Evaluation of safety and tolerability,pharmacokinetics, and pharmacodynamics of BMS-820836 in healthy subjects: a placebo-controlled,ascending single-dose study

Rationale

BMS-820836, a novel triple monoamine reuptake inhibitor, is an experimental monotherapy for sufferers of major depressive disorder who have had an inadequate response to an existing antidepressant treatment.

Objectives

This study was conducted to evaluate the safety and tolerability, pharmacokinetics (PK), and serotonin transporter (SERT) and dopamine transporter (DAT) occupancy for single doses of BMS-820836 in healthy subjects.

Methods

Healthy subjects were assigned to seven BMS-820836 dose panels (0.025, 0.1, 0.5, 1, 2, 3, and 5 mg; n?=?8 each), in which subjects were randomly allocated 3:1 to a single BMS-820836 dose or matched placebo. Serial blood samples were collected on Days 1, 2, 3, 4, 7, and 14 to characterize the PK of BMS-820836. Following evaluation of the maximum tolerated dose, SERT occupancy was determined by applying [¹¹C]DASB positron emission tomography (PET) after single-dose BMS-820836 (0.5 or 3 mg; n?=?3 each) and DAT occupancy by applying [¹¹C]PE2I PET after single-dose BMS-820836 (3 mg; n?=?6).

Results

Single oral doses of BMS-820836 (0.025–3 mg) were generally safe and well tolerated. BMS-820836 had a median T _max of 5.0–7.2 h and a mean apparent terminal T _1/2 of 34–57 h. Mean striatal SERT occupancies were 19?±?9 % and 82?±?8 % after single doses of 0.5 and 3 mg BMS-820836, respectively. The mean striatal DAT occupancy was 19?±?9 % after a single 3 mg BMS-820836 dose.

Conclusions

Single doses of BMS-820836 have meaningful SERT and DAT occupancy and demonstrate an acceptable safety and tolerability profile in healthy control subjects.     

Preclinical to Clinical Translation of CNS Transporter Occupancy of TD-9855, a Novel Norepinephrine and Serotonin Reuptake Inhibitor

Background:

Monoamine reuptake inhibitors exhibit unique clinical profiles that reflect distinct engagement of the central nervous system (CNS) transporters.

Methods:

We used a translational strategy, including rodent pharmacokinetic/pharmacodynamic modeling and positron emission tomography (PET) imaging in humans, to establish the transporter profile of TD-9855, a novel norepinephrine and serotonin reuptake inhibitor.

Results:

TD-9855 was a potent inhibitor of norepinephrine (NE) and serotonin 5-HT uptake in vitro with an inhibitory selectivity of 4- to 10-fold for NE at human and rat transporters. TD-9855 engaged norepinephrine transporters (NET) and serotonin transporters (SERT) in rat spinal cord, with a plasma EC₅₀ of 11.7ng/mL and 50.8ng/mL, respectively, consistent with modest selectivity for NET in vivo.Accounting for species differences in protein binding, the projected human NET and SERT plasma EC₅₀ values were 5.5ng/mL and 23.9ng/mL, respectively. A single-dose, open-label PET study (4–20mg TD-9855, oral) was conducted in eight healthy males using the radiotracers [¹¹C]-3-amino-4- [2-[(di(methyl)amino)methyl]phenyl]sulfanylbenzonitrile for SERT and [¹¹C]-(S,S)-methylreboxetine for NET. The long pharmacokinetic half-life (30–40h) of TD-9855 allowed for sequential assessment of SERT and NET occupancy in the same subject. The plasma EC₅₀ for NET was estimated to be 1.21ng/mL, and at doses of greater than 4mg the projected steady-state NET occupancy is high (>75%). After a single oral dose of 20mg, SERT occupancy was 25 (±8)% at a plasma level of 6.35ng/mL.

Conclusions:

These data establish the CNS penetration and transporter profile of TD-9855 and inform the selection of potential doses for future clinical evaluation.     

Availability of dopamine and serotonin transporters in opioid-dependent users—a two-isotope SPECT study

Rationale and objective

The aims of this study were to examine the differences between 32 opioid-dependent users treated with a very low dose of methadone or undergoing methadone-free abstinence and 32 controls.

Methods

SPECT analysis using [^99mTc] TRODAT-1 to assess striatal dopamine transporter (DAT) availability and [¹²³I] ADAM to assess midbrain serotonin transporter (SERT) availability were performed.

Results

Lower striatal DAT and midbrain SERT availabilities were noted in low-dose methadone users. History of metamphatamine use was associated with the lower striatal DAT. The striatal DAT of methadone-free abstainers was also lower than controls. The midbrain SERT availability tended to be higher in the methadone-free abstainers than the low-dose methadone users. The severity of depressive symptoms was negatively correlated with midbrain SERT availability in the opioid users.

Conclusion

The availability of striatal DAT tended to be, and the availability of midbrain SERT was, lower in the opioid users. History of metamphatamine use may confound the difference in straital DAT between controls and opioid users, as midbrain SERT and depressive symptoms are also associated with opioid use and abstinence.     

Differential inhibitory effects of drugs acting at the noradrenaline and 5-hydroxytryptamine transporters in rat and human neocortical synaptosomes

Background and purpose:

Although the amino acid sequences of rat and human 5-hydroxytryptamine (5-HT) and noradrenaline (NA) transporters (i.e. SERT and NET) are highly homologous, species differences exist in the inhibitory effects of drugs acting at these transporters. Therefore, comparison of the potencies of drugs acting at SERT and NET in native human and rat neocortex may serve to more accurately predict their clinical profile.

Experimental approach:

Synaptosomes prepared from fresh human and rat neocortical tissues were used for [³H]-5-HT and [³H]-NA saturation and competition uptake experiments. The drugs tested included NA reuptake inhibitors (desipramine, atomoxetine and (S,S)-reboxetine), 5-HT reuptake blockers (citalopram, fluoxetine and fluvoxamine) and dual 5-HT/NA reuptake inhibitors (duloxetine and milnacipran).

Key results:

In saturation experiments on synaptosomal [³H]-5-HT and [³H]-NA uptake, the dissociation constants did not indicate species differences although a smaller density of both SERT and NET was observed in human tissues. In competition experiments with the various drugs, marked species differences in their potencies were observed, especially at SERT. The rank order of selectivity ratios (SERT/NET) in human neocortex was as follows: citalopram ≥ duloxetine = fluvoxamine ≥ fluoxetine > milnacipran > desipramine = atomoxetine > (S,S)-reboxetine. Significant species differences in these ratios were observed for duloxetine, atomoxetine and desipramine.

Conclusions and implications:

This study provides the first compilation of drug potency at native human neocortical SERT and NET. The significant species differences (viz., human vs. rat) in drug potency suggest that the general use of rodent data should be limited to predict clinical efficacy or profile.     

NET occupancy by clomipramine and its active metabolite,desmethylclomipramine, in non-human primates in vivo

Rationale  

Norepinephrine transporter (NET) is one of the key targets for antidepressants such as combined serotonin and norepinephrine reuptake inhibitors as well as some of the tricyclic antidepressants. Clomipramine, a tricyclic antidepressant, has been reported to have an active metabolite, desmethylclomipramine, which has high affinity for NET in vitro. However, the NET occupancy of clomipramine and desmethylclomipramine has not fully been evaluated in vivo.     

Differential Potency of Venlafaxine,Paroxetine, and Atomoxetine to Inhibit Serotonin and Norepinephrine Reuptake in Patients With Major Depressive Disorder

BackgroundVenlafaxine is a dual serotonin (5-HT) and norepinephrine reuptake inhibitor. The specific dose at which it begins to efficiently engage the norepinephrine transporter (NET) remained to be determined. Paroxetine is generally considered as a selective 5-HT reuptake inhibitor but exhibits some affinity for NET. Atomoxetine is a NET inhibitor but also has some affinity for the 5-HT reuptake transporter (SERT).MethodsThis study examined the effects of forced titration of venlafaxine from 75 to 300 mg/d, paroxetine from 20 to 50 mg/d, or atomoxetine from 25 to 80 mg/d in 32 patients with major depressive disorder. Inhibition of SERT was estimated using the depletion of whole-blood 5-HT. Inhibition of NET was assessed using the attenuation of the systolic blood pressure produced by i.v. injections of tyramine.ResultsAll 3 medications significantly reduced 5-HT levels at the initiating regimens: venlafaxine and paroxetine by approximately 60% and atomoxetine by 16%. The 3 subsequent regimens of venlafaxine and paroxetine reduced 5-HT levels by over 90%, but the highest dose of atomoxetine only reached a 40% inhibition. Atomoxetine dose dependently inhibited the tyramine pressor response from the lowest dose, venlafaxine from 225 mg/d, and paroxetine left it unaltered throughout.ConclusionThese results confirm that venlafaxine and paroxetine are potent SERT inhibitors over their usual therapeutic range but that venlafaxine starts inhibiting NET only at 225 mg/d, whereas paroxetine remains selective for SERT up to 50 mg/d. Atomoxetine dose dependently inhibits NET from a low dose but does not inhibit SERT to a clinically relevant degree.     

Cognitive function is related to fronto-striatal serotonin transporter levels �C a brain PET study in young healthy subjects

Rationale

Pharmacological manipulation of serotonergic neurotransmission in healthy volunteers impacts on cognitive test performance. Specifically, markers of serotonin function are associated with attention and executive functioning, long-term memory, and general cognitive ability. The serotonin transporter (SERT) protein is a key regulator in the serotonin system. We hypothesized that higher performance on tests sensitive to serotonin would be associated with higher SERT levels in specific fronto-striatal brain regions.

Methods

Thirty-two healthy subjects (25 males, mean age 26.0?years, range 19?C37) underwent positron emission tomography using the SERT ligand [¹¹C]DASB. Subjects underwent the following tests: Stroop Color Word Test, Trail Making Test B, Rey??s Auditory Verbal Learning Test and Complex Figure Test, logical reasoning subtest from Intelligenz-Struktur-Test 2000 R, and a Danish version of National Adult Reading Test.

Results

We found positive associations between performance on the Stroop Color Word Test and right-sided dorsolateral prefrontal SERT binding (R ²?=?0.12, p?=?0.048). Furthermore, scores of logical reasoning (correlating with IQ) and educational level associated positively with SERT binding in the caudate, most prominent on the left side (logical reasoning: R ²?=?0.34, p?=?0.0026 (left), R ²?=?0.2, p?=?0.022 (right), educational level: R ²?=?0.19, p?=?0.012 (left), R ²?=?0.15, p?=?0.027 (right)). Scores of logical reasoning also associated with left-sided ventrolateral prefrontal cortex (R ²?=?0.24, p?=?0.014). There were no significant associations between SERT binding and tests of long-term episodic memory.

Conclusions

The results imply that in healthy subjects, high SERT binding in fronto-striatal regions is associated with better performance on tasks involving executive function and logical reasoning.     

Design and Synthesis of 4-Heteroaryl 1,2,3,4-Tetrahydroisoquinolines as Triple Reuptake Inhibitors

相似文献   

Dose-dependent binding of AZD3783 to brain 5-HT1B receptors in non-human primates and human subjects: a positron emission tomography study with [11C]AZ10419369

Rationale

The serotonin 5-HT_1B receptor is a potential target for the pharmacologic treatment of depression. Positron emission tomography (PET) determination of 5-HT_1B receptor occupancy with drug candidates targeting this receptor in non-human primate and human subjects may facilitate translation of research from animal models and guide dose selection for clinical studies. AZD3783 is a recently developed, orally bioavailable 5-HT_1B receptor antagonist with potential antidepressant properties.

Objectives

To determine the relationship between plasma concentration of AZD3783 and occupancy at primate brain 5-HT_1B receptors using PET and the radioligand [¹¹C]AZ10419369.

Methods

PET studies with [¹¹C]AZ10419369 were performed in three non-human primates at baseline and after intravenous injection of AZD3783. Subsequently, PET measurements were undertaken in six human subjects at baseline and after administration of different single oral doses of AZD3783 (1?C40?mg).

Results

After administration in non-human primates and human subjects, AZD3783 reduced regional [¹¹C]AZ10419369 binding in a dose-dependent and saturable manner. The AZD3783 plasma concentration required for 50% receptor occupancy (K _i,plasma) for monkeys was 25 and 27?nmol/L in occipital cortex and striatum, respectively. Corresponding estimates for human occipital cortex and ventral striatum were 24 and 18?nmol/L, respectively.

Conclusions

The potential antidepressant AZD3783 binds in a saturable manner to brain 5-HT_1B receptors with a similar in vivo affinity for human and monkey receptors. [¹¹C]AZ10419369 can be successfully used to determine occupancy at brain 5-HT_1B receptors in vivo and constitutes a useful tool for dose selection in clinical studies with 5-HT_1B receptor compounds.     

Region-specific regulation of 5-HT1B receptors in the rat brain by chronic venlafaxine treatment

Rationale

Venlafaxine is a non-selective serotonin and noradrenaline reuptake inhibitor antidepressant drug for which clinical studies have suggested a high level efficacy and a possible early action onset compared to the classical antidepressants. Its therapeutic effects might be due, at least in part, to adaptive changes in serotonergic neurotransmission, through the activation of the different 5-HT receptor subtypes. 5-HT_1B receptors are located in the axon terminals of both serotonergic and non-serotonergic neurons, where they act as inhibitory autoreceptors or heteroreceptors, respectively. However, the information about the involvement of this subtype in the mechanism of action of antidepressants is limited and quite controversial.

Objectives

The aim of this study was to evaluate the effect of venlafaxine (10 mg kg^?1 day^?1, p.o.) after 21 days of treatment on the density of 5-HT_1B receptors and their functionality in rat brain.

Methods

Effects of chronic venlafaxine were evaluated at different levels of 5-HT_1B receptor by using receptor autoradiography, [³⁵S]GTPγS binding, and the regulation of body temperature induced by selective 5-HT_1B agonist.

Results

Our results show that venlafaxine induced an increase in sensitivity of 5-HT_1B receptors in hypothalamus both at G-protein level and the control of core temperature without affecting the receptor density.

Conclusions

These results demonstrate that adaptive changes on 5-HT_1B receptors induced by chronic administration of venlafaxine exhibit regional differences suggesting that the hypothalamus might be an important site of drug action.     

Extracellular loop 3 of the noradrenaline transporter contributes to substrate and inhibitor selectivity

The human noradrenaline transporter (NET) and 5-hydroxytryptamine (5-HT) transporter (SERT) are inhibited by antidepressants and psychoactive drugs such as cocaine. Both substrates and inhibitors bind in the transmembrane core of the protein, but molecular divergence at the binding site is not sufficient to account for the NET-selective and SERT-selective inhibition of the antidepressants, desipramine and citalopram, respectively. We considered that the poorly conserved third extracellular loop may contribute to these differences. We substituted single amino acid residues of the third extracellular loop in NET for equivalents from SERT, transiently transfected COS-7 cells with WT NET, 13 mutant NETs and WT SERT, and measured [³H]noradrenaline uptake, [³H]nisoxetine binding and [³H]5-HT uptake. Mutants F299W, Y300Q, R301K and K303L, at the C-terminal end of EL3, all showed significantly decreased [³H]nisoxetine binding, indicative of a reduced cell surface expression. Most mutants differed little, if at all, from WT NET regarding [³H]noradrenaline uptake; however, the I297P mutant showed no significant uptake activity despite intact cell surface expression, and the A293F mutant showed a significantly slower transporter turnover than WT NET in addition to [³H]5-HT uptake that was significantly greater than that of WT NET. The A293F mutation also decreased desipramine potency and increased the inhibition of [³H]noradrenaline uptake by citalopram compared to WT NET. These results suggest that the third extracellular loop allosterically regulates the ability of the transmembrane domains to transport substrates and bind inhibitors and thus contributes to the selectivity of substrates and antidepressants for NET and SERT.     

Milnacipran enhances the control of impulsive action by activating D1-like receptors in the infralimbic cortex

Rationale

Elevated impulsivity is often observed in patients with depression. We recently found that milnacipran, an antidepressant and a serotonin/noradrenaline reuptake inhibitor, could enhance impulse control in rats. However, the neural mechanisms underlying the effects of milnacipran on impulsive action remain unclear. Milnacipran increases not only extracellular serotonin and noradrenaline but also dopamine specifically in the medial prefrontal cortex, which is one of the brain regions responsible for impulsive action.

Objectives

Our goal was to identify whether D₁- and/or D₂-like receptors in the infralimbic cortex (IL), the ventral portion of the medial prefrontal cortex, mediates the milnacipran-enhanced impulse control in a three-choice serial reaction time task.

Methods

The rats were bilaterally injected with SCH23390, a selective D₁-like receptor antagonist (0.3 or 3 ng/side) or eticlopride, a selective D₂-like receptor antagonist (0.3 or 1 μg/side) into the IL after acute intraperitoneal administration of milnacipran (10 mg/kg).

Results

Intra-IL SCH23390 injections reversed the milnacipran-enhanced impulse control, whereas injections of eticlopride into the IL failed to block the effects of milnacipran on impulsive action.

Conclusions

This is the first report that demonstrates a critical role for D₁-like receptors of the IL in milnacipran-enhanced control of impulsive action.     

Interaction of psychoactive tryptamines with biogenic amine transporters and serotonin receptor subtypes

Rationale

Synthetic hallucinogenic tryptamines, especially those originally described by Alexander Shulgin, continue to be abused in the USA. The range of subjective experiences produced by different tryptamines suggests that multiple neurochemical mechanisms are involved in their actions, in addition to the established role of agonist activity at serotonin 2A (5-HT_2A) receptors.

Objectives

This study evaluated the interaction of a series of synthetic tryptamines with biogenic amine neurotransmitter transporters and with serotonin (5-HT) receptor subtypes implicated in psychedelic effects.

Methods

Neurotransmitter transporter activity was determined in rat brain synaptosomes. Receptor activity was determined using calcium mobilization and DiscoveRx PathHunter® assays in HEK293, Gα16-CHO, and CHOk1 cells transfected with human receptors.

Results

Twenty-one tryptamines were analyzed in transporter uptake and release assays, and 5-HT_2A, serotonin 1A (5-HT_1A), and 5-HT_2A β-arrestin functional assays. Eight of the compounds were found to have 5-HT-releasing activity. Thirteen compounds were found to be 5-HT uptake inhibitors or were inactive. All tryptamines were 5-HT_2A agonists with a range of potencies and efficacies, but only a few compounds were 5-HT_1A agonists. Most tryptamines recruited β-arrestin through 5-HT_2A activation.

Conclusions

All psychoactive tryptamines are 5-HT_2A agonists, but 5-HT transporter (SERT) activity may contribute significantly to the pharmacology of certain compounds. The in vitro transporter data confirm structure-activity trends for releasers and uptake inhibitors whereby releasers tend to be structurally smaller compounds. Interestingly, two tertiary amines were found to be selective substrates at SERT, which dispels the notion that 5-HT-releasing activity is limited only to primary or secondary amines.     

Occupancy of dopamine D2 and D3 and serotonin 5-HT1A receptors by the novel antipsychotic drug candidate, cariprazine (RGH-188), in monkey brain measured using positron emission tomography

Rationale

Cariprazine is a novel antipsychotic drug candidate that exhibits high selectivity and affinity to dopamine D₃ and D₂ receptors and moderate affinity to serotonin 5-HT_1A receptors. Targeting receptors other than D₂ may provide a therapeutic benefit for both positive and negative symptoms associated with schizophrenia. Positron emission tomography (PET) can be used as a tool in drug development to assess the in vivo distribution and pharmacological properties of a drug.

Objectives

The objective of this study was to determine dopamine D₂/D₃ and serotonin 5-HT_1A receptor occupancy in monkey brain after the administration of cariprazine.

Methods

We examined three monkeys using the following PET radioligands: [¹¹C]MNPA (an agonist at D₂ and D₃ receptors), [¹¹C]raclopride (an antagonist at D₂ and D₃ receptors), and [¹¹C]WAY-100635 (an antagonist at 5-HT_1A receptors). During each experimental day, the first PET measurement was a baseline study, the second after a low dose of cariprazine, and the third after the administration of a high dose.

Results

We found that cariprazine occupied D₂/D₃ receptors in a dose-dependent and saturable manner, with the lowest dose occupying ~5% of receptors and the highest dose showing more than 90% occupancy. 5-HT_1A receptor occupancy was considerably lower compared with D₂/D₃ occupancy at the same doses, with a maximal value of ~30% for the raphe nuclei.

Conclusions

We conclude that cariprazine binds preferentially to dopamine D₂/D₃ rather than to serotonin 5-HT_1A receptors in monkey brain. These findings can be used to guide the selection of cariprazine dosing in humans.     

Antidepressants share the ability to increase catecholamine output in the bed nucleus of stria terminalis: a possible role in antidepressant therapy?

Rationale

Antidepressants include a relatively wide spectrum of drugs that increase the synaptic concentration of monoamines, mostly through neurotransmitter reuptake blockade. The bed nucleus of stria teminalis (BNST) is considered a relay station in mediating the activation of stress response but also in the acquisition and expression of emotions. BNST is richly innervated by monoamines and sends back projections to the nucleus of origin. We previously showed that the administration of selective blockers of norepinephrine transporter (NET) increases the extracellular concentration (output) of dopamine, suggesting that dopamine could be captured by NET in the BNST.

Objectives

The aim of this study, carried out by means of in vivo microdialysis, was to ascertain the acute effects that antidepressants with varying mechanisms of action have on dopamine and norepinephrine output in the BNST.

Results

We observed that all the antidepressants tested (5–20 mg/kg i.p.) increased the output of catecholamines, dose dependently. In particular, the maximum increases (as a percent of basal) for norepinephrine and dopamine respectively, were as follows: desipramine, 239 and 137; reboxetine, 185 and 128; imipramine, 512 and 359; citalopram, 95 and 122; fluoxetine, 122 and 68; bupropion, 255 and 164.

Conclusions

These results suggest that catecholamine transmission in the BNST may be part of a common downstream pathway that is involved in the action mechanism of antidepressants. Consequently, it is hypothesized that a dysfunction of neuronal transmission in this brain area may have a role in the etiology of affective disorders.     

Effects of milnacipran, a 5-HT and noradrenaline reuptake inhibitor, on C-fibre-evoked field potentials in spinal long-term potentiation and neuropathic pain

BACKGROUND AND PURPOSE

The analgesic action of 5-HT and noradrenaline reuptake inhibitors (SNRIs) on nociceptive synaptic transmission in the spinal cord is poorly understood. We investigated the effects of milnacipran, an SNRI, on C-fibre-evoked field potentials (FPs) in spinal long-term potentiation (LTP), a proposed synaptic mechanism of hypersensitivity, and on the FPs in a neuropathic pain model.

EXPERIMENTAL APPROACH

C-fibre-evoked FPs by electrical stimulation of the sciatic nerve fibres were recorded in the spinal dorsal horn of anaesthetized adult rats, and LTP was induced by high-frequency stimulation of the sciatic nerve fibres. A rat model of neuropathic pain was produced by L5 spinal nerve ligation and transection.

KEY RESULTS

Milnacipran produced prolonged inhibition of C-fibre-evoked FPs when applied spinally after the establishment of LTP of C-fibre-evoked FPs in naïve animals. In the neuropathic pain model, spinal administration of milnacipran clearly reduced the basal C-fibre-evoked FPs. These inhibitory effects of milnacipran were blocked by spinal administration of methysergide, a 5-HT_1/2 receptor antagonist, and yohimbine or idazoxan, α₂-adrenoceptor antagonists. However, spinal administration of milnacipran in naïve animals did not affect the basal C-fibre-evoked FPs and the induction of spinal LTP.

CONCLUSION AND IMPLICATIONS

Milnacipran inhibited C-fibre-mediated nociceptive synaptic transmission in the spinal dorsal horn after the establishment of spinal LTP and in the neuropathic pain model, by activating both spinal 5-hydroxytryptaminergic and noradrenergic systems. The condition-dependent inhibition of the C-fibre-mediated transmission by milnacipran could provide novel evidence regarding the analgesic mechanisms of SNRIs in chronic pain.     

A fatal case of venlafaxine overdose

Introduction

Based on its primary action of serotonin reuptake inhibition, venlafaxine overdose would be expected to result in serotonergic effects.

Case Report

A 40 year old male ingested venlafaxine without co-ingestants in a suicide attempt. The patient developed refractory ventricular fibrillation and expired approximately 9 hours post-ingestion. ECG monitoring revealed significant QRS and QT_C interval prolongation prior to his demise.

Discussion

A literature review of venlafaxine overdose cases and investigation into its mechanism of action was conducted. The potential for sodium channel blockade and implications for therapy are discussed.     

The interaction of escitalopram and R-citalopram at the human serotonin transporter investigated in the mouse

Rationale

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, thereby curtailing the elevation of extracellular 5-hydroxytryptamine (5-HT_Ext), and hence antidepressant 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 hereof.

Objectives

Primary: Investigate at the human (h)SERT, at clinical relevant doses, whether R-citalopram antagonizes escitalopram-induced 5-HT_Ext elevation. Secondary: Investigate whether abolishing the putative allosteric site affects escitalopram-induced 5-HT_Ext elevation and/or modulates the effect of R-citalopram.

Methods

Recombinant generation of hSERT transgenic mice; in vivo microdialysis; SERT binding; pharmacokinetics; 5-HT sensitive behaviors (tail suspension, marble burying).

Results

We generated mice expressing either the wild-type human SERT (hSERT^WT) or hSERT carrying amino acid substitutions (A505V, L506F, I507L, S574T and I575T) collectively abolishing the putative allosteric site (hSERT^{ALI/VFL+SI/TT}). One mg/kg escitalopram yielded clinical relevant plasma levels and brain levels consistent with therapeutic SERT occupancy. The hSERT mice showed normal basal 5-HT_Ext levels. Escitalopram-induced 5-HT_Ext elevation was not decreased by R-citalopram co-treatment and was unaffected by loss of the allosteric site_. The behavioral effects of the clinically relevant escitalopram dose were small and tended to be enhanced by R-citalopram co-administration.

Conclusions

We find no evidence that R-citalopram directly antagonizes escitalopram or that the putative allosteric site is important for hSERT inhibition by escitalopram.