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 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, N(G) -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 N(G) -substituted (S)-argininamides showed Y₂R antagonistic activities and binding affinities similar to those of the parent compound, whereas N(G)-acylated or -carbamoylated analogues with a terminal amine were superior (Y₂R: K(i) and K(B) 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.