Topographic Probes of Angiotensin and Receptor: Potent Angiotensin II Agonist Containing Diphenylalanine and Long-Acting Antagonists Containing Biphenylalanine and 2-Indan Amino Acid in Position 8

A series of phenylalanine-mimicking amino acids with increasing conformational restraint were prepared and incorporated into angiotensin II, in order to develop topographic probes of angiotensin useful for probing receptor boundaries by molecular graphics analysis and for conformational analysis of the ligand by NMR. In binding studies, all analogues displayed high affinity for rat uterus (Kiof 0.74-6.08 nM) and brain (0.46-1.82 nM) receptors. In smooth muscle (rat uterus) contraction assay, the diphenylalanine-containing [Sar1,Dip8]AII and [Sar1,D-Dip8]AII were potent agonists with respectively 284% and 48% activity of [Asn1]AII. In contrast, the diphenylalanine-containing [Sar1,Bip8]AII, [Sar1,D-Bip8]AII, and the 2-indan amino acid containing [Sar1,2-Ind8]AII were potent inhibitors, approximately 9, 2, and 1.4 times more effective than a standard antagonist, [Sar1,Leu8]AII. Their respective pA10values in rat uterus assay were 8.87, 8.70, and 8.82. By comparison, the pA10value for [Sar1,Leu8]AII was 8.35. In rats, a single dose of 10 μg of [Sar1,2-Ind8]AII or [Sar1,Bip8]AII produced prolonged blockade of the pressor response toward angiotensin II for over 90 min. The very different pharmacological profiles of these rigid aromatic analogues suggest that the angiotensin receptor activation site consists of a relatively wide and elongated pocket with a narrow opening.