Structure–Activity Study of <i>N</i>‑((<i>trans</i>)‑4-(2-(7-Cyano-3,4-dihydroisoquinolin-2(1<i>H</i>)‑yl)­ethyl)­cyclohexyl)‑1<i>H</i>‑indole-2-carboxamide (SB269652), a Bitopic Ligand That Acts as a Negative Allosteric Modulator of the Dopamine D₂ Receptor

We recently demonstrated that SB269652 (<b>1</b>) engages one protomer of a dopamine D₂ receptor (D₂R) dimer in a bitopic mode to allosterically inhibit the binding of dopamine at the other protomer. Herein, we investigate structural determinants for allostery, focusing on modifications to three moieties within <b>1</b>. We find that orthosteric “head” groups with small 7-substituents were important to maintain the limited negative cooperativity of analogues of <b>1</b>, and replacement of the tetrahydroisoquinoline head group with other D₂R “privileged structures” generated orthosteric antagonists. Additionally, replacement of the cyclohexylene linker with polymethylene chains conferred linker length dependency in allosteric pharmacology. We validated the importance of the indolic NH as a hydrogen bond donor moiety for maintaining allostery. Replacement of the indole ring with azaindole conferred a 30-fold increase in affinity while maintaining negative cooperativity. Combined, these results provide novel SAR insight for bitopic ligands that act as negative allosteric modulators of the D₂R