Synthesis and neuropeptide Y Y1 receptor antagonistic activity of N,N-disubstituted omega-guanidino- and omega-aminoalkanoic acid amides

Insertion of Nanoluc into the Extracellular Loops as a Complementary Method To Establish BRET-Based Binding Assays for GPCRs

Luminescence-based techniques play an increasingly important role in all areas of biochemical research, including investigations on G protein-coupled receptors (GPCRs). One quite recent and popular addition has been made by introducing bioluminescence resonance energy transfer (BRET)-based binding assays for GPCRs, which are based on the fusion of nanoluciferase (Nluc) to the N-terminus of the receptor and the occurring energy transfer via BRET to a bound fluorescent ligand. However, being based on BRET, the technique is strongly dependent on the distance/orientation between the luciferase and the fluorescent ligand. Here we describe an alternative strategy to establish BRET-based binding assays for GPCRs, where the N-terminal fusion of Nluc did not result in functioning test systems with our fluorescent ligands (e.g., for the neuropeptide Y Y1 receptor (Y1R) and the neurotensin receptor type 1 (NTS1R)). Instead, we introduced Nluc into their second extracellular loop and we obtained binding data for the fluorescent ligands and reported standard ligands (in saturation and competition binding experiments, respectively) comparable to data from the literature. The strategy was transferred to the angiotensin II receptor type 1 (AT1R) and the M1 muscarinic acetylcholine receptor (M1R), which led to affinity estimates comparable to data from radioligand binding experiments. Additionally, an analysis of the binding kinetics of all fluorescent ligands at their respective target was performed using the newly described receptor/Nluc-constructs.

Structure-Based Design of High-Affinity Fluorescent Probes for the Neuropeptide Y Y1 Receptor

The recent crystallization of the neuropeptide Y Y₁ receptor (Y₁R) in complex with the argininamide-type Y₁R selective antagonist UR-MK299 (2) opened up a new approach toward structure-based design of nonpeptidic Y₁R ligands. We designed novel fluorescent probes showing excellent Y₁R selectivity and, in contrast to previously described fluorescent Y₁R ligands, considerably higher (∼100-fold) binding affinity. This was achieved through the attachment of different fluorescent dyes to the diphenylacetyl moiety in 2 via an amine-functionalized linker. The fluorescent ligands exhibited picomolar Y₁R binding affinities (pK_i values of 9.36-9.95) and proved to be Y₁R antagonists, as validated in a Fura-2 calcium assay. The versatile applicability of the probes as tool compounds was demonstrated by flow cytometry- and fluorescence anisotropy-based Y₁R binding studies (saturation and competition binding and association and dissociation kinetics) as well as by widefield and total internal reflection fluorescence (TIRF) microscopy of live tumor cells, revealing that fluorescence was mainly localized at the plasma membrane.

Argininamide-type neuropeptide Y Y1 receptor antagonists: the nature of N ω-carbamoyl substituents determines Y1R binding mode and affinity

The recently resolved crystal structure of the neuropeptide Y Y1 receptor (Y1R), co-crystallized with the high-affinity (pK i: 10.11), argininamide-type Y1R antagonist UR-MK299 (2), revealed that the N ω-carbamoyl substituent (van der Waals volume: 139 Å3) is deeply buried in the receptor, occupying a hydrophobic pocket. We synthesized and characterized a series of argininamides, structurally related to 2. Y1R affinity decreased with increasing size of the carbamoyl residue (minimal pK i: 5.67). Exceeding a critical size of the substituent (van der Waals volume: 212 Å3), the ligands bound in an inverted mode with the carbamoyl side chain located at the surface of the receptor, as suggested by induced-fit docking and MD simulations.

Calcium Signaling Pathways: Key Pathways in the Regulation of Obesity

Nowadays, high epidemic obesity-triggered hypertension and diabetes seriously damage social public health. There is now a general consensus that the body's fat content exceeding a certain threshold can lead to obesity. Calcium ion is one of the most abundant ions in the human body. A large number of studies have shown that calcium signaling could play a major role in increasing energy consumption by enhancing the metabolism and the differentiation of adipocytes and reducing food intake through regulating neuronal excitability, thereby effectively decreasing the occurrence of obesity. In this paper, we review multiple calcium signaling pathways, including the IP3 (inositol 1,4,5-trisphosphate)-Ca²⁺ (calcium ion) pathway, the p38-MAPK (mitogen-activated protein kinase) pathway, and the calmodulin binding pathway, which are involved in biological clock, intestinal microbial activity, and nerve excitability to regulate food intake, metabolism, and differentiation of adipocytes in mammals, resulting in the improvement of obesity.

Split luciferase-based assay for simultaneous analyses of the ligand concentration- and time-dependent recruitment of β-arrestin2

Functional selectivity of agonists has gained increasing interest in G protein-coupled receptor (GPCR) research, e.g. due to expectations of drugs with reduced adverse effects. Different agonist-dependent GPCR conformations are conceived to selectively activate a balanced or imbalanced intracellular signalling response, involving e.g. different Gα subtypes, Gβγ-subunits and β-arrestins. To discriminate between the different signalling pathways (bias), sensitive techniques are needed that do not interfere with signalling. We applied split luciferase complementation to the GPCR/β-arrestin2 interaction and thoroughly analysed the influence of its implementation on intracellular signalling. This led to an assay enabling the functional characterization of ligands at the hH₁R, the hM_1,5R and the hNTS₁R in live HEK293T cells. As demonstrated at the hM_1,5R, the assay was sensitive enough to identify iperoxo as a superagonist. Time-dependent analyses of the recruitment of β-arrestin2 became possible, allowing the identification of class A and class B GPCRs, due to the differential duration of their interaction with β-arrestin2 and their recycling to the cell membrane. The developed β-arrestin2 recruitment assay, which provides concentration- and time-dependent information on the interaction between GPCRs and β-arrestin2 upon stimulation of the receptor, should be broadly applicable and of high value for the analysis of agonist bias.

A split luciferase-based probe for quantitative proximal determination of Gαq signalling in live cells

The earlier an activation of a G protein-dependent signalling cascade at a G protein-coupled receptor (GPCR) is probed, the less amplificatory effects contribute to the measured signal. This is especially useful in case of a precise quantification of agonist efficacies, and is of paramount importance, when determining agonist bias in relation to the β-arrestin pathway. As most canonical assays with medium to high throughput rely on the quantification of second messengers, and assays affording more proximal readouts are often limited in throughput, we developed a technique with a proximal readout and sufficiently high throughput that can be used in live cells. Split luciferase complementation (SLC) was applied to assess the interaction of Gα_q with its effector phospholipase C-β3. The resulting probe yielded an excellent Z' value of 0.7 and offers a broad and easy applicability to various Gα_q-coupling GPCRs (hH₁R, hM_1,3,5R, hNTS₁R), expressed in HEK293T cells, allowing the functional characterisation of agonists and antagonists. Furthermore, the developed sensor enabled imaging of live cells by luminescence microscopy, as demonstrated for the hM₃R. The versatile SLC-based probe is broadly applicable e.g. to the screening and the pharmacological characterisation of GPCR ligands as well as to molecular imaging.

Prototypic 18F-Labeled Argininamide-Type Neuropeptide Y Y1R Antagonists as Tracers for PET Imaging of Mammary Carcinoma

The neuropeptide Y (NPY) Y₁ receptor (Y₁R) selective radioligand (R)-N^α-(2,2-diphenylacetyl)-N^ω-[4-(2-[¹⁸F]fluoropropanoylamino)butyl]aminocarbonyl-N-(4-hydroxybenzyl)argininamide ([¹⁸F]23), derived from the high-affinity Y₁R antagonist BIBP3226, was developed for imaging studies of Y₁R-positive tumors. Starting from the argininamide core bearing amine-functionalized spacer moieties, a series of fluoropropanoylated and fluorobenzoylated derivatives was synthesized and studied for Y₁R affinity. The fluoropropanoylated derivative 23 displayed high affinity (K_i = 1.3 nM) and selectivity toward Y₁R. Radiosynthesis was accomplished via ¹⁸F-fluoropropanoylation, yielding [¹⁸F]23 with excellent stability in mice; however, the biodistribution study revealed pronounced hepatobiliary clearance with high accumulation in the gall bladder (>100 %ID/g). Despite the unfavorable biodistribution, [¹⁸F]23 was successfully used for imaging of Y₁R positive MCF-7 tumors in nude mice. Therefore, we suggest [¹⁸F]23 as a lead for the design of PET ligands with optimized physicochemical properties resulting in more favorable biodistribution and higher Y₁R-dependent enrichment in mammary carcinoma.

Label-free versus conventional cellular assays: Functional investigations on the human histamine H1 receptor

A set of histamine H₁ receptor (H₁R) agonists and antagonists was characterized in functional assays, using dynamic mass redistribution (DMR), electric cell-substrate impedance sensing (ECIS) and various signaling pathway specific readouts (Fura-2 and aequorin calcium assays, arrestin recruitment (luciferase fragment complementation) assay, luciferase gene reporter assay). Data were gained from genetically engineered HEK293T cells and compared with reference data from GTPase assays and radioligand binding. Histamine and the other H₁R agonists gave different assay-related pEC₅₀ values, however, the order of potency was maintained. In the luciferase fragment complementation assay, the H₁R preferred β-arrestin2 over β-arrestin1. The calcium and the impedimetric assay depended on G_q coupling of the H₁R, as demonstrated by complete inhibition of the histamine-induced signals in the presence of the G_q inhibitor FR900359 (UBO-QIC). Whereas partial inhibition by FR900359 was observed in DMR and the gene reporter assay, pertussis toxin substantially decreased the response in DMR, but increased the luciferase signal, reflecting the contribution of both, G_q and G_i, to signaling in these assays. For antagonists, the results from DMR were essentially compatible with those from conventional readouts, whereas the impedance-based data revealed a trend towards higher pK_b values. ECIS and calcium assays apparently only reflect G_q signaling, whereas DMR and gene reporter assays appear to integrate both, G_q and G_i mediated signaling. The results confirm the value of the label-free methods, DMR and ECIS, for the characterization of H₁R ligands. Both noninvasive techniques are complementary to each other, but cannot fully replace reductionist signaling pathway focused assays.

Mimicking of Arginine by Functionalized N(ω)-Carbamoylated Arginine As a New Broadly Applicable Approach to Labeled Bioactive Peptides: High Affinity Angiotensin, Neuropeptide Y, Neuropeptide FF, and Neurotensin Receptor Ligands As Examples

Derivatization of biologically active peptides by conjugation with fluorophores or radionuclide-bearing moieties is an effective and commonly used approach to prepare molecular tools and diagnostic agents. Whereas lysine, cysteine, and N-terminal amino acids have been mostly used for peptide conjugation, we describe a new, widely applicable approach to peptide conjugation based on the nonclassical bioisosteric replacement of the guanidine group in arginine by a functionalized carbamoylguanidine moiety. Four arginine-containing peptide receptor ligands (angiotensin II, neurotensin(8-13), an analogue of the C-terminal pentapeptide of neuropeptide Y, and a neuropeptide FF analogue) were subject of this proof-of-concept study. The N(ω)-carbamoylated arginines, bearing spacers with a terminal amino group, were incorporated into the peptides by standard Fmoc solid phase peptide synthesis. The synthesized chemically stable peptide derivatives showed high receptor affinities with Ki values in the low nanomolar range, even when bulky fluorophores had been attached. Two new tritiated tracers for angiotensin and neurotensin receptors are described.

N(ω)-Carbamoylation of the Argininamide Moiety: An Avenue to Insurmountable NPY Y1 Receptor Antagonists and a Radiolabeled Selective High-Affinity Molecular Tool ([(3)H]UR-MK299) with Extended Residence Time

Analogues of the argininamide-type NPY Y1 receptor (Y1R) antagonist BIBP3226, bearing carbamoyl moieties at the guanidine group, revealed subnanomolar Ki values and caused depression of the maximal response to NPY (calcium assay) by up to 90% in a concentration- and time-dependent manner, suggesting insurmountable antagonism. To gain insight into the mechanism of binding of the synthesized compounds, a tritiated antagonist, (R)-N(α)-diphenylacetyl-N(ω)-[2-([2,3-(3)H]propionylamino)ethyl]aminocarbonyl-(4-hydroxybenzyl)arginin-amide ([(3)H]UR-MK299, [(3)H]38), was prepared. [(3)H]38 revealed a dissociation constant in the picomolar range (Kd 0.044 nM, SK-N-MC cells) and very high Y1R selectivity. Apart from superior affinity, a considerably lower target off-rate (t1/2 95 min) was characteristic of [(3)H]38 compared to that of the higher homologue containing a tetramethylene instead of an ethylene spacer (t1/2 3 min, Kd 2.0 nM). Y1R binding of [(3)H]38 was fully reversible and fully displaceable by nonpeptide antagonists and the agonist pNPY. Therefore, the insurmountable antagonism observed in the functional assay has to be attributed to the extended target-residence time, a phenomenon of relevance in drug research beyond the NPY receptor field.

The neuropeptide y y(1) receptor: a diagnostic marker? Expression in mcf-7 breast cancer cells is down-regulated by antiestrogens in vitro and in xenografts

The neuropeptide Y (NPY) Y(1) receptor (Y(1)R) has been suggested as a tumor marker for in vivo imaging and as a therapeutic target. In view of the assumed link between estrogen receptor (ER) and Y(1)R in mammary carcinoma and with respect to the development of new diagnostic tools, we investigated the Y(1)R protein expression in human MCF-7 cell variants differing in ER content and sensitivity against antiestrogens. ER and Y(1)R expression were quantified by radioligand binding using [(3)H]-17β-estradiol and the Y(1)R selective antagonist [(3)H]-UR-MK114, respectively. The latter was used for cellular binding studies and for autoradiography of MCF-7 xenografts. The fluorescent ligands Cy5-pNPY (universal Y(1)R, Y(2)R and Y(5)R agonist) and UR-MK22 (selective Y(1)R antagonist), as well as the selective antagonists BIBP3226 (Y(1)R), BIIE0246 (Y(2)R) and CGP71683 (Y(5)R) were used to identify the NPY receptor subtype(s) by confocal microscopy. Y(1)R functionality was determined by mobilization of intracellular Ca(2+). Sensitivity of MCF-7 cells against antiestrogen 4-hydroxytamoxifen correlated directly with the ER content. The exclusive expression of Y(1)Rs was confirmed by confocal microscopy. The Y(1)R protein was up-regulated (100%) by 17β-estradiol (EC(50) 20 pM) and the predominant role of ERα was demonstrated by using the ERα-selective agonist "propylpyrazole triol". 17β-Estradiol-induced over-expression of functional Y(1)R protein was reverted by the antiestrogen fulvestrant (IC(50) 5 nM) in vitro. Furthermore, tamoxifen treatment of nude mice resulted in an almost total loss of Y(1)Rs in MCF-7 xenografts. In conclusion, the value of the Y(1)R as a target for therapy and imaging in breast cancer patients may be compromised due to Y(1)R down-regulation induced by hormonal (antiestrogen) treatment.

Synthesis and characterization of DMAP-modified NPY Y1 receptor antagonists as acyl-transfer catalysts

Starting from the working hypothesis that specific chemical labelling may be an attractive approach to detect and study G protein-coupled receptors (GPCRs) we synthesized catalytically active antagonists of the neuropeptide Y1 receptor (Y1R). An argininamide-type Y1R antagonist scaffold was combined with a DMAP moiety via spacers of different length and chemical nature. These hybrid compounds have Y1R affinities in the two-digit nanomolar range and are capable of catalysing acyl-transfer reaction to surrogates of bionucleophiles, as demonstrated in the absence of cells by using esters of fluorescent dyes as substrates in buffer. By contrast, selective staining of Y1Rs on living MCF-7 cells was not achieved due to significant non-catalysed (Y1R ligand independent) reaction with biomolecules and the limited density of Y1R on the cell surface. Although this may also depend on insufficient selectivity of the staining reagents, the results of this study suggest that the general applicability of catalytic staining to GPCRs has to be reconsidered, as this approach is hampered by a very low portion of receptor of interest compared to the total amount of membrane proteins.

Bivalent argininamide-type neuropeptide y y(1) antagonists do not support the hypothesis of receptor dimerisation

Bivalent ligands are potential tools to investigate the dimerisation of G-protein-coupled receptors. Based on the (R)-argininamide BIBP 3226, a potent and selective neuropeptide Y Y(1) receptor (Y(1)R) antagonist, we prepared a series of bivalent Y(1)R ligands with a wide range of linker lengths (8-36 atoms). Exploiting the high eudismic ratio (>1000) of the parent compound, we synthesised sets of R,R-, R,S- and S,S-configured bivalent ligands to gain insight into the "bridging" of two Y(1)Rs by simultaneous interaction with both binding sites of a putative receptor dimer. Except for the S,S isomers, the bivalent ligands are high-affinity Y(1)R antagonists, as determined by Ca(2+) assays on HEL cells and radioligand competition assays on human Y(1)R-expressing SK-N-MC and MCF-7 cells. Whereas the R,R enantiomers are most potent, no marked differences were observed relative to the corresponding meso forms. The difference between R,R and R,S diastereomers was most pronounced (about sixfold) in the case of the Y(1)R antagonist containing a spacer of 20 atoms in length. Among the R,R enantiomers, linker length and structural diversity had little effect on Y(1)R affinity. Although the bivalent ligands preferentially bind to the Y(1)R, the selectivity toward human Y(2), Y(4), and Y(5) receptors was markedly lower than that of the monovalent argininamides. The results of this study neither support the presence of Y(1)R dimers nor the simultaneous occupation of both binding pockets by the twin compounds. However, as the interaction with Y(1)R dimers cannot be unequivocally ruled out, the preparation of a bivalent radioligand is suggested to determine the ligand-receptor stoichiometry. Aiming at such radiolabelled pharmacological tools, prototype twin compounds were synthesised, containing an N-propionylated amino-functionalised branched linker (K(i)> or =18 nM), a tritiated form of which can be easily prepared.

Guanidine-acylguanidine bioisosteric approach in the design of radioligands: synthesis of a tritium-labeled N(G)-propionylargininamide ([3H]-UR-MK114) as a highly potent and selective neuropeptide Y Y1 receptor antagonist

Synthesis and characterization of (R)-N(alpha)-(2,2-diphenylacetyl)-N-(4-hydroxybenzyl)-N(omega)-([2,3-(3)H]-propanoyl)argininamide ([(3)H]-UR-MK114), an easily accessible tritium-labeled NPY Y(1) receptor (Y(1)R) antagonist (K(B): 0.8 nM, calcium assay, HEL cells) derived from the (R)-argininamide BIBP 3226, is reported. The radioligand binds with high affinity (K(D), saturation: 1.2 nM, kinetic experiments: 1.1 nM, SK-N-MC cells) and selectivity for Y(1)R over Y(2), Y(4), and Y(5) receptors. The title compound is a useful pharmacological tool for the determination of Y(1)R ligand affinities, quantification of Y(1)R binding sites, and autoradiography.

Determination of Affinity and Activity of Ligands at the Human Neuropeptide Y Y4Receptor by Flow Cytometry and Aequorin Luminescence

Fluorescence-labeled neuropeptide Y (NPY) has been used in flow cytometric binding assays for the determination of affinity constants of NPY Y1, Y2, and Y5 receptor ligands. Because the binding of fluorescent NPY is insufficient for competition studies at the human Y4 receptor (hY4R), we replaced Glu-4 in hPP with Lys for the derivatization with cyanine-5. Because cy5-[K(4)]hPP has high affinity (Kd 5.6 nM) to the hY4R, it was used as a probe in a flow cytometric binding assay. Specific binding of cy5-[K(4)]hPP to hY4R was visualized by confocal microscopy. The hY(4)R, the chimeric G protein G(qi5) and mitochondrially targeted apoaequorin were stably coexpressed in CHO cells. Aequorin luminescence was quantified in a microplate reader and by a CCD camera. By application of these methods 3-cyclohexyl-N-[(3-1H-imidazol-4-ylpropylamino)(imino)methyl]propanamide (UR-AK49) was discovered as the first nonpeptidic Y4R antagonist (pKi 4.17), a lead to be optimized in terms of potency and selectivity.

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.

Fluorescence- and luminescence-based methods for the determination of affinity and activity of neuropeptide Y2 receptor ligands

With respect to the discovery and characterization of neuropeptide Y(2) receptor ligands as pharmacological tools or potential drugs, fluorescence- and luminescence-based assays were developed to determine both the affinity and the activity of receptor agonists and antagonists. A flow cytometric binding assay is described for the hY(2) receptor stably expressed in CHO cells using cy5-labeled porcine neuropeptide Y and compared with a radioligand binding assay. Binding of the fluorescent ligand was visualized by confocal microscopy. Stable co-transfection with the chimeric G protein Gq(i5) enabled the establishment of a spectrofluorimetric fura-2 and a flow cytometric fluo-4 calcium assay. Further stable expression of apoaequorin targeted to the mitochondria allowed the establishment of an aequorin assay which could be performed in the 96-well format. The shape of the concentration-response curves of porcine neuropeptide Y in the presence of the Y(2)-selective receptor antagonist BIIE0246, characteristic of either competitive or insurmountable antagonism, depended on the period of incubation with the cells. Functional data of Y(2) receptor agonists and antagonists determined in the fluorescence- and luminescence-based assays were in good agreement.

A Simple and Powerful Flow Cytometric Method for the Simultaneous Determination of Multiple Parameters at G Protein-Coupled Receptor Subtypes

The quantification of pharmacological parameters at G protein-coupled receptors (GPCRs) is indispensable in drug research but costly and time-consuming when conventional methods are sequentially applied. With neuropeptide Y (NPY) Y(1), Y(2), and Y(5) receptors as model systems, a homogenous flow cytometric method for the simultaneous determination of the affinity, selectivity, and activity of GPCR ligands was developed. Mixtures of cells expressing the receptors of interest and cyanine-labeled NPY as a universal fluorescent Y(1), Y(2), and Y(5) receptor agonist were used. Calcium mobilization was measured in different channels with the aid of fluo-4 and fura red. A combination of dye-loaded HEL-Y(1) and CHO-Y(2)-Galpha(qi5) cells with unloaded HEC-1B-Y(5) cells allowed the simultaneous determination of Y(1), Y(2), and Y(5) receptor selectivity preceded by the Y(1) and Y(2) receptor-mediated response with one and the same sample. The data are in good agreement with those determined by radioligand binding and spectrofluorimetry. The convenient, robust, and inexpensive multiparametric procedure offers a broad range of applications in the pharmacological characterization of GPCR ligands.

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.

Synthesis and pharmacological activity of fluorescent histamine H1 receptor antagonists related to mepyramine

Fluorescently labeled histamine H(1) receptor antagonists were synthesized starting from N-demethylmepyramine by introduction of omega-aminoalkyl chains (2-8 methylene groups in length) followed by derivatization of the terminal NH(2) group with various fluorophores (fluorescein, naphthofluorescein, rhodamine, tetramethylrhodamine, BODIPY, dansyl, and nitrobenzoxadiazole (NBD)). On the isolated guinea pig ileum and in a Ca(2+) assay on U373MG human glioblastoma cells the highest H(1) antagonistic activities were found in 5- and 6-carboxyfluorescein labeled compounds with hexa- and octamethylene spacers and in an analogous NBD-aminohexanoyl derivative (pA(2) or pK(B) values in the range: 8.3-9.0; compared to 9.3-9.4 for mepyramine).

Structure-activity relationships of neuropeptide Y Y1 receptor antagonists related to BIBP 3226

Analogues of BIBP 3226, (R)-N(alpha)-diphenylacetyl-N-(4-hydroxybenzyl)argininamide, were synthesized and investigated for Y1 antagonism (Ca2+-assay, HEL cells) and binding on Y1, Y2 and Y5 receptors. Replacing the benzylamino by a tetrahydrobenzazepinyl group preserves most of the Y1 activity. Combination with a N(G)-phenylpropyl arginine and a N(alpha)-p-biphenylylacetyl moiety shifted the NPY receptor selectivity towards Y5.

Molecular characterization of the ligand-receptor interaction of the neuropeptide Y family

Neuropeptide Y (NPY), peptide YY (PYY) and pancreatic polypeptide (PP) belong to the NPY hormone family and activate a class of receptors called the Y-receptors, and also belong to the large superfamily of the G-protein coupled receptors. Structure-affinity and structure-activity relationship studies of peptide analogs, combined with studies based on site-directed mutagenesis and anti-receptor antibodies, have given insight into the individual characterization of each receptor subtype relative to its interaction with the ligand, as well as to its biological function. A number of selective antagonists at the Y1-receptor are available whose structures resemble that of the C-terminus of NPY. Some of these compounds, like BIBP3226, BIBO3304 and GW1229, have recently been used for in vivo investigations of the NPY-induced increase in food intake. Y2-receptor selective agonists are the analog cyclo-(28/32)-Ac-[Lys28-Glu32]-(25-36)-pNPY and the TASP molecule containing two units of the NPY segment 21-36. Now the first antagonist with nanomolar affinity for the Y2-receptor is also known, BIIE0246. So far, the native peptide PP has been shown to be the most potent ligand at the Y4-receptor. However, by the design of PP/NPY chimera, some analogs have been found that bind not only to the Y4-, but also to the Y5-receptor with subnanomolar affinities, and are as potent as NPY at the Y1-receptor. For the characterization of the Y5-receptor in vitro and in vivo, a new class of highly selective agonists is now available. This consists of analogs of NPY and of PP/NPY chimera which all contain the motif Ala31-Aib32. This motif has been shown to induce a 3(10)-helical turn in the region 28-31 of NPY and is suggested to be the key motif for high Y5-receptor selectivity. The results of feeding experiments in rats treated with the first highly specific Y5-receptor agonists support the hypothesis that this receptor plays a role in the NPY-induced stimulation of food intake. In conclusion, the selective compounds for the different Y receptor subtypes known so far are promising tools for a better understanding of the physiological properties of the hormones of the NPY family and related receptors.

Pharmacology and quantitative structure-activity relationships of imidazolylpropylguanidines with mepyramine-like substructures as non-peptide neuropeptide Y Y1 receptor antagonists

The design of non-peptide, Y1-selective antagonists of neuropeptide Y (NPY) as pharmacological tools is in progress and is increasingly important as therapeutic applications are expected. Starting from the potent histamine H2 agonist and weak NPY Y1 antagonist arpromidine, 16 imidazolylpropylguanidine derivatives were synthesized and tested for Y1 antagonistic activity (inhibition of NPY-stimulated Ca2+ increase in human erythroleukemic cells), where the pheniramine-like moiety of arpromidine was replaced with 2-pyridylaminoalkyl, benzyl-(2-pyridyl)aminoalkyl, and phenyl-(2-pyridyl)alkylaminoalkyl partial structures derived from mepyramine. The pA2 values of the most active compounds are in the range of 6.2-6.5. Quantitative structure-activity relationships (QSAR) were investigated by fragment regression analysis. Results indicate that a tetramethylene spacer between the guanidino group and the amino nitrogen is optimal. For an at least moderate degree of Y1 antagonistic activity, a second benzyl or phenyl group must be present in addition to the 2-pyridyl ring. At this second group, hydrophobic substituents such as 3,4-di-Cl and 4-Br further enhance Y1 antagonism. The most active derivative additionally bears a 5-Br substituent at the 2-pyridyl moiety. Structure-activity relationships suggest that the compounds might be able to partially imitate the role of NPY when interacting with Y1 receptors and thus behave as moderate non-peptide NPY Y1 antagonists.Key words : neuropeptide Y Y1 antagonists, imidazolylpropylguanidines, quantitative structure-activity relationships.

NPY Y1 antagonists: structure-activity relationships of arginine derivatives and hybrid compounds with arpromidine-like partial structures

Previously, omega-guanidino- and omega-aminoalkanamides, structurally derived from arpromidine-like histamine H2 receptor agonists, were reported as novel neuropeptide Y Y1 antagonists. Regardless of the backbone, they resemble BIBP 3226, an argininamide with high NPY Y1 receptor affinity and selectivity, with respect to nature and arrangement of the 'terminal' diaryl, guanidine, and hydroxyphenyl groups. Hybrid compounds were synthesized combining the argininamide backbone of BIBP 3226 or partial structures derived from the C-terminal dipeptide of NPY with characteristic substructures of arpromidine- or amide-type NPY antagonists. Additionally, some analogs of BIBP 3226 with reduced flexibility were prepared. Structure-activity relationships indicate that, in contrast to alkanamides, homologs and/or isomers of BIBP 3226 with vicinal arrangement of the phenyl rings have decreased Y1 antagonistic activity (Ca2+-assay in HEL cells). Replacement of the hydroxybenzyl group by an imidazole ring further decreases activity. It is concluded that the binding sites of NPY antagonists with one and with two basic groups are not identical. Analogs with a rigid tetrahydro-2-benzazepine or an indan group in place of the benzyl moiety in BIBP 3226 are active, indicating the role of the OH group and supporting the model proposed for the interaction of BIBP 3226 with the Y1 receptor.