Design and Synthesis of 1-(3-(Dimethylamino)propyl)-1-(4-fluorophenyl)-1,3-dihydroisobenzofuran-5-carbonitrile (Citalopram) Analogues as Novel Probes for the Serotonin Transporter S1 and S2 Binding Sites

The serotonin transporter (SERT) is the primary target for antidepressant drugs. The existence of a high affinity primary orthosteric binding site (S1) and a low affinity secondary site (S2) has been described, and their relation to antidepressant pharmacology has been debated. Herein, structural modifications to the N, 4, 5, and 4' positions of (±)citalopram (1) are reported. All of the analogues were SERT-selective and demonstrated that steric bulk was tolerated at the SERT S1 site, including two dimeric ligands (15 and 51). In addition, eight analogues were identified with similar potencies to S-1 for decreasing the dissociation of [(3)H]S-1 from the S1 site via allosteric modulation at S2. Both dimeric compounds had similar affinities for the SERT S1 site (Ki = 19.7 and 30.2 nM, respectively), whereas only the N-substituted analogue, 51, was as effective as S-1 in allosterically modulating the binding of [(3)H]S-1 via S2.