Synthesis and Structure−Activity Relationships of Novel Arylpiperazines as Potent and Selective Agonists of the Melanocortin Subtype-4 Receptor

The melanocortin receptors have been implicated as potential targets for a number of important therapeutic indications, including inflammation, sexual dysfunction, and obesity. We identified compound 1, an arylpiperazine attached to the dipeptide H-d-Tic-d-p-Cl-Phe-OH, as a novel melanocortin subtype-4 receptor (MC4R) agonist through iterative directed screening of nonpeptidyl G-protein-coupled receptor biased libraries. Structure−activity relationship (SAR) studies demonstrated that substitutions at the ortho position of the aryl ring improved binding and functional potency. For example, the o-isopropyl-substituted compound 29 (K i = 720 nM) possessed 9-fold better binding affinity compared to the unsubstituted aryl ring (K i = 6600 nM). Sulfonamide 39 (K i = 220 nM) fills this space with a polar substituent, resulting in a further 2-fold improvement in binding affinity. The most potent compounds such as the diethylamine 44 (K i = 60 nM) contain a basic group at this position. Basic heterocycles such as the imidazole 50 (K i = 110 nM) were similarly effective. We also demonstrated good oral bioavailability for sulfonamide 39.