Application of the guanidine – acylguanidine bioisosteric approach to argininamide-type NPY Y2 receptor antagonists

Abstract

Strongly basic groups such as guanidine moieties are crucial structural elements but compromising drug-likeness of numerous biologically active compounds including ligands of G-protein coupled receptors (GPCRs). As part of a project focusing 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 preferably obtained by guanidinylation using 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 comparable to the parent compound, whereas NG-acylated or -carbamoylated analogs containing a terminal amine were superior (Y2R: Ki and KB values in the low nanomolar range). This demonstrates that the basicity of the compounds, although being by 4-5 orders lower than that of guanidines, suffices to form key interactions with acidic amino acids of the Y2R. The acylguanidines bind with high affinity and selectivity to Y2R compared to Y1, Y4, and Y5 receptors. As derivatization of the amino group is tolerated, these compounds are 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 regardless of reduced basicity.