An investigation of position 3 in arginine vasopressin with aliphatic, aromatic, conformationally-restricted, polar and charged amino acids

We report the solid-phase synthesis and some pharmacological properties of 23 new analogs of arginine vasopressin (AVP) which have the Phe(3) residue replaced by a broad variety of amino acids. Peptides 1-9 have at position 3 : (1) the mixed aromatic/aliphatic amino acid thienylalanine (Thi) and the aliphatic amino acids: (2) cyclohexylalanine (Cha); (3) norleucine (Nle): ( 4) Leu: (5) norvaline (Nva): (6) Val: (7) alpha-aminobutyric acid (Abu); (8 ) Ala; (9) Gly. Peptides 10-23 have at position 3: the aromatic amino acids , (10) homophenylalanine (Hphe): (11)Tyr: (12) Trp: (13) 2-naphthylalanine (2-Nal): the conformationally-restricted amino acids (14) Pro; (15) 2-amino tetraline-2-carboxylic acid (Atc); the polar amino acids (16) Ser; [17) Thr ; (18) Gin: and the charged amino acids (19) Asp: (20) Glu: (21] Arg; (22) Lys; (23) Om. All 23 new peptides were evaluated for agonistic and, where a ppropriate, antagonistic activities in In vivo antidiuretic (V-2-receptor) and vasopressor (V-1a-receptor) assays and in In vitro (no Mg2+) oxytocic a ssays. The corresponding potencies (units/mg) in these assays for AVP are: 323 +/- 16; 369 +/- 6 and 13.9 +/- 0.5. Peptides 1-9 exhibit the following potencies (units/mg) in these three assays: (1) 379 +/- 14; 360 +/- 9; 36.2 +/- 1.9; (2) 294 +/- 21; 73.4 +/- 2.7; 0.33 +/- 0.02; (3) 249 +/- 28; 84.6 +/- 4.3; 4.72 +/- 0.16; (4) 229 +/- 19; 21.4 +/- 0.6; 2.1 +/- 0.2; (5) 134 +/- 5; 31.2 +/- 0.9; 28.4 +/- 0.2; (6) 114 +/- 9; 45.3 +/- 2.3: 11.3 +/- 1 .6; (7) 86.7 +/- 2.5; 4.29 +/- 0.13; 0.45 +/- 0.03; (8) 15.5 +/- 1.5: 0.16 +/- 0.01; similar to 0.02: (9) 3.76 +/- 0.03: < 0.02: in vitro oxytocic ago nism was not detected. These data show that the aliphatic amino acids Cha, Nle, Leu, Nva and Val are well-tolerated at position 3 in AVP with retentio n of surprisingly high levels of antidiuretic activity. Peptides 2-9 exhibi t significant gains in both antidiuretic/vasopressor (A/P) and antidiuretic /oxytocic (A/O) selectivities relative to AVP. [Thi(3)]AVP appears to be a more potent antidiuretic and oxytocic agonist than AVP and is equipotent wi th AVP as a vasopressor agonist. The antidiuretic potencies of peptides 10- 23 exhibit drastic losses relative to AVP. They range from a low of 0.018 /- 0.001 units,mg for the Lys(3) analog (peptide 22) to a high of 24.6 +/- 4.6 units/mg for the Hphe(3) analog (peptide 10). Their vasopressor potenci es are also drastically reduced. These range from a low of < 0.002 units/mg for peptide 22 to a high of 8.99 +/- 0.44 units/mg for the Atc(3) analog ( peptide 15). Peptides 10-23 exhibit negligible or undetectable in vitro oxy tocic agonism. The findings on peptides 10-23 show that position 3 in AVP i s highly intolerant of changes with aromatic, conformationally restricted, polar and charged amino acids. Furthermore, these findings are in striking contrast to our recent discovery that position 3 in the potent V-2/V-1a/OT antagonist d(CH2)(5)D-Tyr(Et)(2)VAVP tolerates a broad latitude of structur al change at position 3 with many of the same amino acids, to give excellen t retention of antagonistic potencies. The data on peptides 1-4 offer promi sing clues to the design of more potent and selective AVP V-2 agonists. Cop yright (C) 1999 European Peptide Society and John Wiley & Sons, Ltd.