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.

From lin‐Benzoguanines to lin‐Benzohypoxanthines as Ligands for Zymomonas mobilis tRNA–Guanine Transglycosylase: Replacement of Protein–Ligand Hydrogen Bonding by Importing Water Clusters

“…For the synthesis of the C(6)N‐monoalkylated lin ‐benzoguanines 11 a , b , the ring closure of an appropriate isatoic anhydride derivative with S‐methylisothioureas 12 a , b , respectively, was envisaged 19. Whereas 12 a was commercially available, the required ethyl derivative 12 b was prepared starting from N,N′‐bis‐Boc‐ protected (Boc= t ‐butyloxycarbonyl) S‐methylisothiourea 13 , which was treated with EtI to afford N‐ethylisothiourea 14 (Scheme ) 20. Deprotection with trifluoroacetic acid gave the desired compound 12 b .…”

[3H]UR‐PLN196: A Selective Nonpeptide Radioligand and Insurmountable Antagonist for the Neuropeptide Y Y2 Receptor

“…[3] Previously, electron-withdrawing substituents, such as acyl and carbamoyl residues, attached to the guanidine group (N G ) of the parent compound were found to be tolerated despite the decrease in basicity by four to five orders of magnitude. [14] Derivatives bearing a terminal amino group (N w ) in the acyl moiety are of special interest with respect to labeling with fluorescent dyes or radioisotopes. Starting from these precursors, we synthesized a small library of potential radioligands ("cold" forms) by propionylation of the terminal amino moiety with respect to selection of an appropriate candidate for radiolabeling.…”

“…Guanidinylating reagents 1-10 (for 5, 6, 8, 10, see Ref. [14]) were prepared by coupling the mono-Boc-protected S-methylisothiourea with the corresponding carboxylic acids (Scheme 2), which were either activated in situ with hydroxybenzotriazole (HOBt)/N,N,N',N'-tetramethyl-O-(benzotriazol-1-yl)uronium tetrafluoroborate (TBTU) or used as commercially available succinimidyl esters (2)(3)(4)6). In the final reaction step, amine precursors 11-20 were acylated with succinimidyl propionate to yield Y 2 R antagonists 21 a and 22-30 (Scheme 3).…”

“…[14]) as well as propionamides 21 a, 22-30 and 34 were investigated for Y 2 R binding and antagonism (Table 1). The Y 2 R binding af-finities were determined in a flow cytometric binding assay, using CHO cells stably expressing the human Y 2 receptor (hY 2 R), [16] and fluorescence-labeled pNPY (Cy5-pNPY or Dy-635-pNPY).…”

Ligands of the neuropeptide Y Y2 receptor

“…1) for Y2 receptor was identified through modification of the guanidine moeity of BIIE0246. 36, 37 The cold UR-PLN196 (Ki=9.9 nM) also inhibited agonist responses in an insurmountable fashion when pre-incubated for 20 min. BIIE0246 did not show binding for Y1, Y4, Y5 and a variety of receptors or enzymes up to 1 μM.…”