Improvement in Potency of an Oxytocin Antagonist after Systematic Substitutions with L-Tryptophan

We report twelve analogues (1-12) of [Pmp¹,D-Trp²,Arg⁸]oxytocin, ANTAG (Pmp = β,β-pentamethylene-β- mercaptopropionic acid), which is a potent antagonist (pA₂ = 7.77) of the uterotonic effect of oxytocin (OT) in rats, as measured in a uterotonic assay. Nine of the following analogues were designed by replacement of each of the nine residues in ANTAG with an L-tryptophan residue: [Ac-Trp¹,D-Trp²,Val⁶,Arg⁸]OT (1), [Pmp¹,Trp²,Arg⁸]OT (2), [Pmp¹, D-Trp²,Trp³,Arg⁸]OT (3), [Pmp¹,D-Trp²,Trp⁴,Arg⁸]OT (4), [Pmp¹,D-Trp²,Trp⁶,Arg⁸]OT (5), [Pmp²,Trp⁶,Arg⁸]OT (6), [Aaa¹,D-Trp²,Val⁶,Arg⁸]OT (7), [Pmp¹,D-Trp²,Ica⁷,Arg⁸]OT (8), [Pmp¹,D-Trp²,Trp⁷Arg⁸]OT (9), [Pmp¹,D-Trp²,Trp⁸]OT (10), [Pmp¹,D-Trp²,Arg⁸,Trp⁹]OT (11), [Pmp¹,D-Trp²,Arg⁸,Trp(For)⁹]OT (12). In these analogues Aaa = 1-adamantaneacetic acid, and Ica = indoline-2-carboxylic acid. All linear analogues and analogues featuring Trp substitutions in the ring sequence of ANTAG were OT antagonists of lower potency than the parent peptide. All the analogues featuring Trp subtitutions in the tail sequence of ANTAG were OT antagonists of equal or better potency than the parent peptide. Replacement with Iea⁷ gave analogue 8, equipotent with ANTAG, but replacement with Trp⁷ gave analogue 9, which shows almost a two-fold increase in potency (pA₂ = 8.06). Replacement with Trp⁹ gave analogue 11 (pA_s = 8.03) which is about 1.8 times more potent than the parent antagonist, although Trp(For)⁹ had lower potency. Of great interest is that substitution with Trp⁸ leads to a more potent analogue, 10 (pÁ₂ = 8.22), which, unlike most antidiuretic hormone anatagonists, lacks any cationic charge in the molecule. The antidiuretic assay shows antagonists 9-11 to be weak antagonists of [Arg⁸]vasopressin, the antidiuretic hormone, with pA₂ ≤ 6.0; hence, they may be interesting leads for future design of more potent and specific OT antagonists.