkaloids, isolated from Pancratium maritimum, lycorine (1), and 2α-10bα-dihydroxy-9-O-demethylhomolycorine (2), were derivatized, giving rise to nineteen derivatives (3 - 21).

METHODS

The main chemical transformation of lycorine resulted from the cleavage of ring E of the diacetylated lycorine derivative (3) to obtain compounds that have carbamate and amine functions (5 - 16), while acylation of compound 2 provided derivatives 17 - 21. Compounds 1 - 21 were evaluated for their effects on cytotoxicity, and drug resistance reversal, using resistant human ovarian carcinoma cells (HOC/ADR), overexpressing P-glycoprotein (P-gp/ABCB1), as model.

RESULTS

Excluding lycorine (1) (IC50 values of 1.2- 2.5 µM), the compounds were not cytotoxic or showed moderate/weak cytotoxicity. Chemo-sensitization assays were performed by studying the in vitro interaction between the compounds and the anticancer drug doxorubicin. Most of the compounds have shown synergistic interactions with doxorubicin. Compounds 5, 6, 9 - 14, bearing both carbamate and aromatic amine moieties, were found to have the highest sensitization rate, reducing the dose of doxorubicin 5-35 times, highlighting their potential to reverse drug resistance in combination chemotherapy. Selected compounds (4 - 6, 9 - 14, and 21), able of re-sensitizing resistant cancer cells, were further evaluated as P-gp inhibitors. Compound 11, which has a para‑methoxy-N-methylbenzylamine moiety, was the strongest inhibitor. In the ATPase assay, compounds 9-11 and 13 behaved as verapamil, suggesting competitive inhibition of P-gp. At the same time, none of these compounds affected P-gp expression at the mRNA or protein level.

CONCLUSIONS

This study provided evidence of the potential of Amaryllidaceae alkaloids as lead candidates for the development of MDR reversal agents.

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.

Using Jupyter Notebooks for re-training machine learning models

Machine learning (ML) models require an extensive, user-driven selection of molecular descriptors in order to learn from chemical structures to predict actives and inactives with a high reliability. In addition, privacy concerns often restrict the access to sufficient data, leading to models with a narrow chemical space. Therefore, we propose a framework of re-trainable models that can be transferred from one local instance to another, and further allow a less extensive descriptor selection. The models are shared via a Jupyter Notebook, allowing the evaluation and implementation of a broader chemical space by keeping most of the tunable parameters pre-defined. This enables the models to be updated in a decentralized, facile, and fast manner. Herein, the method was evaluated with six transporter datasets (BCRP, BSEP, OATP1B1, OATP1B3, MRP3, P-gp), which revealed the general applicability of this approach.

Development of Simple and Accurate in Silico Ligand-Based Models for Predicting ABCG2 Inhibition

The ATP binding cassette transporter ABCG2 is a physiologically important drug transporter that has a central role in determining the ADMET (absorption, distribution, metabolism, elimination, and toxicity) profile of therapeutics, and contributes to multidrug resistance. Thus, development of predictive in silico models for the identification of ABCG2 inhibitors is of great interest in the early stage of drug discovery. In this work, by exploiting a large public dataset, a number of ligand-based classification models were developed using partial least squares-discriminant analysis (PLS-DA) with molecular interaction field- and fingerprint-based structural description methods, regarding physicochemical and fragmental properties related to ABCG2 inhibition. An in-house dataset compiled from recently experimental studies was used to rigorously validated the model performance. The key molecular properties and fragments favored to inhibitor binding were discussed in detail, which was further explored by docking simulations. A highly informative chemical property was identified as the principal determinant of ABCG2 inhibition, which was utilized to derive a simple rule that had a strong capability for differentiating inhibitors from non-inhibitors. Furthermore, the incorporation of the rule into the best PLS-DA model significantly improved the classification performance, particularly achieving a high prediction accuracy on the independent in-house set. The integrative model is simple and accurate, which could be applied to the evaluation of drug-transporter interactions in drug development. Also, the dominant molecular features derived from the models may help medicinal chemists in the molecular design of novel inhibitors to circumvent ABCG2-mediated drug resistance.

Research advances in the role and pharmaceuticals of ATP-binding cassette transporters in autoimmune diseases

Autoimmune diseases are caused by the immune response of the body to its antigens, resulting in tissue damage. The pathogenesis of these diseases has not yet been elucidated. Most autoimmune diseases cannot be cured by effective drugs. The treatment strategy is to relieve the symptoms of the disease and balance the body's autoimmune function. The abnormal expression of ATP-binding cassette (ABC) transporters is directly related to the pathogenesis of autoimmune diseases and drug therapy resistance, which poses a great challenge for the drug therapy of autoimmune diseases. Therefore, this paper reviews the interplay between ABC transporters and the pathogenesis of autoimmune diseases to provide research progress and new ideas for the development of drugs in autoimmune diseases.

Effects of Polyphenols on P-Glycoprotein (ABCB1) Activity

P-glycoprotein (Pgp, ABCB1) is a member of one of the largest families of active transporter proteins called ABC transporters. Thanks to its expression in tissues with barrier functions and its broad substrate spectrum, it is an important determinant of the absorption, metabolism and excretion of many drugs. Pgp and/or some other drug transporting ABC proteins (e.g., ABCG2, MRP1) are overexpressed in nearly all cancers and cancer stem cells by which cancer cells become resistant against many drugs. Thus, Pgp inhibition might be a strategy for fighting against drug-resistant cancer cells. Previous studies have shown that certain polyphenols interact with human Pgp. We tested the effect of 15 polyphenols of sour cherry origin on the basal and verapamil-stimulated ATPase activity of Pgp, calcein-AM and daunorubicin transport as well as on the conformation of Pgp using the conformation sensitive UIC2 mAb. We found that quercetin, quercetin-3-glucoside, narcissoside and ellagic acid inhibited the ATPase activity of Pgp and increased the accumulation of calcein and daunorubicin by Pgp-positive cells. Cyanidin-3O-sophoroside, catechin, naringenin, kuromanin and caffeic acid increased the ATPase activity of Pgp, while they had only a weaker effect on the intracellular accumulation of fluorescent Pgp substrates. Several tested polyphenols including epicatechin, trans-ferulic acid, oenin, malvin and chlorogenic acid were ineffective in all assays applied. Interestingly, catechin and epicatechin behave differently, although they are stereoisomers. We also investigated the effect of quercetin, naringenin and ellagic acid added in combination with verapamil on the transport activity of Pgp. In these experiments, we found that the transport inhibitory effect of the tested polyphenols and verapamil was additive or synergistic. Generally, our data demonstrate diverse interactions of the tested polyphenols with Pgp. Our results also call attention to the potential risks of drug–drug interactions (DDIs) associated with the consumption of dietary polyphenols concurrently with chemotherapy treatment involving Pgp substrate/inhibitor drugs.

Quinoline-based Compounds with Potential Activity against Drugresistant Cancers

Drug resistance is the major cause of the failure of cancer chemotherapy, so one of the most important features in developing effective cancer therapeutic strategies is to overcome drug resistance. Quinoline moiety has become one of the most privileged structural motifs in anticancer agent discovery since its derivatives possess potent activity against various cancers including drug-resistant cancers. Several quinoline-based compounds which are represented by Anlotinib, Bosutinib, Lenvatinib, and Neratinib have already been applied in clinical practice to fight against cancers, so quinoline-based compounds are potential anticancer agents. The present short review article provides an overview of the recent advances of quinoline-based compounds with potential activity against drug-resistant cancers. The structure-activity relationship and mechanisms of action are also discussed.

Polyoxypregnanes as safe, potent, and specific ABCB1-inhibitory pro-drugs to overcome multidrug resistance in cancer chemotherapy in vitro and in vivo

Multidrug resistance (MDR) mediated by ATP binding cassette subfamily B member 1 (ABCB1) is significantly hindering effective cancer chemotherapy. However, currently, no ABCB1-inhibitory drugs have been approved to treat MDR cancer clinically, mainly due to the inhibitor specificity, toxicity, and drug interactions. Here, we reported that three polyoxypregnanes (POPs) as the most abundant constituents of Marsdenia tenacissima (M. tenacissima) were novel ABCB1-modulatory pro-drugs, which underwent intestinal microbiota-mediated biotransformation in vivo to generate active metabolites. The metabolites at non-toxic concentrations restored chemosensitivity in ABCB1-overexpressing cancer cells via inhibiting ABCB1 efflux activity without changing ABCB1 protein expression, which were further identified as specific non-competitive inhibitors of ABCB1 showing multiple binding sites within ABCB1 drug cavity. These POPs did not exhibit ABCB1/drug metabolizing enzymes interplay, and their repeated administration generated predictable pharmacokinetic interaction with paclitaxel without obvious toxicity in vivo. We further showed that these POPs enhanced the accumulation of paclitaxel in tumors and overcame ABCB1-mediated chemoresistance. The results suggested that these POPs had the potential to be developed as safe, potent, and specific pro-drugs to reverse ABCB1-mediated MDR. Our work also provided scientific evidence for the use of M. tenacissima in combinational chemotherapy.

Jatrophane diterpenoids as multidrug resistance modulators from Euphorbia sororia

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

Dihydro-β-agarofuran-Type Sesquiterpenoids from the Seeds of Celastrus virens and Their Multidrug Resistance Reversal Activity against the KB/VCR Cell Line

Twenty-nine dihydro-β-agarofuran-type sesquiterpenoids, including 17 new and 12 known compounds, were obtained from the seeds of Celastrus virens. The structures of the new isolates were characterized by spectroscopic methods and X-ray diffraction analysis. Among these, 20 sesquiterpenoids were evaluated for their multidrug resistance (MDR) reversal activity against the KB/VCR cell line. As a result, compounds 6 and 8 were found to exhibit MDR-reversal activity of more than 10-fold at a concentration of 2 μM, and the reversal fold (RF) ratios of compounds 19, 21, and 24 were >97.9 at a 20 μM nontoxic concentration level.

CD13: A Key Player in Multidrug Resistance in Cancer Chemotherapy

Cancer is one of the most serious diseases that are harmful to human health. Systemic chemotherapy is an optimal therapeutic strategy for the treatment of cancer, but great difficulty has been encountered in its administration in the form of multidrug resistance (MDR). As an enzyme on the outer cell surface, CD13 is documented to be involved in the MDR development of tumor cells. In this review, we will focus on the role of CD13 in MDR generation based on the current evidence.

Poziotinib Inhibits the Efflux Activity of the ABCB1 and ABCG2 Transporters and the Expression of the ABCG2 Transporter Protein in Multidrug Resistant Colon Cancer Cells

Simple Summary

Globally, colorectal cancer (CRC) is a leading cause of cancer deaths and chemotherapy, in combination with radiotherapy when appropriate, is used to treat the majority of CRC patients. However, the acquisition or development of drug resistance can decrease, or even abolish, the efficacy of chemotherapy. ATP-binding cassette (ABC) transporters, particularly, the ABCB1 and ABCG2 transporter, are mediators of multidrug resistance (MDR) in certain types of cancer cells. The aim of our in vitro study was to determine if poziotinib can overcome MDR to certain chemotherapeutic drugs in colon cancer cells. Our results indicated that in MDR CRC cell lines, poziotinib inhibits the transport function of the ABCB1 and ABCG2 transporters, increasing the intracellular accumulation of certain anticancer drugs, and thus, their efficacy. Furthermore, poziotinib decreased the expression of the ABCG2 protein. Therefore, if our results can be translated to humans, they suggest that using poziotinib in combination with certain anticancer drugs may be of therapeutic benefit in colorectal cancer patients.

Abstract

Colorectal cancer (CRC) is a leading cause of cancer deaths in the United States. Currently, chemotherapy is a first-line treatment for CRC. However, one major drawback of chemotherapy is the emergence of multidrug resistance (MDR). It has been well-established that the overexpression of the ABCB1 and/or ABCG2 transporters can produce MDR in cancer cells. In this study, we report that in vitro, poziotinib can antagonize both ABCB1- and ABCG2-mediated MDR at 0.1–0.6 μM in the human colon cancer cell lines, SW620/Ad300 and S1-M1-80. Mechanistic studies indicated that poziotinib increases the intracellular accumulation of the ABCB1 transporter substrates, paclitaxel and doxorubicin, and the ABCG2 transporter substrates, mitoxantrone and SN-38, by inhibiting their substrate efflux function. Accumulation assay results suggested that poziotinib binds reversibly to the ABCG2 and ABCB1 transporter. Furthermore, western blot experiments indicated that poziotinib, at 0.6 μM, significantly downregulates the expression of the ABCG2 but not the ABCB1 transporter protein, suggesting that the ABCG2 reversal effect produced by poziotinib is due to transporter downregulation and inhibition of substrate efflux. Poziotinib concentration-dependently stimulated the ATPase activity of both ABCB1 and ABCG2, with EC₅₀ values of 0.02 μM and 0.21 μM, respectively, suggesting that it interacts with the drug-substrate binding site. Molecular docking analysis indicated that poziotinib binds to the ABCB1 (−6.6 kcal/mol) and ABCG2 (−10.1 kcal/mol) drug-substrate binding site. In summary, our novel results show that poziotinib interacts with the ABCB1 and ABCG2 transporter, suggesting that poziotinib may increase the efficacy of certain chemotherapeutic drugs used in treating MDR CRC.

Chemical molecular-based approach to overcome multidrug resistance in cancer by targeting P-glycoprotein (P-gp)

Multidrug resistance (MDR) remains one of the major impediments for efficacious cancer chemotherapy. Increased efflux of multiple chemotherapeutic drugs by transmembrane ATP-binding cassette (ABC) transporter superfamily is considered one of the primary causes for cancer MDR, in which the role of P-glycoprotein (P-gp/ABCB1) has been most well-established. The clinical co-administration of P-gp drug efflux inhibitors, in combination with anticancer drugs which are P-gp transport substrates, was considered to be a treatment modality to surmount MDR in anticancer therapy by blocking P-gp-mediated multidrug efflux. Extensive attempts have been carried out to screen for sets of nontoxic, selective, and efficacious P-gp efflux inhibitors. In this review, we highlight the recent achievements in drug design, characterization, structure-activity relationship (SAR) studies, and mechanisms of action of the newly synthetic, potent small molecules P-gp inhibitors in the past 5 years. The development of P-gp inhibitors will increase our knowledge of the mechanisms and functions of P-gp-mediated drug efflux which will benefit drug discovery and clinical cancer therapeutics where P-gp transporter overexpression has been implicated in MDR.

Folic Acid-Modified Nanoerythrocyte for Codelivery of Paclitaxel and Tariquidar to Overcome Breast Cancer Multidrug Resistance

The efflux of anticancer agents mediated by P-glycoprotein (P-gp) is one of the main causes of multidrug resistance (MDR) and eventually leads to chemotherapy failure. To overcome this problem, the delivery of anticancer agents in combination with a P-gp inhibitor using nanocarrier systems is considered an effective strategy. On the basis of the physiological compatibility and excellent drug loading ability of erythrocytes, we hypothesized that nanoerythrocytes could be used for the codelivery of an anticancer agent and a P-gp inhibitor to overcome MDR in breast cancer. Herein, a folic acid-modified nanoerythrocyte system (PTX/TQR NPs@NanoRBC-PEG/FA) was prepared to simultaneously transport paclitaxel and tariquidar, and the in vitro and in vivo characteristics of this delivery system were evaluated through several experiments. The results indicated that the average diameter and surface potential of this nanocarrier system were 159.8 ± 1.4 nm and -10.98 mV, respectively. Within 120 h, sustained release of paclitaxel was observed in both pH 6.5 media and pH 7.4 media. Tariquidar release from this nanocarrier suppressed the P-gp function of MCF-7/Taxol cells and significantly increased the intracellular paclitaxel level (p < 0.01 versus the PTX group). The results of the MTT assay indicated that the simultaneous transportation of paclitaxel and tariquidar could significantly inhibit the growth of MCF-7 cells or MCF-7/Taxol cells. After 48 h of incubation with PTX/TQR NPs@NanoRBC-PEG/FA, the viability of MCF-7 cells and MCF-7/Taxol cells decreased to 7.37% and 30.2%, respectively, and the IC50 values were 2.49 μM and 6.30 μM. Pharmacokinetic results illustrated that, compared with free paclitaxel, all test paclitaxel nanoformulations prolonged the drug release time and showed similar plasma concentration-time profiles. The peak concentration (Cmax), area under the curve (AUC0-∞), and half-life (t1/2) of PTX/TQR NPs@NanoRBC-PEG/FA were 3.33 mg/L, 6.02 mg/L·h, and 5.84 h, respectively. Moreover, this active targeting nanocarrier dramatically increased the paclitaxel level in tumor tissues. Furthermore, compared with those of the other paclitaxel formulations, the cellular reactive oxygen species (ROS) and malondialdehyde (MDA) levels of the PTX/TQR NPs@NanoRBC-PEG/FA group increased by 1.38-fold (p < 0.01) and 1.36-fold (p < 0.01), respectively, and the activities of superoxide dismutase (SOD) and catalase (CAT) decreased to 67.8% (p < 0.01) and 65.4% (p < 0.001), respectively. More importantly, in vivo antitumor efficacy results proved that the PTX/TQR NPs@NanoRBC-PEG/FA group exerted an outstanding tumor inhibition effect with no marked body weight loss and fewer adverse effects. In conclusion, by utilizing the inherent and advantageous properties of erythrocytes and surface modification strategies, this biomimetic targeted drug delivery system provides a promising platform for the codelivery of an anticancer agent and a P-gp inhibitor to treat MDR in breast cancer.

ADMET evaluation in drug discovery. 20. Prediction of breast cancer resistance protein inhibition through machine learning

Breast cancer resistance protein (BCRP/ABCG2), an ATP-binding cassette (ABC) efflux transporter, plays a critical role in multi-drug resistance (MDR) to anti-cancer drugs and drug–drug interactions. The prediction of BCRP inhibition can facilitate evaluating potential drug resistance and drug–drug interactions in early stage of drug discovery. Here we reported a structurally diverse dataset consisting of 1098 BCRP inhibitors and 1701 non-inhibitors. Analysis of various physicochemical properties illustrates that BCRP inhibitors are more hydrophobic and aromatic than non-inhibitors. We then developed a series of quantitative structure–activity relationship (QSAR) models to discriminate between BCRP inhibitors and non-inhibitors. The optimal feature subset was determined by a wrapper feature selection method named rfSA (simulated annealing algorithm coupled with random forest), and the classification models were established by using seven machine learning approaches based on the optimal feature subset, including a deep learning method, two ensemble learning methods, and four classical machine learning methods. The statistical results demonstrated that three methods, including support vector machine (SVM), deep neural networks (DNN) and extreme gradient boosting (XGBoost), outperformed the others, and the SVM classifier yielded the best predictions (MCC = 0.812 and AUC = 0.958 for the test set). Then, a perturbation-based model-agnostic method was used to interpret our models and analyze the representative features for different models. The application domain analysis demonstrated the prediction reliability of our models. Moreover, the important structural fragments related to BCRP inhibition were identified by the information gain (IG) method along with the frequency analysis. In conclusion, we believe that the classification models developed in this study can be regarded as simple and accurate tools to distinguish BCRP inhibitors from non-inhibitors in drug design and discovery pipelines.

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

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

Effect of treatment period with LC478, a disubstituted adamantayl derivative, on P-glycoprotein inhibition: its application to increase docetaxel absorption in rats

1. Treatment periods of P-glycoprotein (P-gp) inhibitors have revealed different efficacies. We have previously reported dose-dependent inhibition of P-gp in single-treatment with LC478. However, whether repeated treatment with LC478 can inhibit P-gp even at its ineffective single-treatment dose remains unknown. 2. Therefore, the purpose of this study was to assess the effect of repeated treatment (i.e., 7-day treatment) with LC478 on P-gp known to affect docetaxel bioavailability in rats. Effects of LC478 on P-gp mediated efflux and expression in MDCK-MDR1 cells, P-gp ATPase activity, and binding site with P-gp were evaluated.3. The 7-day treatment with LC478 increased docetaxel absorption via intestinal P-gp inhibition in rats. Intestinal concentrations of LC478 were 8.31-10.3 μM in rats after 7-day treatment of LC478. These concentrations were close to 10 μM that reduced P-gp mediated docetaxel efflux and P-gp expression in MDCK-MDR1 cells. Considering that intestinal LC478 concentrations after 1-day treatment were 2.68-4.19 μM, higher LC478 concentrations after 7-day treatment might have driven P-gp inhibition and increased docetaxel absorption. LC478 might competitively inhibit P-gp considering its stimulated ATPase activity and its binding site with nucleotide binding domain of P-gp. 4. Therefore, repeated treatment with LC478 can determine its feasibility for P-gp inhibition and changing docetaxel bioavailability.

Models for Understanding Resistance to Chemotherapy in Liver Cancer

The lack of response to pharmacological treatment constitutes a substantial limitation in the handling of patients with primary liver cancers (PLCs). The existence of active mechanisms of chemoresistance (MOCs) in hepatocellular carcinoma, cholangiocarcinoma, and hepatoblastoma hampers the usefulness of chemotherapy. A better understanding of MOCs is needed to develop strategies able to overcome drug refractoriness in PLCs. With this aim, several experimental models are commonly used. These include in vitro cell-free assays using subcellular systems; studies with primary cell cultures; cancer cell lines or heterologous expression systems; multicellular models, such as spheroids and organoids; and a variety of in vivo models in rodents, such as subcutaneous and orthotopic tumor xenografts or chemically or genetically induced liver carcinogenesis. Novel methods to perform programmed genomic edition and more efficient techniques to isolate circulating microvesicles offer new opportunities for establishing useful experimental tools for understanding the resistance to chemotherapy in PLCs. In the present review, using three criteria for information organization: (1) level of research; (2) type of MOC; and (3) type of PLC, we have summarized the advantages and limitations of the armamentarium available in the field of pharmacological investigation of PLC chemoresistance.

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.

Discovery of aromatic amides with triazole-core as potent reversal agents against P-glycoprotein-mediated multidrug resistance

P-glycoprotein (P-gp)-mediated multidrug resistance (MDR) is a major impediment for clinical cancer therapy. 19 novel aromatic amides with triazole-core as MDR reversal agents were designed and synthesized via click chemistry to reverse MDR. Among them, compound 42 was identified as the most promising candidate with high potency (EC₅₀ = 78.1 ± 5.4 nM), low cytotoxity (SI > 1282) and persistent duration in reversing doxorubicin (DOX) resistance in K562/A02 cells. 42 also enhanced the potency of other P-gp associated cytotoxic agents with different structures. In further study, remarkably increased intracellular accumulation of Rh123 and DOX in K562/A02 cells was achieved by compound 42, while CYP3A4 activity had no change by compound 42. These results indicate that compound 42 as a relatively safe modulator of P-gp-mediated MDR has good potential for further development.

Novel 3,4-Dihydroisocoumarins Inhibit Human P-gp and BCRP in Multidrug Resistant Tumors and Demonstrate Substrate Inhibition of Yeast Pdr5

Multidrug resistance (MDR) in tumors and pathogens remains a major problem in the efficacious treatment of patients by reduction of therapy options and subsequent treatment failure. Various mechanisms are described to be involved in the development of MDR with overexpression of ATP-binding cassette (ABC) transporters reflecting the most extensively studied. These membrane transporters translocate a wide variety of substrates utilizing energy from ATP hydrolysis leading to decreased intracellular drug accumulation and impaired drug efficacy. One treatment strategy might be inhibition of transporter-mediated efflux by small molecules. Isocoumarins and 3,4-dihydroisocoumarins are a large group of natural products derived from various sources with great structural and functional variety, but have so far not been in the focus as potential MDR reversing agents. Thus, three natural products and nine novel 3,4-dihydroisocoumarins were designed and analyzed regarding cytotoxicity induction and inhibition of human ABC transporters P-glycoprotein (P-gp), multidrug resistance-associated protein 1 (MRP1) and breast cancer resistance protein (BCRP) in a variety of human cancer cell lines as well as the yeast ABC transporter Pdr5 in Saccharomyces cerevisiae. Dual inhibitors of P-gp and BCRP and inhibitors of Pdr5 were identified, and distinct structure-activity relationships for transporter inhibition were revealed. The strongest inhibitor of P-gp and BCRP, which inhibited the transporters up to 80 to 90% compared to the respective positive controls, demonstrated the ability to reverse chemotherapy resistance in resistant cancer cell lines up to 5.6-fold. In the case of Pdr5, inhibitors were identified that prevented substrate transport and/or ATPase activity with IC₅₀ values in the low micromolar range. However, cell toxicity was not observed. Molecular docking of the test compounds to P-gp revealed that differences in inhibition capacity were based on different binding affinities to the transporter. Thus, these small molecules provide novel lead structures for further optimization.

Potentiation of the anticancer effect of doxorubicinin drug-resistant gastric cancer cells by tanshinone IIA

BACKGROUND

Gastric cancer is the fifth commonest cancer and the third cause of cancer-related deaths all over the world. The effectiveness of chemotherapy is still limited by drug resistance in gastric cancer. Tanshinones, abietane diterpenes isolated from the traditional Chinese medicine Danshen (Salvia miltiorrhiza), have exhibited versatile anticancer activities in particular the ability to overcome drug resistance in different cancers.

PURPOSE

The current study aimed to explore the capacity of tanshinone IIA, the most abundant tanshinone found in the plant Danshen, to overcome drug resistance of gastric cancer cells to a commonly used anticancer drug doxorubicin.

STUDY DESIGN

Sensitivity of cell lines to doxorubicin was determined by MTT assay. Doxorubicin resistant gastric cancer cell lines was established by step selection with increasing concentrations of doxorubicin. Cell cycle arrest, apoptosis and doxorubicin efflux were analyzed by flow cytometry. The expression of MRP1 was determined by realtime PCR and western-blot.

RESULTS

Based on the IC₅₀ values of doxorubicin, we identified the doxorubicin-sensitive gastric cancer cell lines SNU-719 and SNU-610 as well as the cell lines relatively resistant to doxorubicin including SNU-638, SNU-668, SNU-216 and SNU-620. We also established two drug-resistant cell lines SNU-719R and SNU-610R. Despite the fact that tanshinone IIA alone showed no cytotoxicity on these gastric cells, we found the potentiation of the anticancer effect of doxorubicin in drug-resistant gastric cancer cells by tanshinone IIA. Furthermore, using doxorubicin-sensitive cell line SNU-719 and doxorubicin-resistant cell lines SNU-719R and SNU-620, we revealed the pivotal roles of MRP1. Its overexpression impaired cell cycle arrest and suppressed apoptosis in the development of both intrinsic and acquired drug resistance in gastric cancer cells to doxorubicin. Importantly, inhibition of MRP1 function enhanced cell cycle arrest, increased apoptosis and induced autophagic cell death which contributed to the capability of tanshinone IIA to potentiate the anticancer effect of doxorubicin in drug-resistant gastric cancer cells.

CONCLUSION

Tanshinone IIA is an interesting agent with potential to treat drug-resistant gastric cancer in combination therapy.

Discovery of Novel Flavonoid Dimers To Reverse Multidrug Resistance Protein 1 (MRP1, ABCC1) Mediated Drug Resistance in Cancers Using a High Throughput Platform with "Click Chemistry"

A 300-member flavonoid dimer library of multidrug resistance-associated protein 1 (MRP1, ABCC1) modulators was rapidly assembled using "click chemistry". Subsequent high-throughput screening has led to the discovery of highly potent (EC₅₀ ranging from 53 to 298 nM) and safe (selective indexes ranging from >190 to >1887) MRP1 modulators. Some dimers have potency about 6.5- to 36-fold and 64- to 358-fold higher than the well-known MRP1 inhibitors, verapamil, and MK571, respectively. They inhibited DOX efflux and restored intracellular DOX concentration. The most potent modulator, Ac3Az11, was predicted to bind to the bipartite substrate-binding site of MRP1 in a competitive manner. Moreover, it provided sufficient concentration to maintain its plasma level above its in vitro EC₅₀ (53 nM for DOX) for about 90 min. Overall, we demonstrate that "click chemistry" coupled with high throughput screening is a rapid, reliable, and efficient tool in the discovery of compounds having potent MRP1-modualting activity.

Structural and functional aspects of P-glycoprotein and its inhibitors

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

Cariprazine, A Dopamine D₂/D₃ Receptor Partial Agonist, Modulates ABCG2-Mediated Multidrug Resistance in Cancer

Multidrug resistance (MDR) is a continuing clinical problem that limits the efficacy of chemotherapy in cancer. The over expression of the ATP-binding cassette (ABC) family G2 (ABCG2) transporter is one of the main mechanisms that mediates MDR in cancer. Molecular modeling data indicated that cariprazine, a dopamine D₂/D₃ receptor partial agonist, had a significant binding affinity for ABCG2 transporter with a Glide XP score of -6.515. Therefore, in this in vitro study, we determined the effect of cariprazine on MDR resulting from the overexpression of ABCG2 transporters. Alone, cariprazine, at concentrations up to 20 μM, did not significantly decrease cell viability. Cariprazine, at concentrations ranging from 1 to 10 μM, did not significantly alter the cytotoxicity of mitoxantrone (MX) in the parental non-small cell cancer cell line, H460 and colon cancer cell S1. However, cariprazine (1⁻20 μM) significantly enhanced the efficacy of ABCG2 substrate antineoplastic drug MX in the ABCG2-overexpressing MDR cell line, H460-MX20 and S1M1-80, by reducing the resistance fold from 28 to 1 and from 93 to 1.33, respectively. Cariprazine, in a concentration-dependent (1⁻20 μM), significantly increased the intracellular accumulation of Rhodamine 123 in S1M1-80. Interestingly, 10 or 20 μM of cariprazine significantly decreased the expression levels of the ABCG2 protein in the colon and lung cancer cell lines, suggesting that cariprazine inhibits both the function and expression of ABCG2 transporters at nontoxic concentrations. Overall, our results suggest that cariprazine, via several distinct mechanisms, can resensitize resistant cancer cells to mitoxantrone.

Design, synthesis, and biological evaluation of 6-methoxy-2-arylquinolines as potential P-glycoprotein inhibitors

Objective(s):

In the present study, a new series of 6-methoxy-2-arylquinoline analogues was designed and synthesized as P-glycoprotein (P-gp) inhibitors using quinine and flavones as the lead compounds.

Materials and Methods:

The cytotoxic activity of the synthesized compounds was evaluated against two human cancer cell lines including EPG85-257RDB, multidrug-resistant gastric carcinoma cells (P-gp-positive gastric carcinoma cell line), and EPG85-257P, drug-sensitive gastric carcinoma cells. Compounds showing low to moderate toxicity in the MTT test were selected to investigate their P-gp inhibition activity. Moreover, trying to explain the results of biological experiments, docking studies of the selected compounds into the homology-modeled human P-gp, were carried out. The physicochemical and ADME properties of the compounds as drug candidate were also predicted.

Results:

Most of our compounds exhibited negligible or much lower cytotoxic effect in both cancer cells. Among the series, 5a and 5b, alcoholic quinoline derivatives were found to inhibit the efflux of rhodamine 123 at the concentration of 10 μM significantly.

Conclusion:

Among the tested quinolines, 5a and 5b showed the most potent P-gp inhibitory activity in the series and were 1.3-fold and 2.1-fold stronger than verapamil, respectively. SAR data revealed that hydroxyl methyl in position 4 of quinolines has a key role in P-gp efflux inhibition of our compounds. ADME studies suggested that all of the compounds included in this study may have a good human intestinal absorption.

Structure-function relationships in ABCG2: insights from molecular dynamics simulations and molecular docking studies

Efflux pumps of the ATP-binding cassette transporters superfamily (ABC transporters) are frequently involved in the multidrug-resistance (MDR) phenomenon in cancer cells. Herein, we describe a new atomistic model for the MDR-related ABCG2 efflux pump, also named breast cancer resistance protein (BCRP), based on the recently published crystallographic structure of the ABCG5/G8 heterodimer sterol transporter, a member of the ABCG family involved in cholesterol homeostasis. By means of molecular dynamics simulations and molecular docking, a far-reaching characterization of the ABCG2 homodimer was obtained. The role of important residues and motifs in the structural stability of the transporter was comprehensively studied and was found to be in good agreement with the available experimental data published in literature. Moreover, structural motifs potentially involved in signal transmission were identified, along with two symmetrical drug-binding sites that are herein described for the first time, in a rational attempt to better understand how drug binding and recognition occurs in ABCG2 homodimeric transporters.

A novel delocalized lipophilic cation-chlorambucil conjugate inhibits P-glycoprotein in HepG2/ADM cells

Multidrug resistance (MDR) limits the application of a large number of cancer-fighting agents in clinical therapy. One reason is that P-glycoprotein (Pgp) efflux pumps are usually overexpressed and lead to drug efflux in the cancer cells, which limits the viability of many chemotherapeutics. Current available inhibitors which block the Pgp pump efflux are usually not widely used in clinical practice, because they change other drug pharmacokinetic profiles or increase side effects. Here, through covalent linkage of cancer-targeting delocalized lipophilic cation FF and DNA-damaging drug nitrogen mustard chlorambucil (CLB), we rationally designed and synthesized a tumor-targeting anticancer agent FFCLB. And we found and proved that the FFCLB was capable of reducing the outflow of Pgp substrates efficiently. This conjugate selectively improves adriamycin uptake and toxicity through reducing MDR1 mRNA and Pgp protein expression. Based on molecular targeted strategy, this study can facilitate the discovery of superior MDR reducing agents to provide a more effective and safer way of resensitizing MDR.

P-gp/ABCB1 exerts differential impacts on brain and fetal exposure to norbuprenorphine

Norbuprenorphine is the major active metabolite of buprenorphine which is commonly used to treat opiate addiction during pregnancy. Norbuprenorphine produces marked respiratory depression and was 10 times more potent than buprenorphine. Therefore, it is important to understand the mechanism that controls fetal exposure to norbuprenorphine, as exposure to this compound may pose a significant risk to the developing fetus. P-gp/ABCB1 and BCRP/ABCG2 are two major efflux transporters regulating tissue distribution of drugs. Previous studies have shown that norbuprenorphine, but not buprenorphine, is a P-gp substrate. In this study, we systematically examined and compared the roles of P-gp and BCRP in determining maternal brain and fetal distribution of norbuprenorphine using transporter knockout mouse models. We administered 1mg/kg norbuprenorphine by retro-orbital injection to pregnant FVB wild-type, Abcb1a-/-/1b-/-, and Abcb1a-/-/1b-/-/Abcg2-/- mice on gestation day 15. The fetal AUC of norbuprenorphine was ∼64% of the maternal plasma AUC in wild-type mice, suggesting substantial fetal exposure to norbuprenorphine. The maternal plasma AUCs of norbuprenorphine in Abcb1a-/-/1b-/- and Abcb1a-/-/1b-/-/Abcg2-/- mice were ∼2 times greater than that in wild-type mice. Fetal AUCs in Abcb1a-/-/1b-/- and Abcb1a-/-/1b-/-/Abcg2-/- mice were also increased compared to wild-type mice; however, the fetal-to-maternal plasma AUC ratio remained relatively unchanged by the knockout of Abcb1a/1b or Abcb1a/1b/Abcg2. In contrast, the maternal brain-to-maternal plasma AUC ratio in Abcb1a-/-/1b-/- or Abcb1a-/-/1b-/-/Abcg2-/- mice was increased ∼30-fold compared to wild-type mice. Protein quantification by LC-MS/MS proteomics revealed significantly higher amounts of P-gp protein in the wild-type mice brain than that in the placenta. These results indicate that fetal exposure to norbuprenorphine is substantial and that P-gp has a minor impact on fetal exposure to norbuprenorphine, but plays a significant role in restricting its brain distribution. The differential impacts of P-gp on norbuprenorphine distribution into the brain and fetus are likely, at least in part, due to the differences in amounts of P-gp protein expressed in the blood-brain and blood-placental barriers. BCRP is not as important as P-gp in determining both the systemic and tissue exposure to norbuprenorphine. Finally, fetal AUCs of the metabolite norbuprenorphine-β-d-glucuronide were 3-7 times greater than maternal plasma AUCs, while the maternal brain AUCs were <50% of maternal plasma AUCs, suggesting that a reversible pool of conjugated metabolite in the fetus may contribute to the high fetal exposure to norbuprenorphine.

Mechanism Underlying the Reversal of Drug Resistance in P-Glycoprotein-Expressing Leukemia Cells by Pinoresinol and the Study of a Derivative

P-glycoprotein (P-gp) is a membrane protein associated with multidrug resistance (MDR) due to its key role in mediating the traffic of chemotherapeutic drugs outside cancer cells, leading to a cellular response that hinders efforts toward successful therapy. With the aim of finding agents that circumvent the MDR phenotype mediated by P-gp, 15 compounds isolated from native and naturalized plants of Argentina were screened. Among these, the non-cytotoxic lignan (±) pinoresinol successfully restored sensitivity to doxorubicin from 7 μM in the P-gp overexpressed human myelogenous leukemia cells, Lucena 1. This resistance-reversing effect was confirmed by competitively increasing the intracellular doxorubicin accumulation and by significantly inhibiting the efflux of doxorubicin and, to a lesser extent, that of rhodamine 123. The activity obtained was similar to that observed with verapamil. No such results were observed in the sensitive parental K562 cell line. To gain deeper insight into the mode of action of pinoresinol, its effect on P-gp function and expression was examined. The docking simulations indicated that the lignan bound to P-gp at the apex of the V-shaped transmembrane cavity, involving transmembrane helices 4, 5, and 6, and partially overlapped the binding region of tariquidar, which was used as a positive control. These results would shed some light on the nature of its interaction with P-gp at molecular level and merit further mechanistic and kinetic studies. In addition, it showed a maximum 29% activation of ATP hydrolysis and antagonized verapamil-stimulated ATPase activity with an IC₅₀ of 20.9 μM. On the other hand, pinoresinol decreased the presence of P-gp in the cell surface. Derivatives of pinoresinol with improved activity were identified by docking studies. The most promising one, the non-cytotoxic 1-acetoxypinoresinol, caused a reversion of doxorubicin resistance from 0.11 μM and thus higher activity than the lead compound. It also caused a significant increase in doxorubicin accumulation. Results were similar to those observed with verapamil. The results obtained positioned these compounds as potential candidates for effective agents to overcome P-gp-mediated MDR, leading to better outcomes for leukemia chemotherapy.

Mechanisms of drug resistance in colon cancer and its therapeutic strategies

Drug resistance develops in nearly all patients with colon cancer, leading to a decrease in the therapeutic efficacies of anticancer agents. This review provides an up-to-date summary on over-expression of ATP-binding cassette (ABC) transporters and evasion of apoptosis, two representatives of transport-based and non-transport-based mechanisms of drug resistance, as well as their therapeutic strategies. Different ABC transporters were found to be up-regulated in colon cancer, which can facilitate the efflux of anticancer drugs out of cancer cells and decrease their therapeutic effects. Inhibition of ABC transporters by suppressing their protein expressions or co-administration of modulators has been proven as an effective approach to sensitize drug-resistant cancer cells to anticancer drugs in vitro. On the other hand, evasion of apoptosis observed in drug-resistant cancers also results in drug resistance to anticancer agents, especially to apoptosis inducers. Restoration of apoptotic signals by BH3 mimetics or epidermal growth factor receptor inhibitors and inhibition of cancer cell growth by alternative cell death pathways, such as autophagy, are effective means to treat such resistant cancer types. Given that the drug resistance mechanisms are different among colon cancer patients and may change even in a single patient at different stages, personalized and specific combination therapy is proposed to be more effective and safer for the reversal of drug resistance in clinics.

The Role of Eukaryotic and Prokaryotic ABC Transporter Family in Failure of Chemotherapy

Over the years chemotherapy failure has been a vital research topic as researchers have been striving to discover reasons behind it. The extensive studies carried out on chemotherapeutic agents confirm that resistance to chemotherapy is a major reason for treatment failure. “Resistance to chemotherapy,” however, is a comprehensive phrase that refers to a variety of different mechanisms in which ATP-binding cassette (ABC) mediated efflux dominates. The ABC is one of the largest gene superfamily of transporters among both eukaryotes and prokaryotes; it represents a variety of genes that code for proteins, which perform countless functions, including drug efflux – a natural process that protects cells from foreign chemicals. Up to date, chemotherapy failure due to ABC drug efflux is an active research topic that continuously provides further evidence on multiple drug resistance (MDR), aiding scientists in tackling and overcoming this issue. This review focuses on drug resistance by ABC efflux transporters in human, viral, parasitic, fungal and bacterial cells and highlights the importance of the MDR permeability glycoprotein being the mutual ABC transporter among all studied organisms. Current developments and future directions to overcome this problem are also discussed.

Mechanisms of drug resistance in colon cancer and its therapeutic strategies

Drug resistance develops in nearly all patients with colon cancer, leading to a decrease in the therapeutic efficacies of anticancer agents. This review provides an up-to-date summary on over-expression of ATP-binding cassette (ABC) transporters and evasion of apoptosis, two representatives of transport-based and non-transport-based mechanisms of drug resistance, as well as their therapeutic strategies. Different ABC transporters were found to be up-regulated in colon cancer, which can facilitate the efflux of anticancer drugs out of cancer cells and decrease their therapeutic effects. Inhibition of ABC transporters by suppressing their protein expressions or co-administration of modulators has been proven as an effective approach to sensitize drug-resistant cancer cells to anticancer drugs in vitro. On the other hand, evasion of apoptosis observed in drug-resistant cancers also results in drug resistance to anticancer agents, especially to apoptosis inducers. Restoration of apoptotic signals by BH3 mimetics or epidermal growth factor receptor inhibitors and inhibition of cancer cell growth by alternative cell death pathways, such as autophagy, are effective means to treat such resistant cancer types. Given that the drug resistance mechanisms are different among colon cancer patients and may change even in a single patient at different stages, personalized and specific combination therapy is proposed to be more effective and safer for the reversal of drug resistance in clinics.

Gingerol Synergizes the Cytotoxic Effects of Doxorubicin against Liver Cancer Cells and Protects from Its Vascular Toxicity

Hydroxyphenylalkanes and diarylheptanoids possess potential therapeutic value in different pathophysiological conditions, such as malignancy. In the current study, naturally isolated hydroxyphenylalkane and diarylheptanoid compounds were investigated for potential chemo-modulatory effects in addition to potential vascular protective roles with doxorubicin. Diarylheptanoids showed stronger antioxidant effects, in comparison to hydroxyphenylalkanes, as demonstrated by DPPH assay and amelioration of CCl₄-induced disturbed intracellular GSH/GSSG balance. Shogaol and 4′-methoxygingerol showed considerable cytotoxic effects against HCT116, HeLa, HepG2 and MCF7 cells, with IC₅₀ values ranging from 3.1 to 19.4 µM. Gingerol significantly enhanced the cytotoxic profile of doxorubicin against HepG₂ and Huh7, cells decreasing its IC₅₀s by 10- and 4-fold, respectively. Cell cycle distribution was studied using DNA cytometry. Doxorubicin alone induced cell accumulation at S-phase and G₂/M-phase, while in combination with gingerol it significantly induced cell cycle arrest at the G₂/M-phase. Additionally, the vascular protective effect of gingerol against doxorubicin (10 µM) was examined on isolated aortic rings. Co-incubation with 6-gingerol (30 µM) completely blocked the exaggerated vasoconstriction and impaired vascular relaxation induced by doxorubicin. In conclusion, despite its relatively weak antioxidant properties, gingerol protected from DOX-induced vascular damage, apparently not through a ROS scavenging mechanism. Besides, gingerol synergized the cytotoxic effects of DOX against liver cancer cells without influencing the cellular pharmacokinetics.

Active targeting co-delivery system based on pH-sensitive methoxy-poly(ethylene glycol)2K-poly(ε-caprolactone)4K-poly(glutamic acid)1K for enhanced cancer therapy

In this paper, we successfully synthesized folate-modified pH-sensitive copolymer methoxy-poly(ethylene glycol)2K-poly(ε-caprolactone)4K-poly(glutamic acid)1K (mPEG2K-PCL4K-PGA1K-FA), which could form the polymeric assembly in an aqueous solution, for co-delivering hydrophilic drugs doxorubicin hydrochloride (DOX) and verapamil hydrochloride (VER) (FA-poly(DOX+VER)). Since VER was an effective P-glycoprotein inhibitor, the combination of DOX and VER could reverse the multidrug resistance efficiently and enhance the therapeutic effect. Therefore, the inhibition ratios of MCF-7/ADR resistant cancer cell treated by FA-poly (DOX+VER) were almost more than 30% higher than those of FA-polyDOX after 48h and 72h. Furthermore, the conjugation of FA could lead the co-delivery systems actively targeting into the FA receptor over-expressing cancer cells in addition to the passive accumulation of the assembly in tumor tissues. Importantly, the prepared mPEG2K-PCL4K-PGA1K-FA assembly showed high pH-sensitive property, which made the drugs mostly released in tumor tissue (acid environment) than in physiological environment (neutral environment). In summary, the as-prepared co-delivery system FA-poly(DOX+VER) demonstrated a high efficiency in reversing the multidrug resistance and targeting FA receptor to improve the anticancer effect of DOX in MCF-7/ADR resistant cells.

3D-QSAR AND CONTOUR MAP ANALYSIS OF TARIQUIDAR ANALOGUES AS MULTIDRUG RESISTANCE PROTEIN-1 (MRP1) INHIBITORS

One of the major obstacles to the successful chemotherapy towards several cancers is multidrug resistance of human cancer cells to anti-cancer drugs. An important contributor to multidrug resistance is the human multidrug resistance protein-1 transporter (MRP1), which is an efflux pump of the ABC (ATP binding cassette) superfamily. Thus, highly efficacious, third generation MRP1 inhibitors, like tariquidar analogues, are promising inhibitors of multidrug resistance and are under clinical trials. To maximize the efficacy of MRP1 inhibitors and to reduce systemic toxicity, it is important to limit the exposure of MRP1 inhibitors and anticancer drugs to normal tissues and to increase their co-localization with tumor cells. Comparative Molecular Field Analysis (CoMFA) and Comparative Molecular Similarity Indices Analysis (CoMSIA) associated with 3D-Quantitiative structure-activity relationship (3D-QSAR) studies were performed on a series of tariquidar analogues, as selective MDR modulators. Best predictability was obtained with CoMFA model r2 (non-cross-validated square of correlation coefficient) = 0.968, F value = 151.768 with five components, standard error of estimate = 0.107 while the CoMSIA yielded r2 = 0.982, F value = 60.628 with six components, and standard error of estimate = 0.154. These results indicate that steric, electrostatic, hydrophobic (lipophilic), and hydrogen bond donor substituents play significant roles in multidrug resistance modulation of tariquidar analogues upon MRP1. The tariquidar analogue and MRP1 binding and stability data generated from CoMFA and CoMSIA based 3D-contour maps may further aid in study and design of tariquidar analogues as novel, potent and selective MDR modulator drug candidates.