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

Three photoaffinity ligands (PALs) for the human serotonin transporter (hSERT) were synthesized based on the selective serotonin reuptake inhibitor (SSRI), (<i>S</i>)-citalopram (<b>1</b>). 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 <b>15</b>, <b>22</b>, and <b>26</b>. These ligands retained high to moderate affinity binding (<i>K</i>_i = 24–227 nM) for hSERT, as assessed by [³H]­5-HT transport inhibition. When tested against Ser438Thr hSERT, all three PALs showed dramatic rightward shifts in inhibitory potency, with <i>K</i>_i values ranging from 3.8 to 9.9 μM, consistent with the role of Ser438 as a key residue for high-affinity binding of many SSRIs, including (<i>S</i>)-citalopram. Photoactivation studies demonstrated irreversible adduction to hSERT by all ligands, but the reduced (<i>S</i>)-citalopram inhibition of labeling by [¹²⁵I]<b>15</b> compared to that by [¹²⁵I]<b>22</b> and [¹²⁵I]<b>26</b> suggests differences in binding mode(s). These radioligands will be useful for characterizing the drug–protein binding interactions for (<i>S</i>)-citalopram at hSERT

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

Three photoaffinity ligands (PALs) for the human serotonin transporter (hSERT) were synthesized based on the selective serotonin reuptake inhibitor (SSRI), (<i>S</i>)-citalopram (<b>1</b>). 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 <b>15</b>, <b>22</b>, and <b>26</b>. These ligands retained high to moderate affinity binding (<i>K</i>_i = 24–227 nM) for hSERT, as assessed by [³H]­5-HT transport inhibition. When tested against Ser438Thr hSERT, all three PALs showed dramatic rightward shifts in inhibitory potency, with <i>K</i>_i values ranging from 3.8 to 9.9 μM, consistent with the role of Ser438 as a key residue for high-affinity binding of many SSRIs, including (<i>S</i>)-citalopram. Photoactivation studies demonstrated irreversible adduction to hSERT by all ligands, but the reduced (<i>S</i>)-citalopram inhibition of labeling by [¹²⁵I]<b>15</b> compared to that by [¹²⁵I]<b>22</b> and [¹²⁵I]<b>26</b> suggests differences in binding mode(s). These radioligands will be useful for characterizing the drug–protein binding interactions for (<i>S</i>)-citalopram at hSERT