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

RATIONALE: Escitalopram is 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, curtailing the elevation of extracellular 5-hydroxytryptamine (5-HT(Ext)), and 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 technology; in vivo microdialysis; receptor 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. Importantly, escitalopram-induced 5-HT(Ext) elevation was not decreased by R-citalopram co-treatment. Further, escitalopram-induced 5-HT(Ext) elevation was not affected by loss of the allosteric site. The behavioral effects of the clinically relevant escitalopram dose were small, tending 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. Our findings points to mechanisms for R-citalopram antagonism of escitalopram’s antidepressant action other than direct antagonistic binding interactions at the hSERT