Structural determinants of AMPA agonist activity in analogues of 2-amino-3-(3-carboxy-5-methyl-4-isoxazolyl)propionic acid: synthesis and pharmacology.

We have previously shown that the 2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA) receptor agonist, 2-amino-3-(3-carboxy-5-methyl-4-isoxazolyl)propionic acid (ACPA, 2), binds to AMPA receptors in a manner different from that of AMPA (1) itself and that 2, in contrast to 1, also binds to kainic acid receptor sites. To elucidate the structural requirements for selective activation of the site/conformation of AMPA receptors recognized by 2, a number of isosteric analogues of 2 have now been synthesized and pharmacologically characterized. The compound 2-amino-3-(5-carboxy-3-methoxy-4-isoxazolyl)propionic acid (3a) (IC(50) = 0.11 microM; EC(50) = 1.2 microM), which is a regioisostere of 2 with a methoxy group substituted for the methyl group, was approximately equipotent with 2 (IC(50) = 0.020 microM; EC(50) = 1.0 microM) as an inhibitor of [(3)H]AMPA binding and as an AMPA agonist, respectively, whereas the corresponding 3-ethoxy analogue 3b (IC(50) = 1.0 microM; EC(50) = 4.8 microM) was slightly weaker. The analogues 3c-e, containing C3 alkoxy groups, were an order of magnitude weaker than 3b, whereas the additional steric bulk of the alkoxy groups of 3f-i or the presence of an acidic hydroxyl group at the 3-position of the isoxazole ring of 3j prevented interaction with AMPA receptor sites. The 2-amino-3-(2-alkyl-5-carboxy-3-oxo-4-isoxazolyl)propionic acids 4a,b, i, which are regioisosteric analogues of 3a,b,i, showed negligible interaction with AMPA recognition sites. Similarly, replacement of the carboxyl group of 3b by isosteric tetrazolyl or 1,2,4-triazolyl groups to give 5 and 6, respectively, or conversion of 3b into analogue 7, in which the diaminosquaric acid group has been bioisosterically substituted for the alpha-aminocarboxylic acid unit, provided compounds completely devoid of effect at AMPA receptors. In contrast to the parent compound ACPA (2) (IC(50) = 6.3 microM), none of the analogues described showed detectable inhibitory effect on [(3)H]kainic acid receptor binding.

Synthesis and muscarinic receptor pharmacology of a series of 4,5,6,7-tetrahydroisothiazolo[4,5-c]pyridine bioisosteres of arecoline.

A series of O- and ring-alkylated derivatives of 4,5,6,7-tetrahydroisothiazolo[4,5-c]pyridin-3-ol was synthesized via treatment of appropriately substituted 4-benzylamino-1,2,5,6-tetrahydropyridine-3-carboxamides with hydrogen sulfide and subsequent ring closure by oxidation with bromine. The muscarinic receptor affinity as well as estimated relative efficacy and subtype selectivity of this series of bicyclic arecoline bioisosteres were determined using rat brain membranes and a number of tritiated muscarinic receptor ligands. The effects at the five cloned human muscarinic receptor subtypes of a selected series of chiral analogues, with established absolute stereochemistry, were studied using receptor selection and amplification technology (R-SAT). The potency, relative efficacy, and receptor subtype selectivity of these compounds were related to the structure of the O-substituents and the position and stereochemical orientation of the piperidine ring methyl substituents.

Heteroaryl analogues of AMPA. Synthesis and quantitative structure-activity relationships.

A number of 3-isoxazolol bioisosteres, 7a-i, of (S)-glutamic acid (Glu), in which the methyl group of (RS)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid (AMPA, 1) was replaced by different 5-membered heterocyclic rings, were synthesized. Comparative in vitro pharmacological studies on this series of AMPA analogues were performed using receptor binding assays (IC50 values) and the electrophysiological rat cortical slice model (EC50 values). None of these compounds showed detectable affinity for the N-methyl-D-aspartic acid subtype of Glu receptors. Some of the compounds were weak inhibitors of [3H]kainic acid binding. The inhibitory effects on [3H]AMPA binding and agonist potencies at AMPA receptors of 7a-i were strictly dependent on the structure, electrostatic potential, and methyl substitution of the heterocyclic 5-substituent. Thus, while 7a (IC50 = 0.094 microM; EC50 = 2.3 microM) was approximately equipotent with AMPA (IC50 = 0.023 microM; EC50 = 5.4 microM), (RS)-2-amino-3-[3-hydroxy-5-(1H-imidazol-2-yl)isoxazol-4-yl]propio nic acid (7b) (IC50 = 48 microM; EC50 = 550 microM) was some 2 orders of magnitude weaker than AMPA, and (RS)-2-amino-3-[3-hydroxy-5-(1-methyl-1H-imidazol-2-yl)-isoxazol-4 -yl] propionic acid (7c) (IC50 > 100 microM; EC50 > 1000 microM) was inactive. Furthermore, (RS)-2-amino-3-[3-hydroxy-5-(2-methyl-2H-tetrazol-5-yl)isoxazol -4-yl] propionic acid (7i) (IC50 = 0.030 microM; EC50 = 0.92 microM) was more potent than AMPA, whereas its N-1 methyl isomer, (RS)-2-amino-3-[3-hydroxy-5-(1-methyl-1H-tetrazol-5-yl)isoxazol -4-yl] propionic acid (7h) (IC50 = 54 microM; EC50 > 1000 microM) was inactive as an AMPA agonist. A quantitative structure-activity relationship (QSAR) analysis revealed a positive correlation between receptor affinity, electrostatic potential near the nitrogen atom at the "ortho" position of the heterocyclic 5-substituent, and the rotational energy barrier around the bond connecting the two rings. We envisage that a hydrogen bond between the protonated amino group and an ortho-positioned heteroatom of the ring substituent at the 5-position stabilize receptor-active conformations of these AMPA analogues.

Enhanced D1 affinity in a series of piperazine ring substituted 1-piperazino-3-arylindans with potential atypical antipsychotic activity.

A study of the effect of aromatic substitution on D1 and D2 affinity in a series of previously reported trans-1-piperazino-3-phenylindans shows similar structure-activity relationships for the two receptor sites. 6-Substituted derivatives have affinity for both receptors, and 6-chloro-or 6-fluoro-substituted derivatives show preference for D1 receptors. D1 affinity and selectivity are significantly increased in a series of new piperazine ring substituted derivatives. Potent D1 and D2 antagonism in vivo are confined to derivatives with relatively small substituents in the 2-position of the piperazine ring (e.g. 2-methyl,2,2-dimethyl, 2-spirocyclobutyl or 2-spirocyclopentyl). Consequently, the effect of aromatic substitution is examined in a series of 1-(2,2-dimethylpiperazino)-3-arylindans. All these compounds except the 4-, 5-, 7- and 4'-chloro-substituted derivatives have potent D1 affinity (IC50's below 10 nM) and the majority of the compounds antagonize SK&F 38393-induced circling in 6-OHDA-lesioned rats with ED50 values about 1 mumol/kg. In vitro all compounds show preference for D1 receptors, but in vivo they are equally effective as D1 and D2 antagonists. The compounds have high affinity for 5-HT2 receptors and selected compounds show high affinity for alpha 1 adrenoceptors. Furthermore, a subgroup consisting of (-)-38, (-)-39, (-)-41, and (-)-54 does not induce catalepsy in rats. These compounds have the potential of being "atypical" antipsychotics and have consequently been selected for further studies. The non-receptor-blocking enantiomers are shown to be inhibitors of DA and NE uptake in accordance with previous observations in compounds unsubstituted in the piperazine ring. Two compounds, (+)-38 and (+)-40, block DA uptake with IC50 values below 10 nM. Finally, the observed structure-activity relationships are discussed in relation to previously published pharmacophore models for D2 and 5-HT2 receptors. It is concluded that the piperazine substituents might induce a different binding mode at the dopamine receptor sites, perhaps only at the D1 receptor site.

Bioisosteres of arecoline: 1,2,3,6-tetrahydro-5-pyridyl-substituted and 3-piperidyl-substituted derivatives of tetrazoles and 1,2,3-triazoles. Synthesis and muscarinic activity.

A series of arecoline bioisosteres, where the ester group is replaced by a 1,2,3-triazole-4-yl or a tetrazole-5-yl group, was synthesized and evaluated in vitro for affinity and efficacy at muscarinic receptors and in vivo for cholinergic side effects. The corresponding piperidine derivatives were also studied. In the 1,2,3,6-tetrahydropyridyl-1,2,3-triazole series, only derivatives with 2-substituents in the 1,2,3-triazole ring exert muscarinic agonist activity. The same trend is seen in the corresponding tetrazole series, where only 2-substituted derivatives display muscarinic agonist activity. The methyl derivatives in both series are full agonists, whereas the derivatives with longer side chains are partial agonists. Introduction of methyl substituents in the 1,2,3,6-tetrahydropyridine ring generally lowers affinity considerably except for the 3-substituted derivatives, where some activity is retained. In both the 1,2,3-triazole and tetrazole series, derivatives without substituents at the basic nitrogen in the 1,2,3,6-tetrahydropyridine ring are unselective full agonists, whereas the methyl-substituted derivatives generally are more M1 selective compared to M2. Larger substituents than methyl abolish activity. The 4-(3-piperidyl)-1,2,3-triazole and 5-(3-piperidyl)-2H-tetrazole derivatives are generally less active than the corresponding 1,2,3,6-tetrahydropyridine derivatives, and only the 2-allyl- and 2-propargyl-1,2,3-triazole derivatives display activities comparable to the most active compounds in the 1,2,3,6-tetrahydropyridine series. The propargyl derivative is an unselective full agonist, and resolution did not reveal any stereoselectivity The allyl derivative is a partial agonist with some selectivity for the M1 receptor, and testing of the enantiomers showed that the (+)-enantiomer is an unselective partial agonist, whereas the (-)-enantiomer is a partial agonist with preference for the M1 receptor. Generally, the structure-activity relationships of the 1,2,3-triazole and tetrazole series are very similar, and two compounds, 2-ethyl-4-(1-methyl-1,2,3,6-tetrahydro-5-pyridyl)-1,2,3-triazole and 2-ethyl-5-(1-methyl-1,2,3,6-tetrahydro-5-pyridyl)-2H-tetrazole, are M1 agonists/M2 antagonists. Muscarinic compounds with this profile are of particular interest as drugs for the treatment of Alzheimer's disease.

Highly potent indanamine serotonin uptake blockers as radiotracers for imaging serotonin uptake sites.

Two highly potent indanamine serotonin (5-HT) uptake blockers, trans-3'-(4'-bromophenyl)-1-indanamine (trans-[11C]DBPI or [11C]Lu 19-056) and its iodo analog, trans-3'(4'-[125I]iodophenyl)-1-indanamine (trans-[125I]DIPI) were evaluated as radiotracers for imaging 5-HT uptake sites in vivo Trans-[11C]DBPI was synthesized by N-methylation of the normethyl precursor with [11C]iodomethane. Trans-[125I]DIPI was synthesized by iododestannylation of the tributyltin precursor with [125I]NaI. Radiochemical yields for the [11C] and [125I] radiotracers were 34 and 40% with specific activities of 4000 and 1800 mCi/mumol, respectively. In vitro, the iodo analog, trans-DIPI, showed an IC50 value of 0.26 nM in inhibition of [3H]paroxetine binding to 5-HT uptake sites in rat cortex. The potency was found to be equivalent to that of paroxetine or McN5652. In vivo, after i.v. injection into mice, both radiotracers showed high uptake in brain (3-4% dose/whole brain at 15 min) and high accumulation into target tissues such as hypothalamus and olfactory tubercles (7-8% dose/g at 60 min). The binding was blocked by pre-injection of 5 mg/kg of peroxetine. While the in vivo distribution agreed with previously reported 5-HT uptake site distribution, the radiotracers showed high uptake in non-target tissues such as cerebellum, resulting in low target-to-non-target ratios (1.5-1.6 at 60 min). Since washout from non-target regions was slower than from target regions, longer-time observation with 125I up to 6 h did not improve the ratios. HPLC analyses of mouse brain homogenates and blocking studies indicated that the high uptake in non-target regions is not the result of metabolism or any interaction of the radiotracers with those tissues via specific binding sites. In spite of low target-to-non-target ratios, target regions with high density of 5-HT uptake sites, such as the raphe nuclei, superior colliculi and substantia nigra, were visualized with trans-[125I]DIPI by ex vivo autoradiography, since the radiotracer showed high specific binding (total mimus nonspecific binding).

Resolution, absolute stereochemistry, and pharmacology of the S-(+)- and R-(-)-isomers of the apparent partial AMPA receptor agonist (R,S)-2-amino-3-(3-hydroxy-5-phenylisoxazol-4-yl)propionic acid [(R,S)-APPA].

(R,S)-2-Amino-3-(3-hydroxy-5-phenylisoxazol-4-yl)propionic acid ((R,S)-APPA) is the only partial agonist at the (R,S)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid (AMPA) subtype of excitatory amino acid receptors so far described. In light of the pharmacological interest in partial agonists, we have now accomplished the resolution of (R,S)-APPA. (S)-(+)-APPA (5) and (R)-(-)-APPA (6) were obtained in high enantiomeric purity using (R)-(+)- and (S)-(-)-1-phenylethylamine, respectively, as resolving agents. The absolute stereochemistry of 6 was established by X-ray analysis of 6.HCl.0.25H2O. Compounds 5 and 6 were tested electropharmacologically using the rat cortical wedge preparation and in receptor-binding assays using [3H]-AMPA, [3H]kainic acid, and the N-methyl-D-aspartic acid (NMDA) receptor ligands [3H]CPP, [3H]MK-801, and [3H]glycine. Whereas 6 did not significantly affect the binding of any of these ligands (IC50 > 100 microM), compound 5 revealed affinity for only the [3H]AMPA-binding site (IC50 = 6 microM). In electropharmacological tests, 5 showed full AMPA receptor agonism (EC50 = 230 microM). This effect of 5 was insensitive to the NMDA antagonist CPP but was inhibited competitively by the non-NMDA antagonist NBQX (pKi = 6.30). Compound 6, on the other hand, turned out to be a non-NMDA receptor antagonist, inhibiting competitively depolarizations induced by AMPA (pKi = 3.54), kainic acid (pKi = 3.07), and 5 (pKi = 3.57).

Development of a receptor-interaction model for serotonin 5-HT2 receptor antagonists. Predicting selectivity with respect to dopamine D2 receptors.

A receptor-interaction model for serotonin 5-HT2 receptor antagonists has been developed by conformational analysis with molecular mechanics (MM2(91)) and superimposition studies of serotonin 5-HT2 receptor antagonists. Substituted 3-(4-piperidinyl)-,1-(4- piperidinyl)-,3-(1,2,3,6-tetrahydropyridin-4-yl)-, and 1-(1,2,3,6-tetrahydropyridin-4-yl)-1H-indoles, substituted 3-(4-fluorophenyl)-1-(4-piperazinyl)indans, cyprohepatadine derivatives, ritanserin, and danitracene have been used as bases for the model. Other serotonin 5-HT2 receptor antagonists, such as ketanserin and MDL 11,939, are well accommodated into the model. Comparison of the model with a recently described receptor-interaction model for dopamine D2 receptor antagonists suggests a common pharmacophore for dopamine D2 and serotonin 5-HT2 receptor antagonists. Important steric differences between 5-HT2 receptor antagonists with additional high affinity for dopamine D2 receptors and serotonin 5-HT2 receptor antagonists with high selectivity versus D2 receptors are described. The geometry of the receptor-interaction model described is significantly different from that of a recently reported receptor-interaction model for 5-HT2 receptor agonists and antagonists developed by use of (+)-LSD as a template, suggesting the existence of two binding modes at the 5-HT2 receptor.

The defibrillating effect of catecholamine reuptake inhibitors.

In previous studies we hypothesized that spontaneous termination of ventricular fibrillation (TVF) requires a high cardiac catecholamine level ([CA]) during VF. During VF, sympathetic activity is enhanced but in the majority of cases [CA] does not reach the level required for self-defibrillation, most likely due to their relatively high reuptake by sympathetic nerve terminals. One possibility of obtaining TVF is by elevation of the [CA] during VF, either by catecholamine intracoronary injection or by treatment with compounds that inhibit catecholamine reuptake. To examine this assumption, we studied the effect of VF on 3 closely related compounds: talopram, talsupram and citalopram, with norepinephrine uptake inhibition (IC50NE) of 2.9, 0.79 and 8800 and dopamine (DA) uptake inhibition (IC50DA) of 44000, 9300 and 41000, respectively, as well as 2 enantiomers of a cis-1-piperazino-3-phenylindan derivative (Lu20-037 and Lu20-036) with IC50NE of 2.5 and 910 and IC50DA of 2.3 and 1700, respectively. The results support our hypothesis relating the defibrillating effect of a compound to its IC50NE, while its inhibitory effect on DA uptake seems to conteract the NE effect.

A structure-activity study of four dopamine D-1 and D-2 receptor antagonists, representing the phenylindan, -indene, and -indole structural classes of compounds.

Representatives of the phenylindan, -indene, and -indole classes of compounds (3-6) have been tested for affinity for the dopamine D-1 and D-2 receptors. The compounds all display high affinities for these receptors. Conformational analysis using MM2(87) and subsequent molecular least-squares superimpositions have been performed in order to determine if the affinities of the compounds can be rationalized by a recently proposed dopamine D-2 receptor-interaction model. In spite of the different geometric and conformational properties, the compounds can be well accommodated into the model in their calculated lowest energy conformations. The molecular superimpositions allow the absolute configurations of the active enantiomers of 4 and 5 to be predicted. The present structure-activity study extends the receptor-interaction model by suggesting that the receptor is not very sensitive to the orientation of the p-fluorophenyl ring in 1 and 3-6 or to the exact spatial location of the associated fluoro substituent.

Stereospecific and selective 5-HT2 antagonism in a series of 5-substituted trans-1-piperazino-3-phenylindans.

A study of the effect of aromatic substitution on 5-HT2, D2, and alpha 1 receptor affinity in a subseries of new and previously synthesized 1-piperazino-3-phenylindans indicated that high 5-HT2 selectivity could be obtained in 5-substituted derivatives. Accordingly, a series of 5-substituted derivatives was synthesized with the goal of obtaining stereospecific and selective, centrally acting 5-HT2 antagonists. This goal was fulfilled in 5-chloro- or 5-fluoro-substituted compounds with 2-(3-alkyl-2-oxoimidazolidin-1-yl)ethyl- or 2-(tetrahydro-2-oxo-1H-pyrimidin-1-yl)ethylpiperazine substituents, as well as in their imidazolidine-2-thione or pyrimidine-2-thione analogues. The most interesting derivatives were resolved either directly via diastereomeric salts or by syntheses from resolved starting materials. Optical purity was determined by a 1H NMR method, using the chiral shift reagent (R)-(-)-2,2,2-trifluoro-1-(9-anthryl)ethanol. The compound (-)-trans-1-[2-[4-[5-chloro-3-(4-fluorophenyl)-2,3-dihydro-1H-inden++ +-1- yl]piperazin-1-yl]ethyl]-3-isopropyl-2-imidazolidinone ((-)-20) had the overall best profile with a high stereoselectivity (eudismic ratio: 68) and a high selectivity versus D2 and alpha 1 receptors (affinity ratios 182 and 191, respectively). It had a potent central effect but was shorter-acting than the tetrahydropyrimidinone or thione derivatives ((-)-39, (+)-40, (-)-41, and (+)-42). The observed activities of the compounds are settled in perspective in relation to a recently proposed D2 receptor interaction model. While there are no indications so far that trans-1-piperazino-3-phenylindans interact with D2 and 5-HT2 receptors in different conformations, the present study shows important differences in aromatic substitution effects. Only 5-HT2 receptors are able to accommodate a 5-substituent in the indan benzene ring, thus allowing syntheses of highly selective compounds.

Noncataleptogenic, centrally acting dopamine D-2 and serotonin 5-HT2 antagonists within a series of 3-substituted 1-(4-fluorophenyl)-1H-indoles.

A series of 1-(4-fluorophenyl)-1H-indoles substituted at the 3-position with 1-piperazinyl, 1,2,3,6-tetrahydro-4-pyridinyl, and 4-piperidinyl was synthesized. Within all three subseries potent dopamine D-2 and serotonin 5-HT2 receptor affinity was found in ligand binding studies. Quipazine-induced head twitches in rats were inhibited by most derivatives as a measure of central 5-HT2 receptor antagonism. Piperazinyl and tetrahydropyridyl indoles were cataleptogenic, while piperidyl substituted indoles surprisingly were found to be noncataleptogenic or only weakly cataleptogenic. Noncataleptogenic piperidyl derivatives also failed to block dopaminergic-mediated stereotypies, that is methyl phenidate-induced gnawing behavior in mice. These profiles resemble that of the atypical neuroleptic clozapine. 1-Ethyl-2-imidazolidinone was found to be the optimal substituent of the basic nitrogen atom in order to avoid catalepsy. The atypical neuroleptic 1-[2-[4-[5-chloro-1-(4-fluorophenyl)-1H-indol-3-yl]-1-piperidinyl] ethyl]-2-imidazolidinone (sertindole, compound 14c) was selected for further development as a result of these structure/activity studies.

The pharmacological effect of citalopram residues in the (S)-(+)-enantiomer.

The enantiomers of citalopram and N-demethylcitalopram have been investigated. Based on the inhibition of 5-HT uptake in vitro and the potentiation of 1-5-HTP in vivo the pharmacological activity resides in the (+)-enantiomers (the eutomers*) with the 1-(S) absolute configuration. In the 5-HT uptake test eudismic ratios of 167 and 6.6 are obtained for the enantiomers of citalopram and N-demethylcitalopram, respectively. The pharmacological profile of the eutomers of citalopram and N-demethylcitalopram very much resembles the profile of the respective racemates.

Octoclothepin enantiomers. A reinvestigation of their biochemical and pharmacological activity in relation to a new receptor-interaction model for dopamine D-2 receptor antagonists.

Octoclothepin (1) was resolved into its R and S enantiomers via the diastereomeric tartaric acid salts. The enantiomers were shown to be of high optical purity by 1H NMR with use of the chiral shift reagent (R)-(-)-2,2,2-trifluoro-1-(9-anthryl)ethanol. Pharmacological and biochemical testing confirmed that (S)-1 is the more potent dopamine (DA) D-2 antagonist both in vitro and in vivo, although the R enantiomer still has significant D-2 antagonistic activity. In contrast, both enantiomers were equally active in test models detecting activity at D-1 receptors, serotonin-2 (5-HT2) receptors and alpha 1 adrenoceptors. Contrary to a previous prediction, it was found that norepinephrine (NE) uptake inhibition was confined solely to the S enantiomer. Overall, (S)-1 has a "classical" neuroleptic profile, while the R enantiomer has a more "atypical" profile. These pharmacological profiles seem to be in agreement with the reported clinical profiles of the two enantiomers. A previous conformational study was revised in light of the biochemical test results with enantiomers of known optical purity. Their relative D-2 receptor affinity corresponded well with the calculated conformational energy difference between their "active conformations" deduced from a previously reported new D-2 receptor model. Also the high enantioselectivity of (S)-1 at the NE uptake site could be explained after a detailed conformational analysis showing strict requirements for the orientation of the piperazine lone-pair direction at the NE uptake site.

Pre- and postsynaptic dopaminergic activities of indolizidine and quinolizidine derivatives of 3-(3-hydroxyphenyl)-N-(n-propyl)piperidine (3-PPP). Further developments of a dopamine receptor model.

Pre- and postsynaptic dopaminergic activities of a series of indolizidine and quinolizidine analogues of 3-(3-hydroxyphenyl)-N-(n-propyl)piperidine (3-PPP) have been studied. The pharmacological data have been interpreted in terms of a previously reported model for interactions with dopamine pre- and postsynaptic D2-receptors and molecular mechanics (MM2(85] calculated geometries and conformational energies. The model has been further developed with respect to the receptor topography in the vicinity of the nitrogen binding site. In particular, a novel spatial orientation of the important "propyl cleft" has been proposed. This cleft is suggested to be located mainly above a plane through the receptor-bound substrate. The biologically active agonist and antagonist conformations of the enantiomers of 3-PPP have been reinvestigated.

Antihypertensive activity in a series of 1-piperazino-3-phenylindans with potent 5-HT2-antagonistic activity.

A series of trans-1-piperazino-3-phenylindans were synthesized with the goal of replacing their established neuroleptic profile with that of peripheral 5-hydroxytryptamine (5-HT2) antagonism. Compounds with an unsubstituted or fluoro-substituted 6-position in the indan ring, and which had a five- or six-membered heterocyclic ring attached by an ethylene chain to the piperazine ring, satisfied this objective. Some of the compounds had potent antihypertensive activity in conscious, spontaneously hypertensive rats (SHR). In pithed rats they antagonized the pressor effect induced by 5-HT in doses 100-1000 times lower than doses needed to antagonize the pressor effect of phenylephrine. The effect was stereoselective and associated with enantiomers with 1R,3S absolute configuration. 1S,3R enantiomers inhibited the uptake of dopamine and norepinephrine in vitro. The compound with the best antihypertensive activity was (+)-(1R,3S)-1-[2-[4-[3-(4-fluorophenyl)-1-indanyl]-1- piperazinyl]ethyl]-2-imidazolidinone (Lu 21-098, irindalone). Its pharmacological profile resembled that of the standard compound ketanserin. There was a close structural correspondence between ketanserin and irindalone in a conformation that we recently identified as a D-2 receptor-relevant configuration of its neuroleptic "parent" tefludazine. This suggests that the dopaminergic (D-2) and the serotonergic (5-HT2) pharmacophores are structurally closely related.

Relative dopamine D1 and D2 receptor affinity and efficacy determine whether dopamine agonists induce hyperactivity or oral stereotypy in rats.

The effects of a range of dopamine (DA) agonists on stereotyped behaviour in rats were analysed and compared both with the affinity of the compounds for D1 and D2 receptor binding sites in vitro and their ability to stimulate the adenylate cyclase activity in rat striatal homogenates. Full and partial agonists at the D1 receptor coupled to adenylate cyclase do not induce sterotypies when given alone, whereas full D2 agonists (e.g. quinpirole) induce hyperactivity but not oral sterotypies. Partial D2 agonists (e.g. (-)-3-PPP) only induce sedation. Mixed D1/D2 agonists (e.g. apomorphine) induce both hyperactivity and oral stereotypies. Maximum stereotypies were induced by combination of SK & F 38393 and a series of D2 agonists, including full agonists and the partial D2 agonist B-HT 920, whereas partial agonists with low intrinsic activity (e.g. (-)-3-PPP, EMD 23448) did not induce stereotypies when given together with SK & F 38393. However, these partial agonists reduced the maximum effect of apomorphine, whereas the full agonists (e.g. quinpirole, (-)-NPA) and B-HT 920 had no apomorphine antagonistic activity. The mixed D1/D2 agonists apomorphine and N,N-dipropyl-5,6-ADTN were only weakly influenced by SK & F 38393, or not at all. D1 agonists with central effects, including SK & F 38393, SK & F 81297 (with relatively high efficacies), and the partial agonist SK & F 75670 with low efficacy, changed the hyperactivity induced by quinpirole into maximum oral stereotypy, whereas the peripheral D1 agonist fenoldopam had no such effect. Inhibition of DA and NA synthesis with alpha-methyl-p-tyrosine depleted striatal DA levels by 72 per cent and antagonized the hyperactivity induced by the D2 agonists quinpirole and (-)-NPA, but not that of apomorphine. Combination of SK & F 38393 and quinpirole induced maximum stereotypy in DA-depleted animals. These results suggest that D1 receptor tonus is a necessary prerequisite for the expression of a DA agonist's effect. The hyperactivity induced by full D2 agonists appears to be mediated by D1 tonus provided by endogenous DA activity, but stronger D1 stimulation is necessary to induce oral stereotypy. A high degree of D1 receptor activation increases the ability of partial D2 agonists to induce hyperactivity or oral stereotypies since treatment with both SK & F 38393 and B-HT 920 had marked effects while B-HT 920 was ineffective.(ABSTRACT TRUNCATED AT 400 WORDS)

Conformational analysis and structural comparisons of (1R,3S)-(+)- and (1S,3R)-(-)-tefludazine, (S)-(+)- and (R)-(-)-octoclothepin, and (+)-dexclamol in relation to dopamine receptor antagonism and amine-uptake inhibition.

Conformational analysis with molecular mechanics (MM2(85] and molecular superimposition studies of (1R,3S)-(+)- and (1S,3R)-(-)-4-[3-(4-fluorophenyl)-6-(trifluoromethyl)indan-1-yl]-1- piperazineethanol (tefludazine) and (S)-(+)- and (R)-(-)-octoclothepin have been employed to identify biologically active conformations of these compounds with respect to dopamine receptor antagonism and amine-uptake inhibition. In contrast to what is commonly assumed, these studies indicate that the conformation of (S)-(+)-octoclothepin responsible for the dopamine receptor antagonism is different from the one observed in the crystal. From least-squares molecular superimpositions with the potent and stereoselective dopamine receptor antagonist (1R,3S)-tefludazine, biologically active conformations for the two compounds on the dopamine receptor have been deduced. This analysis also rationalizes the enantioselectivity of octoclothepin on the dopamine receptor. The X-ray structure of (S)-(+)-octoclothepin is shown to correspond structurally to the 1S,3R enantiomer of tefludazine, which is an amine-uptake inhibitor. This correspondence provides a structural basis for the norepinephrine (NE) uptake blocking properties of octoclothepin. It is predicted that the enantioselectivity of the NE-uptake inhibition of octoclothepin should be low with the S-(+) enantiomer as the more active optical isomer. A comparison of the deduced biologically active conformation of (S)-(+)-octoclothepin with (+)-dexclamol is also discussed on the basis of earlier derived superimposition studies with (+)-dexclamol.

Indolizidine and quinolizidine derivatives of the dopamine autoreceptor agonist 3-(3-hydroxyphenyl)-N-n-propylpiperidine (3-PPP).

Eight indolizidine and quinolizidine derivatives of 3-PPP were synthesized and tested for possible dopamine (DA) autoreceptor activity. The equatorial indolizidine derivative 19e had the profile of a selective autoreceptor agonist and was half as active as 3-PPP. However, resolution of the compound revealed that the 8R enantiomer was an unselective DA agonist with a profile similar to (+)-3-PPP, while the 8S enantiomer was a weak DA antagonist without any DA agonist activity. The unsaturated quinolizidine derivative 21 also had the profile of a DA antagonist while the axial quinolizidine derivative 18a had an amphetamine-like profile in 6-OHDA-lesioned rats. All other derivatives were inactive. The observed structure-activity relationships were in agreement with existing DA receptor models, although these models are not apparently detailed enough to explain why the 8S enantiomer of 19e is inactive as a DA agonist.

3-Phenyl-1-indanamines. Potential antidepressant activity and potent inhibition of dopamine, norepinephrine, and serotonin uptake.

A series of 3-phenyl-1-indanamines was synthesized and tested for potential antidepressant activity and for inhibition of dopamine (DA), norepinephrine (NE), and serotonin (5-HT) uptake. Trans isomers were generally potent inhibitors of DA, NE, and 5-HT uptake, while cis isomers preferentially inhibited the uptake of 5-HT. The affinity for the DA-uptake site was very dependent on the aromatic substitution pattern where highest potency was found for 3',4'-dichloro substituted compounds (45). This substitution pattern also resulted in high affinity for the NE-and 5-HT-uptake sites, but potent 5-HT-uptake inhibiting activity could also be obtained with other substitution patterns. Only small amines could be accommodated at the 5-HT-uptake site while larger amines such as piperazine could be accommodated both at the DA-and NE-uptake sites. The observed structure-activity relationships were explained from the results of superimpositions of a trans (45) and cis (72) isomer with 5-HT and DA, respectively, in relation to a proposed three-point binding of the uptake inhibitors at the uptake sites. Finally, comparison of the structures of the 3-phenyl-1-indanamines with other newer bicyclic catecholamine- and/or serotonin-uptake inhibitors revealed common structural elements important for potent DA-, NE-, and/or 5-HT-uptake inhibition.