Synthesis of 3a,4-dihydro-3H-[1]benzopyrano[4,3-c]isoxazoles,displaying combined 5-HT uptake inhibiting and alpha(2)-adrenoceptor antagonistic activities: a novel series of potential antidepressants

The synthesis of a series of novel 3-piperazinylmethyl-3a,4-dihydro-3H-[1]benzopyrano[4,3-c]isoxazoles as novel dual 5-HT reuptake inhibitors and alpha(2)-adrenoceptor antagonists is described. Their affinity at the three different human alpha(2)-adrenoceptor subtypes and the 5-HT transporter site is reported. The in vivo activity of the compounds was measured in two different assays: (1). inhibition of pCA-induced excitation, which evaluates the ability to block the central 5-HT transporter, and (2). inhibition of xylazine-induced loss of righting, which evaluates the ability to block central alpha(2)-adrenoceptors.     

Studies on the Role of α2-Adrenoceptors in the Control of Synaptosomal [3H]5-Hydroxytryptamine Release: Effects of Antidepressant Drugs

The sensitivity of alpha 2-adrenoceptors on 5-hydroxytryptamine (5-HT) nerve endings obtained from rat cerebral cortex was investigated following treatment with the antidepressant drugs desipramine (10 mg/kg/day for 21-28 days) or clorgyline (1 mg/kg/day for 21-28 days). [3H]5-HT (100 nM) was used to load cortical synaptosomes (P2) after experiments with uptake inhibitors confirmed that this concentration of amine ensured exclusive uptake into 5-HT nerve terminals. The sensitivity of K+-stimulated release of [3H]5-HT to alpha 2-adrenoceptor occupancy was assessed in a superfusion system by means of the dose-dependent inhibition of [3H]5-HT release by clonidine. This is blocked by yohimbine (1 microM), which, when administered alone, enhances release, suggesting that endogenous catecholamines released from other synaptosomes act on these alpha 2-heteroreceptors. The effect of addition of citalopram (1 microM) to superfusates suggests that some reuptake of [3H]5-HT occurs during superfusion. Of the tritium released into superfusates during "background" and K+-stimulated release, 17 and 90%, respectively is [3H]5-HT. The attenuation of K+-stimulated release by clonidine is apparently diminished by the chronic clorgyline regimen but not by desipramine. However, clorgyline elevates catecholamine levels, and this might increase endogenous noradrenaline (NA) efflux, which by competition with clonidine could appear to alter alpha 2-adrenoceptor sensitivity. This possibility was investigated by depleting NA with the neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4). These studies showed that the apparent effect of chronic clorgyline on alpha 2-adrenoceptor sensitivity to clonidine was due to competition with increased levels of endogenous NA.(ABSTRACT TRUNCATED AT 250 WORDS)     

Synthesis of 3a,4-dihydro-3H-[1]benzopyrano[4,3-c]isoxazoles, displaying combined 5-HT uptake inhibiting and alpha2-adrenoceptor antagonistic activities. Part 2: Further exploration on the cinnamyl moiety

In our previous paper we have described the synthesis of a series of 3-piperazinylmethyl-3a,4-dihydro-3H-[1]benzopyrano[4,3-c]isoxazoles, as novel dual 5-HT reuptake inhibitors and alpha2-adrenoceptor antagonists. That investigation led to the identification of the cinnamyl fragment as the most suitable moiety for combined activity. This paper outlines a further optimisation programme, focused on the exploration of the aromatic ring present on the cinnamyl moiety of compounds 1, 2 and 3.     

1,2,4]Triazole derivatives as 5-HT(1A) serotonin receptor ligands.

A series of new 4-amino-3-[3-[4-(2-methoxy or nitro phenyl)-1-piperazinyl] propyl]thio]-5-(substitutedphenyl)[1,2,4]triazoles 11a-t was synthesized in order to obtain compounds with high affinity and selectivity for 5-HT(1A) receptor over the alpha(1)-adrenoceptor. A series of isomeric 4-amino-2-[3-[4-(2-methoxy or nitro phenyl)-1-piperazinyl]propyl]-5-(substitutedphenyl)-2,4-dihydro-3H[1,2,4]triazole-3-thiones 12a-r was also isolated and characterized. New compounds were tested to evaluate their affinity for 5-HT(1A) receptor and alpha(1)-adrenoceptor in radioligand binding experiments. As a general trend, triazoles 11a-t showed a preferential affinity for the 5-HT(1A) receptor whereas isomeric 2,4-dihydro-3H[1,2,4]triazole-3-thiones 12a-r preferentially bind to the alpha(1)-adrenoceptor site. Several molecules showed affinities in the nanomolar range and 4-amino-3-[3-[4-(2-methoxyphenyl)-1-piperazinyl]propyl]thio]-5-(4-propyloxy-phenyl)[1,2,4]triazole (11o) was the most selective derivative for the 5-HT(1A) receptor (K(i) alpha(1)/K(i) 5-HT(1A)=55). The decrease in 5-HT(1A) receptor selectivity in 3-[3-[4-(2-methoxyphenyl)-1-piperazinyl]propyl]thio]-5-(substitutedphenyl)[1,2,4] triazole 14a-b, lacking in the amino group in 4-position of the triazole ring, in comparison with their analogues in the series 11a-t, suggest that the amino function represents a critical structural feature in determining 5-HT(1A) receptor selectivity in this class of compounds.     

Synthesis and biodistribution of [11C]R107474, a new radiolabeled alpha2-adrenoceptor antagonist

R107474, 2-methyl-3-[2-(1,2,3,4-tetrahydrobenzo[4,5]furo[3,2-c]pyridin-2-yl)ethyl]-4H-pyrido[1,2-a]pyrimidin-4-one, was investigated using in vitro and in vivo receptor assays and proved to be a potent and relatively selective alpha(2)-adrenoceptor antagonist. Performed assays in vitro were inhibition of binding to a large number of neurotransmitter receptor sites, drug receptor binding sites, ion channel binding sites, peptide receptor binding sites, and the monoamine transporters in membrane preparations of brain tissue or of cells expressing the cloned human receptors. The compound has subnanomolar affinity for halpha(2A)- and halpha(2C)-adrenoceptors (K(i) = 0.13 and 0.15 nM, respectively) and showed nanomolar affinity for the halpha(2B)-adrenoceptors and 5-hydroxytryptamine(7) (h5-HT(7)) receptors (K(i) = 1 and 5 nM, respectively). R107474 interacted weakly (K(i) values ranging between 81 and 920 nM) with dopamine-hD(2L), -hD(3) and -hD(4), h5-HT(1D)-, h5-HT(1F)-, h5-HT(2A)-, h5-HT(2C)-, and h5-HT(5A) receptors. The compound, tested up to 10 microM, interacted only at micromolar concentrations or not at all with any of the other receptor or transporter binding sites tested in this study. In vivo alpha(2A)- and alpha(2C)-adrenoceptor occupancy was measured by ex vivo autoradiography 1h after subcutaneous (sc) administration of R107474. It was found that R107474 occupies the alpha(2A)- and alpha(2C)-adrenoceptors with an ED(50) (95% confidence limits) of 0.014 mg/kg sc (0.009-0.019) and 0.026 mg/kg sc (0.022-0.030), respectively. Radiolabeled 2-methyl-3-[2-([1-(11)C]-1,2,3,4-tetrahydrobenzo[4,5]furo[3,2-c]pyridin-2-yl)ethyl]-4H-pyrido[1,2-a]pyrimidin-4-one ([(11)C]R107474) was prepared and evaluated as a potential positron emission tomography (PET) ligand for studying central alpha(2)-adrenoceptors. [(11)C]R107474 was obtained via a Pictet-Spengler reaction with [(11)C]formaldehyde in 33 +/- 4% overall decay-corrected radiochemical yield. The total synthesis time was 55 min and the specific activity was 24-28 GBq/micromol. The biodistribution of [(11)C]R107474 in rats revealed that the uptake of [(11)C]R107474 after in vivo intravenous administration is very rapid; in most tissues (including the brain) it reaches maximum concentration at 5 min after tracer injection. In agreement with the known distribution of alpha(2)-adrenoceptors in the brain, highest uptake of radioactivity was observed in septum (3.54 +/- 0.52 ID/g, 5 min pi) and entorhinal cortex (1.57 +/- 0.10 ID/g, 5 min pi). Tissue/cerebellum concentration ratios for septum (5.38 +/- 0.45, 30 min pi) and entorhinal cortex (3.43+/-0.24, 30 min pi) increased with time due to rapid uptake followed by a slow washout. In vivo blocking experiments using the non-selective alpha(2)-adrenoceptor antagonist mirtazapine demonstrated specific inhibition of [(11)C]R107474 binding in selective brain areas. The receptor binding profile of mirtazapine is reported and the selectivity of inhibition of binding is discussed. These results suggest that [(11)C]R107474 deserves further investigation as a potential radioligand for studying alpha(2)-adrenoceptors using PET.     

Modulation of selective serotonin reuptake inhibitor and 5-HT1A antagonist activity in 8-aza-bicyclo[3.2.1]octane derivatives of 2,3-dihydro-1,4-benzodioxane

2,3-Dihydro-1,4-benzodioxanes with aryl 8-aza-bicyclo[3.2.1]oct-3-ene attachments 2 produce compounds with potent 5-HT-T affinity, and weak 5-HT(1A) affinity and alpha(1) affinity. This compares with 2,3-dihydro-1,4-benzodioxanes containing 8-aza-bicyclo[3.2.1] octan-3-ol attachments 4 which possess potent 5-HT(1A) affinity, moderate to good selectivity over alpha(1) and little 5-HT-T affinity. A 3-benzothiophene analogue of 4 (30) was synthesized which possesses potent 5-HT(1A) affinity and especially good selectivity over both alpha(1) and 5-HT-T.     

Development of novel 2-[4-(aminoalkoxy)phenyl]-4(3H)-quinazolinone derivatives as potent and selective histamine H3 receptor inverse agonists

Novel 2-[4-(aminoalkoxy)phenyl]-4(3H)-quinazolinone derivatives were identified as potent human H(3) receptor inverse agonists. After systematic modification of lead 5a, the potent and selective analog 5r was identified. Elimination of hERG K(+) channel and human alpha(1A)-adrenoceptor activities is the main focus of the present study.     

3-Arylpiperazinylethyl-1H-pyrrolo[2,3-d]pyrimidine-2,4(3H,7H)-dione derivatives as novel, high-affinity and selective alpha(1)-adrenoceptor ligands

The discovery of a new series of selective and high-affinity alpha(1)-adrenoceptor (alpha(1)-AR) ligands, characterized by a 1H-pyrrolo[2,3-d]-pyrimidine-2,4(3H,7H)-dione system, is described in this paper. Some synthesized compounds, including 20, 22, and 30, displayed affinity in the nanomolar range for alpha(1)-ARs and substantial selectivity with respect to 5-HT(1A) and dopaminergic D(1) and D(2) receptors. Functional assays, performed on selected derivatives, showed antagonistic properties.     

Synthesis and structure-activity relationship of 2-(aminoalkyl)-3,3a,8,12b-tetrahydro-2H-dibenzocyclohepta[1,2-b]furan derivatives: a novel series of 5-HT(2A/2C) receptor antagonists

Following the program started at Johnson & Johnson Pharmaceutical Research & Development searching for 5-HT(2A/2C) antagonists we now report on the synthesis of a series of substituted 2-(aminomethyl)-3,3a,8,12b-tetrahydro-2H-dibenzocyclohepta[1,2-b]furan derivatives. The 5-HT2A, 5-HT2C and H1 receptor affinities of the described compounds are reported. The mCCP antagonistic activity of a set of selected molecules is also reported.     

Advances toward new antidepressants beyond SSRIs: 1-aryloxy-3-piperidinylpropan-2-ols with dual 5-HT1A receptor antagonism/SSRI activities. Part 1

A series of 1-aryloxy-3-piperidinylpropan-2-ols possessing potent dual 5-HT(1A) receptor antagonism and serotonin reuptake inhibition was discovered. 1-(1H-Indol-4-yloxy)-3-(4-benzo[b]thiophen-2-ylpiperidinyl)propan-2-ols exhibited selective and high affinity at the 5-HT(1A) receptor and serotonin reuptake inhibition at nanomolar concentrations for dual activities.     

Characterization of the 5-Hydroxytryptamine Receptor Modulating the Release of 5-[3H]Hydroxytryptamine in Slices of the Human Neocortex

In the rat brain, the presynaptic 5-hydroxytryptamine (5-HT) autoreceptors located on 5-HT terminals correspond to the 5-HT1B subtype. The presence of a 5-HT receptor probably located on 5-HT nerve endings and modulating transmitter release in the human neocortex has been reported, but its detailed pharmacological characterization is not yet available. On the other hand, receptor binding and autoradiographic results indicate that the 5-HT1B receptor subtype is not present in the human brain. We, therefore, studied the modulation of the electrically evoked release of [3H]5-HT by various 5-HT receptor agonists and antagonists in preloaded slices of human neocortex obtained from 18 patients undergoing neurosurgery. The nonselective 5-HT1A/1B/1D receptor agonist 5-carboxamidotryptamine produced a potent inhibition (70% at 0.03 microM) of the electrically evoked release of [3H]5-HT which was blocked by 5-HT receptor antagonists with the following relative order of potency: methiothepin greater than metergoline = methysergide greater than propranolol. The selective 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin at 0.1 microM did not modify the electrically evoked release of [3H]5-HT. The 5-HT1A/1B receptor agonist RU 24969 was 10 times more potent at inhibiting [3H]5-HT overflow in the rat frontal cortex than in the human neocortex. The potent 5-HT1B receptor antagonist cyanopinodolol did not modify the 5-carboxamidotryptamine-induced inhibition of the electrically evoked release of [3H]5-HT in slices of the human neocortex, but produced by itself a small inhibition of [3H]5-HT overflow.(ABSTRACT TRUNCATED AT 250 WORDS)     

CB1-cannabinoid receptors are involved in the modulation of non-synaptic [3H]serotonin release from the rat hippocampus

In the present study we investigated whether serotonin release in the hippocampus is subject to regulation via cannabinoid receptors. Both rat and mouse hippocampal slices were preincubated with [3H]serotonin ([3H]5-HT) and superfused with medium containing serotonin reuptake inhibitor citalopram hydrobromide (300 nM). The cannabinoid receptor agonist R(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4-benzoxazinyl]-(1-naphthalenyl) methanone mesylate (WIN55,212-2, 1 microM) did not affect either the resting or the electrically evoked [3H]5-HT release. In the presence of the ionotropic glutamate receptor antagonists D(-)-2-amino-5-phosphonopentanoic acid (AP-5, 50 microM) and 6-cyano-7-nitroquinoxaline-2,3-dione-disodium (CNQX, 10 microM) the evoked [3H]5-HT release was decreased significantly. Similar findings were obtained when CNQX (10 microM) was applied alone with WIN55,212-2. This effect was abolished by the selective cannabinoid receptor subtype 1 (CB1) antagonists N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (SR141716, 1 microM) and 1-(2,4-dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-1-piperidinyl-1H-pyrazole-3-carboxamide trifluoroacetate salt (AM251, 1 microM). Similarly to that observed in rats, WIN55,212-2 (1 microM) decreased the evoked [3H]5-HT efflux in wild-type mice (CB1+/+). The inhibitory effect of WIN55,212-2 (1 microM) was completely absent in hippocampal slices derived from mice genetically deficient in CB1 cannabinoid receptors (CB1-/-). Relatively selective degeneration of fine serotonergic axons by the neurotoxin parachloramphetamine (PCA) reduced significantly the tritium uptake and the evoked [3H]5-HT release. In addition, PCA, eliminated the effect of WIN55,212-2 (1 microM) on the stimulation-evoked [3H]5-HT efflux. In contrast to the PCA-treated animals, WIN55,212-2 (1 microM) reduced the [3H]5-HT efflux in the saline-treated group. Our data suggest that a subpopulation of non-synaptic serotonergic afferents express CB1 receptors and activation of these CB1 receptors leads to a decrease in 5-HT release.     

Alpha2-adrenergic agonist modulation of [35S]GTPgammaS binding to guanine-nucleotide-binding-proteins in human platelet membranes.

Alpha2-adrenoceptor (alpha2-AR)-regulated binding of the labelled GTP analog, guanosine 5'-O-(3-[35S]thiotriphosphate) ([35S]GTPgammaS), to guanine-nucleotide-binding proteins (G proteins) was studied in human platelet membranes. Under optimal conditions, the potent alpha2-AR agonist, 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14304), increased the binding of [35S]GTPgammaS up to approximately 1.8 fold, with half-maximal increase at 60 nM and was competitively inhibited by the alpha2-AR antagonist Rauwolscine. The actions of both UK 14304 and Rauwolscine were modulated by monovalent and divalent cation levels, as well as by the concentrations of GDP. [35S]GTPgammaS binding induced by UK 14304 had a Kd value of 4.5 +/- 0.3 nM and a Bmax value of 4.15 +/- 0.40 pmol/mg protein. The rank order of potencies of adrenergic ligands tested in stimulating [3S]GTPgammaS binding and inhibiting forskolin-stimulated c-AMP accumulation was UK 14304> Guanabenz acetate> Oxymetazoline hydrochloride> B-HT 920 dihydrochloride> p-Aminoclonidine hydrochloride> Clonidine hydrochloride. The data presented indicate that enhancement of [35S]GTPgammaS binding by alpha2-AR in human platelet membranes provides a simple functional measure for receptor activation and can be used for determination of potencies and efficacies of ligands at the alpha2-AR.     

Advances toward new antidepressants beyond SSRIs: 1-aryloxy-3-piperidinylpropan-2-ols with dual 5-HT1A receptor antagonism/SSRI activities. Part 5

A series of 1-aryloxy-3-piperidinylpropan-2-ols possessing potent dual 5-HT1A receptor antagonism and serotonin reuptake inhibition was discovered. 1-(1H-Indol-4-yloxy)-3-(4-benzo[b]thiophen-2-ylpiperidinyl)propan-2-ols exhibited selective and high affinities at the 5-HT1A receptor and serotonin reuptake site in vitro. In vivo evaluation of this series of compounds demonstrated elevated extracellular serotonin levels from the basal and quick recovery of neuron firing that was presumably suppressed by the initial acute activation of 5-HT1A somatodendritic autoreceptors.     

Effects of 1-Methyl-4-Phenyl-1,2,5,6-Tetrahydropyridine and Related Compounds on the Uptake of [3H]3,4-Dihydroxyphenylethylamine and [3H]5-Hydroxytryptamine in Neostriatal Synaptosomal Preparations

1-Methyl-4-phenyl-1,2,5,6-tetrahydropyridine (MPTP) is known to cause a destruction of the dopaminergic nigrostriatal pathway in certain animal species including mice. MPTP and some structurally related analogs were tested in vitro for their capacity to inhibit the uptake of [3H]3,4-dihydroxyphenylethylamine-([3H]DA), [3H]5-hydroxytryptamine ([3H]5-HT), and [3H]gamma-aminobutyric acid [( 3H]GABA) in mouse neostriatal synaptosomal preparations. MPTP was a very potent inhibitor of [3H]5-HT uptake (IC50 value 0.14 microM), a moderate inhibitor of [3H]DA uptake (IC50 value 2.6 microM), and a very weak inhibitor of [3H]GABA uptake (no significant inhibition observed at 10 microM MPTP). In other experiments, MPTP caused some release of previously accumulated [3H]DA and [3H]5-HT, but in each case MPTP was considerably better as an uptake inhibitor than as a releasing agent. The 4-electron oxidation product of MPTP, i.e., 1-methyl-4-phenyl-pyridinium iodide (MPP+), was a very potent inhibitor of [3H]DA uptake (IC50 value 0.45 microM) and of [3H]5-HT uptake (IC50 value 0.78 microM) but MPP+ was a very weak inhibitor of [3H]GABA uptake. These data may have relevance to the neurotoxic actions of MPTP.     

Tricyclic isoxazolines: identification of R226161 as a potential new antidepressant that combines potent serotonin reuptake inhibition and alpha2-adrenoceptor antagonism

In previous articles we have described the discovery of a new series of tricyclic isoxazolines combining central serotonin (5-HT) reuptake inhibition with alpha(2)-adrenoceptor antagonistic activity. We report now on the synthesis, the in vitro binding potency and the primary in vivo activity of six enantiomers within this series, one of which was selected for further pharmacological evaluation and assigned as R226161. Some additional in vivo studies in rats are described with this compound, which proved to be centrally and orally active as a combined 5-HT reuptake inhibitor and alpha(2)-adrenoceptor antagonist.     

Synthesis and SAR of 3-arylsulfonyl-pyrazolo[1,5-a]pyrimidines as potent serotonin 5-HT6 receptor antagonists

Syntheses of a series of novel 3-sulfonyl-pyrazolo[1,5-a]pyrimidines and their 5-HT(6) receptor antagonistic structure-activity relationship are disclosed. The nature and position of substituents, which affect their receptor antagonistic activity, are analyzed. Among all synthesized derivatives, {3-(3-chlorophenylsulfonyl)-5,7-dimethyl-pyrazolo[1,5-a]pyrimidin-2-yl}-methyl-amine 33 (K(i)=190 pM), (3-phenylsulfonyl-7-methyl-pyrazolo[1,5-a]pyrimidin-2-yl)-methyl-amine 44 (K(i)=240 pM), (3-phenylsulfonyl-5-metoxymethyl-7-methyl-pyrazolo[1,5-a]pyrimidin-2-yl)-methyl-amine 50 (K(i)=270 pM), and (3-phenylsulfonyl-5-methyl-7-metoxymethyl-pyrazolo[1,5-a]pyrimidin-2-yl)-methyl-amine 52 (K(i)=280 pM) are the most potent antagonists of the 5-HT(6) receptors.     

Synthesis and biological evaluation of novel hexahydro-pyrido[3',2':4,5]pyrrolo[1,2-a]pyrazines as potent and selective 5-HT(2C) receptor agonists

Further lead optimization efforts on previously described 1,2,3,4,10,10a-hexahydro-1H-pyrazino[1,2-a]indoles led to the new class of 5,5a,6,7,8,9-hexahydro-pyrido[3',2':4,5]pyrrolo[1,2-a]pyrazines culminating in the discovery of (5aR,9R)-2-[(cyclopropylmethoxy)methyl]-5,5a,6,7,8,9-hexahydro-9-methyl-pyrido[3', 2':4,5]pyrrolo[1,2-a]pyrazine 18 as a potent, full 5-HT(2C) receptor agonist with an outstanding selectivity profile and excellent hERG and phospholipidosis properties.     

Distribution of [3H]noradrenaline and [3H]serotonin in photophores of Porichthys notatus

Summary Photophores of Porichthys notatus were examined by electron-microscopic radioautography following incubation in tritiated noradrenaline ([³H]NA) or serotonin ([³H]5-HT). Nerve varicosities surrounding the photocytes were found to accumulate [³H]NA but not [³H]5-HT, providing compelling evidence for the catecholaminergic nature of the monoaminergic innervation of photophores. The photocytes themselves appeared selectively labelled with both tracers, but the intensity of labelling after [³H]5-HT incubation was considerably greater than after [³H]NA. Stereological sampling of organelle content in photocytes showed ultrastructural differences between [³H]NA- and [³H]5-HT-labelled cells, probably related to light emission induced by NA. The main changes noted after incubation with [³H]NA were mitochondrial swelling and disorganization, increased coalescence of photocytic vesicles and extrusion of vesicular material into the extracellular matrix. With respect to the subcellular localization of [³H]NA and [³H]5-HT within the photocytes, statistical analysis of the distribution of silver grains disclosed a preferential affinity of both labels for appositional zones between mitochondria and coalescent vesicles. Moreover, in the case of 5-HT, selective affinity was also exhibited by sites comprising vesicular membrane and adjacent cytoplasm, suggesting binding of this biogenic amine to the entire membrane of photocytic vesicles.Supported by grants from the Natural Sciences and Engineering Research Council (M.A.), and Medical Research Council of Canada (L.D.). Dr. Pierre Legendre kindly provided advice on statistical methods     

N-[1-(2-Phenylethyl)pyrrolidin-3-yl]-1-adamantanecarboxamides as novel 5-HT2 receptor antagonists

A series of 1-adamantanecarboxamides was synthesized and examined for their potency as a selective 5-HT2 receptor antagonist. We found (S)-N-[1-[2-(4-fluorophenyl)ethyl]pyrrolidin-3-yl]-1-adamantane carboxamide hydrochloride hydrate (10-(S), Y-39241) to have a high affinity and selectivity for 5-HT2 receptors, and this potent anti-platelet effect of Y-39241 was confirmed both in vitro and in vivo.