Allosteric modulation of the effect of escitalopram, paroxetine and fluoxetine: in-vitro and in-vivo studies

Abstract

Clinical and preclinical studies have shown that the effect of citalopram on serotonin (5-HT) reuptake inhibition and its antidepressant activity resides in the S-enantiomer. In addition, using a variety of in-vivo and in-vitro paradigms, it was shown that R-citalopram counteracts the effect of escitalopram. This effect was suggested to occur via an allosteric modulation at the level of the 5-HT transporter. Using in-vitro binding assays at membranes from COS-1 cells expressing the human 5-HT transporter (hSERT) and in-vivo electrophysiological and microdialysis techniques in rats, the present study was directed at determining whether R-citalopram modifies the action of selective serotonin reuptake inhibitors (SSRIs) known to act on allosteric sites namely escitalopram, and to a lesser extent paroxetine, compared to fluoxetine, which has no affinity for these sites. In-vitro binding studies showed that R-citalopram attenuated the association rates of escitalopram and paroxetine to the 5-HT transporter, but had no effect on the association rates of fluoxetine, venlafaxine or sertraline. In the rat dorsal raphe nucleus, R-citalopram (250 microg/kg i.v.) blocked the suppressant effect on neuronal firing activity of both escitalopram (100 microg/kg i.v.) and paroxetine (500 microg/kg i.v.), but not fluoxetine (10 mg/kg i.v.). Interestingly, administration of R-citalopram (8 mg/kg i.p.) attenuated the increase of extracellular levels of 5-HT ([5-HT]ext) in the ventral hippocampus induced by both escitalopram (0.28 microM) and paroxetine (0.75 microM), but not fluoxetine (10 microM). In conclusion, the present in-vitro and in-vivo studies show that R-citalopram counteracts the activity of escitalopram and paroxetine, but not fluoxetine, by acting at the allosteric binding site of the 5-HT transporter, either located in the dorsal raphe nucleus or post-synaptically in the ventral hippocampus. This conclusion is strengthened by the observation that the inhibitory effect of fluoxetine, which has no stabilizing effect on the radioligand/hSERT complex, was not blocked by co-administration of R-citalopram.

Steric Hindrance Mutagenesis in the Conserved Extracellular Vestibule Impedes Allosteric Binding of Antidepressants to the Serotonin Transporter

“…For example, (S)-CIT has a marked allosteric effect, resulting in dramatic inhibition of the dissociation of a high affinity bound inhibitor, such as (S)-CIT itself or other SSRIs (19,21). Notably, it has been suggested that this putative dual action of (S)-CIT at two binding sites in the SERT is responsible for the higher efficacy and faster onset observed in clinical trials for (S)-CIT as compared with racemic citalopram (CIT) (22)(23)(24)(25)(26). However, despite previous attempts to locate the binding site (27)(28)(29)(30), the molecular mechanism underlying the allosteric effect of SERT inhibitors has remained essentially unknown.…”

Homology Modeling of the Serotonin Transporter: Insights into the Primary Escitalopram‐binding Site

“…[10] The superior performance in terms of efficacy and onset of action of escitalopram compared to the racemic mixture of citalopram (Cipramil, Celexa) has been ascribed to hSERT allosteric effects. [70,71] Escitalopram stabilizes its own binding to the primary binding site by activating/binding to a low-affinity allosteric site. R-citalopram also binds to that site, but its stabilizing effects on the escitalopram-bound primary site are reduced in comparison with escitalopram.…”

Selective Serotonin Reuptake Inhibitors, are They All Equal? A Pharmacoepidemiological Study

“…The superiority of escitalopram over citalopram is explained by the fact that it is the active S-enantiomer of the racemic [41] citalopram drug. The action on serotonin reuptake inhibiting resides in the S-citalopram [17], the R-enantiomer of citalopram may in fact even counteract the action of the S-enantiomer [42] in citalopram. Moreover, the presence of R-citalopram in citalopram can be responsible for antihistaminic effects and a QT increase.…”

“…• Fluoxetine is a 5-HT2c antagonist (which explains the "stimulant" effect), and a weak inhibitor of norepinephrine reuptake • Sertraline is an inhibitor of the dopamine transporter and is also active on the sigma 1 receptor (contributing to an anxiolytic effects) • Paroxetine is anticholinergic and is a norepinephrine reuptake inhibitor as well as a nitric oxide synthetase inhibitor (explaining source of sexual dysfunction) • Fluvoxamine is active on the sigma receptor (contributing to an anxiolytic effect) • Citalopram is a racemate composed of 2 enantiomers R-citalopram and S-citalopram, the efficacy of serotonin reuptake inhibition depends upon the S-citalopram [17]. R-citalopram has a moderate antihistaminic effect and a potential ECG QT increase • Escitalopram is equivalent to the S-citalopram molecule, without the R enantiomer.…”