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A medicinal chemistry approach combining in silico and in vitro methodologies was performed aiming at identifying and characterizing putative allosteric drug-binding sites (aDBSs) at the interface of the transmembrane- and nucleotide-binding domains (TMD-NBD) of P-glycoprotein. Two aDBSs were identified, one in TMD1/NBD1 and another one in TMD2/NBD2, by means of in silico fragment-based molecular dynamics and characterized in terms of size, polarity, and lining residues. From a small library of thioxanthone and flavanone derivatives, experimentally described to bind at the TMD-NBD interfaces, several compounds were identified to be able to decrease the verapamil-stimulated ATPase activity. An IC50 of 81 ± 6.6 μM is reported for a flavanone derivative in the ATPase assays, providing evidence for an allosteric efflux modulation in P-glycoprotein. Molecular docking and molecular dynamics gave additional insights on the binding mode on how flavanone derivatives may act as allosteric inhibitors.

The modulation of drug efflux by P-glycoprotein (P-gp, ABCB1) represents one of the most promising approaches to overcome multidrug resistance (MDR) in cancer cells, however the mechanisms of drug specificity and signal-transmission are still poorly understood, hampering the development of more selective and efficient P-gp modulators. In this study, the impact of four P-gp mutations (G185V, G830V, F978A and ΔF335) on drug-binding and efflux-related signal-transmission mechanism was comprehensively evaluated in the presence of ligands within the drug-binding pocket (DBP), which are experimentally related with changes in their drug efflux profiles. The severe repacking of the transmembrane helices (TMH), induced by mutations and exacerbated by the presence of ligands, indicates that P-gp is sensitive to perturbations in the transmembrane region. Alterations on drug-binding were also observed as a consequence of the TMH repacking, but were not always correlated with alterations on ligands binding mode and/or binding affinity. Finally, and although all P-gp variants holo systems showed considerable changes in the intracellular coupling helices/nucleotide-binding domain (ICH-NBD) interactions, they seem to be primarily induced by the mutation itself rather than by the presence of ligands within the DBP. The data further suggest that the changes in drug efflux experimentally reported are mostly related with changes on drug specificity rather than effects on signal-transmission mechanism. We also hypothesize that an increase in the drug-binding affinity may also be related with the decreased drug efflux, while minor changes in binding affinities are possibly related with the increased drug efflux observed in transfected cells.

A medicinal chemistry approach combining in silico and in vitro methodologies was performed aiming at identifying and characterizing putative allosteric drug-binding sites (aDBS) at the interface of the transmembrane- and nucleotide-binding domains (TMD-NBD) of P-glycoprotein. Two aDBSs were identified, one in each NBD, by means of in silico fragment-based molecular dynamics, and characterized in terms of size, polarity, and lining residues. From a small library of thioxanthone and flavanone derivatives, experimentally described to bind at the TMD-NBD interfaces, several compounds were identified to be able to decrease the verapamil-stimulated ATPase activity. An IC50 of 81 ± 6.6 μM is reported for a flavanone derivative in the ATPase assays, providing evidence for an allosteric efflux modulation in P-glycoprotein. Molecular docking and Molecular Dynamics gave additional insights on the binding mode on how flavanone derivatives may act as allosteric inhibitors.

P-glycoprotein (P-gp) is a key efflux pump involved in cellular multidrug resistance (MDR), lowering the concentration of many anticancer drugs in tumor cells by pumping them into the extracellular milieu. While previous studies identified 1,4-dihydropyridines (DHP) as putative P-gp allosteric inhibitors, none reported the effect of stereochemistry on the ability of DHPs to bind P-gp. In the present study both symmetric (

The development of multidrug resistance (MDR) is a major cause of failure in cancer chemotherapy. Several abietane diterpenes with antitumoral activities have been isolated from Plectranthus spp. such as 6,7-dehydroroyleanone (DHR, 1) and 7α-acetoxy-6β-hydroxyroyleanone (AHR, 2). Several royleanone derivatives were prepared through hemisynthesis from natural compounds 1 and 2 to achieve a small library of products with enhanced anti-P-glycoprotein activity. Nonetheless, some derivatives tend to be unstable. Therefore, to reason such lack of stability, the electron density based local reactivity descriptors condensed Fukui functions and dual descriptor were calculated for several derivatives of DHR. Additionally, molecular docking and molecular dynamics studies were performed on several other derivatives to clarify the molecular mechanisms by which they may exert their inhibitory effect in P-gp activity. The analysis on local reactivity descriptors was important to understand possible degradation pathways and to guide further synthetic approaches toward new royleanone derivatives. A molecular docking study suggested that the presence of aromatic moieties increases the binding affinity of royleanone derivatives toward P-gp. It further suggests that one royleanone benzoylated derivative may act as a noncompetitive efflux modulator when bound to the M-site. The future generation of novel royleanone derivatives will involve (i) a selective modification of position C-12 with chemical moieties smaller than unsubstituted benzoyl rings and (ii) the modification of the substitution pattern of the benzoyloxy moiety at position C-6. This document is the unedited Author’s version of a Submitted Work that was subsequently accepted for publication in ACS Medicinal Chemistry Letters, copyright © American Chemical Society after peer review. To access the final edited and published work see [https://pubs.acs.org/doi/10.1021/acsmedchemlett.9b00642]. Related to: [http://ibiss-r.rcub.bg.ac.rs/handle/123456789/3817].

Crossing the gram-negative bacterial membrane poses a major barrier to antibiotic development, as many small molecules that can biochemically inhibit key bacterial processes are rendered microbiologically ineffective by their poor cellular uptake. The outer membrane is the major permeability barrier for many drug-like molecules, and the chemical properties that enable efficient uptake into mammalian cells fail to predict bacterial uptake. We have developed a computational method for accurate prospective prediction of outer-membrane uptake of drug-like molecules, which we combine with a new medium-throughput experimental assay. Parallel molecular dynamics simulations are used to successfully and quantitatively predict experimental permeabilities. For most polar molecules we test, outer membrane permeability also correlates well with whole-cell uptake. The ability to accurately predict and measure outer-membrane uptake of a wide variety of small molecules will enable simpler determination of which molecular scaffolds and which derivatives are most promising prior to extensive chemical synthesis. It will also assist in formulating a more systematic understanding of the chemical determinants of outer-membrane permeability.

In silico simulations of biological systems are of the significant importance to obtain insights on specific processes that experimental protocols have difficulty to elucidate. More particularly, and to ensure that a given molecule is able to reach its cellular target, the development of computational methods able to quickly estimate the cellular permeabilities for small molecules can become an important tool in the early stages of drug development. Herein, a computational protocol for predicting permeability coefficients, concerning both membranes and proteins, is presented and discussed.

In silico simulations of biological systems are of the significant importance to obtain insights on specific processes that experimental protocols have difficulty to elucidate. More particularly, and to ensure that a given molecule is able to reach its cellular target, the development of computational methods able to quickly estimate the cellular permeabilities for small molecules can become an important tool in the early stages of drug development. Herein, a computational protocol for predicting permeability coefficients, concerning both membranes and proteins, is presented and discussed.

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The Gram-negative bacterial outer membrane poses a major obstacle to the development of much-needed antibiotics against drug-resistant infections. Its chemical composition and porin proteins differ from Gram-positive bacteria and mammalian cells, and heuristics developed for mammalian cell uptake apply poorly. Recently, machinelearning methods have predicted small-molecule uptake into Gram-negative bacteria, offering the possibility to rationally optimize this aspect of antibiotic lead development. Here, we report physics-based methods to prospectively predict Gram-negative bacterial uptake, select, and synthesize promising chemical derivatives targetingE. coliDNA gyrase B. Our methods do not require empirical parameterization and are readily adaptable to new chemical scaffolds. These physics-based predictions well capture experimentally measured uptake (r > 0.95) and are indeed predictive of antimicrobial activity (r > 0.92). These methods can be used prospectively in combination with target-binding simulations to optimize both bacterial uptake and target binding, overcoming important barriers to antibiotic lead generation before small-molecule synthesis.

Background: Multidrug resistance (MDR) has been regarded as one of the major hurdles for the successful outcome of cancer chemotherapy. The collateral sensitivity (CS) effect is one the most auspicious anti-MDR strategies. Epoxylathyrane derivatives 1-16 were obtained by derivatization of the macrocyclic diterpene epoxyboetirane A (17), a lathyrane-type macrocyclic diterpene isolated from Euphorbia boetica. Some of these compounds were found to strongly modulate P-glycoprotein (P-gp/ABCB1) efflux. Purpose: The main goal was to develop lathyrane-type macrocyclic diterpenes with improved MDR-modifying activity, by targeting more than one anti-MDR mechanism. Study design/methods: In this study, the potential CS effect of compounds 1-16 was evaluated against gastric (EPG85-257), pancreatic (EPP85-181), and colon (HT-29) human cancer cells and their drug-resistant counterparts, respectively selected against mitoxantrone (EPG85-257RNOV; EPP85-181RNOV; HT-RNOV) or daunorubicin (EPG85-257RDB; EPP85-181RDB; HT-RDB). The most promising compounds (8, 15, and 16) were investigated as apoptosis inducers, using the assays annexin V/PI and active caspase-3. Results: The compounds were more effective against the resistant gastric cell lines, being the CS effect more significant in EPG85-257RDB cells. Taking together the IC50 values and the CS effect, compounds 8, 15, and 16 exhibited the best results. Epoxyboetirane P (8), with the strongest MDR-selective antiproliferative activity against gastric carcinoma EPG85-257RDB cells (IC50 of 0.72 mu M), being 10-fold more active against this resistant subline than in sensitive gastric carcinoma cells. The CS effect elicited by compounds 15 and 16 appeared to be by inducing apoptosis via caspase-3 activation. Structure-activity relationships of the compounds were additionally obtained through regression models to clarify the structural determinants associated to the CS effect. Conclusions: This study reinforces the importance of lathyrane-type diterpenes as lead molecules for the research of MDR-modifying agents.

Cancer is among the leading causes of death worldwide. One of the most challenging obstacles in cancer treatment is multidrug resistance (MDR). Overexpression of P-glycoprotein (P-gp) is associated with MDR. The growing incidence of cancer and the development of MDR drive the search for novel and more effective anticancer drugs to overcome the MDR problem. Royleanones are natural bioactive compounds frequently found in Plectranthus spp. The cytotoxic diterpene 6,7-dehydroroyleanone (1) is the main component of the P. madagascariensis (Pers.) Benth. essential oil, while 7α-acetoxy-6β-hydroxyroyleanone (2) can be isolated from acetonic extracts of P. grandidentatus Gürke. The reactivity of the natural royleanones 1 and 2 was explored to obtain a small library of new P-gp inhibitors. Four new derivatives (6,7-dehydro-12-O-tert-butyl-carbonate-royleanone (20), 6,7-dehydro-12-O-methylroyleanone (21), 6,7-dehydro-12-O-benzoylroyleanone (22), and 7α-acetoxy-6β-hydroxy-12-O-benzoylroyleanone (23) were obtained as pure with overall modest to excellent yields (21–97%). P-gp inhibition potential of the derivatives 20–23 was evaluated in human non-small cell lung carcinoma NCI-H460 and its MDR counterpart NCI-H460/R with the P-gp overexpression, through MTT assay. Previously prepared diterpene 7α-acetoxy-6β-benzoyloxy-12-O-(4-chloro)benzoylroyleanone (4), has also been tested. The P-gp inhibiting effects of compounds 1–4 were also assessed through a Rhodamine 123 accumulation assay. Derivatives 4 and 23 have significant P-gp inhibitory potential. Regarding stability and P-gp inhibition potential, results suggest that the formation of benzoyl esters is a more convenient approach for future derivatives with enhanced effect on the cell viability decrease. Compound 4 presented higher anti-P-gp potential than the natural diterpenes 1, 2, and 3, with comparable inhibitory potential to Dexverapamil. Moreover, derivative 4 showed the ability to sensitize the resistant NCI-H460/R cells to doxorubicin.

Aim: Naringenin (1), isolated in large amount from the aerial parts of Euphorbia pedroi, was chemically derivatized to yield 18 imine derivatives (2–19) and three alkylated derivatives through a Mannich-type reaction (20–22) that were tested as multidrug resistance (MDR) reversers in cancer cells. Results/methodology: While hydrazone (2–4) and azine (5–13) derivatives showed an improvement in their MDR reversal activities against the breast cancer resistance protein, carbohydrazides 14–19 revealed an enhancement in MDR reversal activity toward the multidrug resistance protein 1. Conclusion: The observed activities, together with pharmacophoric analysis and molecular docking studies, identified the spatial orientation of the substituents as a key structural feature toward a possible mechanism by which naringenin derivatives may reverse MDR in cancer.

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

P-glycoprotein (P-gp, ABCB1) overexpression is, currently, one of the most important multidrug resistance (MDR) mechanisms in tumor cells. Thus, modulating drug efflux by P-gp has become one of the most promising approaches to overcome MDR in cancer. Yet, more insights on the molecular basis of drug specificity and efflux-related signal transmission mechanism between the transmembrane domains (TMDs) and the nucleotide binding domains (NBDs) are needed to develop molecules with higher selectivity and efficacy. Starting from a murine P-gp crystallographic structure at the inward-facing conformation (PDB ID: 4Q9H), we evaluated the structural quality of the herein generated human P-gp homology model. This initial human P-gp model, in the presence of the “linker” and inserted in a suitable lipid bilayer, was refined through molecular dynamics simulations and thoroughly validated. The best human P-gp model was further used to study the effect of four single-point mutations located at the TMDs, experimentally related with changes in substrate specificity and drug-stimulated ATPase activity. Remarkably, each P-gp mutation is able to induce transmembrane α-helices (TMHs) repacking, affecting the drug-binding pocket volume and the drug-binding sites properties (e.g. volume, shape and polarity) finally compromising drug binding at the substrate binding sites. Furthermore, intracellular coupling helices (ICH) also play an important role since changes in the TMHs rearrangement are shown to have an impact in residue interactions at the ICH-NBD interfaces, suggesting that identified TMHs repacking affect TMD-NBD contacts and interfere with signal transmission from the TMDs to the NBDs.

Background: From a dataset obtained by chemical derivatization of a macrocyclic diterpenic scaffold, in silico approaches identified which structural features correlate with experimental modulation of P-gp activity. Results/methodology: Ninety-two percent of the strongest MDR modulators were positively identified within the dataset by virtual screening. Quantitative structure–activity relationships models with high robustness and predictability were obtained for both MDR1-transfected L5178Y mouse lymphoma T-cells (q2 0.875, R2 pred 0.921) and human colon adenocarcinoma (q2 0.820, R2 pred 0.951) cell lines. A new pharmacophoric model suggests that charge distribution within the molecule is important for biological activity. Conclusion: For the studied diterpenes, the conformation of the macrocyclic scaffold and its substitution pattern are the main determinants for the biological activity, being related with steric and electrostatic factors.

Naringenin (1), isolated from Euphorbia pedroi, was previously derivatized yielding compounds 2-13. In this study, aiming at expanding the pool of analogues of the flavanone core towards better multidrug resistance (MDR) reversal agents, alkylation reactions and chemical modification of the carbonyl moiety was performed (15-39). Compounds structures were assigned mainly by 1D and 2D NMR experiments. Compounds 1-39 were assessed as MDR reversers, in human ABCB1-transfected mouse T-lymphoma cells, overexpressing P-glycoprotein (P-gp). The results revealed that O-methylation at C-7, together with the introduction of nitrogen atoms and aromatic moieties at C-4 or C-4', significantly improved the activity, being compounds 27 and 37 the strongest P-gp modulators and much more active than verapamil. In combination assays, synergistic interactions of selected compounds with doxorubicin substantiated the results. While molecular docking suggested that flavanone derivatives act as competitive modulators, molecular dynamics showed that dimethylation promotes binding to a modulator-binding site. Moreover, flavanones may also interact with a vicinal ATP-binding site in both nucleotide-binding domains, hypothesizing an allosteric mode of action.

Non-enveloped icosahedral double-stranded RNA (dsRNA) viruses possess multifunctional capsids required for their proliferation. Whereas protozoan/fungal dsRNA viruses have a relatively simple capsid structure, which suffices for the intracellular phase in their life cycle, metazoan dsRNA viruses have acquired additional structural features as an adaptation for extracellular cell-to-cell transmission in multicellular hosts. Here, we present the first atomic model of a metazoan dsRNA totivirus-like virus and the structure reveals three unique structural traits: a C-terminal interlocking arm, surface projecting loops, and an obstruction at the pore on the 5-fold symmetry axis. These traits are keys to understanding the capsid functions of metazoan dsRNA viruses, such as particle stability and formation, cell entry, and endogenous intraparticle transcription of mRNA. On the basis of molecular dynamics simulations of the obstructed pore, we propose a possible mechanism of intraparticle transcription in totivirus-like viruses, which dynamically switches between open and closed states of the pore(s).

The phytochemical study of Euphorbia pedroi led to the isolation of a new tetracyclic triterpenoid with an unusual spiro scaffold, spiropedroxodiol (1), along with seven known terpenoids (2-8). Aiming at obtaining compounds with improved multidrug-resistance (MDR) reversal activity, compound 8, an ent-abietane diterpene, was derivatized by introducing nitrogen-containing and aromatic moieties, yielding compounds 9-14. The structures of compounds were characterized by detailed spectroscopic analysis, including 2D NMR experiments (COSY, HMQC/HSQC, HMBC, and NOESY). Compounds 1-14 were evaluated for their MDR-reversing activity on human ABCB1 gene transfected mouse lymphoma cells (L5178Y-MDR) through a combination of functional and chemosensitivity assays. The natural compounds 1-8 were further evaluated on resistant human colon adenocarcinoma cells (Colo320), and, additionally, their cytotoxicity was assessed on noncancerous mouse (NIH/3T3) and human (MRC-5) embryonic fibroblast cell lines. While spiropedroxodiol (1) was found to be a very strong MDR reversal agent in both L5178Y-MDR and Colo320 cells, the chemical modifications of helioscopinolide E (8) at C-3 positively contributed to increase the MDR reversal activity of compounds 10, 12, and 13. Furthermore, in combination assays, compounds 1 and 7-14 enhanced synergistically the cytotoxicity of doxorubicin. Finally, by means of molecular docking, the key residues and binding modes by which compounds 1-14 may interact with a murine P-glycoprotein model were identified, allowing additional insights on the efflux modulation mechanism of these compounds.

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

Multidrug-resistance (MDR) phenomena are a worldwide health concern. ATP-binding cassette efflux pumps as P-glycoprotein have been thoroughly studied in a frantic run to develop new efflux modulators capable to reverse MDR phenotypes. The study of efflux pumps has provided some key aspects on drug extrusion, however the answers could not be found solely on ATP-binding cassette transporters. Its counterpart – the plasma membrane – is now emerging as a critical structure able to modify drug behavior and efflux pump activity. Alterations in the membrane surrounding P-glycoprotein are now known to modulate drug efflux, with membrane-related biophysical, biochemical and mechanical aspects further increasing the complexity of an already multifaceted phenomena. This review summarizes the main knowledge comprising the plasma membrane role in MDR.

P-glycoprotein (P-gp) has been considered an important molecular target in the reversal of multidrug resistance (MDR). As such, the development of P-gp modulators able to restore drug sensitivity in resistant cells is still considered one of the most promising strategies for overcoming MDR. Since the identification of the P-gp's role in MDR, several studies have been performed in order to develop effective P-gp modulators and understand the efflux mechanism. However, no efflux modulator is still clinically available for treating multidrug-resistant cancers. Nevertheless, recent experimental studies suggest that MDR can be surpassed by targeting a specific region within the ABC transporter structure rather than the polyspecific drug-binding pocket. This article will focus on the information available about this new target region and on a brief overview of which scaffolds would be suitable for modulating P-gp at this new location. WIREs Comput Mol Sci 2017, 7:e1316. doi: 10.1002/wcms.1316 For further resources related to this article, please visit the WIREs website.

Herein, collateral sensitivity effect was exploited as a strategy to select effective compounds to overcome multidrug resistance in cancer. Thus, eleven macrocyclic diterpenes, namely jolkinol D (1), isolated from Euphorbia piscatoria, and its derivatives (2-11) were evaluated for their activity on three different Human cancer entities: gastric (EPG85-257), pancreatic (EPP85-181) and colon (HT-29) each with a variant selected for resistance to mitoxantrone (EPG85-257RN; EPP85-181RN; HT-29RN) and one to daunorubicin (EPG85-257RD; EPP85-181RD; HT-29RD). Jolkinol D (1) and most of its derivatives (2-11) exhibited significant collateral sensitivity effect towards the cell lines EPG85-257RN (associated with P-glycoprotein overexpression) and HT-29RD (altered topoisomerase II expression). The benzoyl derivative, jolkinoate L (8) demonstrated ability to target different cellular contexts with concomitant high antiproliferative activity. These compounds were previously assessed as P-glycoprotein modulators, at non-cytotoxic doses, on MDR1-mouse lymphoma cells. A regression analysis between the antiproliferative activity presented herein and the previously assessed P-glycoprotein modulatory effect showed a strong relation between the compounds that presented both high P-glycoprotein modulation and cytotoxicity.

P-Glycoprotein (P-gp) is often involved in multidrug resistance (MDR) to the pharmacological action of a wide number of anticancer agents. In this article, a series of molecular dynamics simulations of murine's P-gp were developed, elucidating the importance of the lipid membrane and linker sequence in the protein structure stability. The behavior of several molecules inside the drug-binding pocket revealed a striking difference between substrates or modulators, and motion patterns were identified that could be correlated with conformational alterations due to substrate binding, corresponding to the initial step in the efflux mechanism. Only one "entrance gate" to the drug-binding pocket was found and, in the presence of a substrate, leads to changes in the motion patterns of the transporter into an efflux-like movement.

Multidrug resistance related to the increased expression of P-glycoprotein (P-gp) by cancer cells is the major contributor for the failure of chemotherapeutic treatments. Starting from pharmacophores and data already published and in macrocyclic diterpenes isolated from Euphorbia species, a comprehensive study of pharmacophore definitions of features was performed in order to obtain a new improved four-point pharmacophore able to detect literature and in-house modulators and simultaneously specific enough to avoid the detection of most nonactive molecules in a universe of 152 (literature), 74 (in-house), and 46 (inactive) molecules. This pharmacophore detects 84.2% of the molecules described in the literature, along with 100% detection of in-house isolated compounds and 19.5% of false positives. The importance of the hydrophobic and electron acceptor moieties as essential features for recognition of different molecules by the P-gp drug-binding site is clarified. The best combination of acceptor, donor, hydrophobic, and aromatic characteristics that contribute for the increased selectivity shown by the described pharmacophore is evaluated, and the protonation state of the molecules is also addressed.

The P-glycoprotein efflux mechanism is being studied since its identification as a leading protagonist in multidrug resistance. Recently, it was suggested that drugs enter the drug-binding pocket (DBP) through gates located between the transmembrane domains. For both a substrate and a modulator, the potential of mean force curves along the reaction coordinate obtained with the WHAM approach were similar, with no activation energy required for crossing the gate. Moreover, drug transit from bulk water into the DBP was characterized as an overall free-energy downhill process.

The membrane biophysical aspects by which multidrug resistance (MDR) relate to the ABC transporter function still remain largely unknown. Notwithstanding the central role that efflux pumps like P-glycoprotein have in MDR onset, experimental studies classified additionally the lipid micro-environment where P-gp is inserted as a determinant for the increased efflux capability demonstrated in MDR cell lines. Recently, a nonlinear model for drug–membrane interactions showed that, upon drug adsorption, long-range mechanical alterations are predicted to affect the P-gp ATPase function at external drug concentrations of ∼10–100 μM. However, our results also show that drug adsorption may also occur at P-gp nucleotide-binding domains where conformational changes drive the efflux cycle. Thus, we assessed the effect of drug adsorption to both protein–water and lipid–water interfaces by means of molecular dynamics simulations. The results show that free energies of adsorption are lower for modulators in both lipid/water and protein/water interfaces. Important differences in drug–protein interactions, protein dynamics and membrane biophysical characteristics were observed between the different classes. Therefore, we hypothesize that drug adsorption to the protein and lipid–water interface accounts for a complex network of events that affect the ability of transporters to efflux drugs.

The phytochemical study of Euphorbia piscatoria yielded jolkinol D (1) in a large amount, whose derivatization gave rise to 12 ester derivatives (2–13) and hydrolysis to compound 14. The in vitro modulation of P-gp of compounds 1–14 was evaluated through a combination of transport and chemosensitivity assays, using the L5178 mouse T lymphoma cell line transfected with the human MDR1 gene. Apart from jolkinol D, all derivatives (2–14) showed potential as MDR reversal agents. In this small library of novel bioactive macrocyclic lathyrane diterpene derivatives, designed to evaluate structure–activity relationships essential in overcoming multidrug resistance (MDR), some correlations between MDR reversal and molecular weight, accessible solvent areas, and octanol/water partition coefficient were identified that can contribute to the development of new selective P-gp reversal agents., This study was supported by FCT (Fundação para a Ciência e a ̂Tecnologia, Portugal) by Research Projects PTDC/QUI-QUI/099815/2008 and PEst-OE/SAU/UI4013/2011. M.R. acknowledges FCT for her Ph.D. Grant SFRH/BD/72915/2010. The authors thank Dr. Teresa Vasconcelos, Instituto Superior de Agronomia, Universidade de Lisboa, Portugal, for the taxonomic work on the plant material.

This paper focuses on the importance of the intermediate linker sequence for the stabilization of the cytoplasmic portion of murine P-glycoprotein, an ABC transporter involved in Multidrug Resistance (MDR) in cancer. Three putative protein-protein interaction areas were predicted to exist, two of them next to the C-terminal nucleotide-binding domain (NBD2) and the third one next to the inner leaflet interface of the lipid bilayer. These contact spots were confirmed by detailed contact maps from structures obtained before and after a 100 ns molecular dynamics production run, allowing a more thorough characterization of the type and number of residues involved in protein-protein contacts. It was found that these contact surfaces are located next to several highly conserved motifs of ABC transporters, serving as anchor points and assisting the linker's 'damper' function.

Radiotherapy (RT) is nowadays, after surgery, among the most commonly used treatment regarding cancer. For predicted clinical outcomes it is of importance assuring that the treatment plan is correctly delivered without dose deviations, namely target underdosage and/or organ-at-risk overdosage. For that, a new multi-pixel imaging technique for real-time dose verification during photon RT has been proposed (RTmon). First scanned, single-pixel experimental results are presented here. The principle of operation of RTmon relies on the detection of scattered photons emitted perpendicularly to the beam direction. The first experimental results were obtained with a collimated γ-ray detector operated under a 6-MV clinical linac beam. A large contaminating background originating from the head of the linac was found. We show that its mitigation is possible by (1) using appropriate shielding, (2) decreasing the scintillator volume pointing towards out-of-field, background-prone regions, and (3) optimizing the collimation by means of detector positioning and differential signal sampling. Simulation and experimental data showed that multiple scattering inside a phantom target constitutes an additional source of background that increases along the beam direction due to momentum conservation in collisions between the incoming photons and target electrons. This second source of background can be reduced effectively by optimizing the distance between the collimated detector and the beam axis, at the expense of decreased collimation sensitivity. Despite the harsh therapeutic irradiation conditions, background and noise suppression by appropriate shielding and differential signal sampling renders the scanned experimental results quite similar to those obtained by means of Geant4 simulations. Such experimental results suggest therefore that this detector system may be useful for real-time dose verification in external X-ray radiotherapy.

The failure of chemotherapeutics in cancer is mainly imputed to multidrug resistance (MDR). Among several proposed approaches to overcome MDR, the modulation of P-glycoprotein (Pgp) is the most highlighted [1]. Another valuable strategy is to take advantage of the collateral sensitivity effect, where some compounds may selectively exert a more pronounced cytotoxicity on cells with MDR phenotype than in the parental ones [1]. Macrocyclic diterpenes isolated from Euphorbia species, have been recognized as potent Pgp modulators and as having a collateral sensitivity effect on MDR cell variants [2]. Our main goal is to explore these two approaches, since tackling more than one MDR mechanism simultaneously might be a solution to control MDR in its complexity. The fractionation of the methanol extract of the aerial parts of E. piscatoria led to the isolation of a macrocyclic diterpene that allowed to build a small library of derivatives [3]. In this work, eleven compounds were evaluated for their potential collateral sensitivity effect on nine cell lines, gastric, pancreatic and colon cancer cell models (drug sensitive and drug resistant sublines), using the SRB assay. Most of the derivatives exhibited significant collateral sensitivity effect towards the MDR gastric cell line (with Pgp overexpression) and the MDR colon cells. The most significant results were obtained with a benzoyl derivative that demonstrated ability to target different cellular contexts with concomitant high antiproliferative activity. These compounds were previously assessed as Pgp modulators, at non-cytotoxic doses, on MDR1-mouse lymphoma cells [3]. A regression analysis between the antiproliferative activity presented herein and the previously assessed Pgp modulatory effect showed a strong relation between the compounds that presented both high Pgp modulation and cytotoxicity. Acknowledgements: FCT, Portugal (PTDC/QUI-QUI/099815/2008; PTDC/QEQ-MED/0905/2012; SFRH/BD/72915/2010). References: [1] Hall MD, Handley MD, Gottesman MM. Is resistance useless? Multidrug resistance and collateral sensitivity. Trends Pharmacol Sci 2009; 30: 546 – 556. [2] Ferreira M-JU, Duarte N, Reis M, Madureira AM, Molnar J. Euphorbia and Momordica metabolites for overcoming multidrug resistance. Phytochem Rev 2014. [3] Reis M, Ferreira RJ, Santos MMM, dos Santos DJV a, Molnar J, Ferreira M-JU. Enhancing macrocyclic diterpenes as multidrug-resistance reversers: structure-activity studies on jolkinol D derivatives. J Med Chem 2013; 56: 748 – 760.

Multidrug resistance (MDR) is a limiting step on the success of cancer chemotherapy. The drug efflux mediated by P-gp (Pglycoprotein) is one of the best studied mechanisms of MDR. This paper focuses on the inhibitory P-gp efflux activity, pharmacophore modeling and structure-activity relationships studies of sixteen macrocyclic diterpenes and polycyclic derivatives obtained from Euphorbia species. The MDR human colon adenocarcinoma cells (COLO 320 MDR) overexpressing P-gp were used as the biological model to screen for P-gp dependent efflux inhibitors. Most of the compounds showed potential as MDR reversal agents. Combined analysis of two different statistic algorithms, K-means clustering and Principal Component Analysis discriminated two clusters and showed a strong correlation between log P and MDR reversal activity for compounds 1-5. The most effective compounds (1-4 and 11-12) were tested in combination with doxorubicin and all potentiated its activity lowering the ID50. Pharmacophore modeling allowed the definition of an aromatic moiety as an additional feature to a previous published P-gp pharmacophore, creating a new five-point pharmacophore with enhanced selectivity for the most active compounds of the present study. Docking results also show the importance of an aromatic moiety, positively identifying the most relevant residues that can be linked to an inhibitory activity increase.