Structure-based design of novel melanin-concentrating hormone receptor-1 ligands based on saturated nitrogen-containing heterocycles.

Melanin Concentrating Hormone (MCH) receptor is a G protein-coupled receptor (GPCR) with two subtypes R1 and R2. MCH-R1 is involved in the control of energy homeostasis, feeding behavior and body weight. Many studies have proved that administration of MCH-R1 antagonists significantly reduces food intake and causes weight loss in animal models. Herein, we report the optimization of our previously reported virtual screening hits into novel MCH-R1 ligands with chiral aliphatic nitrogen-containing scaffolds. The activity was improved from the micromolar range of the initial leads to 7 nM. We also disclose the first MCH-R1 ligands based on a diazaspiro[4.5]decane nucleus with sub-micromolar activity. A potent MCH-R1 antagonist with acceptable pharmacokinetic profile could represent a new hope for the management of obesity.

Selective Inhibition of Plasmodium falciparum ATPase 6 by Artemisinins and Identification of New Classes of Inhibitors after Expression in Yeast.

Treatment failures with artemisinin combination therapies (ACTs) threaten global efforts to eradicate malaria. They highlight the importance of identifying drug targets and new inhibitors and of studying how existing antimalarial classes work. Here, we report the successful development of a heterologous expression-based compound-screening tool. The validated drug target Plasmodium falciparum ATPase 6 (PfATP6) and a mammalian orthologue (sarco/endoplasmic reticulum calcium ATPase 1a [SERCA1a]) were functionally expressed in Saccharomyces cerevisiae, providing a robust, sensitive, and specific screening tool. Whole-cell and in vitro assays consistently demonstrated inhibition and labeling of PfATP6 by artemisinins. Mutations in PfATP6 resulted in fitness costs that were ameliorated in the presence of artemisinin derivatives when studied in the yeast model. As previously hypothesized, PfATP6 is a target of artemisinins. Mammalian SERCA1a can be mutated to become more susceptible to artemisinins. The inexpensive, low-technology yeast screening platform has identified unrelated classes of druggable PfATP6 inhibitors. Resistance to artemisinins may depend on mechanisms that can concomitantly address multitargeting by artemisinins and fitness costs of mutations that reduce artemisinin susceptibility.

Structure-Activity Relationships of the Antimalarial Agent Artemisinin 10. Synthesis and Antimalarial Activity of Enantiomers of rac-5ß-Hydroxy-d-Secoartemisinin and Analogs: Implications Regarding the Mechanism of Action.

An efficient synthesis of rac-6-desmethyl-5ß-hydroxy-d-secoartemisinin 2, a tricyclic analog of R-(+)-artemisinin 1, was accomplished and the racemate was resolved into the (+)-2b and (-)-2a enantiomers via their Mosher Ester diastereomers. Antimalarial activity resided with only the artemisinin-like enantiomer R-(-)-2a. Several new compounds 9-16, 19a, 19b, 22 and 29 were synthesized from rac-2 but the C-5 secondary hydroxyl group was surprisingly unreactive. For example, the formation of carbamates and Mitsunobu reactions were unsuccessful. In order to assess the unusual reactivity of 2, a single crystal X-ray crystallographic analysis revealed a close intramolecular hydrogen bond from the C-5 alcohol to the oxepane ether oxygen (O-11). All products were tested in vitro against the W-2 and D-6 strains of Plasmodium falciparum. Several of the analogs had moderate activity in comparison to the natural product 1. Iron (II) bromide-promoted rearrangement of 2 gave, in 50% yield, the ring-contracted tetrahydrofuran 22, while the 5-ketone 15 provided a monocyclic methyl ketone 29 (50%). Neither 22 nor 29 possessed in vitro antimalarial activity. These results have implications in regard to the antimalarial mechanism of action of artemisinin.

Isolation and Synthesis of Veranamine, an Antidepressant Lead from the Marine Sponge Verongula rigida.

The natural product veranamine was isolated from the marine sponge Verongula rigida. It contains a unique heterocyclic scaffold and demonstrates in vivo antidepressant activity and selective affinity for 5HT2B and sigma-1 receptors. The first total synthesis of veranamine is reported. Our scalable synthesis offers veranamine in six steps and 25% yield via an unprecedented vinylogous Pictet-Gams pyridine formation strategy. Veranamine is a promising new lead compound for antidepressant drug development.

Identification of a new small molecule chemotype of Melanin Concentrating Hormone Receptor-1 antagonists using pharmacophore-based virtual screening.

MCH receptor is a G protein-coupled receptor with two subtypes R1 and R2. Many studies have demonstrated the role of MCH-R1 in feeding and energy homeostasis. It has been proven that oral administration of small molecule MCH-R1 antagonists significantly reduces food intake and causes a dose-dependent weight loss. In this study, two ligand-based pharmacophores were developed and validated based on recently published MCH-R1 antagonists with diverse structures. Successful pharmacophores had one hydrogen bond acceptor, one positive ionizable, one ring aromatic and two or three hydrophobic groups. These 3D-QSAR models were used for virtual screening of the ZINC chemical database resulting in the identification of nine compounds with more than 50% displacement of radiolabeled MCH at a 20 µM concentration. Moreover, four of these compounds showed antagonistic activities in Aequorin functional assay, including MH-3 which is the first MCH-R1 antagonist based on a diazaspiro[4.5]decane scaffold. The most active compounds were also docked into our previously published MCH-R1 homology model to gain insights into their binding determinants. These compounds could represent a viable starting scaffold for the design of potent MCH-R1 antagonists with improved pharmacokinetic properties as an effective treatment for obesity.

Computationally Assisted Discovery and Assignment of a Highly Strained and PANC-1 Selective Alkaloid from Alaska's Deep Ocean.

We report here the orchestration of molecular ion networking and a set of computationally assisted structural elucidation approaches in the discovery of a new class of pyrroloiminoquinone alkaloids that possess selective bioactivity against pancreatic cancer cell lines. Aleutianamine represents the first in a new class of pyrroloiminoquinone alkaloids possessing a highly strained multibridged ring system, discovered from Latrunculia ( Latrunculia) austini Samaai, Kelly & Gibbons, 2006 (class Demospongiae, order Poecilosclerida, family Latrunculiidae) recovered during a NOAA deep-water exploration of the Aleutian Islands. The molecule was identified with the guidance of mass spectrometry, nuclear magnetic resonance, and molecular ion networking (MoIN) analysis. The structure of aleutianamine was determined using extensive spectroscopic analysis in conjunction with computationally assisted quantifiable structure elucidation tools. Aleutianamine exhibited potent and selective cytotoxicity toward solid tumor cell lines including pancreatic cancer (PANC-1) with an IC50 of 25 nM and colon cancer (HCT-116) with an IC50 of 1 µM, and represents a potent and selective candidate for advanced preclinical studies.

Design, Synthesis, and Biological Evaluation of Peptidomimetic N-Substituted Cbz-4-Hyp-Hpa-Amides as Novel Inhibitors of Plasmodium falciparum.

A new series of peptidomimetic N-substituted Cbz-4-Hyp-Hpa-amides were designed, synthesized, and evaluated for inhibition of the Plasmodium falciparum. Substituents on the N-atom of the amide group were selected alkyl-, allyl-, aryl-, 2-hydroxyethyl-, 2-cyanoethyl-, cyanomethyl-, 2-hydroxyethyl-, 2,2-diethoxyethyl-, or 2-ethoxy-2-oxoethylamino groups, and about of 40 new compounds were synthesized and evaluated for antiplasmodial activity in vitro. Antimalarial activity has been investigated as for the final peptide mimetics, and their immediate predecessors, carrying TBDMS or TBDPS protecting groups on 4-hydroxyproline residue and 18 derivatives exhibited toxicity against P. falciparum. Of these agents, compound 23e was shown to have potent antimalarial activity with IC50 528 ng/ml.

Total synthesis of macrodiolide ionophores aplasmomycin A and boromycin via double ring contraction.

The half structure of the symmetrical macrodiolide aplasmomycin A was synthesized by alkylation of a C3-C10 α-sulfonyl ketone subunit, prepared from (R)-pulegone and protected as a C3 ortholactone with (2R,3R)-butanediol, by a protected 15,16-dihydroxy (12E)-allylic chloride representing C11-C17. The latter was obtained from (2S,3R)-1,2-epoxy-3-butanol and propargyl alcohol. Regio- and stereoselective 5-exo-trig cyclization of the ene diol moiety in this segment, mediated by N-bromosuccinimide, led to the (2R,3S,5R)-tetrahydrofuran substructure of aplasmomycin A. Attachment of an α-acetic ester at the C3 carboxylic acid and esterification of the 3'-hydroxyl group of the tetrahydrofuran as its α-bromoacetate enabled coupling of two aplasmomycin half structures as an α-acyloxy acetate. Mukaiyama macrolactonization of this hydroxy acid afforded a symmetrical 36-membered diolide. Base-mediated double Chan rearrangement of this bis α-acyloxy dilactone caused ring contraction to the 34-membered macrocycle of desboroaplasmomycin A while generating the transannular 2-hydroxy-3-hemiketal motif of the natural product in the correct configuration. Final incorporation of boron into the tetraol core produced aplasmomycin A, isolated as its sodium borate. Extension of this route to the unsymmetrical macrodiolide boromycin was accomplished by modifications that included reversal of C12-C13 olefin geometry to (Z) for the southern half structure along with stereoselective hydride reductions of the C9 ketone that produced (9R) and (9S) alcohols for northern and southern half structures, respectively. Coupling of these half structures was made using an α-acyloxy ester linkage as for aplasmomycin A, but ring closure in this case was orchestrated via a blocked C16 alcohol that left open the C15 hydroxyl group of the southern half for Mukaiyama macrolactonization. A double Chan rearrangement of the resulting 35-membered macrocycle produced the 33-membered diolide of desborodesvalinylboromycin which had been obtained previously by degradation of natural boromycin. Insertion of boron into the tetraol core followed by esterification of the C16 alcohol with a masked d-valine and final deprotection furnished boromycin as its zwitterionic (Böeseken) complex.

UPLC-MS-ELSD-PDA as a powerful dereplication tool to facilitate compound identification from small-molecule natural product libraries.

The generation of natural product libraries containing column fractions, each with only a few small molecules, using a high-throughput, automated fractionation system, has made it possible to implement an improved dereplication strategy for selection and prioritization of leads in a natural product discovery program. Analysis of databased UPLC-MS-ELSD-PDA information of three leads from a biological screen employing the ependymoma cell line EphB2-EPD generated details on the possible structures of active compounds present. The procedure allows the rapid identification of known compounds and guides the isolation of unknown compounds of interest. Three previously known flavanone-type compounds, homoeriodictyol (1), hesperetin (2), and sterubin (3), were identified in a selected fraction derived from the leaves of Eriodictyon angustifolium. The lignan compound deoxypodophyllotoxin (8) was confirmed to be an active constituent in two lead fractions derived from the bark and leaves of Thuja occidentalis. In addition, two new but inactive labdane-type diterpenoids with an uncommon triol side chain were also identified as coexisting with deoxypodophyllotoxin in a lead fraction from the bark of T. occidentalis. Both diterpenoids were isolated in acetylated form, and their structures were determined as 14S,15-diacetoxy-13R-hydroxylabd-8(17)-en-19-oic acid (9) and 14R,15-diacetoxy-13S-hydroxylabd-8(17)-en-19-oic acid (10), respectively, by spectroscopic data interpretation and X-ray crystallography. This work demonstrates that a UPLC-MS-ELSD-PDA database produced during fractionation may be used as a powerful dereplication tool to facilitate compound identification from chromatographically tractable small-molecule natural product libraries.

Synthesis, biological evaluation, hydration site thermodynamics, and chemical reactivity analysis of α-keto substituted peptidomimetics for the inhibition of Plasmodium falciparum.

A new series of peptidomimetic pseudo-prolyl-homophenylalanylketones were designed, synthesized and evaluated for inhibition of the Plasmodium falciparum cysteine proteases falcipain-2 (FP-2) and falcipain-3 (FP-3). In addition, the parasite killing activity of these compounds in human blood-cultured P. falciparum was examined. Of twenty-two (22) compounds synthesized, one peptidomimetic comprising a homophenylalanine-based α-hydroxyketone linked Cbz-protected hydroxyproline (39) showed the most potency (IC50 80 nM against FP-2 and 60 nM against FP-3). In silico analysis of these peptidomimetic analogs offered important protein-ligand structural insights including the role, by WaterMap, of water molecules in the active sites of these protease isoforms. The pseudo-dipeptide 39 and related compounds may serve as a promising direction forward in the design of competitive inhibitors of falcipains for the effective treatment of malaria.

Click chemistry decoration of amino sterols as promising strategy to developed new leishmanicidal drugs.

A series of 1,2,3-triazolylsterols was prepared from pregnenolone through reductive amination and copper(I)-catalyzed azide-alkyne cycloaddition (click chemistry). The newly generated stereocenter of the key propargylamino intermediate provided a mixture of diastereomers which were separated chromatographically, and the configuration of the R isomer was determined by X-ray crystallography. Ten triazolyl sterols were prepared, and the products and intermediates were screened in vitro against different parasites, with some compounds presenting IC50 values in the low micromolar range against Leishmania donovani.

Directed hydrogenation of acyclic homoallylic alcohols: enantioselective syntheses of (+)- and (-)-laurenditerpenol.

Laurenditerpenol is the first marine natural product shown to inhibit hypoxia-inducible factor 1 (HIF-1) activation. Preclinical studies support that the inhibition of HIF-1 is one of the molecular targets for antitumor drug discovery. The synthetically challenging molecular architecture of laurenditerpenol, its absolute stereostructure, and the biological activity of several diastereoisomers were accomplished by our group in 2007 by diastereoselective synthesis. Herein, we report enantioselective syntheses of both enantiomers of laurenditerpenol involving sequential Michael addition and remote homoallylic hydroxyl group-directed asymmetric hydrogenation at ambient temperature and pressure as key reaction steps. The current approach is elegant and overall more efficient than the ones previously reported in the literature.

Expression in yeast links field polymorphisms in PfATP6 to in vitro artemisinin resistance and identifies new inhibitor classes.

BACKGROUND: The mechanism of action of artemisinins against malaria is unclear, despite their widespread use in combination therapies and the emergence of resistance. RESULTS: Here, we report expression of PfATP6 (a SERCA pump) in yeast and demonstrate its inhibition by artemisinins. Mutations in PfATP6 identified in field isolates (such as S769N) and in laboratory clones (such as L263E) decrease susceptibility to artemisinins, whereas they increase susceptibility to unrelated inhibitors such as cyclopiazonic acid. As predicted from the yeast model, Plasmodium falciparum with the L263E mutation is also more susceptible to cyclopiazonic acid. An inability to knockout parasite SERCA pumps provides genetic evidence that they are essential in asexual stages of development. Thaperoxides are a new class of potent antimalarial designed to act by inhibiting PfATP6. Results in yeast confirm this inhibition. CONCLUSIONS: The identification of inhibitors effective against mutated PfATP6 suggests ways in which artemisinin resistance may be overcome.

Stevioside methanol tetra-solvate.

Stevioside is a naturally occurring diterpenoid glycoside in Stevia rebaudiana Bertoni. The title compound, C38H60O18·4CH3OH, crystallized as its methanol tetrasolvate. Stevioside consists of an aglycone steviol (a tetra-cyclic diterpene in which the four-fused-ring system consists of three six-membered rings and one five-membered ring) and a sugar part (three glucose units). A weak intra-molecular O-H⋯O hydrogen bond occurs. In the crystal, the methanol mol-ecules participate in a two-dimensional hydrogen-bonded network parallel to b axis with the sugars and together they form a hydrophilic tunnel which encloses the lipophilic part of the molecule.

Combined receptor-based and ligand-based approach to delineate the mode of binding of guaianolide-endoperoxides to PfATP6.

Plasmodium falciparum calcium-ATPase (PfATP6) has been reported to be a target of artemisinin and related endoperoxides. In this study, a series of previously reported guaianolide-endoperoxides (thaperoxides) were docked into a homology model of PfATP6 and also used to develop a pharmacophore model. This combined approach led to useful insights into the binding determinants of thaperoxides to the malarial enzyme. In addition, in silico mutagenesis and molecular dynamics suggested the importance of Phe264 and the electrostatic interactions between Lys260 in helix H3 and Lys1036 and Asp1038 in L6/7 loop for the binding of thaperoxides. These results could help in the design of more potent inhibitors of PfATP6.

Determination of antimalarial compound, ARB-89 (7ß-hydroxy-artemisinin carbamate) in rat serum by UPLC/MS/MS and its application in pharmacokinetics.

Among all the antimalarial agents, artemisinin and its semi synthetic family of analogs are the most potent antimalarials available for the treatment of Plasmodium falciparum infections. But these analogs have a few issues such as shorter half-lives and low oral bioavailability values. In order to overcome these inherent problems, novel artemisinin analogs were synthesized from 7ß-hydroxy artemisinin by the Department of Medicinal Chemistry, University of Mississippi using a new synthesis mechanism. Out of all the 7ß-hydroxy artemisinin analogs synthesized, 7ß-hydroxy artemisinin carbamate (ARB-89) was chosen as a lead compound because of its high in vitro and in vivo activity. In this manuscript, a sensitive and rapid ultra-performance liquid chromatography tandem mass spectrometry (UPLC/MS/MS) method was developed and validated for the quantification of ARB-89 in rat serum. The analysis was carried out on an Acquity™ UPLC BEH C(18) column (1.7 µm, 2.1 mm × 50 mm) with a flow rate of 0.3 mL/min. The detection was performed on a triple quadrupole tandem mass spectrometer in positive electrospray ionization (ESI) mode. The selected mass-to-charge (m/z) ratio transitions used in the multiple reaction monitoring (MRM) for ARB-89 and artemisinin (internal standard) were m/z 778.4>253.4 and m/z 283.4>151.1 respectively. The calibration curve was linear from 1.00 ng/mL to 10.0 µg/mL (r(2)=0.999). A simple protein precipitation method was used for extraction. Moreover, the inter-day and intra-day precision values were found to be less than 15%. The recoveries of the method ranged from 94.0% to 96.7% at three concentrations. ARB-89 in rat serum was found to be stable at room temperature for 12h. This method was successfully used to quantitate the novel antimalarial compound ARB-89 after intravenous and oral administration to rats.

Computer-aided drug design of falcipain inhibitors: virtual screening, structure-activity relationships, hydration site thermodynamics, and reactivity analysis.

Falcipains (FPs) are hemoglobinases of Plasmodium falciparum that are validated targets for the development of antimalarial chemotherapy. A combined ligand- and structure-based virtual screening of commercial databases was performed to identify structural analogs of virtual screening hits previously discovered in our laboratory. A total of 28 low micromolar inhibitors of FP-2 and FP-3 were identified and the structure-activity relationship (SAR) in each series was elaborated. The SAR of the compounds was unusually steep in some cases and could not be explained by a traditional analysis of the ligand-protein interactions (van der Waals, electrostatics, and hydrogen bonds). To gain further insights, a statistical thermodynamic analysis of explicit solvent in the ligand binding domains of FP-2 and FP-3 was carried out to understand the roles played by water molecules in binding of these inhibitors. Indeed, the energetics associated with the displacement of water molecules upon ligand binding explained some of the complex trends in the SAR. Furthermore, low potency of a subset of FP-2 inhibitors that could not be understood by the water energetics was explained in the context of poor chemical reactivity of the reactive centers of these compounds. The present study highlights the importance of considering energetic contributors to binding beyond traditional ligand-protein interactions.

New insights into the binding mode of melanin concentrating hormone receptor-1 antagonists: homology modeling and explicit membrane molecular dynamics simulation study.

Melanin concentrating hormone (MCH) is a cyclic 19-amino-acid peptide expressed mainly in the hypothalamus. It is involved in the control of feeding behavior, energy homeostasis, and body weight. Administration of MCH-R1 antagonists has been proved to reduce food intake and cause weight loss in animal models. In the present study, a homology model of the human MCH-R1 was constructed using the crystal structure of bovine rhodopsin (PDB: 1u19) as a template. Based on the observation that MCH-R1 can bind ligands of high chemical diversity, the initial model was subjected to an extensive ligand-supported refinement using antagonists of different chemotypes. The refinement process involved stepwise energy minimizations and molecular dynamics simulations. The refined model was inserted into a pre-equilibrated DPPC/TIP3P membrane system and then simulated for 20 ns in complex with structurally diverse antagonists. This protocol was able to explain the SAR of MCH-R1 antagonists with diverse chemical structures. Moreover, it reveals new insights into the critical recognition sites within the receptor. This work represents the first detailed study of molecular dynamics of MCH-R1 inserted into a membrane-aqueous environment.

Identification of novel malarial cysteine protease inhibitors using structure-based virtual screening of a focused cysteine protease inhibitor library.

Malaria, in particular that caused by Plasmodium falciparum , is prevalent across the tropics, and its medicinal control is limited by widespread drug resistance. Cysteine proteases of P. falciparum , falcipain-2 (FP-2) and falcipain-3 (FP-3), are major hemoglobinases, validated as potential antimalarial drug targets. Structure-based virtual screening of a focused cysteine protease inhibitor library built with soft rather than hard electrophiles was performed against an X-ray crystal structure of FP-2 using the Glide docking program. An enrichment study was performed to select a suitable scoring function and to retrieve potential candidates against FP-2 from a large chemical database. Biological evaluation of 50 selected compounds identified 21 diverse nonpeptidic inhibitors of FP-2 with a hit rate of 42%. Atomic Fukui indices were used to predict the most electrophilic center and its electrophilicity in the identified hits. Comparison of predicted electrophilicity of electrophiles in identified hits with those in known irreversible inhibitors suggested the soft-nature of electrophiles in the selected target compounds. The present study highlights the importance of focused libraries and enrichment studies in structure-based virtual screening. In addition, few compounds were screened against homologous human cysteine proteases for selectivity analysis. Further evaluation of structure-activity relationships around these nonpeptidic scaffolds could help in the development of selective leads for antimalarial chemotherapy.

Chemical characterization of a commercial Commiphora wightii resin sample and chemical profiling to assess for authenticity.

The gum resin of Commiphora wightii [(Hook. ex Stocks) Engl.] is an ayurvedic medicine for the treatment of arthritis, inflammation, obesity, lipid disorders, and cardiovascular diseases and is known as guggul. Morphologically, it is not easy to distinguish guggul from closely related gum resins of other plants. Reliability of the commercially available guggul is critical due to the high risk of adulteration. To check authenticity, a commercial guggul sample was investigated for its chemical markers and 17 metabolites were identified, including three new, 20(S),21-epoxy-3-oxocholest-4-ene (1), 8 ß-hydroxy-3,20-dioxopregn-4,6-diene (2), and 5-(13' Z-nonadecenyl)resorcinol (17) from the ethyl acetate soluble part. During the current study, compounds 14- 17 were identified as constituents of Mangifera indica gum, as an adulterant in the commercial guggul sample. This discovery highlighted the common malpractices in the trade of medicinal raw material in the developing world. The structures of the compounds were deduced by the spectroscopic technique and chemical methods, as well as by comparison with the reported data. The structure of 20(S),21-epoxy-3-oxocholest-4-ene (1) was also unambiguously deduced by single-crystal X-ray diffraction technique.