Synthesis and biological evaluation of flavones and benzoflavones as inhibitors of BCRP/ABCG2

Enhanced reversal of ABCG2-mediated drug resistance by replacing a phenyl ring in baicalein with a meta-carborane

Success of chemotherapy is often hampered by multidrug resistance. One mechanism for drug resistance is the elimination of anticancer drugs through drug transporters, such as breast cancer resistance protein (BCRP; also known as ABCG2), and causes a poor 5-year survival rate of human patients. Co-treatment of chemotherapeutics and natural compounds, such as baicalein, is used to prevent chemotherapeutic resistance but is limited by rapid metabolism. Boron-based clusters as meta-carborane are very promising phenyl mimetics to increase target affinity; we therefore investigated the replacement of a phenyl ring in baicalein by a meta-carborane to improve its affinity towards the human ABCG2 efflux transporter. Baicalein strongly inhibited the ABCG2-mediated efflux and caused a fivefold increase in mitoxantrone cytotoxicity. Whereas the baicalein derivative 5,6,7-trimethoxyflavone inhibited ABCG2 efflux activity in a concentration of 5 μm without reversing mitoxantrone resistance, its carborane analogue 5,6,7-trimethoxyborcalein significantly enhanced the inhibitory effects in nanomolar ranges (0.1 μm) and caused a stronger increase in mitoxantrone toxicity reaching similar values as Ko143, a potent ABCG2 inhibitor. Overall, in silico docking and in vitro studies demonstrated that the modification of baicalein with meta-carborane and three methoxy substituents leads to an enhanced reversal of ABCG2-mediated drug resistance. Thus, this seems to be a promising basis for the development of efficient ABCG2 inhibitors.

Synthesis and Characterization of New Pyrano[2,3-c]pyrazole Derivatives as 3-Hydroxyflavone Analogues

In this paper, an efficient synthetic route from pyrazole-chalcones to novel 6-aryl-5-hydroxy-2-phenylpyrano[2,3-c]pyrazol-4(2H)-ones as 3-hydroxyflavone analogues is described. The methylation of 5-hydroxy-2,6-phenylpyrano[2,3-c]pyrazol-4(2H)-one with methyl iodide in the presence of a base yielded a compound containing a 5-methoxy group, while the analogous reaction of 5-hydroxy-2-phenyl-6-(pyridin-4-yl)pyrano[2,3-c]pyrazol-4(2H)-one led to the zwitterionic 6-(N-methylpyridinium)pyrano[2,3-c]pyrazol derivative. The treatment of 5-hydroxy-2,6-phenylpyrano[2,3-c]pyrazol-4(2H)-one with triflic anhydride afforded a 5-trifloylsubstituted compound, which was further used in carbon-carbon bond forming Pd-catalyzed coupling reactions to yield 5-(hetero)aryl- and 5-carbo-functionalized pyrano[2,3-c]pyrazoles. The excited-state intramolecular proton transfer (ESIPT) reaction of 5-hydroxypyrano[2,3-c]pyrazoles from the 5-hydroxy moiety to the carbonyl group in polar protic, polar aprotic, and nonpolar solvents was observed, resulting in well-resolved two-band fluorescence. The structures of the novel heterocyclic compounds were confirmed by 1H-, 13C-, 15N-, and 19F-NMR spectroscopy, HRMS, and single-crystal X-ray diffraction data.

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.

A curated binary pattern multitarget dataset of focused ATP-binding cassette transporter inhibitors

Multitarget datasets that correlate bioactivity landscapes of small-molecules toward different related or unrelated pharmacological targets are crucial for novel drug design and discovery. ATP-binding cassette (ABC) transporters are critical membrane-bound transport proteins that impact drug and metabolite distribution in human disease as well as disease diagnosis and therapy. Molecular-structural patterns are of the highest importance for the drug discovery process as demonstrated by the novel drug discovery tool ‘computer-aided pattern analysis’ (‘C@PA’). Here, we report a multitarget dataset of 1,167 ABC transporter inhibitors analyzed for 604 molecular substructures in a statistical binary pattern distribution scheme. This binary pattern multitarget dataset (ABC_BPMDS) can be utilized for various areas. These areas include the intended design of (i) polypharmacological agents, (ii) highly potent and selective ABC transporter-targeting agents, but also (iii) agents that avoid clearance by the focused ABC transporters [e.g., at the blood-brain barrier (BBB)]. The information provided will not only facilitate novel drug prediction and discovery of ABC transporter-targeting agents, but also drug design in general in terms of pharmacokinetics and pharmacodynamics.

Measurement(s)	Influx • Efflux • Tracer • Transport velocity
Technology Type(s)	Fluorometry • Radioactivity • Plate reader • Flow cytometer • Tracer distribution
Factor Type(s)	half-maximal inhibition concentration
Sample Characteristic - Organism	Homo sapiens
Sample Characteristic - Environment	cell culture
Sample Characteristic - Location	Kingdom of Norway • Germany • Australia • Latvia

Binding mode analysis of ABCA7 for the prediction of novel Alzheimer's disease therapeutics

The adenosine-triphosphate-(ATP)-binding cassette (ABC) transporter ABCA7 is a genetic risk factor for Alzheimer's disease (AD). Defective ABCA7 promotes AD development and/or progression. Unfortunately, ABCA7 belongs to the group of 'under-studied' ABC transporters that cannot be addressed by small-molecules. However, such small-molecules would allow for the exploration of ABCA7 as pharmacological target for the development of new AD diagnostics and therapeutics. Pan-ABC transporter modulators inherit the potential to explore under-studied ABC transporters as novel pharmacological targets by potentially binding to the proposed 'multitarget binding site'. Using the recently reported cryogenic-electron microscopy (cryo-EM) structures of ABCA1 and ABCA4, a homology model of ABCA7 has been generated. A set of novel, diverse, and potent pan-ABC transporter inhibitors has been docked to this ABCA7 homology model for the discovery of the multitarget binding site. Subsequently, application of pharmacophore modelling identified the essential pharmacophore features of these compounds that may support the rational drug design of innovative diagnostics and therapeutics against AD.

Anticancer potential of Naringenin, Biosynthesis, Molecular target, and structural perspectives

Numerous novel medicinal agents isolated from plant sources were used as indigenous remedies for the management and treatment of various types of cancer diseases. Naringenin is a naturally occurring flavanone glycoside and aglycone (genin) moiety of naringin, predominantly found in citrus and grapefruits, has emerged as a potential therapeutic agent for the management of a variety of diseases. A huge number of scientific papers have been published on naringenin describing its detailed studies and its therapeutic application in different diseases. The current study highlights, a comprehensive study on naringenin concerning its biosynthesis, molecular targets/pathways involved in carcinogenesis, mechanism of actions (MOAs), and structure-activity relationships (SARs), and patents granted have been highlighted. Naringenin and its derivatives has remarkable anti-cancer activity due to their inhibitory potential against diverse targets namely ABCG2/P-gp/BCRP, 5a-reductase, 17-bhydroxysteroid dehydrogenase, aromatase, proteasome, HDAC/Situin-1, VEGF, VEGFR-2 kinase, MMP-2/9, JAK/STAT signaling pathways, CDC25B, tubulin, topoisomerase-II, cathepsin-K, Wnt, NF-kB, B-Raf and mTOR, etc. With the huge knowledge of molecular targets, structural intuition, and SARs, the current study may be beneficial to design more potent, safe, effective, and economic anti-cancer naringenin. This is concluded that naringenin is a promising natural product for the management and therapy of cancer. Further evolution for pharmacological importance, clinical research, and trials are required to manifest its therapeutic action on metabolic syndrome in the human community.

Tetrahydroquinoline/4,5-Dihydroisoxazole Molecular Hybrids as Inhibitors of Breast Cancer Resistance Protein (BCRP/ABCG2)

Multidrug resistance (MDR) is one of the major factors in the failure of many chemotherapy approaches. In cancer cells, MDR is mainly associated with the expression of ABC transporters such as P-glycoprotein, MRP1 and ABCG2. Despite major efforts to develop new selective and potent inhibitors of ABC drug transporters, no ABCG2-specific inhibitors for clinical use are yet available. Here, we report the evaluation of sixteen tetrahydroquinoline/4,5-dihydroisoxazole derivatives as a new class of ABCG2 inhibitors. The affinity of the five best inhibitors was further investigated by the vanadate-sensitive ATPase assay. Molecular modelling data, proposing a potential binding mode, suggest that they can inhibit the ABCG2 activity by binding on site S1, previously reported as inhibitors binding region, as well targeting site S2, a selective region for substrates, and by specifically interacting with residues Asn436, Gln398, and Leu555. Altogether, this study provided new insights into THQ/4,5-dihydroisoxazole molecular hybrids, generating great potential for the development of novel most potent ABCG2 inhibitors.

Structure-Based Discovery of ABCG2 Inhibitors: A Homology Protein-Based Pharmacophore Modeling and Molecular Docking Approach

ABCG2 is an ABC membrane protein reverse transport pump, which removes toxic substances such as medicines out of cells. As a result, drug bioavailability is an unexpected change and negatively influences the ADMET (absorption, distribution, metabolism, excretion, and toxicity), leading to multi-drug resistance (MDR). Currently, in spite of promising studies, screening for ABCG2 inhibitors showed modest results. The aim of this study was to search for small molecules that could inhibit the ABCG2 pump. We first used the WISS MODEL automatic server to build up ABCG2 homology protein from 655 amino acids. Pharmacophore models, which were con-structed based on strong ABCG2 inhibitors (IC₅₀ < 1 μM), consist of two hydrophobic (Hyd) groups, two hydrogen bonding acceptors (Acc2), and an aromatic or conjugated ring (Aro|PiR). Using molecular docking method, 714 substances from the DrugBank and 837 substances from the TCM with potential to inhibit the ABCG2 were obtained. These chemicals maybe favor synthesized or extracted and bioactivity testing.

Scaffold fragmentation and substructure hopping reveal potential, robustness, and limits of computer-aided pattern analysis (C@PA)

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Exploratory changes in substructure patterns are well tolerated by C@PA.

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Extended positive substructures support prediction capability.

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Increased biological hit rate of 40% for multitarget pan-ABC transporter inhibition.

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Contribution to major understanding of pattern analysis and multitarget activity.

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Pan-ABC transporter inhibitors as tool for elucidation of multitarget binding site.

Computer-aided pattern analysis (C@PA) was recently presented as a powerful tool to predict multitarget ABC transporter inhibitors. The backbone of this computational methodology was the statistical analysis of frequently occurring molecular features amongst a fixed set of reported small-molecules that had been evaluated toward ABCB1, ABCC1, and ABCG2. As a result, negative and positive patterns were elucidated, and secondary positive substructures could be suggested that complemented the multitarget fingerprints. Elevating C@PA to a non-statistical and exploratory level, the concluded secondary positive patterns were extended with potential positive substructures to improve C@PA’s prediction capabilities and to explore its robustness. A small-set compound library of known ABCC1 inhibitors with a known hit rate for triple ABCB1, ABCC1, and ABCG2 inhibition was taken to virtually screen for the extended positive patterns. In total, 846 potential broad-spectrum ABCB1, ABCC1, and ABCG2 inhibitors resulted, from which 10 have been purchased and biologically evaluated. Our approach revealed 4 novel multitarget ABCB1, ABCC1, and ABCG2 inhibitors with a biological hit rate of 40%, but with a slightly lower inhibitory power than derived from the original C@PA. This is the very first report about discovering novel broad-spectrum inhibitors against the most prominent ABC transporters by improving C@PA.

C@PA: Computer-Aided Pattern Analysis to Predict Multitarget ABC Transporter Inhibitors

Based on literature reports of the last two decades, a computer-aided pattern analysis (C@PA) was implemented for the discovery of novel multitarget ABCB1 (P-gp), ABCC1 (MRP1), and ABCG2 (BCRP) inhibitors. C@PA included basic scaffold identification, substructure search and statistical distribution, as well as novel scaffold extraction to screen a large virtual compound library. Over 45,000 putative and novel broad-spectrum ABC transporter inhibitors were identified, from which 23 were purchased for biological evaluation. Our investigations revealed five novel lead molecules as triple ABCB1, ABCC1, and ABCG2 inhibitors. C@PA is the very first successful computational approach for the discovery of promiscuous ABC transporter inhibitors.

Molecular mechanism and pharmacokinetics of flavonoids in the treatment of resistant EGF receptor-mutated non-small-cell lung cancer: A narrative review

Here, we review the molecular mechanism and pharmacokinetics of flavonoids in the treatment of resistant EGF receptor (EGFR)-mutated non-small-cell lung cancer (NSCLC) and particularly the possible mechanism(s) of delicaflavone, a biflavonoid extracted from Selaginella doederleinii Hieron. EGFR TK inhibitors (EGFR-TKI) are ubiquitously used in the treatment of NSCLC bearing EGFR mutations. However, patients treated with EGFR-TKI inevitably and continuously develop resistance. In laboratory studies, flavonoids, as potential adjuvants for cancer chemotherapy, exhibited anti-cancer properties such as inhibition of chemoresistance by interference with ABC transporters-induced drug efflux, curbing of c-MET amplification, or reversal of T790M mutation-mediated resistance. The current review aims at summarizing the association between the anti-cancer potentials of flavonoids and their possible regulatory roles in certain types of mutation that could trigger EGFR-TKI resistance in NSCLC. Potential practical applications of these phytochemicals, as well as the relevant pharmacokinetics, are also discussed.

Rational drug design of 6-substituted 4-anilino-2-phenylpyrimidines for exploration of novel ABCG2 binding site

In the search for novel, highly potent, and nontoxic adjuvant chemotherapeutics to resolve the major issue of ABC transporter-mediated multidrug resistance (MDR), pyrimidines were discovered as a promising compound class of modern ABCG2 inhibitors. As ABCG2-mediated MDR is a major obstacle in leukemia, pancreatic carcinoma, and breast cancer chemotherapy, adjuvant chemotherapeutics are highly desired for future clinical oncology. Very recently, docking studies of one of the most potent reversers of ABCG2-mediated MDR were reported and revealed a putative second binding pocket of ABCG2. Based on this (sub)pocket, a series of 16 differently 6-substituted 4-anilino-2-phenylpyrimidines was designed and synthesized to explore the potential increase in inhibitory activity of these ABCG2 inhibitors. The compounds were assessed for their influence on the ABCG2-mediated pheophorbide A transport, as well as the ABCB1- and ABCC1-mediated transport of calcein AM. They were additionally evaluated in MDR reversal assays to determine their half-maximal reversal concentration (EC₅₀). The 6-substitution did not only show increased toxicity against ABCG2-overexpressing cells in combination with SN-38 but also a negative influence on cell viability in general. Nevertheless, several candidates had EC₅₀ values in the low double-digit nanomolar concentration range, qualifying them as some of the most potent reversers of ABCG2-mediated MDR. In addition, five novel multitarget ABCB1, ABCC1, and ABCG2 inhibitors were discovered, four of them exerting their inhibitory power against the three stated transporters at least in the single-digit micromolar concentration range.

Superior Pyrimidine Derivatives as Selective ABCG2 Inhibitors and Broad-Spectrum ABCB1, ABCC1, and ABCG2 Antagonists

In the search for highly effective modulators addressing ABCG2-mediated MDR, 23 pyrimidines were synthesized and biologically assessed. Seven derivatives with (a) nitrogen- and/or halogen-containing residue(s) had extraordinary potencies against ABCG2 (IC₅₀ < 150 nM). The compounds competitively inhibited ABCG2-mediated Hoechst 33342 transport but were not substrates of ABCG2. The most potent MDR reverser, compound 19, concentration-dependently increased SN-38-mediated cancer cell death at 11 nM (EC₅₀), time-dependently doubled SN-38 toxicity in a period of 7 days at 10 nM, and half-maximally accelerated cell death combined with SN-38 at 17 nM. No induction of ABCG2 was observed. Furthermore, 11 pyrimidines were revealed as triple ABCB1/ABCC1/ABCG2 inhibitors. Five possessed IC₅₀ values below 10 μM against each transporter, classifying them as some of the 50 most potent multitarget ABC transporter inhibitors. The most promising representative, compound 37, reversed ABCB1-, ABCC1-, and ABCG2-mediated MDR, making it one of the three most potent ABC transporter inhibitors and reversers of ABC transporters-mediated MDR.

Design and synthesis of novel Flavone-based histone deacetylase inhibitors antagonizing activation of STAT3 in breast cancer

Histone deacetylases (HDACs) inhibitors have demonstrated a great clinical achievement in hematological malignancies. However, the efficacy of HDACs inhibitors in treating solid tumors remains limited due to the complicated tumor microenvironment. In this study, we designed and synthesized a class of novel HDACs inhibitors based on the structure of flavones and isoflavones, followed by biological evaluation. To be specific, a lead compound 15a was discovered with strong anti-proliferative effects on a variety of solid tumor cells, especially for breast cancer cells with resistance to SAHA. Studies demonstrated that 15a could significantly inhibit the activity of HDAC 1, 2, 3 (class I) and 6 (class IIB), leading to a dose-dependent accumulation of acetylated histones and α-Tubulin, cell cycle arrest (G1/S phase) and apoptosis in breast cancer cells. Furthermore, the lead compound 15a could also antagonize the activation of STAT3 induced by HDACs inhibition in some breast cancer cells, which further reduced the level of pro-survive proteins in tumor cells and enhanced anti-tumor activity regulated by STAT3 signaling in vivo. Overall, our findings demonstrated that the novel compound 15a might be a HDACs inhibitor candidate, which could be used as promising chemotherapeutic agent for 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.

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.

Identification of Thienopyrimidine Scaffold as an Inhibitor of the ABC Transport Protein ABCC1 (MRP1) and Related Transporters Using a Combined Virtual Screening Approach

A virtual screening protocol with combination of similarity search and pharmacophore modeling was applied to virtually screen a large compound library to gain new scaffolds regarding ABCC1 inhibition. Biological investigation of promising candidates revealed four compounds as ABCC1 inhibitors, three of them with scaffolds not associated with ABCC1 inhibition until now. The best hit molecule-a thienopyrimidine-was a moderately potent, competitive inhibitor of the ABCC1-mediated transport of calcein AM which also sensitized ABCC1-overexpressing cells toward daunorubicin. Further evaluation showed that it was a moderately potent, competitive inhibitor of the ABCB1-mediated transport of calcein AM, and noncompetitive inhibitor of the ABCG2-mediated pheophorbide A transport. In addition, the thienopyrimidine could also sensitize ABCB1- as well as ABCG2-overexpressing cells toward daunorubicin and SN-38, respectively, in concentration ranges that qualified it as one of the ten best triple ABCC1/ABCB1/ABCG2 inhibitors in the literature. Besides, three more new multitarget inhibitors were identified by this virtual screening approach.

Flavonoids: New Frontier for Immuno-Regulation and Breast Cancer Control

Breast cancer (BC) remains the second most common cause of cancer-related deaths in women in the US, despite advances in detection and treatment. In addition, breast cancer survivors often struggle with long-term treatment related comorbidities. Identifying novel therapies that are effective while minimizing toxicity is critical in curtailing this disease. Flavonoids, a subclass of plant polyphenols, are emerging as promising treatment options for the prevention and treatment of breast cancer. Recent evidence suggests that in addition to anti-oxidant properties, flavonoids can directly interact with proteins, making them ideal small molecules for the modulation of enzymes, transcription factors and cell surface receptors. Of particular interest is the ability of flavonoids to modulate the tumor associated macrophage function. However, clinical applications of flavonoids in cancer trials are limited. Epidemiological and smaller clinical studies have been largely hypothesis generating. Future research should aim at addressing known challenges with a broader use of preclinical models and investigating enhanced dose-delivery systems that can overcome limited bioavailability of dietary flavonoids. In this review, we discuss the structure-functional impact of flavonoids and their action on breast tumor cells and the tumor microenvironment, with an emphasis on their clinical role in the prevention and treatment of breast cancer.

Synthesis of 5-Hydroxy-3',4',7-trimethoxyflavone and Related Compounds and Elucidation of Their Reversal Effects on BCRP/ABCG2-Mediated Anticancer Drug Resistance

3',4',7-Trimethoxyflavone (TMF) has been reported to show a potent reversal effect on drug resistance mediated by breast cancer resistance protein (BCRP)/ATP-binding cassette subfamily G member 2 (ABCG2). In this study, we designed and synthesized five derivatives with either a hydroxy group or a fluorine atom at C-5 and several kinds of capping moiety at the C-7 hydroxy group, on the same 3',4'-dimethoxy-substituted flavone skeleton. We subsequently evaluated the efficacies of these compounds against BCRP-expressing human leukaemia K562/BCRP cells. Reversal of drug resistance was expressed as the concentration of compound causing a twofold reduction in drug sensitivity (RI₅₀ ). Of the synthesized compounds, the reversal effect of 5-hydroxy-3',4',7-trimethoxyflavone (HTMF, RI₅₀ 7.2 nm) towards 7-ethyl-10-hydroxycamptothecin (SN-38) was stronger than that of TMF (RI₅₀ 18 nm). Fluoro-substituted 5-fluoro-3',4',7-trimethoxyflavone (FTMF, RI₅₀ 25 nm) and monoglycosylated 7-(β-glucosyloxy)-5-hydroxy-3',4'-dimethoxyflavone (GOHDMF, 91 nm) also exhibited reversal effects, whereas the di- and triglycoside derivatives did not. TMF, HTMF and FTMF at 0.01-10 μm upregulated the K562/BCRP cellular accumulation of Hoechst 33342 nuclear staining dye. In addition, western blotting revealed that treatment of K562/BCRP cells with 0.1 μm TMF, HTMF or FTMT suppressed the expression of BCRP. HTMF showed the strongest inhibition of BCRP-mediated efflux and suppression of BCRP expression of the three effective synthesized flavones.

A simple and effective preparation of quercetin pentamethyl ether from quercetin

Among the five hydroxy (OH) groups of quercetin (3,5,7,3',4'-pentahydroxyflavone), the OH group at 5 position is the most resistant to methylation due to its strong intramolecular hydrogen bonding with the carbonyl group at 4 position. Thus, it is generally difficult to synthesize the pentamethyl ether efficiently by conventional methylation. Here, we describe a simple and effective per-O-methylation of quercetin with dimethyl sulfate in potassium (or sodium) hydroxide/dimethyl sulfoxide at room temperature for about 2 hours, affording quercetin pentamethyl ether (QPE) quantitatively as a single product. When methyl iodide was used in place of dimethyl sulfate, the C-methylation product 6-methylquercetin pentamethyl ether was also formed. A computational study provided a rationale for the experimental results.

Novel chalcone and flavone derivatives as selective and dual inhibitors of the transport proteins ABCB1 and ABCG2

During cancer chemotherapy, certain cancers may become cross-resistant to structurally diverse antineoplastic agents. This so-called multidrug resistance (MDR) is highly associated with the overexpression of ATP-binding cassette (ABC) transport proteins. These membrane-bound efflux pumps export a broad range of structurally diverse endo- and xenobiotics, including chemically unrelated anticancer agents. This translocation of drugs from the inside to the outside of cancer cells is mediated at the expense of ATP. In the last 40 years, three ABC transporters - ABCB1 (P-gp), ABCC1 (MRP1), and ABCG2 (BCRP) - have mainly been attributed to the occurrence of MDR in cancer cells. One of the strategies to overcome MDR is to inhibit the efflux transporter function by small-molecule inhibitors. In this work, we investigated new chalcone- and flavone-based compounds for selective as well as broad-spectrum inhibition of the stated transport proteins. These include substituted chalcones with variations at rings A and B, and flavones with acetamido linker at position 3. The synthesized molecules were evaluated for their inhibitory potential against ABCB1, ABCC1, and ABCG2 in calcein AM and pheophorbide A assays. In further investigations with the most promising candidates from each class, we proved that ABCB1- and ABCG2-mediated MDR could be reversed by the compounds. Moreover, their intrinsic toxicity was found to be negligible in most cases. Altogether, our findings contribute to the understanding of ABC transport proteins and reveal new compounds for ongoing evaluation in the field of ABC transporter-mediated MDR.

Design, synthesis, and biological evaluation of Helicobacter pylori inosine 5'-monophosphate dehydrogenase (HpIMPDH) inhibitors

Inosine 5'-monophosphate dehydrogenase (IMPDH) catalyzes a crucial step in the biosynthesis of guanine nucleotides. Being a validated target for immunosuppressive, antiviral, and anticancer drug development, lately it has been exploited as a promising target for antimicrobial therapy. Extending our previous work on Mycobacterium tuberculosis IMPDH, GuaB2, inhibitor development, we screened a set of 23 new chemical entities (NCEs) with substituted flavone (Series 1) and 1,2,3-triazole (Series 2) core structures for their in vitro Helicobacter pylori IMPDH (HpIMPDH) and human IMPDH2 (hIMPDH2) inhibitory activities. All the NCEs possessed acceptable molecular, physicochemical, and toxicity property profiles. The ranges for HpIMPDH and hIMPDH2 inhibition were 9-99.9% and 16-57%, respectively, at 10 μM concentration. The most potent HpIMPDH inhibitor, 25c, exhibited IC₅₀ value of 1.27 μM with no hIMPDH2 inhibitory activity. The moderately potent, structurally novel hit molecule, 25c, may serve as a lead for further design and development of highly potent HpIMPDH inhibitors.

Synthesis and biological evaluation of quinazoline derivatives - A SAR study of novel inhibitors of ABCG2

Multidrug resistance (MDR) is a major obstacle for effective chemotherapeutic treatment of cancer frequently leading to failure of the therapy. MDR is often associated with the overexpression of ABC transport proteins like ABCB1 or ABCG2 which efflux harmful substances out of cells at the cost of ATP hydrolysis. One way to overcome MDR is to apply potent inhibitors of ABC transporters to restore the sensitivity of the cells toward cytostatic agents. This study focusses on the synthesis and evaluation of novel 2,4-disubstituted quinazoline derivatives regarding the structure-activity-relationship (SAR), their ability to reverse MDR and their mode of interaction with ABCG2. Hence, the inhibitory potency and selectivity toward ABCG2 was determined. Moreover, the intrinsic cytotoxicity and the reversal of MDR were investigated. Interaction type studies with the substrate Hoechst 33342 and conformational analyses of ABCG2 with 5D3 monoclonal antibody were performed for a better understanding of the underlying mechanisms. In our study we could further enhance the inhibitory effect against ABCG2 (compound 31, IC₅₀: 55 nM) and identify the structural features that are crucial for inhibitory potency, the impact on transport activity and binding to the protein.

Discovery of novel human inosine 5'-monophosphate dehydrogenase 2 (hIMPDH2) inhibitors as potential anticancer agents

The enzyme inosine 5'-monophosphate dehydrogenase (IMPDH) catalyzes an essential step in the de novo biosynthesis of guanine nucleotides, and thus regulates the guanine nucleotide pool required for cell proliferation. Of the two isoforms, human IMPDH type 2 (hIMPDH2) is a validated molecular target for potential immunosuppressive, antiviral and anticancer chemotherapy. In search of newer hIMPDH2 inhibitors as potential anticancer agents, three novel series (A: 5-aminoisobenzofuran-1(3H)-one, B: 3,4-dimethoxyaniline and C: benzo[d]-[1,3]dioxol-5-ylmethanamine) were synthesized and evaluated for in vitro and cell-based activities. A total of 37 molecules (29-65) were screened for their in vitro hIMPDH2 inhibition, with particular emphasis on establishing their structure-activity relationship (SAR) trends. Eight compounds (hits, 30, 31, 33-35, 37, 41 and 43) demonstrated significant enzyme inhibition (>70% @ 10 μM); especially the A series molecules were more potent than B series (<70% inhibition @ 10 μM), while C series members were found to be inactive. The hIMPDH2 IC₅₀ values for the hits ranged from 0.36 to 7.38 μM. The hits displaying >80% hIMPDH2 inhibition (30, 33, 35, 41 and 43) were further assessed for their cytotoxic activity against cancer cell lines such as MDA-MB-231 (breast adenocarcinoma), DU145 (prostate carcinoma), U87 MG (glioblastoma astrocytoma) and a normal cell line, NIH-3T3 (mouse embryonic fibroblast) using MTT assay. Most of the compounds exhibited higher cellular potency against cancer cell lines and notably lower toxicity towards NIH-3T3 cells compared to mycophenolic acid (MPA), a prototypical hIMPDH2 inhibitor. Two of the series A hits (30 and 35) were evaluated in human peripheral blood mononuclear cells (hPBMC) assay and found to be better tolerated than MPA. The calculated/predicted molecular and physicochemical properties were satisfactory with reference to drug-likeness. The molecular docking studies clearly demonstrated crucial interactions of the hits with the cofactor-binding site of hIMPDH2, further providing critical information for refining the design strategy. The present study reports the design and discovery of structurally novel hIMPDH2 inhibitors as potential anticancer agents and provides a guide for further research on the development of safe and effective anticancer agents, especially against glioblastoma.

2,4,6-Substituted Quinazolines with Extraordinary Inhibitory Potency toward ABCG2

Several members of the ABC transporter superfamily play a decisive role in the development of multidrug resistance (MDR) in cancer. One of these MDR associated efflux transporters is ABCG2. One way to overcome this MDR is the coadministration of potent inhibitors of ABCG2. In this study, we identified novel inhibitors containing a 2,4,6-substituted quinazoline scaffold. Introduction of a 6-nitro function led to extraordinarily potent compounds that were highly selective for ABCG2 and also able to reverse the MDR toward the chemotherapeutic drugs SN-38 and mitoxantrone. The binding of substrate Hoechst 33342 and the two potent inhibitors 31 and 41 which differ in their mechanism of inhibition was rationalized using the recently published cryo-EM structures of ABCG2. For a better understanding of the interaction between the inhibitors and ABCG2, additional investigations regarding the ATPase activity, the interaction with Hoechst 33342, and with the conformational sensitive 5D3 antibody were carried out.

New Inhibitors of Breast Cancer Resistance Protein (ABCG2) Containing a 2,4-Disubstituted Pyridopyrimidine Scaffold

Multidrug resistance (MDR) occurring during cancer chemotherapy is a major obstacle for effectiveness and response to therapy and is often caused by ATP-binding cassette (ABC) efflux transporters. Belonging to the family of ABC transporters, breast cancer resistance protein is getting more and more in the spotlight of research. As a strategy to overcome MDR, inhibitors of ABC transporters were synthesized, which could be applied in combination with cytostatic drugs. For this purpose, 2,4-disubstituted pyridopyrimidine derivatives were synthesized. The investigations confirmed three key characteristics of good inhibitors: a low intrinsic cytotoxicity and a high potency and selectivity toward ABCG2. For selected compounds the interaction with ABCG2 was elucidated and their effect on ATPase activity and conformation sensitive 5D3 antibody binding was investigated. Their ability to reverse MDR in coadministration with the active metabolite of irinotecan and mitoxantron was confirmed.

5,6-Benzoflavones as cholesterol esterase inhibitors: synthesis, biological evaluation and docking studies

In a continued effort to develop potent cholesterol esterase (CEase) inhibitors, a series of 5,6-benzoflavone derivatives was rationally designed and synthesized by changing the position of the benzene ring attached to the flavone skeleton in previously reported 7,8-benzoflavones. All the synthesized compounds were checked for their inhibitory potential against cholesterol esterase (CEase) using a spectrophotometric assay. Among the series of forty compounds, seven derivatives (B-10 to B-16) exhibited above 90 percent inhibition against CEase in an in vitro enzymatic assay. Compound B-16 showed the most promising activity with an IC50 value of 0.73 nM against cholesterol esterase. To determine the type of inhibition, enzyme kinetic studies were carried out for B-16, which revealed its mixed-type inhibition approach. Moreover, to figure out the key binding interactions of B-16 with the amino acid residues of the enzyme's active site, molecular protein-ligand docking studies were also performed. B-16 completely blocks the catalytic assembly of CEase and prevents it from participating in the ester hydrolysis mechanism. The favorable binding conformation of B-16 suggests its prevailing role as a CEase inhibitor. Overall, the study showed that the cis-orientation of ring A with respect to the carbonyl group of ring C is responsible for the potent CEase inhibitory activity of the newly synthesized compounds.

The lead optimization of the polyamine conjugate of flavonoid with a naphthalene motif: Synthesis and biological evaluation

Polyamine conjugated flavonoid with a naphthalene moiety (ZYY14) displayed excellent therapeutic activity against hepatocellular carcinoma. In this study, three different series of novel flavonoid-polyamine conjugates were designed and screened against tumor cell lines. The structure-activity relationship study demonstrated the importance of the naphthalene moiety (as the B-ring), the basic side chains in the A-ring, and the methoxy group linked to the C-ring. The optimized compound 9b displayed better antitumor potency in vitro and in vivo than the lead compound ZYY14. Fluorescent assays revealed that 9b could enter cancer cells via polyamine transporter (PAT) and locate in mitochondria and endoplasmic reticulum. Compound 9b and ZYY14 demonstrated similar apoptotic mechanism in the cytotoxicity studies and stimulated the expression of apoptosis-related proteins, such as p-p38, p-JNK, p53 and Bax. In addition, 9b can initiate autophagy which inhibited the occurrence of apoptosis. Thus, 9b can be used as a valuable lead for the future development of antitumor agents.

9-Deazapurines as Broad-Spectrum Inhibitors of the ABC Transport Proteins P-Glycoprotein, Multidrug Resistance-Associated Protein 1, and Breast Cancer Resistance Protein

P-Glycoprotein (P-gp, ABCB1), multidrug resistance-associated protein 1 (MRP1, ABCC1), and breast cancer resistance protein (BCRP, ABCG2) are the three major ABC transport proteins conferring resistance to many structurally diverse anticancer agents, leading to the phenomenon called multidrug resistance (MDR). Much effort has been put into the development of clinically useful compounds to reverse MDR. Broad-spectrum inhibitors of ABC transport proteins can be of great use in cancers that simultaneously coexpress two or three transporters. In this work, we continued our effort to generate new, potent, nontoxic, and multiply effective inhibitors of the three major ABC transporters. The best compound was active in a very low micromolar concentration range against all three transporters and restored sensitivity toward daunorubicin (P-gp and MRP1) and SN-38 (BCRP) in A2780/ADR (P-gp), H69AR (MRP1), and MDCK II BCRP (BCRP) cells. Additionally, the compound is a noncompetitive inhibitor of daunorubicin (MRP1), calcein AM (P-gp), and pheophorbide A (BCRP) transport.

3'-Hydroxy-3,4'-dimethoxyflavone blocks tubulin polymerization and is a potent apoptotic inducer in human SK-MEL-1 melanoma cells

Flavonoids are naturally occurring polyphenolic compounds and are among the most promising anticancer agents. A series of flavonols and their 3-methyl ether derivatives were synthesized and assessed for cytotoxicity. It was found that 3'-hydroxy-3,4'-dimethoxyflavone (flavonoid 7a) displayed strong cytotoxicity against human SK-MEL-1 melanoma cells and blocked tubulin polymerization, but had no significant cytotoxic effects against quiescent or proliferating human peripheral blood mononuclear cells. Our analyses showed that flavonoid 7a induces G₂-M cell cycle arrest and apoptosis in melanoma cells which is associated with cytochrome c release and activation of both extrinsic and intrinsic apoptotic pathways of cell death.

Synthesis and biological investigation of 2,4-substituted quinazolines as highly potent inhibitors of breast cancer resistance protein (ABCG2)

Expression of ABCG2, a member of the ABC transporter superfamily, has been correlated to the clinical outcome of multiple cancers and is often associated with the occurrence of multidrug resistance (MDR) in chemotherapy. Inhibition of the transport protein by potent and selective inhibitors might be a way to treat cancer more efficiently and improve the therapy of cancer patients. Recently we reported the synthesis of new inhibitors based on a quinazoline scaffold. In the present study more structural variations were explored. Compounds with 3,4-dimethoxy groups and meta or para nitro substituents were found to be highly potent inhibitors of ABCG2. The most potent compound was more than five-fold more potent than Ko143, one of the best inhibitors of ABCG2. To determine the new compounds selectivity toward ABCG2 their inhibitory effects on ABCB1 and ABCC1 were also investigated identifying selective as well as broadspectrum inhibitors. Furthermore, intrinsic cytotoxicity and efficacy regarding the reversal of multidrug resistance toward SN-38 and mitoxantrone were explored. The most potent compounds were able to reverse the resistance toward the cytostatic agents with EC₅₀ values below 20 nM. Additionally, the type of interaction between inhibitors and the ABCG2 substrate Hoechst 33342 was investigated yielding competitive and non-competitive interactions suggesting different modes of binding. Finally the effect of the derivatives on vanadate-sensitive ATPase activity of ABCG2 was determined. According to the different effects on ATPase activity we conclude the existence of different binding sites. This study provides the structural requirements for high potency inhibition and elucidates the interaction with ABCG2 setting the basis for further studies.

Research Progress in the Modification of Quercetin Leading to Anticancer Agents

The flavonoid quercetin (3,3′,4′,5,7-pentahydroxyflavone) is widely distributed in plants, foods, and beverages. This polyphenol compound exhibits varied biological actions such as antioxidant, radical-scavenging, anti-inflammatory, antibacterial, antiviral, gastroprotective, immune-modulator, and finds also application in the treatment of obesity, cardiovascular diseases and diabetes. Besides, quercetin can prevent neurological disorders and exerts protection against mitochondrial damages. Various in vitro studies have assessed the anticancer effects of quercetin, although there are no conclusive data regarding its mode of action. However, low bioavailability, poor aqueous solubility as well as rapid body clearance, fast metabolism and enzymatic degradation hamper the use of quercetin as therapeutic agent, so intense research efforts have been focused on the modification of the quercetin scaffold to obtain analogs with potentially improved properties for clinical applications. This review gives an overview of the developments in the synthesis and anticancer-related activities of quercetin derivatives reported from 2012 to 2016.

4-Anilino-2-pyridylquinazolines and -pyrimidines as Highly Potent and Nontoxic Inhibitors of Breast Cancer Resistance Protein (ABCG2)

Multidrug resistance (MDR) mediated by ATP-binding cassette (ABC) transport proteins remains a major problem in the chemotherapeutic treatment of cancer and might be overcome by inhibition of the transporter. Because of the lack of understanding, the complex mechanisms involved in the transport process, in particular for breast cancer resistance protein (BCRP/ABCG2), there is a persistent need for studies of inhibitors of ABCG2. In this study, we investigated a systematic series of 4-substituted-2-pyridylquinazolines in terms of their inhibitory potency as well as selectivity toward ABCG2. For comparison, the quinazoline scaffold was reduced to the significantly smaller 4-methylpyrimidine basic structure. Furthermore, the cytotoxicity and the ability to reverse MDR was tested with the chemotherapeutic agents SN-38 and mitoxantrone (MX). Interaction of the compounds with ABCG2 was investigated by a colorimetric ATPase assay. Enzyme kinetic studies were carried out with Hoechst 33342 as fluorescent dye and substrate of ABCG2 to elucidate the compounds binding modes.

Insect deterrent and nematicidal screening of microfungi from Mexico and anti-aphid compounds from Gliomastix masseei

Fungal metabolites are promising alternatives for the development of biorational pesticides. In this sense, microfungi from tropical regions are valuable sources of natural compounds for pest management. With the aim of broadening the search for new eco-friendly products to manage plant pests, this study was carried out to evaluate the biological activity of 23 tropical fungal extracts on three species of phytophagous insects and a plant parasitic nematode. In addition, the active principles of the most effective extract were identified. The insect deterrent activity of fungal extracts was evaluated on the settling of aphids Myzus persicae and Rhopalosiphum padi, and on the feeding of lepidoptera larva Spodoptera littoralis; the nematostatic activity was evaluated on the mobility of Meloidogyne javanica. Active metabolites from Gliomastix masseei were identified by GC-MS techniques and by comparison with commercial standards. Results showed seven extracts with strong effect on the settling of M. persicae and R. padi (settling inhibition >80%). The calculated median of effective concentration (EC₅₀) values ranged from 8 to 38μg/cm² for the extracts of Clonostachys rosea and G. masseei, respectively. Bioassay-guided separation of the ethyl acetate extract of G. masseei revealed the presence of fatty acids and their derivatives, where methyl 9-octadecenoate was the most active compound with EC₅₀ values of 16μg and 35μg/cm² for M. persicae and R. padi, respectively. Extracts of C. rosea and G. masseei could be a promising option in the control of pest aphids in agriculture.

ABCG2/BCRP: Specific and Nonspecific Modulators

Multidrug resistance (MDR) in cancer cells is the development of resistance to a variety of structurally and functionally nonrelated anticancer drugs. This phenomenon has become a major obstacle to cancer chemotherapy seriously affecting the clinical outcome. MDR is associated with increased drug efflux from cells mediated by an energy-dependent mechanism involving the ATP-binding cassette (ABC) transporters, mainly P-glycoprotein (ABCB1), the MDR-associated protein-1 (ABCC1), and the breast cancer resistance protein (ABCG2). The first two transporters have been widely studied already and reviews summarized the results. The ABCG2 protein has been a subject of intense study since its discovery as its overexpression has been detected in resistant cell lines in numerous types of human cancers. To date, a long list of modulators of ABCG2 exists and continues to increase. However, little is known about the clinical consequences of ABCG2 modulation. This makes the design of novel, potent, and nontoxic inhibitors of this efflux protein a major challenge to reverse MDR and thereby increase the success of chemotherapy. The aim of the present review is to describe and highlight specific and nonspecific modulators of ABCG2 reported to date based on the selectivity of the compounds, as many of them are effective against one or more ABC transport proteins.

Pyrrolopyrimidine derivatives and purine analogs as novel activators of Multidrug Resistance-associated Protein 1 (MRP1, ABCC1)

Multidrug resistance (MDR) is the main cause of diminished success in cancer chemotherapy. ABC transport proteins are considered to be one important factor of MDR. Besides P-glycoprotein (P-gp, ABCB1) and Breast Cancer Resistance Protein (BCRP, ABCG2), Multidrug Resistance-associated Protein 1 (MRP1, ABCC1) is associated with non-response to chemotherapy in different cancers. While considerable effort was spent in overcoming MDR during the last two decades, almost nothing is known with respect to activators of transport proteins. In this work we present certain pyrrolo[3,2-d]pyrimidine derivatives with variations at positions 4 and 5 and purine analogs with variations at position 6 as novel activators of MRP1-mediated transport of the MRP1 substrate calcein AM and the anticancer drug daunorubicin in low nanomolar concentration range. Two different MRP1 overexpressing cell lines were used, the doxorubicin-selected human lung cancer cell line H69 AR and the transfected Madin-Darby Canine Kidney cell line MDCK II MRP1. No effect was observed in the sensitive counterparts H69 and MDCK II wild type (wt). Derivatives with higher molecular weight possessed also inhibitory properties at low micromolar concentrations, although most compounds were rather poor MRP1 inhibitors. Purine analogs derived from potent MRP1 inhibitors of the pyrrolopyrimidine class showed equal activating, but no inhibiting effects at all. All tested compounds were non-toxic and had only minor impact on P-gp or BCRP, showing no inhibition or activation.

Acryloylphenylcarboxamides: A New Class of Breast Cancer Resistance Protein (ABCG2) Modulators

Chalcones are easily synthesized natural precursors of secondary plant metabolites, and their derivatives show various biological activities including inhibition of ABC transporters. Especially, their role as inhibitors of ABCG2, the most recently discovered ABC transporter involved in multidrug resistance, inspired the synthesis of new structurally diverse derivatives. Therefore, we combined the typical chalcone moiety with several acid chlorides by using an amide linker at position 2', 3', or 4' on ring A of the chalcone. The resulting 35 compounds covered a wide spectrum of substitution patterns, which allowed development of structure-activity relationships and to find the optimal structural features for further investigations. Synthesized acryloylphenylcarboxamides were investigated for their inhibitory activity against ABCG2 and their behavior toward ABCB1 and ABCC1. Furthermore, for the most promising compounds, their intrinsic cytotoxicity and their ability to reverse ABCG2-mediated multidrug resistance were determined.

Novel ABCG2 Antagonists Reverse Topotecan-Mediated Chemotherapeutic Resistance in Ovarian Carcinoma Xenografts

Chemotherapeutic resistance remains a challenge in the treatment of ovarian carcinoma, especially in recurrent disease. Despite the fact that most patients with newly diagnosed tumors attain complete remission following cytoreductive surgery and chemotherapy, ovarian carcinoma has a recurrence rate that exceeds 75%. The ATP-binding cassette family G member 2 (ABCG2) efflux protein has been described as one mechanism that confers multiple-drug resistance to solid tumors and contributes to topotecan resistance in ovarian carcinoma. In fact, one clinical trial demonstrated ABCG2 expression in all patients with primary or recurrent ovarian carcinoma. On the basis of our previous work, we hypothesized that three compounds (CID44640177, CID1434724, and CID46245505), which represent a new piperazine-substituted pyrazolo[1,5]pyrimidine substructure class of ABCG2-specific antagonists, would restore chemosensitivity to drug-resistant ovarian cancer in vitro and in vivo To address the treatment difficulties associated with chemotherapeutic resistance in ovarian cancer, we combined each compound (CID44640177, CID1434724, and CID46245505) with topotecan and administered the mixture to chemoresistant Igrov1/T8 ovarian cancer cells in vitro and Igrov1/T8 xenografts in CB-17 SCID mice. We found that only nanomolar concentrations of each ABCG2 inhibitor in combination with topotecan were required to restore chemosensitivity to Igrov1/T8 cells in vitro In vivo, substantial tumor reduction was achieved with each compound in 4 days, with CID1434724 causing the largest reduction in excess of 60%. No signs of secondary toxic effects were observed with the ABCG2 antagonists. These novel compounds should be viewed as promising drug candidates to reverse ABCG2-mediated chemoresistance. Mol Cancer Ther; 15(12); 2853-62. ©2016 AACR.

Profiling of Flavonol Derivatives for the Development of Antitrypanosomatidic Drugs

Flavonoids represent a potential source of new antitrypanosomatidic leads. Starting from a library of natural products, we combined target-based screening on pteridine reductase 1 with phenotypic screening on Trypanosoma brucei for hit identification. Flavonols were identified as hits, and a library of 16 derivatives was synthesized. Twelve compounds showed EC50 values against T. brucei below 10 μM. Four X-ray crystal structures and docking studies explained the observed structure-activity relationships. Compound 2 (3,6-dihydroxy-2-(3-hydroxyphenyl)-4H-chromen-4-one) was selected for pharmacokinetic studies. Encapsulation of compound 2 in PLGA nanoparticles or cyclodextrins resulted in lower in vitro toxicity when compared to the free compound. Combination studies with methotrexate revealed that compound 13 (3-hydroxy-6-methoxy-2-(4-methoxyphenyl)-4H-chromen-4-one) has the highest synergistic effect at concentration of 1.3 μM, 11.7-fold dose reduction index and no toxicity toward host cells. Our results provide the basis for further chemical modifications aimed at identifying novel antitrypanosomatidic agents showing higher potency toward PTR1 and increased metabolic stability.