The S-enantiomer of R,S-citalopram, increases inhibitor binding to the human serotonin transporter by an allosteric mechanism. Comparison with other serotonin transporter inhibitors (original) (raw)
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Psychopharmacology, 2014
Escitalopram appears to be a superior antidepressant to racemic citalopram. It has been hypothesized that binding of R-citalopram to the serotonin transporter (SERT) antagonizes escitalopram binding to and inhibition of the SERT, there by curtailing the elevation of extracellular 5-hydroxytryptamine (5-HTExt), and hence anti-depressant efficacy. Further, it has been suggested that a putative allosteric binding site is important for binding of escitalopram to the primary, orthosteric, site, and for R-citalopram's inhibition here of. Primary: Investigate at the human (h)SERT, at clinical relevant doses, whether R-citalopram antagonizes escitalopram-induced 5-HTExt elevation. Secondary: Investigate whether abolishing the putative allosteric site affects escitalopram-induced 5-HTExt elevation and/or modulates the effect of R-citalopram. Recombinant generation of hSERT transgenic mice; in vivo microdialysis; SERT binding; pharmacokinetics; 5-HT sensitive behaviors (tail suspension, m...
British Journal of Pharmacology, 2004
1 Clinical observations with the selective serotonin reuptake inhibitor (SSRI), S-citalopram, indicate that S-citalopram is more efficacious and produces earlier symptom relief than RS-citalopram. Since R-citalopram is at least 20-fold weaker than S-citalopram as inhibitor of the 5-HT transporter (SERT) in preclinical studies, the clinical data suggest an unexpected antagonistic interaction between the two enantiomers. We therefore characterised the interaction of R-and S-citalopram with the SERT in in vivo and in vitro assays. 2 In both behavioural (potentiation of 5-hydroxytryptophan (5-HTP)-induced behaviour) and electrophysiological studies (inhibition of 5-HT-elicited ion currents in Xenopus oocytes expressing the human SERT (hSERT) R-citalopram inhibited the effects of S-citalopram in a dose-dependent manner. With S-citalopram : R-citalopram ratios of 1 : 2 and 1 : 4, 5-HTP potentiation was significantly smaller than with S-citalopram alone. 3 R-citalopram did not antagonise the effects of another SSRI (fluoxetine) in either behavioural or electrophysiological studies. 4 In oocytes, inhibition of hSERT-mediated currents by R-citalopram was almost completely reversible and characterised by fast on-and off-sets of action. In contrast, the off-set for S-citalopram was 35-fold slower than for R-citalopram. 5 Kinetic analysis of the oocyte experiments suggests that S-citalopram binding to SERT induces a long-lasting, inhibited state of the transporter and that coapplication of R-citalopram partially relieves SERT of this persistent inhibition. 6 We propose that the kinetic interaction of R-and S-citalopram with SERT is a critical factor contributing to the antagonistic effects of R-citalopram on S-citalopram in vitro and in vivo.
The International Journal of Neuropsychopharmacology, 2006
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 mg/kg i.v.) blocked the suppressant effect on neuronal firing activity of both escitalopram (100 mg/kg i.v.) and paroxetine (500 mg/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 mM) and paroxetine (0.75 mM), but not fluoxetine (10 mM). 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.
International Clinical Psychopharmacology, 2009
Escitalopram the S-enantiomer of the racemate citalopram, is clinically more effective than citalopram in the treatment of major depressive disorder. However, the precise mechanism by which escitalopram achieves superiority over citalopram is yet to be determined. It has been hypothesized that the therapeutically inactive R-enantiomer competes with the serotonin-enhancing S-enantiomer at a low-affinity allosteric site on serotonin reuptake transporters (SERTs), and reduces the effectiveness of the S-enantiomer at the primary, highaffinity serotonin-binding site. This study summarizes the results of two recent single-photon emission computerized tomography studies measuring SERT occupancy in citalopram-treated and escitalopram-treated healthy volunteers, after a single dose and multiple doses (i.e. under steady-state conditions). The single-dose study showed no attenuating effect of R-citalopram. After multiple dosing, however, SERT occupancy was significantly reduced in the presence of R-citalopram. Under steady-state conditions, R-enantiomer concentrations were greater than for the S-enantiomer because of slower clearance of R-citalopram. A pooled analysis suggests that build-up of the R-enantiomer after repeated citalopram dosing may lead to increased inhibition of S-enantiomer occupancy of SERT. This review adds to the growing body of evidence regarding differences in the dynamics of SERT occupancy, that is, molecular mechanisms underlying the often-observed superior clinical efficacy of escitalopram compared with citalopram in major depressive disorder.
European Journal of Pharmacology, 2007
The human 5-HT transporter (hSERT) has two binding sites for 5-HT and 5-HT uptake inhibitors: the orthosteric high-affinity site and a lowaffinity allosteric site. Activation of the allosteric site increases the dissociation half-life for some uptake inhibitors. The objectives of this study were 1) to identify hSERT mutations that inactivate the high-affinity site without affecting the allosteric site and 2) to observe allosteric effects in which hSERT binds R-citalopram with higher affinity than S-citalopram. Wild-type and mutant (Y95F, I172M, and Y95F/I172M) hSERTs were expressed in COS-7 cells, and their 5-HT uptake and uptake inhibitor-binding abilities were studied. The hSERT mutations did not alter affinities for 5-HT or paroxetine, but high-affinity binding of S-citalopram was severely affected, particularly by the I172M, and Y95F/I172M mutations -K i respectively 4 nM (wild-type), 35 nM, 1000 nM, and 17.100 nM (mutants). The allosteric site however, in wild-type hSERT and the three mutants was unaffected by the mutations as attenuation of the dissociation rate of the [ 3 H]-paroxetine:hSERT complex in the presence of Scitalopram or paroxetine was the same for wild-type hSERT and the three mutants. Further, R-citalopram previously thought of as an inactive enantiomer strongly attenuated dissociation of the wild-type [ 3 H]-imipramine:hSERT complex, whereas S-citalopram had almost no effect on this complex. These results suggest that 1: The allosteric site on hSERT is distinct from the site to which S-citalopram binds with high affinity. 2: The allosteric effects of R-citalopram on the dissociation of [ 3 H]-imipramine from hSERT indicate that R-citalopram introduces a conformational change in hSERT.
Biological Psychiatry, 2001
Background: Single isomers of the selective serotonin reuptake inhibitors citalopram (escitalopram, S-citalopram) and fluoxetine (R-fluoxetine) are currently under development for the treatment of depression and other psychiatric disorders. Previous studies conducted in laboratory animals have revealed that the biological effects on serotonin reuptake for citalopram reside in the S enantiomer. In contrast, both enantiomers of fluoxetine contribute to its biological activity. Methods: In the present study, the potency and selectivity of escitalopram, R-fluoxetine, and all of the other currently available selective serotonin reuptake inhibitors were compared for binding affinity at the human serotonin, norepinephrine, and dopamine transporters and several select neurotransmitter receptors using radioligand binding assays. Results: Both escitalopram and R-fluoxetine were potent inhibitors of the serotonin transporter (K i ϭ 1.1 and 1.4 nmol/L, respectively). Escitalopram was the most serotonin transporter-selective compound tested and was ϳ30fold more potent than R-citalopram. Conclusions: As noted previously, paroxetine and sertraline possess moderate affinity (Ͻ50 nmol/L) for the human norepinephrine transporter and dopamine transporter, respectively. R-Fluoxetine, unlike the other selective serotonin reuptake inhibitors, possesses moderate affinity (K i ϭ 64 nmol/L) for the serotonin 2C receptor. Potential clinical correlates of these unique attributes of escitalopram and R-fluoxetine are discussed. Biol Psychiatry 2001;50: 345-350
Improved Potency of Escitalopram on the Human Serotonin Transporter
Journal of Clinical Psychopharmacology, 2004
The potency of escitalopram (''Lexapro,'' s-citalopram, LU-26-054) was compared with that of racemic citalopram (''Celexa'') using plasma samples from drug-treated normal controls applied to an assay of human serotonin [5-hydroxytryptamine (5-HT)] transport inhibition in blood platelets. Samples were available for both 4-hour and 24-day drug administration. The data indicated that 5-HT transport inhibition was fully manifest for each drug within 4 hours of administration, without significant increase in platelet transport inhibition by 24-day treatment. In addition, a doseresponse relationship could be seen for escitalopram and citalopram with increasing 5-HT transport inhibition observed with increasing dose. It was evident from the data that escitalopram was significantly more potent than its racemate in inhibiting human platelet 5-HT transport. Thirty milligrams of escitalopram approximated the effect of 60 mg of racemic citalopram, and similarly, 10 mg of escitalopram approximated that of 20 mg of its racemate. This is the first demonstration of escitalopram's pharmacodynamic effect on the human 5-HT transporter. The results demonstrate its superior potency at the human 5-HT transporter site.
Psychopharmacology, 2007
Objectives Previous studies have investigated the occupancy of the serotonin reuptake transporter (SERT) after clinical doses of citalopram and other selective serotonin reuptake inhibitors. In the present study, the occupancies of SERT after multiple doses of escitalopram and citalopram were compared using the radioligand [123I]ADAM and single photon emission computed tomography (SPECT). Methods Fifteen healthy subjects received escitalopram 10 mg/day (n = 6) or citalopram 20 mg/day (n = 9) for a total of 10 days. SERT occupancies in midbrain were determined with SPECT and [123I]ADAM at three different time points: at baseline (no medication) and at 6 and 54 h after last drug intake. Results At 6 h after the last dose, mean SERT occupancies were 81.5 ± 5.4% (mean±SD) for escitalopram and 64.0 ± 12.7% for citalopram (p < 0.01). At 54 h after the last dose, mean SERT occupancies were 63.3 ± 12.1% for escitalopram and 49.0 ± 11.7% for citalopram (p < 0.05). The plasma concentrations of the S-enantiomer were of the same magnitude in both substances. For both drugs, the elimination rate of the S-enantiomer in plasma was markedly higher than the occupancy decline rate in the midbrain. Conclusion The significantly higher occupancy of SERT after multiple doses of escitalopram compared to citalopram indicates an increased inhibition of SERT by escitalopram. The results can also be explained by an attenuating effect of R-citalopram on the occupancy of S-citalopram at the SERT.
Psychopharmacology, 2006
Background Escitalopram is a dual serotonin reuptake inhibitor (SSRI) approved for the treatment of depression and anxiety disorders. It is the S-enantiomer of citalopram, and is responsible for the serotonin reuptake activity, and thus for its pharmacological effects. Previous studies pointed out that clinically efficacious doses of other SSRIs produce an occupancy of the serotonin reuptake transporter (SERT) of about 80% or more. The novel radioligand [ 123 I]ADAM and single photon emission computer tomography (SPECT) were used to measure midbrain SERT occupancies for different doses of escitalopram and citalopram. Methods Twenty-five healthy subjects received a single dose of escitalopram [5 mg (n=5), 10 mg (n=5), and 20 mg (n=5)] or citalopram [(10 mg (n=5) and 20 mg (n=5)]. Midbrain SERT binding was measured with [ 123 I]ADAM and SPECT on two study days, once without study drug and once 6 h after single dose administration of the study drug. The ratio of midbrain-cerebellum/cerebellum was the outcome measure (V3") for specific binding to SERT in midbrain. Subsequently, SERT occupancy levels were calculated using the untreated baseline level for each subject. An E max model was used to describe the relationship between S-citalopram concentrations and SERT occupancy values. Additionally, four subjects received placebo to determine test-retest variability. Results Single doses of 5, 10, or 20 mg escitalopram led to a mean SERT occupancy of 60±6, 64±6, and 75±5%, respectively. SERT occupancies for subjects treated with single doses of 10 and 20 mg citalopram were 65±10 and 70±6%, respectively. A statistically significant difference was found between SERT occupancies after application of 10 and 20 mg escitalopram, but not for 10 and 20 mg citalopram. There was no statistically significant difference between the SERT occupancies of either 10 mg citalopram or 10 mg escitalopram, or between 20 mg citalopram and 20 mg escitalopram. E max was slightly higher after administration of citalopram (84%) than escitalopram (79%). In the test-retest study, a mean SERT "occupancy" of 4% was found after administration of placebo, the intraclass correlation coefficient was 0.92, and the repeatability coefficient was 0.25. Conclusion SPECT and [ 123 I]ADAM were used to investigate SERT occupancies after single doses of escitalopram or citalopram. The test-retest study revealed good reproducibility of SERT quantification. Similar SERT occupancies were found after administration of equal doses (in respect to mg) of escitalopram and citalopram, giving indirect evidence for a fractional blockade of SERT by the inactive R-citalopram.