Design of Oxytocin Antagonists, which are more Selective than Atosiban

Abstract

We report the solid phase synthesis of four pairs of 𝓁- and 𝒹-thienylalanine (Thi/𝒹-Thi) position two modified analogues of the following four oxytocin (OT) antagonists: des-9-glycinamide [1-(β-mercapto-β,β-pentamethylene propionic acid), 2-O-methyltyrosine, 4-threonine]ornithine-vasotocin (desGly(NH₂)⁹,d(CH₂)₅[Tyr(Me)²,Thr⁴]OVT) (A); the Tyr-(NH₂)⁹ analogue of (A), d(CH₂)₅[Tyr(Me)²,Thr⁴,Tyr-(NH₂)⁹]OVT (B); the Eda⁹ analogue (where Eda=ethylenediamine) of (A), d(CH₂)₅[Tyr(Me)², Thr⁴, Eda⁹]OVT (C); and the retro Tyr¹⁰ modified analogue of (C), d(CH₂)₅[Tyr(Me)², Thr⁴, Eda⁹←Tyr¹⁰]OVT (D). The eight new analogues of A–D are (1) desGly(NH₂),d(CH₂)₅[Thi²,Thr⁴]OVT, (2) desGly(NH₂),d(CH₂)₅[𝒹-Thi²,Thr⁴]OVT, (3) d(CH₂)₅[Thi²,Thr⁴,Tyr-(NH₂)⁹]OVT, (4) d(CH₂)₅[𝒹-Thi²,Thr⁴,Tyr-(NH₂)9¯]OVT (5) d(CH₂)₅[Thi²,Thr⁴Eda⁹]OVT, (6) d(CH₂)₅[𝒹-Thi²,Thr⁴,Eda⁹]OVT, (7) d(CH₂)₅[Thi²,Thr⁴,Eda⁹←Tyr¹⁰]OVT, (8) d(CH₂)₅[𝒹-Thi²,Thr⁴,Eda⁹←Tyr¹⁰]OVT. We also report the synthesis of (C). Peptides 1¯–8¯ and C were evaluated for agonistic and antagonistic activities in in vitro and in vivo OT assays, in in vivo vasopressor (V_1a receptor) assays and in in vivo antidiuretic (V₂ receptor) assays. None of the eight peptides nor C exhibit oxytocic or vasopressor agonism. Peptides 1¯–8¯ are extremely weak V₂ agonists (antidiuretic activities range from 2 agonist/antagonist. Peptides 1¯–8¯ and C exhibit potent in vitro (no Mg²⁺) OT antagonism (anti-OT pA₂ values range from 7.76 to 8.05). Peptides 1¯– 8¯ are all OT antagonists in vivo (estimated in vivo anti-OT pA₂ values range from 6.54–7.19). With anti-V_1a pA₂ values of ∼5–5.80, peptides 1¯– 8¯ exhibit marked reductions in anti-V_1a potencies relative to those of the parent peptides A–D (anti-V_1a pA₂ range from 6.48 to 7.10) and to 1-deamino[𝒹-Tyr(Et)², Thr⁴]OVT (Atosiban, trade name Tractocile) (anti-V_1a pA₂-6.14). Atosiban has recently been approved in Europe for clinical use for the prevention of premature labour (Pharm. J. 264(7-100): 871). Peptides 1¯– 8¯ exhibit striking gains in in vitro anti-OT/anti-V_1a selectivities with respect to the parent peptides A, B, C and D and to Atosiban. Peptides 1¯– 8¯ exhibit anti-OT (in vitro)/anti-V_1a selectivities of 450, 525, 550, 450, ∼1080, 116, 355, 227 respectively. The corresponding values for A–D and Atosiban are 30, 4.2, 4.3, 2.6 and 37. With the exception of peptide 6¯, the remaining seven peptides exhibit 3–18-fold gains in anti-OT (in vivo)/anti-V_1a selectivity with respect to Atosiban, peptides 1¯– 8¯ exhibit anti-OT (in vivo)/anti-V_1a selectivities of 22, ∼82, ∼82, 147, ∼83, 11, 31 and 42. By comparison, Atosiban exhibits an anti-OT (in vivo)/anti-V_1a selectivity=8. With an estimated in vivo anti-OT pA₂ value=7.19±0.06, peptide 4¯ is equipotent with Atosiban (pA₂=7.05±0.05). However, with its significantly reduced anti-vasopressor potency, pA₂=∼5, it is ∼18 times more selective for OT receptors with respect to VP V_1a receptors than Atosiban. Since we have shown that V_1a antagonism could be an unwanted side-effect in tocolytics, peptide 4¯ and some of the OT antagonists reported here have advantages over Atosiban and thus may be suitable candidates for evaluation as potential tocolytic agents for the treatment of preterm labour. Copyright © 2001 European Peptide Society and John Wiley & Sons, Ltd.