Vinyl sulfide cyclized analogues of angiotensin II with high affinity and full agonist activity at the AT1 receptor

Vinyl sulfide cyclized analogues of the octapeptide angiotensin II that are structurally related to the cyclic disulfilde agonist c[HCy3.5]Ang II have been prepared. The synthesis relies on the reaction of the mercapto group of a cysteine residue in position 3 with the formyl group of allysine incorporated in position 5 of angiotensin II. A mixture of the cis and the trans isomers was formed, and these were separated and isolated by RP-HPLC. Thus, the three-atom CH2- S-S element of the AT(1) receptor agonist c[Hcy(3.5)]Ang II has been displaced by a bioisosteric three-atom S-CH=CH element. A comparative conformational analysis of the 13-membered ring systems of c[Hcy(3.5)]Ang II and the 13-membered cyclic vinyl sulfides with cis and trans configuration, respectively, suggested that all three systems adopted very similar low-energy conformations. This similarity was also reflected in the bioactivity. Both of the compounds that contained the ring systems encompassing the cis or trans vinyl sulfide elements between positions 3 and 5 exhibited K-i values less than 2 nM and exerted full agonism at the AT(1) receptor. In contrast, vinyl sulfide cyclization involving the amino acid residues 5 and 7 rendered inactive compounds. The cyclic vinyl sulfides that have agonist activity were both shown to possess low-energy conformers compatible with the previously proposed 3D model for the bioactive conformation of Ang II.