Synthesis and Molecular Modeling of New 1-Aryl-3-[4-arylpiperazin-1-yl]-1-propane Derivatives with High Affinity at the Serotonin Transporter and at 5-HT<INF>1A</INF> Receptors

It has been proposed that 5-HT<INF>1A</INF> receptor antagonists augment the antidepressant efficacy of selective serotonin (5-HT) reuptake inhibitors. In a search toward new and efficient antidepressants, 1-(aryl)-3-[4-arylpiperazin-1-yl]-1-propane molecular hybrids were designed, synthesized, and evaluated for 5-HT reuptake inhibition and 5-HT<INF>1A</INF> receptor affinity. The design was based in coupling structural moieties related to inhibition of serotonin reuptake, such as benzo[b]thiophene derivatives to arylpiperazines, typical 5-HT<INF>1A</INF> receptor ligands. In binding studies, several compounds showed affinity at the 5-HT transporter and at 5-HT<INF>1A</INF> receptors. Molecular modeling studies predicted the pharmacophore elements required for high affinity binding and the features that enable to discriminate between agonist, partial agonist, or antagonist action at 5-HT<INF>1A</INF> receptors and 5-HT transporter inhibition. Solvent interactions in desolvation prior to the binding step along with enthalpy and enthropy compensations might be responsible to explain agonist, partial agonist, and antagonist character. Hydrogen-bonding capability seems to be important to break hydrogen interhelical hydrogen bonds or alternatively to form other bonds upon ligand binding. Partial agonists and antagonists are unable to do this as the full agonist, which interacts closely by long-range forces or directly. The compounds showing the higher affinity at both the 5-HT transporter (K<INF>i</INF> &lt; 50 nM) and the 5-HT<INF>1A</INF> receptors (K<INF>i</INF> &lt; 20 nM) were further explored for their ability to stimulate [³⁵S]GTPγS binding or to antagonize 8-hydroxy-2-di-n-propylamino-tetralin (8-OH-DPAT)-stimulated [³⁵]GTPγS binding to rat hippocampal membranes, an index of agonist/antagonist action at 5-HT<INF>1A</INF> receptors, respectively. Compound <BO>8g</BO> exhibited agonist activity (EC<INF>50</INF> = 30 nM) in this assay, whereas compounds <BO>7g</BO> and <BO>8h</BO>,<BO>i</BO> behaved as weak partial agonists and <BO>7h</BO>−<BO>j</BO> and <BO>8j</BO>,<BO>l</BO> antagonized the R(+)-8-OH-DPAT-stimulated GTPγS binding. Functional characterization was performed by measuring the antagonism to 8-OH-DPAT-induced hypothermia in mice.