Effects of a D-Cys6/L-Cys6 interchange in nonselective and selective vasopressin and oxytocin antagonists

We report the solid-phase synthesis of the D-Cys6 analogues of arginine-vasopressin (AVP), peptide 1, of the selective AVP vasopressor (V1a receptor) antagonist [1-(beta-mercapto-beta,beta-pentamethylenepropionic acid),2-O-methyltyrosine]arginine-vasopressin (d(CH2)5[Tyr(Me)2]-AVP, (A)), peptide 2, of the three nonselective antidiuretic/vasopressor (V2/V1a receptor) AVP antagonists d(CH2)5[Tyr(Et)2]VAVP (B), d(CH2)5[D-Tyr(Et)2]VAVP (C), and d(CH2)5[D-Phe2]VAVP (D) (where V = Val4), peptides 3-5, of the nonselective oxytocin (OT) antagonists d(CH2)5-[Tyr(Me)2]OVT (E) and d(CH2)5[Tyr(Me)2,Thr4,Tyr-NH2(9)]OVT (F) (where OVT = ornithine-vasotocin), peptides 6 and 7, and of the selective OT antagonists desGly-NH2,d(CH2)5[Tyr(Me)2,Thr4]OVT (G) and d(CH2)5]D-Trp2,Thr4]OVT (H), peptides 8 and 9. We also present the repeat syntheses of the previously reported d(CH2)5[D-Trp2]AVT (peptide 10) and its D-Cys6 analogue (peptide 11) (where AVT = arginine-vasotocin). Peptides 1-11 were assayed for agonistic and antagonistic activities in in vivo V1a, V2, and oxytocic assays and in in vitro oxytocic assays without and with 0.5 mM Mg2+. With V2 and V1a agonistic potencies of 0.82 and 0.41 units/mg, [D-Cys6]AVP has retained less than 0.3% of the V2 and V1a potencies of AVP. It exhibits no oxytocic activity and is an in vitro OT antagonist. pA2 = 6.67 (no Mg2+); pA2 = 5.24 (0.5 mM Mg2+). By contrast, with one or two exceptions, a D-Cys6/L-Cys6 interchange in antagonists 2-9, although resulting in reductions of antagonistic potencies in all assays for virtually all peptides 2-9 relative to A-H, has been well tolerated. For peptides 2-5, the anti-V2 and anti-V1a pA2 values range from approximately 5.54 to 7.33 and from 7.19 to 8.06, respectively; the range of in vitro anti-OT pA2 values (no Mg2+) is 7.35-7.87; with 0.5 mM Mg2+, the range is 7.24-8.21. Peptides 2 and 4 have in vivo anti-OT pA2s = 6.60 and 7.16, respectively. For peptides 6-9, the range of in vitro anti-OT pA2 values (no Mg2+) is 7.65-7.96; with 0.5 mM Mg2+, the range is 7.41-7.65, and the in vivo anti-OT pA2 values range from 6.85 to 7.33. With an in vivo anti-OT pA2 = 7.33, peptide 6 is equipotent with its parent E. The in vivo anti-OT potencies of peptides 7-9 are significantly reduced relative to those of F-H. The in vitro anti-OT (0.5 mM Mg2+) pA2 values of 10 and 11 are 7.54 and 7.50, both significantly lower than those previously reported.(ABSTRACT TRUNCATED AT 400 WORDS)