Application of the Guanidine-Acylguanidine Bioisosteric Approach to Argininamide-Type NPY Y2 Receptor Antagonists

Strongly basic groups such as guanidine moieties are crucial structural elements, but they compromise the drug‐likeness of numerous biologically active compounds, including ligands of G‐protein‐coupled receptors (GPCRs). As part of a project focused on the search for guanidine bioisosteres, argininamide‐type neuropeptide Y (NPY) Y2 receptor (Y2R) antagonists related to BIIE0246 were synthesized. Starting from ornithine derivatives, NG‐acylated argininamides were obtained by guanidinylation with tailor‐made mono‐Boc‐protected N‐acyl‐S‐methylisothioureas. The compounds were investigated for Y2R antagonism (calcium assays), Y2R affinity, and NPY receptor subtype selectivity (flow cytometric binding assays). Most of the NG‐substituted (S)‐argininamides showed Y2R antagonistic activities and binding affinities similar to those of the parent compound, whereas NG‐acylated or ‐carbamoylated analogues with a terminal amine were superior (Y2R: Ki and KB values in the low nanomolar range). This demonstrates that the basicity of the compounds, although 4–5 orders of magnitude lower than that of guanidines, is sufficient to form key interactions with acidic amino acids of the Y2R. The acylguanidines bind with high affinity and selectivity to Y2R over the Y1, Y4, and Y5 receptors. As derivatization of the amino group is tolerated, these compounds can be considered building blocks for the preparation of versatile fluorescent and radiolabeled pharmacological tools for in vitro studies of the Y2R. The results support the concept of bioisosteric guanidine–acylguanidine exchange as a broadly applicable approach to retain pharmacological activity despite decreased basicity. Bioisosteric replacement of the strongly basic guandino group in argininamides derived from BIIE0246 with guanidines bearing electron‐withdrawing groups (Z), in particular NG‐(ω‐aminoalkylcarbamoyl), resulted in potent and selective Y2R antagonists (Ki and KB values in the low nanomolar range). The results support the hypothesis that the guanidine–acylguanidine exchange is a broadly applicable bioisosteric approach to GPCR ligands with decreased basicity.