DESIGN AND SYNTHESIS OF HIGHLY SELECTIVE IN-VITRO AND IN-VIVO UTERINERECEPTOR ANTAGONISTS OF OXYTOCIN - COMPARISONS WITH ATOSIBAN

We report the solid phase synthesis and some pharmacological propertie s of seven position two analogues (peptides 1-7) of one of our lead ox ytocin antagonists, 1-beta-mercapto-beta,-beta-pentamethylenepropionic acid), tocin(desGly-NH2,d(CH2)(5)-[Tyr(Me)(2),Thr(4)]OVT) (A). Peptid es 1-7 have the following substituents at position two (1) D-Tyr(Me); (2) L-Tyr(Et); (3) D-Tyr(Et); (4) L-Tyr; (5) D-Tyr; (6) D-Phe and (7) D-Trp. These were evaluated for agonistic and antagonistic activities in in vitro and in vivo OT assays, in vivo vasopressor (V-1a-receptor) assays and in vivo antidiuretic (V-2-receptor) assays. None of the se ven peptides exhibits oxytocic or vasopressor agonism, Peptides 1, 2, 4, 6 and 7 are extremely weak V-2 agonists (V-2 activities range from 0.001 to 0.02 U/mg), Peptides 3 and 5 exhibit weak V-2 antagonism (pA( 2)<6.0 and <5.5, respectively). Peptides 1-7 exhibit potent in vitro, (no Mg2+) OT antagonism (anti-OT pA(2) values range from 7.66 to 8.03) . Peptides 1 and 4-7 exhibit potent in vivo OT antagonism. Estimated i n vivo anti-OT pA(2) values range from 7.06 to 7.79 (peptides 2 and 3 were not tested), With anti-V-1a pA(2) values of 5.17-6.25 all seven p eptides exhibit reduced anti-V-1a potencies relative to the parent pep tide (A) (anti-V-1a pA(2) = 6.46). Four of these peptides (4-7) exhibi t striking gains in in vitro and in vivo anti-OT/anti-V-1a selectiviti es compared to (A) which has an in vitro selectivity of 30 and an in v ivo selectivity of 18. The D-Tyr(2) (5), D-Trp(2) (7), D-Phe(2) (6) an d L-Tyr(2) (4) analogues of (A) exhibit anti-OT (in vitro)/anti-V-1a s electivities = 240, 390, 404 and 540, respectively, The L-Tyr(2) (4), D-Trp(2) (7), D-Phe(2) (6) and D-Tyr(2) (5) analogues exhibited anti-O T (in vivo)/anti-V-1a, selectivities of 72, 80, 88 and 95, respectivel y, Peptides 4-7 appear to be the most selective peptide OT antagonists reported to date. In this regard it may be noted that they appear to be as or more potent and much more selective than the closely related OT antagonist 1-deamino[D-Tyr(Et)(2)Thr(4)]OVT (Atosiban) which is cur rently undergoing clinical trial as a potential therapeutic agent for the prevention of premature labor. Atosiban (peptide 8) was resynthesi zed and pharmacologically evaluated in our laboratories. Atosiban exhi bits the following antagonistic potencies. Anti-OT in vitro, no Mg2+) pA(2) = 7.71; anti-OT in vivo pA(2) = 7.05; anti-V-1a pA(2) = 6.14 and anti-V-2 pA(2) approximate to 5.9. Its anti-OT (inn vivo)/anti-V-1a s electivity is 8. Some of these antagonists may be suitable candidates for evaluation as potential tocolytic agents for use in the treatment of pre-term labor, They could also serve as useful new pharmacological tools for studies on the physiological roles of oxytocin. Finally, th e findings presented here provide useful clues for the design of more potent and more selective OT antagonists. (C) Munksgaard 1995.