P-Glycoprotein (Pgp) is one of the best characterized ABC transporters, often involved in the multidrug-resistance phenotype overexpressed by several cancer cell lines. Experimental studies contributed to important knowledge concerning substrate polyspecificity, efflux mechanism, and drug-binding sites. This information is, however, scattered through different perspectives, not existing a unifying model for the knowledge available for this transporter. Using a previously refined structure of murine Pgp, three putative drug-binding sites were hereby characterized by means of molecular docking. The modulator site (M-site) is characterized by cross interactions between both Pgp halves herein defined for the first time, having an important role in impairing conformational changes leading to substrate efflux. Two other binding sites, located next to the inner leaflet of the lipid bilayer, were identified as the substrate-binding H and R sites by matching docking and experimental results. A new classification model with the ability to discriminate substrates from modulators is also proposed, integrating a vast number of theoretical and experimental data.

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.

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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.

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.

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.

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