Discovery of a Novel Class of Negative Allosteric Modulator of the Dopamine D₂ Receptor Through Fragmentation of a Bitopic Ligand

Recently, we have demonstrated that <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) (<b>1</b>) adopts a bitopic pose at one protomer of a dopamine D₂ receptor (D₂R) dimer to negatively modulate the binding of dopamine at the other protomer. The 1<i>H</i>-indole-2-carboxamide moiety of <b>1</b> extends into a secondary pocket between the extracellular ends of TM2 and TM7 within the D₂R protomer. To target this putative allosteric site, we generated and characterized fragments that include and extend from the 1<i>H</i>-indole-2-carboxamide moiety of <b>1</b>. <i>N</i>-Isopropyl-1<i>H</i>-indole-2-carboxamide (<b>3</b>) displayed allosteric pharmacology and sensitivity to mutations of the same residues at the top of TM2 as was observed for <b>1</b>. Using <b>3</b> as an “allosteric lead”, we designed and synthesized an extensive fragment library to generate novel SAR and identify <i>N</i>-butyl-1<i>H</i>-indole-2-carboxamide (<b>11d</b>), which displayed both increased negative cooperativity and affinity for the D₂R. These data illustrate that fragmentation of extended compounds can expose fragments with purely allosteric pharmacology