Synthesis and characterization of the first fluorescent antagonists for human 5-HT4 receptors

Rational Design, Synthesis, and Evaluation of Fluorescent CB2 Receptor Ligands for Live-Cell Imaging: A Comprehensive Review

The cannabinoid receptors CB1 and CB2 are class A G protein-coupled receptors (GPCRs) that are activated via endogenous lipids called endocannabinoids. The endocannabinoid system (ECS) plays a critical role in the regulation of several physiological states and a wide range of diseases. In recent years, drug discovery approaches targeting the cannabinoid type 2 receptor (CB2R) have gained prominence. Particular attention has been given to selective agonists targeting the CB2 receptors to circumvent the neuropsychotropic side effects associated with CB1 receptors. The pharmacological modulation of CB2R holds therapeutic promise for various diseases, such as inflammatory disorders and immunological conditions, as well as pain management and cancer treatment. Recently, the utilization of fluorescent probes has emerged as a valuable technique for investigating the interactions between ligands and proteins at an exceptional level of spatial and temporal precision. In this review, we aim to examine the progress made in the development of fluorescent probes targeting CB2 receptors and highlight their significance in facilitating the successful clinical translation of CB2R-based therapies.

Lighting Up the Plasma Membrane: Development and Applications of Fluorescent Ligands for Transmembrane Proteins

Despite the fact that transmembrane proteins represent the main therapeutic targets for decades, complete and in-depth knowledge about their biochemical and pharmacological profiling is not fully available. In this regard, target-tailored small-molecule fluorescent ligands are a viable approach to fill in the missing pieces of the puzzle. Such tools, coupled with the ability of high-precision optical techniques to image with an unprecedented resolution at a single-molecule level, helped unraveling many of the conundrums related to plasma proteins' life-cycle and druggability. Herein, we review the recent progress made during the last two decades in fluorescent ligand design and potential applications in fluorescence microscopy of voltage-gated ion channels, ligand-gated ion channels and G-coupled protein receptors.

Photochemical Reactivity of Naphthol-Naphthalimide Conjugates and Their Biological Activity

Quinone methide precursors 1a–e, with different alkyl linkers between the naphthol and the naphthalimide chromophore, were synthesized. Their photophysical properties and photochemical reactivity were investigated and connected with biological activity. Upon excitation of the naphthol, Förster resonance energy transfer (FRET) to the naphthalimide takes place and the quantum yields of fluorescence are low (Φ_F ≈ 10⁻²). Due to FRET, photodehydration of naphthols to QMs takes place inefficiently (Φ_R ≈ 10⁻⁵). However, the formation of QMs can also be initiated upon excitation of naphthalimide, the lower energy chromophore, in a process that involves photoinduced electron transfer (PET) from the naphthol to the naphthalimide. Fluorescence titrations revealed that 1a and 1e form complexes with ct-DNA with moderate association constants K_a ≈ 10⁵–10⁶ M⁻¹, as well as with bovine serum albumin (BSA) K_a ≈ 10⁵ M⁻¹ (1:1 complex). The irradiation of the complex 1e@BSA resulted in the alkylation of the protein, probably via QM. The antiproliferative activity of 1a–e against two human cancer cell lines (H460 and MCF 7) was investigated with the cells kept in the dark or irradiated at 350 nm, whereupon cytotoxicity increased, particularly for 1e (>100 times). Although the enhancement of this activity upon UV irradiation has no imminent therapeutic application, the results presented have importance in the rational design of new generations of anticancer phototherapeutics that absorb visible light.

The impact of naphthalimide derivative on the mitigation of mild steel corrosion in sulfamic acid medium: experimental and theoretical insights

The deterioration of iron-based alloys, especially mild steel (MS) is one amongst the most challenging problems faced in various chemical industries. The present work focuses on the potential activity of a naphthalimide derivative namely 2-(2-hydroxyethyl)benzo[de]isoquinoline-1,3-dione (HBIQ) as corrosion inhibitor for MS in sulfamic acid (SA) medium in the temperature range from 303 to 323 K. Potentiodynamic polarization (PP) and electrochemical impedance spectroscopy (EIS) were employed in the experimental measurement and HBIQ exhibited 89% inhibition at its optimum concentration. HBIQ demonstrated electrostatic interactions with MS surface and behaved as a mixed type of inhibitor by obeying Langmuir’s isotherm model. Surface characterization of uninhibited and inhibited MS specimens combined with elemental analysis data provided clear evidences for the formation of a protective adsorption layer of HBIQ on MS surface. Spectral analysis such as Ultraviolet visible and Fourier Transform Infra-red spectral analyses were carried out in order to confirm the adsorption of HBIQ on to the metal surface. The density functional theory calculations supported the experimental results and indicated the contribution of delocalized π-electrons in the naphthalimide unit and the lone-pair electrons of oxygen in the carbonyl and hydroxyl group for improved adsorption of HBIQ onto MS surface, thereby reducing the corrosion of the alloy in SA environment.

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Synthesis of naphthalimide derivatives with potential anticancer activity, their comparative ds- and G-quadruplex-DNA binding studies and related biological activities

Two new cytotoxic 1,8-naphthalimide derivatives have been synthesized and characterized. Their biological activities as cytotoxicity and antimicrobial activities and inhibitory activities against DNA-polymerase were evaluated. The interactions of compounds with double-stranded- and quadruple-DNA have been studied by UV-Vis, fluorescent intercalator displacement, competition dialysis, circular dichroism and the findings were compared with the parent naphthalimide and the other compounds. The results show that both compounds (1 and 2) and the parent compound NI have strong cytotoxic activities against Beas-2B, MCF-7, HepG2 and MDA-MB-231 cancer cell lines, antimicrobial activities against Staphylococcus aureus ATCC 29213, Enterococcus faecalis ATCC 29212 and inhibitory activities towards Taq-polymerase and transcriptase. These novel cationic compounds 1 and 2 can stabilize G-quadruplexes DNA according to thermal denaturation experiments, they change the 3D structure of the DNA (see details in CD experiments) and they exhibit different binding affinities for q-DNA and ds-DNA revealed by spectrophotometric titrations and competitive dialysis studies.

Naphthalimides in fluorescent imaging of tumor hypoxia - An up-to-date review

Hypoxia is a distinctive characteristic of advanced solid malignancies that results from a disparity between oxygen supply and its consumption. The degree of hypoxia is believed to have adverse prognostic significance. Therefore detecting cellular hypoxia can potentially offer insights into the grade of tumour as well as its evolution towards a progressive malignant phenotype, which clinically translates to greater metastatic potential and treatment resistance. Fluorescence imaging to visualize hypoxia in biological systems is a minimally-invasive method. Recently there are several reports on interdisciplinary research that aims at developing functional probes that can be efficiently used for non-invasive imaging of hypoxic tumours. Upregulated levels of nitroreductase (NTR) is detected in hypoxic solid malignancies, and this characteristic feature is increasingly utilized in the development of NTR-targeted fluorescent molecules to selectively sense hypoxia in vivo. The present review summarizes various reports published on the design concepts of nitro naphthalimide-based bio-reductive fluorescent sensors that can be applied noninvasively to image hypoxia in cancer.

Less toxic zinc(ii), diorganotin(iv), gallium(iii) and cadmium(ii) complexes derived from 2-benzoylpyridine N,N-dimethylthiosemicarbazone: synthesis, crystal structures, cytotoxicity and investigations of mechanisms of action

Four metal complexes based on 2-benzoylpyridine N,N-dimethylthiosemicarbazone (Bp44mT) were designed. Free ligand and zinc(ii), diorganotin(iv), gallium(iii) and cadmium(ii) complexes all demonstrated pronounced activity, which was indicated using the growth inhibition test in vitro. Interestingly, most of the compounds were found to be selective against hepatocellular carcinoma (HepG2) cells but had little effect on normal hepatocyte (QSG7701) cells. In particular, Zn(Bp44mT)₂ (1) exhibited toxicity on QSG7701 cells which approximately 12-fold lower than that on HepG2 cells. The studies of mechanisms of action indicated that 1 induced reactive oxygen species (ROS) generation in a dose-dependent manner via the mitochondria transduction pathway. Protein analyses showed that 1 significantly promoted p21 and p53 gene expression, causing caspase-3 activation.

1,8-Naphthalimide: A Potent DNA Intercalator and Target for Cancer Therapy

The poor pharmacokinetics, side effects and particularly the rapid emergence of drug resistance compromise the efficiency of clinically used anticancer drugs. Therefore, the discovery of novel and effective drugs is still an extremely primary mission. Naphthalimide family is one of the highly active anticancer drug based upon effective intercalator with DNA. In this article, we review the discovery and development of 1,8-naphthalimide moiety, and, especially, pay much attention to the structural modifications and structure activity relationships. The review demonstrates how modulation of the moiety affecting naphthalimide compound for DNA binding that is achieved to afford a profile of antitumor activity. The DNA binding of imide and ring substitution at naphthalimide, bisnaphthalimide, naphthalimide-metal complexes is achieved by molecular recognition through intercalation mode. Thus, this synthetic/natural small molecule can act as a drug when activation or inhibition of DNA function, is required to cure or control the cancer disease. The present study is a review of the advances in 1,8-naphthalimide-related research, with a focus on how such derivatives are intercalated into DNA for their anticancer activities.

Multivalent ligands for the serotonin 5-HT4 receptor

5-HT₄ receptors are known to form constitutive dimers in membranes. To explore whether multivalency can enhance ligand interactions and/or efficacy in 5-HT₄ receptors, the structure of the partial agonist ML10302 was modified with oligo(ethylene glycol) chains, thus generating, by a gradual approach, short and long tethered bivalent or tetravalent ligands and the corresponding spanner-linked monovalent controls. Both bivalent and tetravalent ligands displayed a 10-20-fold increase in binding affinity compared to appropriate controls, but no multivalent ligand showed greater binding energy than ML10302 itself. Furthermore, the direct assessment of receptor-Gs interaction and studies of cAMP signalling indicated that multivalency does not enhance the efficacy of ML10302.

Modular chemical probes for visualizing and tracking endogenous ion channels

Fluorescently labeled, small molecule ligands designed for the labeling and tracking of neuronal receptors have become an increasingly popular tool in neurobiology. The small size of these probes allows for subcellular imaging of proteins in their native state with minimal perturbation of the system. Several factors such as the selectivity of the pharmacophore, the size and composition of linkers used, and the fluorescence stability of the fluorophore can all influence the effectiveness of the small molecule probe. Here we discuss a few key molecular targets of this technology including the NMDA receptor, serotonin transporter, dopamine transporter, and adenosine receptor due to their involvement in numerous neurodegenerative diseases. Future iterations of these probes will allow for a better understanding of many important neurological proteins as well as the development of new and potent therapeutic drugs. This review will cover probe design considerations and discuss examples of specific small molecule fluorescent ligands that have been used to study a multitude of neuronal receptors through fluorescent imaging. This article is part of the Special Issue entitled 'Fluorescent Tools in Neuropharmacology'.

Toward fluorescent probes for G-protein-coupled receptors (GPCRs)

G-protein-coupled receptors (GPCRs), a superfamily of cell-surface receptors that are the targets of about 40% of prescription drugs on the market, can sense numerous critical extracellular signals. Recent breakthroughs in structural biology, especially in holo-form X-ray crystal structures, have contributed to our understanding of GPCR signaling. However, actions of GPCRs at the cellular and molecular level, interactions between GPCRs, and the role of protein dynamics in receptor activities still remain controversial. To overcome these dilemmas, fluorescent probes of GPCRs have been employed, which have advantages of in vivo safety and real-time monitoring. Various probes that depend on specific mechanisms and/or technologies have been used to study GPCRs. The present review focuses on surveying the design and applications of fluorescent probes for GPCRs that are derived from small molecules or using protein-labeling techniques, as well as discussing some design strategies for new probes.

Fluorescent approaches for understanding interactions of ligands with G protein coupled receptors

G protein coupled receptors are responsible for a wide variety of signaling responses in diverse cell types. Despite major advances in the determination of structures of this class of receptors, the underlying mechanisms by which binding of different types of ligands specifically elicits particular signaling responses remain unclear. The use of fluorescence spectroscopy can provide important information about the process of ligand binding and ligand dependent conformational changes in receptors, especially kinetic aspects of these processes that can be difficult to extract from X-ray structures. We present an overview of the extensive array of fluorescent ligands that have been used in studies of G protein coupled receptors and describe spectroscopic approaches for assaying binding and probing the environment of receptor-bound ligands with particular attention to examples involving yeast pheromone receptors. In addition, we discuss the use of fluorescence spectroscopy for detecting and characterizing conformational changes in receptors induced by the binding of ligands. Such studies have provided strong evidence for diversity of receptor conformations elicited by different ligands, consistent with the idea that GPCRs are not simple on and off switches. This diversity of states constitutes an underlying mechanistic basis for biased agonism, the observation that different stimuli can produce different responses from a single receptor. It is likely that continued technical advances will allow fluorescence spectroscopy to play an important role in continued probing of structural transitions in G protein coupled receptors. This article is part of a Special Issue entitled: Structural and biophysical characterisation of membrane protein-ligand binding.

The Concise Guide to PHARMACOLOGY 2013/14: G protein-coupled receptors

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. G protein-coupled receptors are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

Synthesis of Closo-1,7-Carboranyl Alkyl Amines

Of the three closo-carborane isomers (C(2)B(10)H(12)), closo-1,2-carborane has been used most widely in the synthesis of carboranyl amines. However, closo-1,2-carboranes are prone to deboronation to nido-7,8-carborane under various conditions including attack by basic amino groups. In order to overcome this problem, closo-1,7-carboranyl ethyl-, propyl-, and butylamine were synthesized, which should be more stable towards basic deboronation than their closo-1,2-carboranyl counterparts. These closo-1,7-carboranyl amines (5, 18 and 19) were synthesized using two different methods, both starting from the corresponding closo-1,7-carboranyl alkyl iodides (3, 14 and 15). One of the carboranyl alkyl amine (5) was conjugated with folic acid to form a closo-1,7-carborane-folic acid bioconjugate (20).

Design, synthesis, and biological evaluation of new 5-HT4 receptor agonists: application as amyloid cascade modulators and potential therapeutic utility in Alzheimer's disease

Serotonin 5-HT(4) receptor (5-HT(4)R) agonists are of particular interest for the treatment of Alzheimer's disease because of their ability to ameliorate cognitive deficits and to modulate production of amyloid beta-protein (Abeta). However, despite the range of 5-HT(4)R agonists synthesized to date, potent and selective 5-HT(4)R agonists are still lacking. In the present study, two libraries of molecules based on the scaffold of ML10302, a highly specific and partial 5-HT(4)R agonist, were efficiently prepared by parallel supported synthesis and their binding affinities and agonist activities evaluated. Furthermore, we showed that, in vivo, the two best candidates exhibited neuroprotective activity by increasing the level of the soluble form of the amyloid precursor protein (sAPPalpha) in the cortex and hippocampus of mice. Interestingly, one of these compounds could also inhibit Abeta fibril formation in vitro.

Design, Synthesis, and Pharmacology of Fluorescently Labeled Analogs of Serotonin: Application to Screening of the 5-HT2C Receptor

Novel fluorescent derivatives of serotonin have been synthesized and used as tracers for the development of a 5-HT2C fluorescence polarization assay. Serotonin analogs that feature a fluorescent probe attached through an ether linkage at the tryptamine 5-position have high affinity for the 5-HT2C receptor, and affinity is dependent on both linker length and pendent dye. These variables have been optimized to generate Cy3B derivative 5a, which has 10-fold higher 5-HT2C affinity relative to serotonin (Kd = 0.23 nM). In receptor activation experiments, 5a acts as a full agonist of 5-HT2C. Upon binding to 5-HT2C cell membranes, 5a shows a robust increase in fluorescence polarization (FP) signal. In an FP binding assay using 5a as a tracer ligand, Ki values for known 5-HT2C agonists and antagonists showed excellent agreement with Ki values from radioligand binding ( r2 = 0.93). The FP ligand assay is suitable for high-throughput drug screening applications with respect to speed of analysis, displaceable signal, precision, and sensitivity to various reagents. A 384-well-based high-throughput assay that is rapid, economical, and predictive of test compounds' ability to bind to the 5-HT2C receptor has been compiled and validated. ( Journal of Biomolecular Screening 2009:360-370)

Developments in fluorescent probes for receptor research

Early reports on the identification of fluorescent probes for receptors date back to mid-1970s. Fluorescent probes were initially used to visualize molecular targets in an analogous way to the use of fluorescent antibodies but with the same resolution as isotopically labelled ligands. In parallel to the rapid development of techniques, such as fluorescence correlation spectroscopy, multi-photon excitation fluorescence microscopy, fluorescence polarization and in vivo fluorescence imaging, fluorescent probes are becoming multifaceted tools in life science. The present review will focus on how the design of fluorescent ligands for receptors has evolved to meet the needs of most recent fluorescence applications.

A versatile amino acid analogue of the solvatochromic fluorophore 4-N,N-dimethylamino-1,8-naphthalimide: a powerful tool for the study of dynamic protein interactions

We have developed a new unnatural amino acid based on the solvatochromic fluorophore 4-N,N-dimethylamino-1,8-naphthalimide (4-DMN) for application in the study of protein-protein interactions. The fluorescence quantum yield of this chromophore is highly sensitive to changes in the local solvent environment, demonstrating "switch-like" emission properties characteristic of the dimethylaminophthalimide family of fluorophores. In particular, this new species possesses a number of significant advantages over related fluorophores, including greater chemical stability under a wide range of conditions, a longer wavelength of excitation (408 nm), and improved synthetic accessibility. This amino acid has been prepared as an Fmoc-protected building block and may readily be incorporated into peptides via standard solid-phase peptide synthesis. A series of comparative studies are presented to demonstrate the advantageous properties of the 4-DMN amino acid relative to those of the previously reported 4-N,N-dimethylaminophthalimidoalanine and 6-N,N-dimethylamino-2,3-naphthalimidoalanine amino acids. Other commercially available solvatochromic fluorophores are also include in these studies. The potential of this new probe as a tool for the study of protein-protein interactions is demonstrated by introducing it into a peptide that is recognized by calcium-activated calmodulin. The binding interaction between these two components yields an increase in fluorescence emission greater than 900-fold.

Synthesis and characterization of the first fluorescent nonpeptide NPY Y1 receptor antagonist

Cyanine-5-labelled neuropeptide Y (NPY) was demonstrated to be an ideal universal fluorescent ligand for the combined investigation of NPY Y(1), Y(2) and Y(5) receptors. With respect to improved stability, detection of receptor subtypes in cells and tissues, and prevention of receptor internalization, small nonpeptidic fluorescent antagonists should be superior. Here we present a set of four fluorescent nonpeptide NPY Y(1) receptor (Y(1)R) antagonists. The highest affinity was obtained by labelling an N(G)-(6-aminohexanoyl)argininamide derived from the Y(1)R antagonist BIBP 3226, with Py-1, a small pyrylium dye. The fluorescent pyridinium-type Y(1)R antagonist, compound 4 had K(i) values of 29 nM and 2.7 nM, which were determined by radioligand binding and flow cytometry under equilibrium conditions, respectively; 4 had a K(b) value of 0.6 nM (Ca(2+) assay). The large Stoke's shift (541 vs. 615 nm) in buffer (PBS, pH 7.4) in the presence of 1% BSA and the red emission (quantum yield 56%) are advantageous with respect to the signal-to-noise ratio. The new probe was successfully used in fluorescence-based binding experiments evaluated by flow cytometry and confocal microscopy; this demonstrates the potential of pyrylium dyes for the preparation of fluorescent ligands that are applicable for the study of G protein-coupled receptors on living cells.

4-[ω-[4-Arylpiperazin-1-yl]alkoxy]phenyl)imidazo[1,2-a]pyridine Derivatives: Fluorescent High-Affinity Dopamine D3Receptor Ligands as Potential Probes for Receptor Visualization

Sixteen long-chain arylpiperazines bearing the fluorescent moiety 2-phenylimidazo[1,2-a]pyridine were synthesized as fluorescent dopamine D3 receptors ligands (385 nM < Ki < 0.72 nM). The most potent D3 compounds 15a and 19a (Ki = 1.6 and 0.72 nM, respectively) showed good Stokes shift and high quantum yield in ethanol (Phi = 0.74 and 0.66, respectively). In the first attempt, 15a was unable to visualize D3 receptors expressed in CHO cells by epifluorescence microscopy.

Fluorescently Labeled Analogues of Dofetilide as High-Affinity Fluorescence Polarization Ligands for the Human Ether-a-go-go-Related Gene (hERG) Channel

Novel fluorescent derivatives of dofetilide (1) have been synthesized. Analogues that feature a fluorescent probe attached through an aliphatic spacer to the central tertiary nitrogen of 1 have high affinity for the hERG channel, and affinity is dependent on both linker length and pendent dye. These variables have been optimized to generate Cy3B derivative 10e, which has hERG channel affinity equivalent to that of dofetilide. When bound to cell membranes expressing the hERG channel, 10e shows a robust increase in fluorescence polarization (FP) signal. In a FP binding assay using 10e as tracer ligand, Ki values for several known hERG channel blockers were measured and excellent agreement with the literature Ki values was observed over an affinity range of 2 nM to 3 muM. 10e blocks hERG channel current in electrophysiological patch clamp experiments, and computational docking experiments predict that the dofetilide core of 10e binds hERG channel in a conformation similar to that previously predicted for 1. These analogues enable high-throughput hERG channel binding assays that are rapid, economical, and predictive of test compounds' potential for prolonged QT liabilities.

Design and cellular kinetics of dansyl-labeled CADA derivatives with anti-HIV and CD4 receptor down-modulating activity

A new class of anti-retrovirals, cyclotriazadisulfonamide (CADA) and its derivatives, specifically down-regulate CD4, the main receptor of HIV, and prevent HIV infection in vitro. In this work, several CADA derivatives, chemically labeled with a fluorescent dansyl group, were evaluated for their biological features and cellular uptake kinetics. We identified a derivative KKD-016 with antiviral and CD4 down-modulating capabilities similar to those of the parental compound CADA. By using flow cytometry, we demonstrated that the dose-dependent cellular uptake of this derivative correlated with CD4 down-modulation. The uptake and activity of the dansyl-labeled compounds were not dependent on the level of expression of CD4 at the cell surface. Removal of the CADA compounds from the cell culture medium resulted in their release from the cells followed by a complete restoration of CD4 expression. The inability of several fluorescent CADA derivatives to down-modulate CD4 was not associated with their lower cellular uptake and was not reversed by facilitating their cell penetration by a surfactant. These results prove the successful integration of the dansyl fluorophore into the chemical structure of a CD4 down-modulating anti-HIV compound, and show the feasibility of tracking a receptor and its down-modulator simultaneously. These fluorescent CADA analogs with reversible CD4 down-regulating potency can now be applied in further studies on receptor modulation, and in the exploration of their potentials as preventive and therapeutic anti-HIV drugs.

Synthesis of fluorescent-labeled aeruginosin derivatives for high-throughput fluorescence correlation spectroscopy assays

The design and solid-phase synthesis of effective fluorescent-labeled aeruginosin derivatives and their application to the fluorescence correlation spectroscopy (FCS)-based competitive binding assay of an aeruginosin library are described. The phenolic hydroxyl group on the (R)-3-(4-hydroxyphenyl)lactic acid (d-Hpla) residue was observed to be suitable for connecting Rhodamine green derivative with minimum loss of biological activity. In addition, the FCS-based binding assay of the library using fluorescent-labeled chemical probes was also achieved.

Rhodamine-labeled 2beta-carbomethoxy-3beta-(3,4-dichlorophenyl)tropane analogues as high-affinity fluorescent probes for the dopamine transporter

Novel fluorescent ligands were synthesized to identify a high-affinity probe that would enable visualization of the dopamine transporter (DAT) in living cells. Fluorescent tags were extended from the N- or 2-position of 2beta-carbomethoxy-3beta-(3,4-dichlorophenyl)tropane, using an ethylamino linker. The resulting 2-substituted (5) and N-substituted (9) rhodamine-labeled ligands provided the highest DAT binding affinities expressed in COS-7 cells (Ki= 27 and 18 nM, respectively) in the series. Visualization of the DAT with 5 and 9 was demonstrated by confocal fluorescence laser scanning microscopy in stably transfected HEK293 cells.

Design and synthesis of specific probes for human 5-HT4 receptor dimerization studies

Recently, human 5-HT4 receptors have been demonstrated to form constitutive dimers in living cells. To evaluate the role of dimerization on the 5-HT4 receptor function, we investigated the conception and the synthesis of bivalent molecules able to influence the dimerization process. Their conception is based on a model of the 5-HT4 receptor dimer derived from protein/protein docking experiments. These bivalent ligands are constituted by two ML10302 units, a specific 5-HT4 ligand, linked through a spacer of different sizes and natures. These synthesized bivalent ligands were evaluated in binding assays and cyclic AMP production on the 5-HT4(e/g) receptor isoform stably transfected in C6 glial cells. Our data showed that bivalent ligands conserved a similar affinity compared to the basal ML10302 unit. Nevertheless, according to the nature and the size of the spacer, the pharmacological profile of ML10302 is more or less conserved. In view of the interest of bivalent ligands for investigating the GPCR dimerization process, these 5-HT4 specific bivalent ligands constitute valuable pharmacological tools for the study of 5-HT4 receptor dimerization.

Fluorophore-tagged GPCR ligands

Fluorescently tagged drug molecules can be successfully employed to visualize the location of their receptor target at the single-cell level. Furthermore, if their binding to the receptor is reversible, one can now obtain detailed pharmacological information such as affinity using single-molecule detection techniques. When coupled to the growing exploitation of fluorescence-based read-outs in high throughput and high content screening, it is clear that fluorescent molecules offer a safer, more powerful and more versatile alternative to radioligands in molecular pharmacology and drug discovery. GPCR pharmacology has benefited enormously from the application of fluorescence-based technologies and we now possess a much greater understanding of this receptor family's basic molecular mechanisms of action through the careful design and judicious use of fluorescent peptide and small-molecule-based ligands.

Utilization of a nitrobenzoxadiazole (NBD) fluorophore in the design of a Grb2 SH2 domain-binding peptide mimetic

Fluorescence labeling has become a general technique for studying the intracellular accumulation and localization of exogenously administered materials. Reported herein is a low nanomolar affinity Grb2 SH2 domain-binding antagonist that utilizes the environmentally-sensitive nitrobenzoxadiazole (NBD) fluorophore as a naphthyl replacement. This novel agent should serve as a useful tool to visualize the actions of this class of Grb2 SH2 domain-binding antagonists in whole cell systems.

Synthesis and pharmacological evaluation of benzamide derivatives as selective 5-HT4 receptor agonists

It is thought that selective 5-HT(4) receptor agonists-such as 4-amino-5-chloro-2-methoxy-N-[1-(6-oxo-6-phenylhexyl)piperidin-4ylmethyl]benzamide (2)-have the ability to enhance both upper and lower gastrointestinal motility without any significant adverse effects. Modification of 2 was performed. Variation of the piperidin-4ylmethyl moiety of 2 led to a decrease in the binding affinity for the 5-HT(4) receptor. Following conversion of the carbonyl group on the benzoyl part to a hydroxyl or sulfoxide group, the binding affinity for the 5-HT(4) receptor was retained although the effect on defecation was reduced. Many of the 4-amino-5-chloro-2-methoxy-N-(piperidin-4ylmethyl)benzamides that had a ether or sulfide moiety in the side-chain part at the 1-position of the piperidine exhibited high affinity for the 5-HT(4) receptor. Among these, phenylthio 41c and benzylthio derivative 44 were selective 5-HT(4) receptor agonists, and had a similar effect on defecation to compound 2.

New insights into the human 5-HT4 receptor binding site: exploration of a hydrophobic pocket

A body of evidences suggests that a hydrophobic pocket of the human 5-HT(4) receptor contributes to the high affinity of some bulky 5-HT(4) ligands. A thorough study of this pocket was performed using mutagenesis and molecular modeling. Ligand binding or competition studies with selected bulky ligands (RS39604, RS100235, [(3)H]GR113808 and ML11411) and small ligands (5-HT and ML10375) were carried out on wild-type and mutant receptors (W7.40A/F, Y7.43F, R3.28L) transiently transfected in COS-7 cells. The functional activity of the mutated receptors was evaluated by measuring the ability of 5-HT to stimulate adenylyl cyclase. For W7.40F mutation, no changes in the affinity of studied ligands and in the functional activity of the mutant receptor were observed, in contrary to W7.40A mutation, which abolished both binding of ligands and 5-HT-induced cAMP production. Mutation R3.28L revealed a totally silent receptor with a basal level of cAMP production similar to the mock control despite its ability to product cAMP in the presence of 5-HT. Moreover, a one order loss of affinity of RS39604 and a 45-fold increase of ML11411 affinity were observed. Mutation Y7.43F modified the affinity of GR113808, which displays a 13-fold lower affinity for the mutant than for the wild-type receptor. In conclusion, in the hydrophobic pocket, two polar amino acids are able to interact through hydrogen bonds with bulky ligands depending on their chemical properties. Moreover, these experimental data may validate the proposed new three-dimensional model of the human 5-HT(4) receptor.

Bridgehead-methyl analog of SC-53116 as a 5-HT4 agonist

Pyrrolizidine benzamide (+/-)-2, the bridgehead-methyl analog of SC-53116, was prepared and evaluated for 5-HT(4) agonism activity in the rat tunica muscularis (TMM) mucosae assay. Compound (+/-)-2 has an EC(50) of 449 nM in the TMM assay, as compared to 23 nM for SC-53116, and 66 nM for the racemate of SC-53116.