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

Abstract

BACKGROUND AND PURPOSEThe 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. EXPERIMENTAL APPROACHWe performed a systematic structure-activity relationship study based on the scaffold of citalopram and the structurally closely related congener, talopram, which shows low-affinity S1 binding in SERT. The role of the four chemical substituents, which distinguish citalopram from talopram in conferring selectivity towards the S1 and S2 site, respectively, was assessed by determining the binding of 14 citalopram/talopram analogous to the S1 and S2 binding sites in SERT using membranes of COS7 cells transiently expressing SERT. KEY RESULTSThe structure-activity relationship study revealed that dimethyl citalopram possesses the highest affinity for the allosteric site relative to the S1 site in SERT and has approximately twofold selectivity for the allosteric site relative to the S1 site in SERT. CONCLUSIONS AND IMPLICATIONSThe compound could be a useful lead for future synthesis of drugs with high affinity and high selectivity towards the allosteric binding site.

Monoamine transporters: structure, intrinsic dynamics and allosteric regulation

“…The allosteric site, proposed for SERT almost three decades ago 53 , has been confirmed in subsequent studies of SERT 5,54 and the structural homologs LeuT [55][56][57][58][59] and the NSS MhsT 60 . This site (S2) is more than 10-12 Å away from S1, at the periphery of the EC vestibule, between the EC loops EL4a-EL4b, EL6 and TM1b, TM6a and TM11 6 (Fig.…”

“…Another mechanism of inhibition is the binding of drugs to the allosteric site S2 (Fig. 1e), which may help in the synthesis of high-affinity and high-selectivity inhibitors of SERT 54 . Occupancy of S2 by a substrate not only sterically hinders unbinding of the substrate from S1 6 but also facilitates its cytoplasmic release 58,79,80 by triggering cooperative movements that engage the two sites.…”

The mechanism of a high-affinity allosteric inhibitor of the serotonin transporter

“…We further report on a compound with nanomolar affinity at the S2 site of hSERT. Lu AF60097 has a >100-fold gain in allosteric potency relative to any other reported S2-bound hSERT ligand 22 . Our MD simulations suggest that Lu AF60097 can stably bind in the EV with its S-CIT scaffold in a similar pose as that of S2:S-CIT revealed by the hSERT crystal structure 11 .…”

“…The key observation was that certain SERT inhibitors could impede the dissociation of a pre-bound radiolabeled ligand [16][17][18][19] . The most potent impedance being by S-CIT on [ 3 H]S-CIT dissociation, however, only displays a low potency (IC 50 is~5 µM), and in spite of intensive investigations 20,21 no other compound has shown to possess higher potency 20,22 . Based on computational modeling and experimental binding studies, we previously located the lowaffinity second binding site for S-CIT and clomipramine to the extracellular vestibule (EV), the entry pathway toward the S1 site 21 .…”

Palladium Nanoparticles Supported on Smopex-234® as Valuable Catalysts for the Synthesis of Heterocycles

“…The 1,3-dihydroisobenzofuran [3,4] (phthalan, Figure 1a) nucleus is present in many natural and synthetic molecules, such as pestacin [5], citalopram [6,7], escitalopram [8][9][10], talopram [11], and egenine [12,13], which possess antidepressant, antioxidant, antimycotic, antihistaminic, antibacterial and antitumoral properties [5,8,14]. The phthalan derivative FR198248 was found to act as an anti-influenza agent [15,16], carbonylmethyleneisobenzofuran-1-imines have shown promising potential in herbicidal activity [17], and alkylidene functionalized 1,3-dihydroisobenzofurans have recently been tested as luminophores showing good fluorescence properties and remarkable Stokes shifts [18].…”