Prucalopride and donepezil act synergistically to reverse scopolamine-induced memory deficit in C57Bl/6j mice.

It is known that 5-HT(4) receptor agonists increase sAPPalpha levels in the cortex and hippocampus of mice as well as in a model of Alzheimer's disease (AD). As sAPPalpha is thought to have pro-mnesic properties, we assessed whether its increase induces cognitive improvement in a spatial memory task and whether it reverses a scopolamine-induced memory deficit. Mice treated or not treated with scopolamine were trained in the Morris water maze for 3 days. Before the probe test, they received an injection of either a 5-HT(4) receptor agonist (prucalopride or RS 67333), or an acetylcholinesterase inhibitor (donepezil), or both drugs. As expected, scopolamine decreased performance, an effect that was not reversed by the drugs tested when injected alone. However, prucalopride (5 mg kg(-1), s.c.) acted synergistically with donepezil (0.75 mg kg(-1), s.c.) to counteract completely scopolamine-induced amnesia. Western blot analysis of tissue homogenates in the cortex and hippocampus shows that sAPPalpha levels did not differ between saline- and scopolamine-treated mice. Furthermore, a region-dependent drug action was observed since the scopolamine-treated mice display a tendency to increase sAPPalpha levels in the hippocampus after donepezil or in the cortex after prucalopride. Our results suggest that a combined treatment with a 5-HT(4) receptor agonist with an acetylcholinesterase inhibitor has beneficial effects on memory in mice. Moreover, it seems to enhance sAPPalpha levels in two brain regions highly affected in AD. Thus, a drug polytherapy could be interesting not only to enhance cognitive performance and decrease drawbacks but also to get the best action in each brain region.

5-HT4 receptor agonists increase sAPPalpha levels in the cortex and hippocampus of male C57BL/6j mice.

BACKGROUND AND PURPOSE: A strategy to treat Alzheimer's disease (AD) is to increase the soluble form of amyloid precursor protein (sAPPalpha), a promnesic protein, in the brain. Because strong evidence supports beneficial effects of 5-hydroxytryptamine 5-HT(4) receptor agonists in memory and learning, we investigated the role of 5-HT(4) receptors on APP processing in 8 weeks-old male C57BL/6j mice. EXPERIMENTAL APPROACH: Mice were given, subcutaneously, prucalopride or ML 10302 (s.c.), two highly selective 5-HT(4) receptor agonists and, up to 240 min later, the hippocampus and cortex were analysed by Western blot for sAPPalpha determination. KEY RESULTS: Prucalopride (5 or 10 mg kg(-1)) significantly increased sAPPalpha levels in the hippocampus and cortex, but did not modify the expression level of APP mRNA as detected by quantitative RT-PCR. A selective 5-HT(4) receptor antagonist, GR125487 (1 mg kg(-1), s.c.) inhibited prucalopride induced- increase in sAPPalpha levels. In addition, levels of sAPPalpha were increased by ML10302 only at 20 mg kg(-1) and was limited to the cortex. Also, prucalopride increased sAPPalpha levels in the cortex of a transgenic mouse model of AD, expressing the London mutation of APP. Furthermore, the combined injection of a selective acetylcholinesterase inhibitor, donepezil and prucalopride induced a synergic increase in sAPPalpha levels in the cortex and hippocampus. CONCLUSIONS AND IMPLICATIONS: Our results demonstrate that the 5-HT(4) receptor plays a key role in the non-amyloidogenic pathway of APP metabolism in vivo and give support to the beneficial use of 5-HT(4) agonists for AD treatment.

New arylpiperazine derivatives as antagonists of the human cloned 5-HT(4) receptor isoforms.

New derivatives of arylpiperazine 9 were designed from ML 10302, a potent 5-HT(4) receptor agonist in the gastrointestinal system. Compounds were synthesized by condensation of a number of available arylpiperazines or heteroarylpiperazines with 2-bromoethyl 4-amino-5-chloro-2-methoxybenzoate. They were evaluated in binding assays on the recently cloned human 5-HT(4(e)) isoform stably expressed in C6 glial cells with [(3)H]GR 113808 as the radioligand. The affinity values (K(i)) depended upon the substituent on the aromatic ring. A chlorine atom produced a marked drop in activity (K(i) > 100 nM), while a m-methoxy group gave a compound with nanomolar affinity (K(i) = 3 nM). The most potent compounds were the heterocyclic derivatives with pyrimidine, pyrazine, pyridazine, or pyridine moieties (compounds 9r, 9t, 9u, 9x, respectively). K(i) values for 9a and 9r were determined for the 5-HT(4(a)), 5-HT(4(b)), 5-HT(4(c)), and 5-HT(4(d)) receptor isoforms transiently expressed in COS cells. The results indicated that the compounds were not selective. They produced an inhibition of the 5-HT-stimulated cyclic AMP synthesis in the C6 glial cells stably expressing the 5-HT(4(e)) receptor and shifted the 5-HT concentration-effect curve on adenylyl cyclase activity with pK(D) values of 7.44 and 8.47, respectively. In isolated human atrial myocytes, 9r antagonized the stimulatory effect of 5-HT on the L-type calcium current (I(Ca)) with a K(D) value of 0.7 nM.

Pharmacological characterization of the human 5-HT(4(d)) receptor splice variant stably expressed in Chinese hamster ovary cells.

The recently identified C-terminal splice variant of the human 5-HT(4) receptor, the h5-HT(4(d)) receptor, was stably expressed in a CHO cell line at 493+/-25 fmol mg(-1) protein. We analysed its pharmacological properties by measuring binding affinities and 5-HT(4) ligand-induced cyclic AMP production. The pharmacological binding profile determined in competition studies with the specific antagonist [(3)H]-GR113808 revealed a rank order of affinity of 5-HT(4) ligands for the h5-HT(4(d)) receptor that was consistent with those previously reported for other 5-HT(4) receptor isoforms. In adenylyl cyclase functional assays, the h5-HT(4(d)) receptor displayed equipotent coupling for all 5-HT(4) agonists tested (EC(50) in the range of 1 - 6 nM). EC(50) values were lower than those previously obtained with the 5-HT(4(e)) receptor stably expressed in CHO cells indicating that the 5-HT(4(d)) receptor was more efficiently coupled to its effector than the 5-HT(4(e)) receptor isoform. Moreover, in terms of agonist efficacy (E(max)), the benzamide derivative, renzapride displayed full agonist properties at the h5-HT(4(d)) receptor (same E(max) as 5-HT) whereas it was previously shown to be a partial agonist at the h5-HT(4(e)) receptor. A constitutive activity of the h5-HT(4(d)) receptor was observed in CHO cells in the absence of any 5-HT(4) ligand. Surprisingly, two 5-HT(4) ligands, SB204070 and RS39604 which are described as highly potent antagonists in various biological models, revealed partial agonist properties at the h5-HT(4(d)) receptor. We conclude that C-terminal tails of 5-HT(4) receptor isoforms may directly influence their functional properties.

Exploration of the ligand binding site of the human 5-HT(4) receptor by site-directed mutagenesis and molecular modeling.

Among the five human 5-HT(4) (h5-HT(4)) receptor isoforms, the h5-HT(4(a)) receptor was studied with a particular emphasis on the molecular interactions involved in ligand binding. For this purpose, we used site-directed mutagenesis of the transmembrane domain. Twelve mutants were constructed with a special focus on the residue P4.53 of helix IV which substitutes in h5-HT(4) receptors the highly conserved S residue among the rhodopsin family receptors. The mutated receptors were transiently expressed in COS-7 cells. Ligand binding or competition studies with two h5-HT(4) receptor agonists, serotonin and ML10302 and two h5-HT(4) receptor antagonists, [(3)H]-GR113808 and ML10375 were performed on wild type and mutant receptors. Functional activity of the receptors was evaluated by measuring the ability of serotonin to stimulate adenylyl cyclase. Ligand binding experiments revealed that [(3)H]-GR113808 did not bind to mutants P4.53A, S5.43A, F6.51A, Y7.43A and to double mutant F6.52V/N6.55L. On the other hand mutations D3.32N, S5.43A and Y7.43A appeared to promote a dramatic decrease of h5-HT(4(a)) receptor functional activity. From these studies, S5.43 and Y7.43 clearly emerged as common anchoring sites to antagonist [(3)H]-GR113808 and to serotonin. According to these results, we propose ligand-receptor complex models with serotonin and [(3)H]-GR113808. For serotonin, three interaction points were selected including ionic interaction with D3.32, a stabilizing interaction of this ion pair by Y7.43 and a hydrogen bond with S5.43. [(3)H]-GR113808 was also docked, based on the same type of interactions with S5.43 and D3.32: the proposed model suggested a possible role of P4.53 in helix IV structure allowing the involvement of a close hydrophobic residue, W4.50, in a hydrophobic pocket for hydrophobic interactions with the indole ring of [(3)H]-GR113808.

Isolation of the serotoninergic 5-HT4(e) receptor from human heart and comparative analysis of its pharmacological profile in C6-glial and CHO cell lines.

RT - PCR technique was used to clone the human 5-HT(4(e)) receptor (h5-HT(4(e))) from heart atrium. We showed that this h5-HT(4(e)) receptor splice variant is restricted to brain and heart atrium. Recombinant h5-HT(4(e)) receptor was stably expressed in CHO and C6-glial cell lines at 347 and 88 fmol mg(-1) protein, respectively. Expression of h5-HT(4(e)) receptors at the cell membrane was confirmed by immunoblotting. The receptor binding profile, determined by competition with [(3)H]-GR113808 of a number of 5-HT(4) ligands, was consistent with that previously reported for other 5-HT(4) receptor isoforms. Surprisingly, we found that the rank order of potencies (EC(50)) of 5-HT(4) agonists obtained from adenylyl cyclase functional assays was inversely correlated to their rank order of affinities (K(i)) obtained from binding assays. Furthermore, EC(50) values for 5-HT, renzapride and cisapride were 2 fold lower in C6-glial cells than in CHO cells. ML10302 and renzapride behaved like partial agonists on the h5-HT(4(e)) receptor. These results are in agreement with the reported low efficacy of the these two compounds on L-type Ca(2+) currents and myocyte contractility in human atrium. A constitutive activity of the h5-HT(4(e)) receptor was observed in CHO cells in the absence of any 5-HT(4) ligand and two 5-HT(4) antagonists, GR113808 and ML10375, behaved as inverse agonists. These data show that the h5-HT(4(e)) receptor has a pharmacological profile which is close to the native h5-HT(4) receptor in human atrium with a functional potency which is dependent on the cellular context in which the receptor is expressed.

Design, synthesis, and evaluation of conformationally constrained tongs, new inhibitors of HIV-1 protease dimerization.

The active form of human immunodeficiency virus type 1 protease (HIV-1 PR) is a homodimeric structure in which two subunits are linked through a two-stranded antiparallel beta-sheet consisting of the N- and C-termini of each monomer. To inhibit the dimerization process or disrupt the dimeric interface leading to inactive enzyme, conformationally constrained "molecular tongs" have been designed and synthesized to interfere with one monomer end in a beta-sheet fashion. These molecules are based on two peptidic strands attached to an aromatic scaffold. Inhibitions (submicromolar range) were obtained with molecular tongs containing tripeptidic or tetrapeptidic arms attached to a pyridinediol- or naphthalenediol-based scaffold (Kid = 0.56-4.5 microM at pH 4.7 and 30 degrees C). Kinetic studies are in agreement with an interface inhibition mechanism.

Synthesis of benzocycloalkane derivatives as new conformationally restricted ligands for melatonin receptors.

Benzocycloalkane derivatives 1-4 were synthesized as new conformationally restricted melatoninergic ligands. They were prepared by the reaction of the ketones 5 with diethylcyanophosphonate and the reduction of the corresponding cyano compounds or by the Wittig reaction and Curtius degradation to obtain the amines 8. The 1-Cyanobenzocyclobutane derivative was obtained by the benzyne cyclisation reaction. The amines 8 were acylated with acetyl, propionyl or butyryl groups. The affinity of the compounds for chicken brain melatonin receptors was evaluated using 2-[125I]-iodomelatonin as the radioligand. The indanyl (2b,c), tetralin (3a-c) and benzocycloheptane (4c) derivatives were potent compounds with nanomolar affinity and an important enantioselectivity of the receptor was observed with the (+) enantiomers 2b and 3b.

New esters of 4-amino-5-chloro-2-methoxybenzoic acid as potent agonists and antagonists for 5-HT4 receptors.

A number of benzoates derived from 4-amino-5-chloro-2-methoxybenzoic acid and substituted 1-piperidineethanol were synthesized and found to be potent 5-HT4 receptor agonists in the electrically-stimulated myenteric plexus and longitudinal muscle of the guinea pig ileum and the rat esophagus muscle. Monosubstitution of the piperidine ring with Me, OH, NH-Ac, or CONH2 groups gave compounds equipotent to 7a (ML 10302), a 5-HT4 receptor agonist previously reported to have nanomolar affinity. 7a,k were as potent as serotonin (5-HT) but had maximal responses which were only 60-80% of that of 5-HT, suggesting a partial agonist profile for these compounds. Binding assays were performed with [3H]GR 113808 in the rat striatum, and several of these compounds were found to have nanomolar affinity for 5-HT4 receptors (7a, Ki = 1.07 +/- 0.5 nM; 7k, Ki = 1.0 +/- 0.3 nM). The introduction of two methyl groups on the piperidine ring brought about a dramatic change in the pharmacological profile of 2-[(cis- and trans-3,5-dimethylpiperidinyl)ethyl]-4-amino-5-chloro-2- methoxybenzoate, 7g,h. 7g (Ki = 0.26 +/- 0.06 nM) inhibited the relaxant action of 5-HT in the rat esophagus muscle with a pA2 value of 8.6. The advantage of the ester function was demonstrated by comparing the activity of several such compounds at 5-HT4 receptors with those of the corresponding amidic derivatives. This difference was less marked when the basic moiety was sterically constrained as in the quinuclidine and tropane moieties. Structural analyses of 7a,g were performed by determining their X-ray crystal structures and by molecular modeling (SYBYL). A relatively limited number of minimum energy conformers was found for both compounds. They were characterized by the cis folded conformation of the ethyl chain and by the orientation of the lone pair of the nitrogen atom pointing out of the molecule as seen in conformationally-constrained benzamides such as zacopride and renzapride. A hypothetical model for the 5-HT4 receptor with two sites for the binding of agonist and antagonist molecules was proposed.

Three-dimensional quantitative structure-activity relationship of melatonin receptor ligands: a comparative molecular field analysis study.

A three-dimensional quantitative structure-activity relationship using the comparative molecular field analysis (CoMFA) paradigm applied to 57 melatonin receptor ligands belonging to diverse structural families was performed. The compounds studied which have been synthesized previously and reported to be active at chicken brain melatonin receptors were divided into a training set of 48 molecules and a test set of 9 molecules. As most of these compounds have a highly flexible ethylamido side chain, the alignments were based on the most sterically constrained molecule which contains a tricyclic phenalene structure. This tricyclic compound can adopt an axial and an equatorial conformation. Two different molecular superpositions representing possible positioning within the receptor site have been suggested previously. CoMFA was tentatively used to discriminate between alternate hypothetical biologically active conformation and between possible positionings. The best 3D quantitative structure-activity relationship model found yields significant cross-validated, conventional, and predictive r2 values equal to 0.798, 0.967, and 0.76, respectively, along with an average absolute error of prediction of 0.25 log units. These results suggest that the active conformation of the most flexible molecules including melatonin is in a folded form if we consider the spatial position of the ethylamido side chain relative to the aromatic ring.

Synthesis of 2-amido-2,3-dihydro-1H-phenalene derivatives as new conformationally restricted ligands for melatonin receptors.

Tetrahydroanthracene, tetrahydrophenanthrene, and tetrahydrophenalene moieties were used to design novel constrained melatoninergic agents. Compounds 1 and 2 were synthesized from the cyclization of the aryl succinic acids 6a,b followed by catalytic reduction, Curtius degradation to the amino derivatives, and acetylation. The phenalene derivatives 3 were prepared by cyclization of the aza lactones of the corresponding alpha-N-acetyl amino acids. The ketone derivatives were reduced directly by catalytic hydrogenation to produce the compounds 3. The different compounds were evaluated in vitro in binding assays using 2-[125I] iodomelatonin and chicken brain membranes. Melatonin and 2-acetamido-8-methoxytetralin were used as the reference compounds. The results showed the superiority of the dihydrophenalene framework 3 over those of tetrahydroanthracene and tetrahydrophenanthrene. 3a had relatively good affinity for melatonin receptors (Ki = 28.7 nM). Introduction of an additional methoxy group gave a derivative (3c) with nanomolar affinity (Ki = 0.7 nM), confirming the existence of a secondary binding site in the receptor which has been described previously. An increase in the affinity was also observed with the propionamido derivative 3e (Ki = 6.0 nM). The potential agonist properties of the compound 3e were evaluated on the dermal melanocytes of Xenopus laevis tadpoles. At the concentration of 2.3 nM (5 x Ki), melatonin gave a melanophore index value of 1. Similarly to melatonin, 3e was shown to be a potent agonist of the melanosome aggregation.

Design and synthesis of new naphthalenic derivatives as ligands for 2-[125I]iodomelatonin binding sites.

New melatonin-like agents were designed from the frameworks of 2,5-dimethoxyphenethylamine, an important structural moiety for the 5-HT receptor, and (2-methoxynaphthyl)-ethylamine. The compounds were synthesized by classical methods and evaluated in binding assays with chicken brain membranes using 2-[125I]iodomelatonin as the radioligand. Preliminary studies on the series of N-acyl-disubstituted phenethylamines showed the favorable role of the methoxy group in the ortho position of the side chain on the affinity for the receptor (Ki = 8 +/- 0.2 nM) for N-[2-(2-methoxy-5-bromophenyl)ethyl]propionamide (3o). This effect was confirmed in a series of the naphthalene derivatives, a bioisosteric moiety of the indole ring, and several potent ligands for melatonin binding sites were prepared such as N-[2-(2-methoxynaphthyl)ethyl]propionamide (4b) (Ki = 0.67 +/- 0.05 nM) and N-[2-(2,7-dimethoxynaphthyl)ethyl]cyclopropylformamide (Ki = 0.05 +/- 0.004 nM) (4k). Structure-activity relationships are discussed with regard to melatonin and bioisosteric naphthalenic compound 2. The Ki value for 4b was affected to a similar extent to that of melatonin by GTP-gamma-S or Mn2+ in competition experiments, suggesting an agonist profile for this compound.

Synthesis and biological activity of N6-(N[(4-azido-3,5,6-trifluoro)-pyridin-2-yl]-2-aminoethyl)-adenosine 5'-monophosphate, a new AMP photoactivatable derivative. Covalent modification of horse liver alcohol dehydrogenase.

N6-(N-[(4-Azido-3,5,6-trifluoro)pyridin-2-yl]-2-aminoethyl)- adenosine 5'-monophosphate has been synthesized and evidence presented for its structural assignment by ultraviolet and 19F-NMR spectroscopies. Its photolysis was shown to occur within 5 min. This AMP derivative behaves as a competitive inhibitor of NAD+ in horse-liver-alcohol-dehydrogenase-promoted oxidation of ethanol, with a Ki (0.95 mM) comparable to the Ki of AMP (1.9 mM). Moreover it is an activator of the enzyme when nicotinamide ribose is used as the oxidation cofactor. This activation is as good as that promoted by AMP or by the well known 8-azido-AMP. Upon photolysis of this new derivative in the presence of horse liver alcohol dehydrogenase, a covalent enzyme--analogue complex was isolated and assayed as a catalyst in the oxidation of ethanol using nicotinamide ribose as the cofactor. The reaction took place without complementation of AMP, indicating clearly that the AMP analogue is mainly covalently bound in the AMP-binding site, and that the linkage formed between the enzyme and the azido derivative has not dramatically altered the active site of the enzyme. A similar experiment with 8-azido-AMP produced a completely inactive complex.

Activity of NMN+, nicotinamide ribose and analogs in alcohol oxidation promoted by horse-liver alcohol dehydrogenase. Improvement of this activity and structural requirements of the pyridine nucleotide part of the NAD+ coenzyme.

Kinetic constants of horse-liver alcohol-dehydrogenase-mediated oxidation of alcohol by nicotinamide mononucleotide and nicotinamide ribose were determined and the role of different adenine moieties complementing the reaction mixtures was investigated. Five nicotinamide ribose analogs were synthesized and their activities as NAD+ inhibitors and as cofactors in this dehydrogenase-mediated oxidation of alcohol were assayed. In the light of these results, structural requirements of the pyridine ribofuranosyl part of the NAD+ are discussed.

A new approach for using cofactor dependent enzymes: example of alcohol dehydrogenase.

The use of enzymes requiring a cofactor as substrate in organic synthesis is still a problem since the cofactors are expensive. This study deals with a new approach consisting of using fragments of NAD+. Three fragments of NAD(H) are examined. The activities of NMN+ and NMNH are greatly improved by the addition of adenosine in ethanol oxidation and in cyclohexanone reduction, respectively. Nicotinamide mononucleoside is not active in the ethanol oxidation but the addition of AMP promotes this reaction.

[Hydrosoluble polymers of NAD+ and ADP. Study of their coenzymatic properties as a function of their size]. / Polymères hydrosolubles de NAD+ et d'ADP. Etude de leurs propriétés coenzymatiques en fonction de leur taille.

Hydrosoluble polymers of NAD+ and ADP were synthesized according to a described method with some modifications. The cofactor was bound to the matrix by a spacer group of five atoms at the exocyclic adenine C-6 amino group. Cofactor incorporations were very high. The loading of NAD+ polymers were: 420 mumol NAD+/g polymer and 330 mumol ADP ribose/g polymer arising from degradation of NAD+; the loading of ADP polymers were: 1.40 and 1.43 mmol ADP/g polymer. Each polymer obtained in a same polymerization step was fractionated in two parts by gel filtration in such a manner that these two parts had the same framework but different molecular weights. The biological properties of the fractionated polymers were compared. The small sized polymers of NAD+, assayed with four dehydrogenases, were more reduced enzymatically than the larger ones and had the higher reduction rates relative to free NAD+. In a coupled system (L-lactate dehydrogenase, formate dehydrogenase) with coenzyme regeneration, the rate of production of L-lactate at the steady state of the system using NAD+ polymers of small size was 80% that of the system using free NAD+. The small-sized polymers of ADP were more phosphorylated by pyruvate kinase and creatine kinase than the larger ones. But the relative rates of transformation by pyruvate kinase of large sized polymers were higher than those of smaller ones.

Effects of N-alkylation and n-acylation on tobramycin activity.

The activities of tobramycin derivatives acetylated and ethylated on the 6'-N,2'-N and 3-N positions were examined. The MICs of these derivatives against tobramycin sensitive strains indicated that 2'-N-ethylated and 6'-N-ethylated derivatives have a fairly good activity, and confirmed that the 3-N position is the most important one for antibiotic activity since 3-N derivatives were less active. The MICs of these derivatives against tobramycin resistant strains, and their inactivation by tobramycin modifying enzymes were examined. These results showed that 2'-N or 6'-N ethylation protects the drug against inactivation by AAC(2') or AAC(6'), respectively, and 2'-N-ethyltobramycin and 6'-N-ethyltobramycin were active against strains containing these modifying enzymes. On the other hand, 3-N ethylation protects the drug against inactivation by AAC(3) but 3-N-ethyl tobramycin does not inhibit strains containing this enzyme.

A two-step synthesis of new water-soluble polymers of NAD+ and ADP. The biological properties of these polymers.

Alkylation at the N-1 position of the adenine moiety of NAD+, ADP or ATP with 2,3-epoxypropyl acrylate, followed by polymerization with or without acrylamide at pH 8, gave water-soluble polymers of NAD+ and ADP where the alkyl chain was located at the exocyclic adenine C-6 amino group. Cofactor incorporations were good to high: 145-447 mumol NAD+/g polymer and 667 mumol ADP/g polymer. About 30% of the bound NAD+ could be reduced with rabbit muscle lactae dehydrogenase, yeast alcohol dehydrogenase and Bacillus subtilis alanine dehydrogenase; 84% of the bound ADP was phosphorylated with rabbit muscle creatine kinase. High cofactor activities were obtained with polymerized NAD+ with alcohol dehydrogenase as enzyme: the initial rate of NAD+ polymer reduction was 35-81% that of free NAD+. These values remained substantially high with agarose-immobilized alcohol dehydrogenase (15-36%) and should eventually allow their use in continuous enzymatic reactors. Enzymatic phosphorylation of ADP polymer by creatine kinase gave an ATP polymer with high biological activity: 480 mumol ATP/g polymer were transformed with yeast hexokinase.