Novel Azido-Iodo Photoaffinity Ligands for the Human Serotonin Transporter Based on the Selective Serotonin Reuptake Inhibitor (S)-Citalopram

Three photoaffinity ligands (PALs) for the human serotonin transporter (hSERT) were synthesized based on the selective serotonin reuptake inhibitor (SSRI), (S)-citalopram (1). The classic 4-azido-3-iodo-phenyl group was appended to either the C-1 Or C-5 position of the parent molecule, with variable-length linkers, to generate ligands 15, 22, and 26: These ligands retained high to moderate affinity binding (K-i = 24-227 nM) for hSERT, as assessed by [H-3]5-HT transport inhibition. When tested against Ser438Thr hSERT, all three PALs showed dramatic rightward shifts in inhibitory potency, with K-i values ranging from 3.8 to 9.9 mu M, consistent with the role of Ser438 as a key residue for high-affinity binding of many SSRIs, including (S)-citalopram. Photoactivation studies demonstrated irreversible adduction to hSERT by all ligands, but the reduced (S)-citalopram inhibition of labeling by [I-125]15 compared to that by [I-125]22 and [I-125]26 suggests differences in binding mode(s). These radioligands will be useful for characterizing the drug protein binding interactions for (S)-citalopram at hSERT.