Design of oxytocin antagonists, which are more selective than Atosiban

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-(beta -mercapto-beta,beta -pentamethylene propionic acid), 2-O-methyltyrosine, 4-threonine]ornithine-vasotocin (desGly(NH(2))(9),d (CH(2))(5)[Tyr(Me)(2),Thr(4)]OVT) (A). the Tyr-(NH(2))(9) analogue of (A), d(CH(2))(5)[Tyr(Me)(2) Thr(4),Tyr-(NH(2))(9)OVT (B): the Eda(9) analogue (where Eda = ethylenediamine) of (A), d(CH(2))(5)[Tyr(Me)(2), Thr(4), Eda(9)]OVT (C). and the retro Tyr(10) modified analogue of (C), d(CH(2))(5)[Tyr(Me)(2), Thr(4), Eda(9)<-- Tyr(10)]OVT (D). The eight new analogues of A-D are (1) desGly(NH(2)),d(CH(2))(5)[Thi(2),Thr(4)]OVT, (2) desGly(NH(2)),d(CH(2))(5)[D-Thi(2),Thr(4)]OVT, (3) d(CH(2)) (5)[Thi(2), Thr(4),Tyr-(NH(2))(9)]OVT, (4) d(CH(2))(5)[D-Thi(2),Thr(4),Tyr-(NH(2))(9)]OVT (5) d(CH(2))(5)[Thi(2)Thr(4)Eda(9)]OVT, (6) d(CH(2))(5)[D-Thi(2) Thr(4),Eda(9)]OVT, (7) d(CH(2))(5) [Thi(2),Thr(4), Eda(9)<-- Tyr(10)]OVT, (8) d(CH(2))(5)[D-Thi(2),Thr(4) Eda(9) <-- Tyr(10)]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 (Via receptor) assays and in in vivo antidiuretic (V(2) receptor) assays. None of the eight peptides nor C exhibit oxytocic or vasopressor agonism. Peptides 1-8 are extremely weak V(2) agonists (antidiuretic activities range from < 0.0005 to 0.20 U/mg). Peptide C is a weak mixed V(2) agonist/antagonist. Peptides 1 - 8 and C exhibit potent tn vitro (no Mg(2+)) OT antagonism (anti-OT pA(2) values range from 7.76 to 8.05). Peptides 1-8 are all OT antagonists in vivo (estimated in vivo anti-OT pA(2) values range from 6.54-7.19). With anti-V(1a) pA(2) values of similar to 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) pA(2) range from 6.48 to 7.10) and to 1-deamino[D-Tyr(Et)(2), Thr(4)]OVT (Atosiban, trade name Tractocile) (anti-V(1a) pA(2)-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)llanti-V(1a) selectivities of 450, 525, 550, 450, similar to 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, similar to 82, similar to 82, 147, similar to 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 pA(2) value =7.19 +/- 0.06, peptide 4 is equipotent with Atosiban (pA(2)=7.05 +/- 0.05). However, with its significantly reduced anti -vasopressor potency. pA(2) = similar to 5, It Is similar to 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 (C) 2001 European Peptide Society and John Wiley & Sons, Ltd.