Design of peptide oxytocin antagonists with strikingly higher affinities and selectivities for the human oxytocin receptor than atosiban

The peptide oxytocin (OT) antagonist atosiban, approved for tocolytic use in Europe (under the tradename Tractocile), represents an important new therapeutic advance for the treatment of premature labor. This paper presents some new peptide OT antagonists which offer promise as superior tocolytics. The solid phase synthesis is reported of four pairs of L and D‐2‐naphthylalanine (L/D‐2Nal) position‐2 modified analogs 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,d(CH2)5[Tyr(Me)2,Thr4]OVT) (A); the Tyr‐NH29 analog of (A), d(CH2)5[Tyr(Me)2,Thr4,Tyr‐NH29]OVT (B); the Eda9 analog of (A), d(CH2)5[Tyr(Me)2,Thr4,Eda9]OVT (C); and the retro COCH2Ph(4‐0H)10 modified analog of (C), d(CH2)5[Tyr(Me)2,Thr4,Eda9← COCH2Ph(4‐0H)10]OVT (D). The eight new analogs of A–D are (1) desGly‐NH2,d(CH2)5[D‐2Nal2,Thr4]OVT, (2) desGly‐NH2,d(CH2)5[2‐Nal2,Thr4]OVT, (3) d(CH2)5[D‐2Nal2,Thr4,Tyr‐NH29]OVT, (4) d(CH2)5[2Nal2,Thr4,Tyr‐NH29]OVT, (5) d(CH2)5[D‐2Nal2,Thr4,Eda9]OVT, (6) d(CH2)5[2Nal2,Thr4,Eda9]OVT, (7) d(CH2)5[D‐2Nal2,Thr4,Eda9← COCH2Ph(4‐0H)10]OVT, (8) d(CH2)5[2Nal2,Thr4,Eda9← COCH2Ph(4‐OH)10]OVT. Peptides 1–8 were evaluated for agonistic and antagonistic activities in in vitro and in vivo rat bioassays, in rat OT receptor (rOTR) binding assays and in human OT receptor (hOTR) and human vasopressin (VP) vasopressor (V1a) receptor (hV1aR) binding assays. Also reported are the hOTR and hV1aR affinity data for atosiban and for B. None of the eight peptides exhibit oxytocic or vasopressor agonism. Peptides 1–8 exhibit weak antidiuretic agonism (activities in the range 0.014–0.21 U/mg). Peptides 1–6 exhibit potent in vitro (no Mg2+) OT antagonism (anti‐OT pA2 values range from 7.63 to 8.08). Peptides 7 and 8 are weaker OT antagonists. Peptides 1–6 are all OT antagonists in vivo (estimated in vivo anti‐OT pA2 values in the range 6.94–7.23). Peptides 1–8 exhibit vasopressor antagonism, anti‐V1a pA2 values in the range 5.1–7.65. Peptides 1–8 exhibit high affinities for the rOTR (Ki values = 0.3–7.8 nM). Peptides 1–4 and B exhibit surprisingly very high affinities for the hOTR; their Ki values are 0.17, 0.29, 0.07, 0.14 and 0.59 nM, respectively. Peptides 1–4 and B exhibit respectively 449, 263, 1091, 546 and 129 times greater affinity for the hOTR than atosiban (Ki = 76.4 nM). Peptides 1–4 exhibit high affinities for the hV1aR (Kis = 1.1 nM, 1.3 nM, 0.1