Emília Sousa

Three Decades of P-gp Inhibitors: Skimming Through Several Generations and Scaffolds

Many tumor cells become resistant to commonly used cytotoxic drugs due to the overexpression of ATP-binding cassette (ABC) transporters, namely P-glycoprotein (P-gp). The discovery of the reversal of multidrug resistance (MDR) by verapamil occured in 1981, and in 1968 MDR Chinese hamster cell lines were isolated for the first time. Since then, P-gp inhibitors have been intensively studied as potential MDR reversers. Initially, drugs to reverse MDR were not specifically developed for inhibiting P-gp; in fact, they had other pharmacological properties, as well as a relatively low affinity for MDR transporters. An example of this first generation P-gp inhibitors is verapamil. The second generation included more specific with less side-effect inhibitors, such as dexverapamil or dexniguldipine. A third generation of P-gp inhibitors comprised compounds such as tariquidar, with high affinity to P-gp at nanomolar concentrations. These generations of inhibitors of P-gp have been examined in preclinical and clinical studies; however, these trials have largely failed to demonstrate an improvement in therapeutic efficacy. Therefore, new and innovative strategies, such as the fallback to natural products, the design of peptidomimetics and dual activity ligands emerged as a fourth generation of P-gp inhibitors. The chemistry of P-gp inhibitors, as well as their in vitro, in vivo and clinical trials are discussed, and the most recent advances concerning Pgp modulators are reviewed.

Dual inhibitors of P-glycoprotein and tumor cell growth: (Re)discovering thioxanthones

For many pathologies, there is a crescent effort to design multiple ligands that interact with a wide variety of targets. 1-Aminated thioxanthone derivatives were synthesized and assayed for their in vitro dual activity as antitumor agents and P-glycoprotein (P-gp) inhibitors. The approach was based on molecular hybridization of a thioxanthone scaffold, present in known antitumor drugs, and an amine, described as an important pharmacophoric feature for P-gp inhibition. A rational approach using homology modeling and docking was used, to select the molecules to be synthesized by conventional or microwave-assisted Ullmann C-N cross-coupling reaction. The obtained aminated thioxanthones were highly effective at inhibiting P-gp and/or causing growth inhibition in a chronic myelogenous leukemia cell line, K562. Six of the aminated thioxanthones had GI(50) values in the K562 cell line below 10 μM and 1-{[2-(diethylamino)ethyl]amino}-4-propoxy-9H-thioxanthen-9-one (37) had a GI(50) concentration (1.90 μM) 6-fold lower than doxorubicin (11.89 μM) in the K562Dox cell line. The best P-gp inhibitor found was 1-[2-(1H-benzimidazol-2-yl)ethanamine]-4-propoxy-9H-thioxanthen-9-one (45), which caused an accumulation rate of rhodamine-123 similar to that caused by verapamil in the K562Dox resistant cell line, and a decrease in ATP consumption by P-gp. At a concentration of 10 μM, compound 45 caused a decrease of 12.5-fold in the GI(50) value of doxorubicin in the K562Dox cell line, being 2-fold more potent than verapamil. From the overall results, the aminated thioxanthones represent a new class of P-gp inhibitors with improved efficacy in sensitizing a resistant P-gp overexpressing cell line (K562Dox) to doxorubicin.

New uses for old drugs: pharmacophore-based screening for the discovery of P-glycoprotein inhibitors.

P‐glycoprotein (P‐gp) is one of the best characterized transporters responsible for the multidrug resistance phenotype exhibited by cancer cells. Therefore, there is widespread interest in elucidating whether existing drugs are candidate P‐gp substrates or inhibitors. With this aim, a pharmacophore model was created based on known P‐gp inhibitors and it was used to screen a database of existing drugs. The P‐gp modulatory activity of the best hits was evaluated by several methods such as the rhodamine‐123 accumulation assay using K562Dox cell line, and a P‐gp ATPase activity assay. The ability of these compounds to enhance the cytotoxicity of doxorubicin was assessed with the sulphorhodamine‐B assay. Of the 21 hit compounds selected in silico, 12 were found to significantly increase the intracellular accumulation of Rhodamine‐123, a P‐gp substrate. In addition, amoxapine and loxapine, two tetracyclic antidepressant drugs, were discovered to be potent non‐competitive inhibitors of P‐gp, causing a 3.5‐fold decrease in the doxorubicin GI50 in K562Dox cell line. The overall results provide important clues for the non‐label use of known drugs as inhibitors of P‐gp. Potent inhibitors with a dibenzoxazepine scaffold emerged from this study and they will be further investigated in order to develop new P‐gp inhibitors.

Emerging Sulfated Flavonoids and other Polyphenols as Drugs: Nature as an Inspiration

Nature uses sulfation of endogenous and exogenous molecules mainly to avoid potential toxicity. The growing importance of natural sulfated molecules, as modulators of a number of physiological and pathological processes, has inspired the synthesis of non‐natural sulfated scaffolds. Until the 1990s, the synthesis of sulfated small molecules was almost restricted to derivatives of flavonoids and aimed mainly at structure elucidation and plant biosynthesis studies. Currently, the synthesis of this type of compounds concerns structurally diverse scaffolds and is aimed at the development of potential drugs and/or exploitation of the biological effects of sulfated metabolites. Some important hit compounds are emerging from sulfated flavonoids and other polyphenols mainly as anticoagulant and antiviral agents. When compared with polymeric macromolecules such as heparins, sulfated small molecules could be of value in therapeutics due to their hydrophobic nature that can contribute to improve the bioavailability. This review highlights the synthetic approaches that were applied to obtain monosulfated or polysulfated phenolic small molecules and compiles the diverse biological activities already reported for this type of derivatives. Toxicity and pharmacokinetic parameters of this emerging class of derivatives will also be considered, emphasizing their value for therapeutic applications.

LZSA glass ceramic foams prepared by replication process

Abstract In this study the replication process was applied to produce porous glass ceramic foams. Polyurethane (PU) sponges were impregnated by immersion in aqueous slurry of a parent glass (mean particle size around 3 μm) from the system Li2O–ZrO2–SiO2–Al2O3 and submitted to heat treatment. From TGA and IR analyses, two main paths of thermal decomposition of PU sponges were observed at 312 and 393°C, which were associated with the decomposition products CO2, CO, NH3 and isocyanide. According to linear thermal shrinkage and DTA measurements, sintering starts at around 570°C and is completed below 700°C, when crystallisation takes place. The main crystalline phases identified by XRD analysis were zirconium silicate, lithium metasilicate and β-spodumene. The morphology and properties of the glass ceramic foams obtained from PU sponges were quite similar for all samples tested. From XCT measurements, a mean cell diameter of around 260 μm and mean strut thickness of 185 μm were calculated, corresponding to a porosity of 75–78%. Compressive strength values under 500 kPa and permeability coefficient in air of about 1000 Darcy were also measured for the samples tested. LZSA glass ceramic foams can be a very interesting alternative to ceramic gas filters, owing to low temperature processing, and to superior properties such as corrosion and thermal shock resistance.

Insights into the In Vitro Antitumor Mechanism of Action of a New Pyranoxanthone

Naturally occurring xanthones have been documented as having antitumor properties, with some of them presently undergoing clinical trials. In an attempt to improve the biological activities of dihydroxyxanthones, prenylation and other molecular modifications were performed. All the compounds reduced viable cell number in a leukemia cell line K‐562, with the fused xanthone 3,4‐dihydro‐12‐hydroxy‐2,2‐dimethyl‐2H,6H‐pyrano[3,2‐b]xanthen‐6‐one (5) being the most potent. The pyranoxanthone 5 was particularly effective in additional leukemia cell lines (HL‐60 and BV‐173). Furthermore, the pyranoxanthone 5 decreased cellular proliferation and induced an S‐phase cell cycle arrest. In vitro, the pyranoxanthone 5 increased the percentage of apoptotic cells which was confirmed by an appropriate response at the protein level (e.g., PARP cleavage). Using a computer screening strategy based on the structure of several anti‐ and pro‐apoptotic proteins, it was verified that the pyranoxanthone 5 may block the binding of anti‐apoptotic Bcl‐xL to pro‐apoptotic Bad and Bim. The structure‐based screening revealed the pyranoxanthone 5 as a new scaffold that may guide the design of small molecules with better affinity profile for Bcl‐xL.

Discovery of a new small-molecule inhibitor of p53-MDM2 interaction using a yeast-based approach.

The virtual screening of a library of xanthone derivatives led us to the identification of potential novel MDM2 ligands. The activity of these compounds as inhibitors of p53-MDM2 interaction was investigated using a yeast phenotypic assay, herein developed for the initial screening. Using this approach, in association with a yeast p53 transactivation assay, the pyranoxanthone (3,4-dihydro-12-hydroxy-2,2-dimethyl-2H,6H-pyrano[3,2-b]xanthen-6-one) (1) was identified as a putative small-molecule inhibitor of p53-MDM2 interaction. The activity of the pyranoxanthone 1 as inhibitor of p53-MDM2 interaction was further investigated in human tumor cells with wild-type p53 and overexpressed MDM2. Notably, the pyranoxanthone 1 mimicked the activity of known p53 activators, leading to p53 stabilization and activation of p53-dependent transcriptional activity. Additionally, it led to increased protein levels of p21 and Bax, and to caspase-7 cleavage. By computational docking studies, it was predicted that, like nutlin-3a, a known small-molecule inhibitor of p53-MDM2 interaction, pyranoxanthone 1 binds to the p53-binding site of MDM2. Overall, in this work, a novel small-molecule inhibitor of p53-MDM2 interaction with a xanthone scaffold was identified for the first time. Besides its potential use as molecular probe and possible lead to develop anticancer agents, the pyranoxanthone 1 will pave the way for the structure-based design of a new class of p53-MDM2 inhibitors.

Polysulfated xanthones: multipathway development of a new generation of dual anticoagulant/antiplatelet agents.

A multipathway strategy was used to evaluate the in vitro and in vivo antithrombotic effects of a new synthetic family of sulfated small molecules. Polysulfated xanthonosides showed highly effective anticoagulation effects in vitro, both in plasma (clotting times) and in whole human blood (thromboelastography), as well as in vivo (ip administration, mice). Physicochemical properties were assessed for mangiferin heptasulfate (7), which showed high solubility and stability in water and in human plasma and no putative hepatotoxicity in vivo. Mangiferin heptasulfate (7) was found to be a direct inhibitor of FXa, while persulfated 3,6-(O-β-glucopyranosyl)xanthone (13) acted as a dual inhibitor of FXa (directly and by antithrombin III activation). By impedance aggregometry, compounds 7 and 13 exhibited the antiplatelet effect by inhibition of both arachidonic acid and ADP-induced platelet aggregation. Dual anticoagulant/antiplatelet agents, such as sulfated xanthonosides 7 and 13, are expected to lead to a new therapeutic approach for the treatment of both venous and arterial thrombosis.

Isomeric Kielcorins and Dihydroxyxanthones: Synthesis, Structure Elucidation, and Inhibitory Activities of Growth of Human Cancer Cell Lines and on the Proliferation of Human Lymphocytes In Vitro

The synthesis, structure elucidation, and biological activities of five isomeric xanthonolignoids, (±)-trans-kielcorin C, (±)-cis-kielcorin C, (±)-trans-kielcorin D, (±)-trans-isokielcorin D, and (±)-trans-kielcorin E, are reported. The synthetic approach is based on the oxidative coupling of coniferyl alcohol with an appropriate xanthone. The influence of different oxidizing agents was studied, and the best results were obtained with potassium hexacyanoferrate(III). The structure elucidation was achieved by 2D-NMR techniques such as COSY, HETCOR, HSQC, and HMBC. Long-range C,H connectivities were used to establish the orientation of the substituents on the 1,4-dioxine rings, while NOE experiments were used to determine the configurations of these rings. These xanthonolignoids, as well as (±)-trans-kielcorin, (±)-trans-kielcorin B, (±)-trans-isokielcorin B, and the xanthonic building blocks 3,4-, 1,2-, and 2,3-dihydroxy-9H-xanthen-9-ones, and 2,3-dihydroxy-4-methoxy-9H-xanthen-9-one, were evaluated for their in vitro effect on the growth of three human cancer cell lines, MCF-7 (breast), TK-10 (renal), and UACC-62 (melanoma), and on the proliferation of human lymphocytes.

Antifungal activity of xanthones: evaluation of their effect on ergosterol biosynthesis by high-performance liquid chromatography.

The increasing resistance of pathogenic fungi to antifungal compounds and the reduced number of available drugs led to the search for therapeutic alternatives among natural products, including xanthones. The antifungal activity of 27 simple oxygenated xanthones was evaluated by determination of their minimal inhibitory concentration on clinical and type strains of Candida, Cryptococcus, Aspergillus and dermatophytes, and their preponderance on the dermatophytic filamentous fungi was observed. Furthermore, a simple and efficient HPLC method with UV detection to study the effect of the active xanthones on the biosynthesis of ergosterol was developed and validated. Using this methodology, the identification and quantification of fungal sterols in whole cells of Candida albicans, Cryptococcus neoformans, Aspergillus fumigatus, and Trichophyton mentagrophytes were accomplished. In summary, 1,2‐dihydroxyxanthone was found to be the most active compound against all strains tested, showing its effect on sterol biosynthesis by reducing the amount of ergosterol detected.