Klaus Peter Bogeso

The pharmacological effect of citalopram resides 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.

Escitalopram versus citalopram: the surprising role of the R-enantiomer

RationaleCitalopram is a racemate consisting of a 1:1 mixture of the R(−)- and S(+)-enantiomers. Non-clinical studies show that the serotonin reuptake inhibitory activity of citalopram is attributable to the S-enantiomer, escitalopram. A series of recent non-clinical and clinical studies comparing escitalopram and citalopram to placebo found that equivalent doses of these two drugs, i.e. containing the same amount of the S-enantiomer, showed better effect for escitalopram. These results suggested that the R-citalopram in citalopram inhibits the effect of the S-enantiomer.ObjectiveTo review the pharmacological and non-clinical literature that describes the inhibition of escitalopram by R-citalopram, as well as the implications of this inhibition for the clinical efficacy of escitalopram compared to citalopram.MethodsThe information in this review was gathered from published articles and abstracts.ResultsIn appropriate neurochemical, functional, and behavioural non-clinical experiments, escitalopram shows greater efficacy and faster onset of action than comparable doses of citalopram. The lower efficacy of citalopram in these studies is apparently due to the inhibition of the effect of the S-enantiomer by the R-enantiomer, possibly via an allosteric interaction with the serotonin transporter. Data from randomised clinical trials consistently show better efficacy with escitalopram than with citalopram, including higher rates of response and remission, and faster time to symptom relief.ConclusionThe R-enantiomer present in citalopram counteracts the activity of the S-enantiomer, thereby providing a possible basis for the pharmacological and clinical differences observed between citalopram and escitalopram.

Dopamine receptor agonistic and antagonistic effects of 3-PPP enantiomers

The pharmacological profile of the enantiomers of the proposed selective dopamine (DA) autoreceptor agonist 3-PPP [3-(3-hydroxyphenyl)-N-n-propylpiperidine] has been studied. In vitro both enantiomers showed weak DA agonistic activity, and (-)-3-PPP some DA antagonistic effect on DA-stimulated adenylate cyclase activity. Both enantiomers in low doses had a similar profile in vivo: Inhibition of locomotor activity of mice and rats, induction of contralateral circling behaviour in 6-hydroxy-DA-lesioned rats and an emetic effect in dogs. At higher doses, differential effects of the enantiomers were found: (+)-3-PPP induced hyperactivity, weak stereotyped behaviour and ipsilateral circling in hemitransected rats. (-)-3-PPP had depressant effects in high doses, inhibited d-amphetamine-induced hyperactivity and d-amphetamine-, methylphenidate- and apomorphine-induced stereotyped licking/biting in rats and antagonized apomorphine-induced emesis in dogs. However, (+)-3-PPP also showed a weak antagonistic activity against d-amphetamine-induced hyperactivity and d-amphetamine-and apomorphine-induced stereotypy in rats and inhibited apomorphine-induced emesis in dogs.It is suggested that both enantiomers have significant effects on postsynaptic DA receptors in high doses: (-)-3-PPP with weak antagonistic activity in some test models and (+)-3-PPP with agonistic and antagonistic effect. Since these effects of (+)-3-PPP were of low intensity at high doses, (+)-3-PPP may be a partial DA agonist at postsynaptic receptors in high doses. Interaction experiments with neuroleptics indicated that only (-)-3-PPP significantly increased the antistereotypic effect of neuroleptics in rats. Therefore, the proposed DA autoreceptor stimulation is possibly unrelated to the neuroleptic potentiation.

Molecular determinants for selective recognition of antidepressants in the human serotonin and norepinephrine transporters

Inhibitors of the serotonin transporter (SERT) and norepinephrine transporter (NET) are widely used in the treatment of major depressive disorder. Although SERT/NET selectivity is a key determinant for the therapeutic properties of these drugs, the molecular determinants defining SERT/NET selectivity are poorly understood. In this study, the structural basis for selectivity of the SERT selective inhibitor citalopram and the structurally closely related NET selective inhibitor talopram is delineated. A systematic structure-activity relationship study allowed identification of the substituents that control activity and selectivity toward SERT and NET and revealed a common pattern showing that SERT and NET have opposite preference for the stereochemical configuration of these inhibitors. Mutational analysis of nonconserved SERT/NET residues within the central substrate binding site was performed to determine the molecular basis for inhibitor selectivity. Changing only five residues in NET to the complementary residues in SERT transferred a SERT-like affinity profile for R- and S-citalopram into NET, showing that the selectivity of these compounds is determined by amino acid differences in the central binding site of the transporters. In contrast, the activity of R- and S-talopram was largely unaffected by any mutations within the central substrate binding site of SERT and NET and in the outer vestibule of NET, suggesting that citalopram and talopram bind to distinct sites on SERT and NET. Together, these findings provide important insight into the molecular basis for SERT/NET selectivity of antidepressants, which can be used to guide rational development of unique transporter inhibitors with fine-tuned transporter selectivity.

Molecular dynamics simulations of Na+/Cl(-)-dependent neurotransmitter transporters in a membrane-aqueous system.

We have performed molecular dynamics simulations of a homology model of the human serotonin transporter (hSERT) in a membrane environment and in complex with either the natural substrate 5‐HT or the selective serotonin reuptake inhibitor escitalopram. We have also included a transporter homologue, the Aquifex aeolicus leucine transporter (LeuT), in our study to evaluate the applicability of a simple and computationally attractive membrane system. Fluctuations in LeuT extracted from simulations are in good agreement with crystallographic B factors. Furthermore, key interactions identified in the X‐ray structure of LeuT are maintained throughout the simulations indicating that our simple membrane system is suitable for studying the transmembrane protein hSERT in complex with 5‐HT or escitalopram. For these transporter complexes, only relatively small fluctuations are observed in the ligand‐binding cleft. Specific interactions responsible for ligand recognition, are identified in the hSERT–5HT and hSERT–escitalopram complexes. Our findings are in good agreement with predictions from mutagenesis studies.

Homology modeling of the serotonin transporter: insights into the primary escitalopram-binding site.

The serotonin transporter (SERT) is one of the neurotransmitter transporters that plays a critical role in the regulation of endogenous amine concentrations and therefore is an important target for therapeutic agents affecting the central nervous system. The recently published, high resolution X‐ray structure of the closely related amino acid transporter, Aquifex aeolicus leucine transporter (LeuT), provides an opportunity to develop a three‐dimensional model of the structure of SERT. We present herein a homology model of SERT using LeuT as the template and containing escitalopram as a bound ligand. Our model explains selectivities known from mutational studies and varying ligand data, which are discussed and illustrated in the paper.

Effects of S (+)-3-phenethyl-PP, a putative dopamine autoreceptor agonist with greater autoreceptor selectivity than 3-PPP enantiomers

The experiments concerned the pharmacology of the enantiomers of the phenethyl-analogue (3-phenethyl-PP) of the putative dopamine (DA) autoreceptor agonist 3-PPP. In contrast to the almost equipotency of 3-PPP enantiomers, the phenethyl enantiomers showed marked stereoselectivity. S(+)-3-Phenethyl-PP had 25 times higher affinity to D-2 DA receptors in vitro than the R(-)-enantiomer. In vivo a similar potency difference was seen for the inhibition of motility, induction of circling behaviour in 6-OHDA-lesioned rats and emetic effect in dogs. None of the enantiomers induced stereotypy and hypermotility in normal rats or in rats pretreated with reserpine and alpha-methyl-p-tyrosine. In test models for antidopaminergic activity only slight activity of either enantiomer was observed. The S(+)-enantiomer had no antagonistic effect against apomorphine- and amphetamine-induced stereotypies, no cataleptogenic activity and only partially antagonized amphetamine-induced hypermotility. Apomorphine-induced emesis was weakly antagonized. The results indicate a greater and higher selectivity of S(+)-3-phenethyl-PP for DA receptors mediating sedation compared with 3-PPP enantiomers which previously have been shown to exert significant effects on postsynaptic DA receptors. Thus S(+)-3-phenethyl-PP may be a more selective model compound for the differential study of effects elicited by stimulation of pre- and postsynaptic DA receptors.

Structure–activity relationship studies of citalopram derivatives: examining substituents conferring selectivity for the allosteric site in the 5‐HT transporter

The 5‐HT transporter (SERT) is a target for antidepressant drugs. SERT possesses two binding sites: the orthosteric (S1) binding site, which is the presumed target for current SERT inhibitors, and an allosteric (S2) site for which potential therapeutic effects are unknown. The antidepressant drug citalopram displays high‐affinity S1 binding and low‐affinity S2 binding. To elucidate a possible therapeutic role of allosteric inhibition of SERT, a drug that specifically targets the allosteric site is required. The purpose of this study was to find a compound having higher selectivity towards the S2 site.

X-ray structure based evaluation of analogs of citalopram: Compounds with increased affinity and selectivity compared with R-citalopram for the allosteric site (S2) on hSERT.

The recent publication of X-ray structures of SERT includes structures with the potent antidepressant S-Citalopram (S-Cit). Earlier predictions of ligand binding at both a primary (S1) and an allosteric modulator site (S2), were confirmed. We provide herein examples of a series of Citalopram analogs, showing distinct structure-activity relationship (SAR) at both sites that is independent of the SAR at the other site. Analogs with a higher affinity and selectivity than benchmark R-Citalopram (R-Cit) for the S2 versus the S1 site were identified. We deploy structural and computational analyses to explain this SAR and demonstrate the potential utility of the newly emerging X-ray structures within the neurotransmitter:sodium Symporter family for drug design.

Muscarinic agonists. Syntheses and structure—activity relationships of bicyclic isoxazole ester bioisosteres of norarecoline

Summary ( RS )-3-Methoxy-8-methyl-5,6,7,8-tetrahydro-4 H -isoxazolo[4,5- c ]azepine ( O ,8-di-Me-THAO, 2c) and ( RS )-8-methyl-3-propargyloxy-5,6,7,8-tetrahydro-4 H -isoxazolo[4,5- c ]azepine (8-Me- O -propargyl-THAO, 2d ) have been synthesized and evaluated as muscarinic receptor ligands in receptor binding assays and in in vitro functional assays. The corresponding compounds without methyl groups at C-8, ie O -Me-THAO ( 2a ) and O -propargyl-THAO ( 2b ), have previously been shown to exhibit muscarinic agonistic profiles with very little discrimination between M 1 and M 2 -receptor sites. Based on functional assays, 2c and 2d were found to be less efficacious than 2a and 2b , respectively, and 2d proved to be an M 1 -selective partial agonist. The affinity and M 1 efficacy of 2c and 2d were comparable to those of the corresponding six-membered ring analogues, ( RS ) -3-methoxy-7-methyl-4,5,6,7-tetrahydroisoxazolo [4,5- c ]pyridine (0,7-di-Me-THPO, 1c ) and ( RS )-7-methyl-3-propargyloxy-4,5,6,7-tetrahydroisoxazolo[4,5- c ]pyridine (7-Me- O -propargyl-THPO, 1d ). However, neither compound 1c nor compound 1d displayed M 1 selectivity. In summary, within this class of bicyclic muscarinic agonists, replacement of 3-methoxy by 3-propargyloxy groups generally increases muscarinic affinity without affecting the efficacy at M 1 receptors significantly. Introduction of a methyl group into the saturated ring at the position α to the C-5 of the isoxazole ring (α-position) leads to compounds exerting lower efficacy and, in the case of compound 2d , an increased selectivity with respect to M 1 agonism.