Multi-drug-resistance-reverting agents: 2-aryloxazole and 2-arylthiazole derivatives as potent BCRP or MRP1 inhibitors

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.

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

Structural feature-driven pattern analysis for multitarget modulator landscapes

MOTIVATION

Multitargeting features of small molecules have been of increasing interest in recent years. Polypharmacological drugs that address several therapeutic targets may provide greater therapeutic benefits for patients. Furthermore, multitarget compounds can be used to address proteins of the same (or similar) protein families for their exploration as potential pharmacological targets. In addition, the knowledge of multitargeting features is of major importance in the drug selection process; particularly in ultra-large virtual screening procedures to gain high-quality compound collections. However, large-scale multitarget modulator landscapes are almost non-existent.

RESULTS

We implemented a specific feature-driven computer-aided pattern analysis (C@PA) to extract molecular-structural features of inhibitors of the model protein family of ATP-binding cassette (ABC) transporters. New molecular-structural features have been identified that successfully expanded the known multitarget modulator landscape of pan-ABC transporter inhibitors. The prediction capability was biologically confirmed by the successful discovery of pan-ABC transporter inhibitors with a distinct inhibitory activity profile.

AVAILABILITY AND IMPLEMENTATION

The multitarget dataset is available on the PANABC web page (http://www.panabc.info) and its use is free of charge.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

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.

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

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.

Chemical validation of a druggable site on Hsp27/HSPB1 using in silico solvent mapping and biophysical methods

Destabilizing mutations in small heat shock proteins (sHsps) are linked to multiple diseases; however, sHsps are conformationally dynamic, lack enzymatic function and have no endogenous chemical ligands. These factors render sHsps as classically "undruggable" targets and make it particularly challenging to identify molecules that might bind and stabilize them. To explore potential solutions, we designed a multi-pronged screening workflow involving a combination of computational and biophysical ligand-discovery platforms. Using the core domain of the sHsp family member Hsp27/HSPB1 (Hsp27c) as a target, we applied mixed solvent molecular dynamics (MixMD) to predict three possible binding sites, which we confirmed using NMR-based solvent mapping. Using this knowledge, we then used NMR spectroscopy to carry out a fragment-based drug discovery (FBDD) screen, ultimately identifying two fragments that bind to one of these sites. A medicinal chemistry effort improved the affinity of one fragment by ~50-fold (16 µM), while maintaining good ligand efficiency (~0.32 kcal/mol/non-hydrogen atom). Finally, we found that binding to this site partially restored the stability of disease-associated Hsp27 variants, in a redox-dependent manner. Together, these experiments suggest a new and unexpected binding site on Hsp27, which might be exploited to build chemical probes.

PB28, the Sigma-1 and Sigma-2 Receptors Modulator With Potent Anti-SARS-CoV-2 Activity: A Review About Its Pharmacological Properties and Structure Affinity Relationships

These unprecedented times have forced the scientific community to gather to face the COVID-19 pandemic. Efforts in diverse directions have been made. A multi-university team has focused on the identification of the host (human) proteins interacting with SARS-CoV-2 viral proteins, with the aim of hampering these interactions that may cause severe COVID-19 symptoms. Sigma-1 and sigma-2 receptors surprisingly belong to the “druggable” host proteins found, with the pan-sigma receptor modulator PB28 displaying the most potent anti–SARS-CoV-2 activity in in vitro assays. Being 20-fold more active than hydroxychloroquine, without cardiac side effects, PB28 is a promising antiviral candidate worthy of further investigation. Our research group developed PB28 in 1996 and have thoroughly characterized its biological properties since then. Structure–affinity relationship (SAfiR) studies at the sigma receptor subtypes were also undertaken with PB28 as the lead compound. We herein report our knowledge of PB28 to share information that may help to gain insight into the antiviral action of this compound and sigma receptors, while providing structural hints that may speed up the translation into therapeutics of this class of ligands.

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.

Paris saponin VII enhanced the sensitivity of HepG2/ADR cells to ADR via modulation of PI3K/AKT/MAPK signaling pathway

To find the effect of Paris saponin VII (PS VII)-mediated PI3K/AKT/MAPK signaling pathway on the sensitivity of ADR-resistant HepG2 cell (HepG2/ADR) cells to ADR. The proliferation inhibitory rates were detected by using MTT assay. Flow cytometry was employed to examine the intracellular accumulation of ADR. The expressions of drug-resistant genes (P-gp, MRP and BCRP) were detected by qRT-PCR, cell apoptosis by Annexin-V-FITC/PI staining, and the expressions of drug-resistance-related proteins, apoptosis-related proteins, and PI3K/AKT/MAPK pathway-related proteins were determined by Western blotting. HepG2/ADR and HepG2 cells treated with PS VII (0.88, 1.32, 1.98, and 2.97 μM) for 48 hours showed increased proliferation inhibitory rate in a dose-dependent manner. HepG2/ADR cells treated PS VII (0.88, 1.32, 1.98 μM) for 48 hours showed decreased IC50 of ADR. Compared with HepG2/ADR cells treated with ADR (5 nM), those treated with PS VII (≤1.98 μM) and ADR (5 nM) showed enhanced ADR accumulation, decreased drug-resistant gene expressions, increased cell apoptosis with unregulated Bax and cleaved caspase-3 and downregulated Bcl-2, as well as the inhibition of PI3K/AKT/MAPK pathway. Moreover, the combination of ADR (5 nM), PS VII (1.98 μM), and LY294002 (PI3K/AKT inhibitor, 20 μM)/SB203580 (P38 inhibitor, 20 μM) for 48 hours could further decreased the HepG2/ADR cell viability, but induced cell apoptosis, accompanying with the decreased expressions of drug-resistant genes. PS VII could downregulate the expressions of drug-resistance genes, increase intracellular accumulation of ADR, promote cell apoptosis, and enhance the sensitivity of HepG2/ADR cells to ADR via PI3K/AKT/MAPK.

Development of an Aryloxazole Derivative as a Brain-Permeable Anti-Glioblastoma Agent

Glioblastoma drug development has been difficult due to the extremely low blood brain barrier (BBB) penetration of conventional anti-cancer agents. P-glycoprotein, an efflux membrane transporter, is responsible for the poor brain uptake of small and hydrophobic drug substances. To develop brain-penetrable anti-tumor agents, we designed colchicine derivatives containing an aryloxazole moiety, which is known to inhibit P-glycoprotein. Among those tested, an aryloxazole derivative named KIST-G1 showed the strongest anti-glioblastoma cell proliferation activity (IC₅₀ = 3.2 ± 0.8 nM). Compared to colchicine, KIST-G1 showed dramatically increased BBB-permeable properties presenting 51.7 ± 0.5 (10^-6 cm/s) parallel artificial membrane permeability assay (PAMPA) permeability and 45.0 ± 6.0% of P-gp inhibition. Aid by the BBB-permeable properties, KIST-G1 (5 mg/kg) suppressed glioblastoma cell growth and migration almost completely in the brain of glioblastoma xenograft models by showing 98.2 ± 0.1% reduced tumor area compared with phosphate buffered saline (PBS)-injected control. In comparison, temozolomide, which is the most widely used drug for glioblastoma, showed only moderate effects. Our results demonstrate the effectiveness of an aryloxazole moiety in targeting brain tumors and suggest KIST-G1 as a potent anti-glioblastoma agent.

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.

Novel fluorescent probes for relay detection copper/citrate ion and application in cell imaging

Two novel fluorescent probes, 2‑(2'‑hydroxyphenyl)‑4‑(2'‑hydroxymethyl‑8‑quinolinamino)methyloxazole (L1), and 2‑(2'‑hydroxyphenyl)‑4‑(2'‑methyl‑8‑quinolinamino)methyloxazole (L2), exhibited colorimetric and "turn off" fluorometric response to Cu²⁺ ion in DMSO/H₂O solution (v/v = 1/1, 0.01 M, Tris-HCl buffer, pH 7.20) and the corresponding detection limit were found to be 2.14 × 10^-8 and 2.70 × 10^-8 M, which were much lower than drinking water permission concentrations by the United States Environmental Protection Agency (U.S. EPA) and World Health Organization (WHO). The L1-Cu²⁺ and L2-Cu²⁺ complexes ensemble detected citrate anions (CA) sequentially through fluorescence recovery response due to the extrusion of Cu²⁺ ion from the complexes. The binding processes were investigated by UV-vis, fluorescence, IR and DFT calculation. Furthermore, the vivo sensitivity experiments of Cu²⁺ ion and CA was demonstrated through fluorescence imaging in living cells.

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.

New tetrahydroisoquinoline-based P-glycoprotein modulators: decoration of the biphenyl core gives selective ligands

P-glycoprotein (P-gp, MDR1) is a membrane transporter expressed in several regions of our body. It plays a crucial defense role as it mediates the efflux of hundreds of potentially toxic substances. However, P-gp is one of the main causes of failure in cancer chemotherapy, as a number of chemotherapeutic agents are P-gp substrates. Another interesting implication concerns the correlation between P-gp expression impairment and the onset of several central nervous system pathologies such as Alzheimer's and Parkinson's diseases. In view of these considerations, in the present study, a new series of P-gp modulators have been designed, synthesized and evaluated for their activity towards P-gp and two other sister proteins (BCRP and MRP1). The compounds, structurally correlated to the potent but non-selective P-gp inhibitor MC70 [4'-(6,7-dimethoxy-3,4-dihydro-1H-isoquinolin-2-ylmethyl)biphenyl-4-ol], proved fairly selective towards P-gp, with a potency in the micromolar range. Compounds 5a, 5d and 12d proved capable of restoring doxorubicin toxicity in resistant cancer cells.

An innovative small molecule for promoting neuroreparative strategies

In this study, a new regenerative strategy to treat several neurodegenerative diseases is suggested by the use of a multitarget approach induced by our small molecule, MC111. Considering the importance of P-gp and BCRP expression on stem cell differentiation and the involvement of TLR4 on neurodegeneration processes, we investigated the effect of MC111, belonging to our library of P-gp active compounds on: (i) TLR4 signaling; (ii) P-gp and BCRP activity and expression; (iii) neurite sprouting. The observed findings exerted by MC111, open a new scenario for a multitarget and regenerative approach in neurodegenerative diseases encouraging the in vivo evaluation of MC111 as new tool in neuroreparative medicine.

In this study, a new regenerative strategy to treat several neurodegenerative diseases is suggested by the use of a multitarget approach induced by our small molecule, MC111.

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.

The Structure-Antimicrobial Activity Relationships of a Promising Class of the Compounds Containing the N-Arylpiperazine Scaffold

This research was focused on in silico characterization and in vitro biological testing of the series of the compounds carrying a N-arylpiperazine moiety. The in silico investigation was based on the prediction of electronic, steric and lipohydrophilic features. The molecules were screened against Mycobacterium avium subsp. paratuberculosis CIT03, M. smegmatis ATCC 700084, M. kansasii DSM 44162, M. marinum CAMP 5644, Staphylococcus aureus ATCC 29213, methicillin-resistant S. aureus 63718, Escherichia coli ATCC 25922, Enterococcus faecalis ATCC 29212, Candida albicans CCM 8261, C. parapsilosis CCM 8260 and C. krusei CCM 8271, respectively, by standardized microdilution methods. The eventual antiproliferative (cytotoxic) impact of those compounds was examined on a human monocytic leukemia THP-1 cell line, as a part of the biological study. Promising potential against M. kansasii was found for 1-[3-(3-ethoxyphenylcarbamoyl)oxy-2-hydroxypropyl]-4-(3-trifluoromethylphenyl)piperazin-1-ium chloride (MIC = 31.75 μM), which was comparable to the activity of isoniazid (INH; MIC = 29.17 μM). Moreover, 1-{2-hydroxy-3-(3-methoxyphenylcarbamoyl)oxy)propyl}-4-(4-fluorophenyl)piperazin-1-ium chloride was even more effective (MIC = 17.62 μM) against given mycobacterium. Among the tested N-arylpiperazines, 1-{2-hydroxy-3-(4-methoxyphenylcarbamoyl)oxy)propyl}-4-(3-trifluoromethylphenyl)piperazin-1-ium chloride was the most efficient against M. marinum (MIC = 65.32 μM). One of the common features of all investigated substances was their insignificant antiproliferative (i.e., non-cytotoxic) effect. The study discussed structure–antimicrobial activity relationships considering electronic, steric and lipophilic properties.

Synthesis and Preclinical Evaluation of Three Novel Fluorine-18 Labeled Radiopharmaceuticals for P-Glycoprotein PET Imaging at the Blood-Brain Barrier

P-Glycoprotein (P-gp), along with other transporter proteins at the blood-brain barrier (BBB), limits the entry of many pharmaceuticals into the brain. Altered P-gp function has been found in several neurological diseases. To study the P-gp function, many positron emission tomography (PET) radiopharmaceuticals have been developed. Most P-gp radiopharmaceuticals are labeled with carbon-11, while labeling with fluorine-18 would increase their applicability due to longer half-life. Here we present the synthesis and in vivo evaluation of three novel fluorine-18 labeled radiopharmaceuticals: 4-((6,7-dimethoxy-3,4-dihydroisoquinolin-2(1H)-yl)methyl)-2-(4-fluorophenyl)oxazole (1a), 2-biphenyl-4-yl-2-fluoroethoxy-6,7-dimethoxy-1,2,3,4-tetrahydro-isoquinoline (2), and 5-(1-(2-fluoroethoxy))-[3-(6,7-dimethoxy-3,4-dihydro-1H-isoquinolin-2-yl)-propyl]-5,6,7,8-tetrahydronaphthalen (3). Compounds were characterized as P-gp substrates in vitro, and Mdr1a/b((-/-))Bcrp1((-/-)) and wild-type mice were used to assess the substrate potential in vivo. Comparison was made to (R)-[(11)C]verapamil, which is currently the most frequently used P-gp substrate. Compound [(18)F]3 was performing the best out of the new radiopharmaceuticals; it had 2-fold higher brain uptake in the Mdr1a/b((-/-))Bcrp1((-/-)) mice compared to wild-type and was metabolically quite stable. In the plasma, 69% of the parent compound was intact after 45 min and 96% in the brain. Selectivity of [(18)F]3 to P-gp was tested by comparing the uptake in Mdr1a/b((-/-)) mice to uptake in Mdr1a/b((-/-))Bcrp1((-/-)) mice, which was statistically not significantly different. Hence, [(18)F]3 was found to be selective for P-gp and is a promising new radiopharmaceutical for P-gp PET imaging at the BBB.

SAR studies on tetrahydroisoquinoline derivatives: the role of flexibility and bioisosterism to raise potency and selectivity toward P-glycoprotein

The development of P-glycoprotein (P-gp) ligands remains of considerable interest, mostly for investigating the protein's structure and transport mechanism. In recent years, many different generations of ligands have been tested for their ability to modulate P-gp activity. The aim of the present work is to perform SAR studies on tetrahydroisoquinoline derivatives in order to design potent and selective P-gp ligands. For this purpose, the effect of bioisosteric replacement and the role of flexibility have been investigated, and four series of tetrahydroisoquinoline ligands have been developed: (a) 2-aryloxazole bioisosteres, (b) elongated analogues, (c) 2H-chromene, and (d) 2-biphenyl derivatives. The results showed that both 2-biphenyl derivative 20b and elongated derivative 6g behaved as strong P-gp substrates. In conclusion, important aspects for developing potent and selective P-gp ligands have been highlighted, providing a solid starting point for further optimization.

Deconstruction of 6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline moiety to separate P-glycoprotein (P-gp) activity from σ2 receptor affinity in mixed P-gp/σ2 receptor agents

6,7-Dimethoxytetrahydroisoquinoline is widely used as basic moiety in σ2 receptor ligands, in order to provide σ2versus σ1 selectivity. This same moiety is also widely exploited in modulators of P-glycoprotein (P-gp) efflux pump, so that mixed σ2/P-gp agents are often obtained. Deconstruction of 6,7-dimethoxytetrahydroisoquinoline moiety present in the potent mixed σ2/P-gp agent 6,7-dimethoxy-2-[4-[1-(4-fluorophenyl)-1H-indol-3-yl]butyl]-1,2,3,4-tetrahydroisoquinoline (1) could lead to the separation of σ2 affinity from P-gp activity. Therefore, phenethylamino-, benzylamino- and indanamine series were obtained. The NH group was also methylated in the N-phenethylamino series, and ethylated in the benzylamino series, to better match 6,7-dimethoxytetrahydroisoquinoline. The σ2 affinity drastically decreased with the increase of conformational freedom, whereas alkylation of the NH-group was beneficial for σ2 receptor interaction. By contrast, deconstruction of 6,7-dimethoxytetrahydroisoquinoline slightly reduced P-gp activity, with dimethoxy-substituted derivatives displaying potent P-gp interaction. Therefore, 'ring-opened' 6,7-dimethoxytetrahydroisoquinoline derivatives represent a promising strategy to obtain P-gp selective agents devoid of σ2 receptor affinity.

Crystallographic study of PET radio-tracers in clinical evaluation for early diagnosis of Alzheimers

The title compound, C₂₄H₂₅NO₃·2CH₃OH, which crystallized as a methanol disolvate, has applications as a PET radiotracer in the early diagnosis of Alzheimer’s disease. The dihedral angle between the biphenyl rings is 8.2 (2)° and the heterocyclic ring adopts a half-chair conformation with the N atom adopting a pyramidal geometry (bond-angle sum = 327.6°). The C atoms of both meth­oxy groups lie close to the plane of their attached ring [deviations = 0.107 (6) and 0.031 (6) Å]. In the crystal, the components are linked by O—H⋯O and O—H⋯N hydrogen bonds, generating [010] chains. C—H⋯O inter­actions are also observed.

Thieno[2,3-b]pyridines--a new class of multidrug resistance (MDR) modulators

To identify new potent multidrug resistance modulators, we have synthesized a series of novel thieno[2,3-b]pyridines and furo[2,3-b]pyridines, and examined their structure-activity relationships. All synthesized compounds were tested to determine BCRP1, P-gp, and MRP1 inhibitor activity, and most potent MDR modulators were also screened for their toxicity, cytotoxicity and Ca(2+) channel antagonist activity. Among these compounds, thieno[2,3-b]pyridine (6r) was found to exhibit a potent P-gp inhibitory action with EC50 = 0.3 ± 0.2 μM, MRP1 inhibitory action with EC50 = 1.1 ± 0.1 μM and BCRP1 inhibitory action with EC50 = 0.2 ± 0.05 μM and may represent suitable candidate for further pharmacological studies.

ABC transporters in CSCs membranes as a novel target for treating tumor relapse

CSCs are responsible for the high rate of recurrence and chemoresistance of different types of cancer. The current antineoplastic agents able to inhibit bulk replicating cancer cells and radiation treatment are not efficacious toward CSCs since this subpopulation has several intrinsic mechanisms of resistance. Among these mechanisms, the expression of ATP-Binding Cassette (ABC) transporters family and the activation of different signaling pathways (such as Wnt/β-catenin signaling, Hedgehog, Notch, Akt/PKB) are reported. Therefore, considering ABC transporters expression on CSCs membranes, compounds able to modulate MDR could induce cytotoxicity in these cells disclosing an exciting and alternative strategy for targeting CSCs in tumor therapy. The next challenge in the cure of cancer relapse may be a multimodal strategy, an approach where specific CSCs targeting drugs exert simultaneously the ability to circumvent tumor drug resistance (ABC transporters modulation) and cytotoxic activity toward CSCs and the corresponding differentiated tumor cells. The efficacy of suggested multimodal strategy could be probed by using several scaffolds active toward MDR pumps on CSCs isolated by tumor specimens.

Small and Innovative Molecules as New Strategy to Revert MDR

Multidrug resistance (MDR) is a complex phenomenon principally due to the overexpression of some transmembrane proteins belonging to the ATP binding cassette (ABC) transporter family. Among these transporters, P-glycoprotein (P-gp) is mostly involved in MDR and its overexpression is the major cause of cancer therapy failure. The classical approach used to overcome MDR is the co-administration of a P-gp inhibitor and the classic antineoplastic drugs, although the results were often unsatisfactory. Different classes of P-gp ligands have been developed and, among them, Tariquidar has been extensively studied both in vitro and in vivo. Although Tariquidar has been considered for several years as the lead compound for the development of P-gp inhibitors, recent studies demonstrated it to be a substrate and inhibitor, in a dose-dependent manner. Moreover, Tariquidar structure-activity relationship studies were difficult to carry out because of the complexity of the structure that does not allow establishing the role of each moiety for P-gp activity. For this purpose, SMALL molecules bearing different scaffolds such as tetralin, biphenyl, arylthiazole, furoxane, furazan have been developed. Many of these ligands have been tested both in in vitro assays and in in vivo PET studies. These preliminary evaluations lead to obtain a library of P-gp interacting agents useful to conjugate chemotherapeutic agents displaying reduced pharmacological activity and appropriate small molecules. These molecules could get over the limits due to the antineoplastic-P-gp inhibitor co-administration since pharmacokinetic and pharmacodynamic profiles are related to a dual innovative drug.

Sigma-2 receptor agonists as possible antitumor agents in resistant tumors: hints for collateral sensitivity

With the aim of contributing to the development of novel antitumor agents, high-affinity σ2 receptor agonists were developed, with 6,7-dimethoxy-2-[4-[1-(4-fluorophenyl)-1H-indol-3-yl]butyl]-1,2,3,4-tetrahydroisoquinoline (15) and 9-[4-(6,7-dimethoxy-1,2,3,4-tetrahydroisoquinolin-2-yl)butyl]-9H-carbazole (25) showing exceptional selectivity for the σ2 subtype. Most of the compounds displayed notable antiproliferative activity in human MCF7 breast adenocarcinoma cells, with similar activity in the corresponding doxorubicin-resistant MCF7adr cell line. Surprisingly, a few compounds, including 25, displayed enhanced activity in MCF7adr cells over parent cells, recalling the phenomenon of collateral sensitivity, which is under study for the treatment of drug-resistant tumors. All of the compounds showed interaction with P-glycoprotein (P-gp), and 15 and 25, with the greatest activity, were able to revert P-gp-mediated resistance and reestablish the antitumor effect of doxorubicin in MCF7adr cells. We therefore identified a series of σ2 receptor agonists endowed with intriguing antitumor properties; these compounds deserve further investigation for the development of alternate strategies against multidrug- resistant cancers.

(99m)Tc(N)-DBODC(5), a potential radiolabeled probe for SPECT of multidrug resistance: in vitro study

[(99m)Tc(N)(DBODC)(PNP5)](+) [DBODC is bis(N-ethoxyethyl)dithiocarbamato; PNP5 is bis(dimethoxypropylphosphinoethyl)ethoxyethylamine], abbreviated as (99m)Tc(N)-DBODC(5), is a lipophilic cationic mixed compound investigated as a myocardial imaging agent. The findings that this tracer accumulates in mitochondrial structures through a mechanism mediated by the negative mitochondrial membrane potential and that the rapid efflux of (99m)Tc(N)-DBODC(5) from nontarget tissues seems to be associated with the multidrug resistance (MDR) P-glycoprotein (P-gp) transport function open up the possibility to extend its clinical applications to tumor imaging and noninvasive MDR studies. The rate of uptake at 4 and 37 °C of (99m)Tc(N)-DBODC(5) was evaluated in vitro in selected human cancer cell lines and in the corresponding sublines before and after P-gp and/or MDR-associated protein (MRP) modulator/inhibitor treatment using (99m)Tc-sestamibi as a reference. The results indicated that (1) the uptake of both (99m)Tc(N)-DBODC(5) and (99m)Tc-sestamibi is correlated to metabolic activity of the cells and (2) the cellular accumulation is connected to the level of P-gp/MRP expression; in fact, an enhancement of uptake in resistant cells was observed after treatment with opportune MDR inhibitor/modulator, indicating that the selective blockade of P-gp/MRP prevented efflux of the tracers. This study provides a preliminary indication of the applicability of (99m)Tc(N)-DBODC(5) in tumor imaging and in detecting P-gp/MRP-mediated drug resistance in human cancer. In addition, the possibility to control the hydrophobicity and pharmacological activity of this heterocomplex through the variation of the substituents on the ligands backbone without affecting the P2S2 coordinating sphere makes (99m)Tc(N)-DBODC(5) a suitable scaffold for the preparation of a molecular probe for single photon emission computed tomography of MDR.

Potent and selective tariquidar bioisosters as potential PET radiotracers for imaging P-gp

Compounds 8a-d have been designed as bioisosters of tariquidar for imaging P-gp expression and density by PET. The results displayed that compounds 8b and 8d could be considered potential P-gp/BCRP ligands suitable as (11)C and (18)F radiotracers, respectively.

Naphthalenyl derivatives for hitting P-gp/MRP1/BCRP transporters

Substituted naphthalenyl derivatives bearing oxazole, or thiazole or furyl heteronuclei have been carried out as bioisosters of aryl-oxazoles and -thiazoles derivatives previously reported in order to investigate the role of the hindrance on the activity towards P-gp/BCRP/and MRP1 transporters. In addition, the role of naphthalenyl group to modulate P-gp intrinsic activity of these compounds was ascertained. The results demonstrated that all naphthalenyl derivatives displayed comparable P-gp activity with respect to lead compounds previously characterized in our SAR studies but were less active towards BCRP and MRP1 pumps. In terms of intrinsic activity, the replacement of aryl with naphthalenyl moiety led to P-gp inhibitors, unambiguous or ambiguous substrates on the base of the heteronucleus and the substituent on the naphthalenyl fragment. Indeed, oxazole derivatives were: inhibitors (R=H, F, OH), unambiguous substrates (R=OCH(3)), or ambiguous substrate (R=Br); thiazole derivatives were: unambiguous substrates (R=OCH(3), Br), or ambiguous substrates (R=H, F). Finally furyl derivatives were ambiguous substrates.

Potent galloyl-based selective modulators targeting multidrug resistance associated protein 1 and P-glycoprotein

The multifactorial nature of chemotherapy failure in controlling cancer is often associated with the occurrence of multidrug resistance (MDR), a phenomenon likely related to the increased expression of members of the ATP binding cassette (ABC) transporter superfamily. In this respect, the most extensively characterized MDR transporters include ABCB1 (also known as MDR1 or P-glycoprotein) and ABCC1 (also known as MRP1) whose inhibition remains a priority to circumvent drug resistance. Herein, we report how the simple galloyl benzamide scaffold can be easily and properly decorated for the preparation of either MRP1 or P-gp highly selective inhibitors. In particular, some gallamides and pyrogallol-1-monomethyl ethers showed remarkable affinity and selectivity toward MRP1. On the other hand, trimethyl ether galloyl anilides, with few exceptions, exhibited moderate to very high and selective P-gp inhibition.

1-Cyclohexyl-4-(4-arylcyclohexyl)piperazines: Mixed σ and human Δ(8)-Δ(7) sterol isomerase ligands with antiproliferative and P-glycoprotein inhibitory activity

Many new chemotherapeutic agents are under preclinical investigation and, despite efforts to more selectively target cancer cells, limitations such as toxicity and inherent resistance are often encountered. Therefore, alternative strategies are needed to treat cancer and overcome such limitations. We describe novel cyclohexylpiperazine derivatives, designed as mixed affinity ligands for sigma (σ) receptors and human Δ₈-Δ₇ sterol isomerase (HSI) ligands, which also exhibit P-glycoprotein (P-gp) inhibitory activity, with the aim of exploiting the antiproliferative effects mediated by σ and HSI sites while overcoming P-gp-mediated resistance. All of the compounds displayed high affinities for σ receptors and HSI sites, P-gp inhibitory activity, and σ₂ receptor agonist antiproliferative activity. Antiproliferative activity was also tested in PC-3 cells to establish σ₁ and HSI contribution. Compound cis-11, which displayed the best antiproliferative and P-gp inhibitory activities, was co-administered with 0.1 μM doxorubicin in MDCK-MDR1 cells. Compound cis-11 caused 70 % and 90 % cell death when co-administered at 30 μM and 50 μm, respectively. When administered alone, cis-11 resulted in 50 % cell death, demonstrating its single agent antitumor properties in a tumor cell line overexpressing P-gp.

Lentinan enhances cisplatin-mediated inhibition of cell proliferation in human gastric cancer cell line SGC-7901

AIM: To investigate whether lentinan enhances cisplatin-mediated inhibition of cell proliferation in human gastric cancer cell line SGC-7901 and to explore its effect on the expression of multidrug resistance genes.

METHODS: SGC-7901 cells were divided into four groups: untreated cells (control group), those treated with lentinan alone (lentinan group), those treated with cisplatin alone (cisplatin group), and those treated with both lentinan and cisplatin (lentinan + cisplatin group). RT-PCR was applied to detect the mRNA expression of MDR1, MRP1, and LRP in SGC-7901 cells. The proliferation of SGC-7901 cells was detected using the Cell Counting Kit-8.

RESULTS: High expression of multidrug resistance genes MDR1, MRP1 and LRP was detected in untreated SGC-7901 cells. Treatment with lentinan significantly decreased the mRNA expression of multidrug resistance genes but had no effect on cell proliferation (P > 0.05). Cisplatin treatment lessened cell proliferation and promoted the expression of multidrug resistance genes. Treatment with lentinan + cisplatin completely suppressed the mRNA expression of MDR1 and MRP1 and significantly decreased LRP expression and cell proliferation compared with the control group, lentinan group, and cisplatin group (10 d: 0.54 vs 1.90, 1.88, 0.92, all P < 0.05).

CONCLUSION: Lentinan combined with cisplatin can significantly inhibit the expression of multidrug resistance genes and strongly enhance cisplatin-mediated inhibition of the proliferation of SGC-7901 cells.

Analogues of σ receptor ligand 1-cyclohexyl-4-[3-(5-methoxy-1,2,3,4-tetrahydronaphthalen-1-yl)propyl]piperazine (PB28) with added polar functionality and reduced lipophilicity for potential use as positron emission tomography radiotracers

1-Cyclohexyl-4-[3-(5-methoxy-1,2,3,4-tetrahydronaphthalen-1-yl)propyl]piperazine 1 (PB28) represents an excellent lead candidate for therapeutic and/or diagnostic applications in oncology. However, because its utility is limited by its relatively high degree of lipophilicity, novel analogues of 1 with reduced lipophilic character were designed by substituting methylene groups with more polar functional groups in the propylene linker and at the tetralin C4 position. For the chiral analogues, separate enantiomers exhibited substantial and roughly equal affinities within a given receptor subtype, with the greatest difference observed for compound 9 at σ(1) (7.5-fold; (-)-(S)-9 K(i) = 94.6 nM, (+)-(R)-9 K(i) = 12.6 nM). Compound (-)-(S)-9 was also found to be the most σ(2)-selective agent (σ(2) K(i) = 5.92 nM), to possess a lipophilicity consistent with entry into tumor cells (log D(7.4) = 2.38), and to show minimal antiproliferative activity. However, (-)-(S)-9 exhibited moderate activity (EC(50) = 8.1 μM) at the P-gp efflux pump.

Phenylsulfonylfuroxans as modulators of multidrug-resistance-associated protein-1 and P-glycoprotein

A series of furoxan derivatives were studied for their ability to interact with P-gp and MRP1 transporters in MDCK cells overexpressing these proteins. 3-Phenylsulfonyl substituted furoxans emerged as the most interesting compounds. All of them were capable of inhibiting P-gp, and a few also were capable of inhibiting MRP1. Substituents at the 4-position of 3-phenylsulfonylfuroxan scaffold were able to modulate the selectivity and the intensity of inhibition. In some cases, they reverted MRP1 inhibitor activity, namely, they were capable of potentiating MRP1 dependent efflux. When compounds 16 and 17 were coadministered with doxorubicin, they restored a high degree of the activity of the antibiotic. Preliminary immunoblotting studies carried out on these two compounds indicate that they are capable of nitrating P-gp, which in this form is likely unable to efflux the antibiotic.