A Selective AT2 Receptor Ligand with a γ-Turn-Like Mimetic replacing the Amino Acid Residues 4-5 of Angiotensin II

• Three angiotensin II (Ang II) analogues encompassing a benzodiazepine-based γ-turn-like scaffold have been synthesized. Evaluation of the compounds in a radioligand binding assay showed that they had no affinity to the rat liver AT1 receptor. However, one of the compounds displayed considerable affinity to the pig uterus AT2 receptor (Ki = 3.0 nM) while the other two lacked affinity to this receptor. It was hypothesized that the reason for the inactivity of one of these analogues to the AT2 receptor was that the guanidino group of the Arg2 residue and/or the N-terminal end of the pseudopeptide could not interact optimally with the receptor. To investigate this hypothesis, a conformational analysis was performed and a comparison was carried out with the monocyclic methylenedithioether analogue cyclo(S−CH2−S)[Cys3,5]Ang II which is known to bind with high affinity to the AT2 receptor (Ki = 0.62 nM). This comparison showed that, in the compounds with high AT2 receptor affinity, the guanidino group of the Arg2 residue and the N-terminal end could access common regions of space that were not accessible to the inactive compound. To examine the importance of the guanidino group for binding, the Arg side chain was removed by substituting Arg2 for Ala2 in the analogue having the high affinity. This analogue lacked affinity to AT2 receptors, which supports the role of the guanidino group in receptor binding.

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