Design of oxytocin antagonists with prolonged action: Potential tocolytic agents for the treatment of preterm labor

In our continuing effort to produce more potent and specific oxytocin (OT) antagonists that may have value as tocolytic agents, we have synthesized a number of new OT antagonists. Our previous studies have shown that rigid conformational structure and restricted dynamic properties are associated with antagonistic activity of the [1-penicillamine]OT ([Pen¹]OT) analogs. We therefore synthesized a series of structural analogs of [Pen¹,]OT; [Pen¹,Thr⁴]-OT and [Pen¹,Phe²,Thr⁴]OT with greater restricted conformational features. They are [Pen¹,Δ^3,4-Pro⁷]OT; [Pen¹,Thr⁴,Δ^3,4-Pro⁷]OT; [Pen¹,Phe²,Thr⁴,Δ^3,4-Pro⁷]OT; [Pen¹,Orn⁸]OT; [Pen¹,Phe²,Thr⁴,Δ^3,4-Pro⁷,Orn⁸]OT; [Pen¹,Tyr(OMethyl)²,-Thr⁴,Orn⁸]OT; [Pen¹,Tyr(OEthyl)²,Thr⁴,Orn⁸]OT; [Pen¹,Phe(Methyl)²,Thr⁴,Orn⁸]OT and [Pen¹Phe(Ethyl)², Thr⁴,Orn⁸]OT. As expected, all were found to be potent OT antagonists, with in vitro pA₂ values ranging from 5.32 to 7.67. They were also effective OT antagonists in vivo in the term pregnant rats. Structural modifications in the above analogs produced various and interesting effects. Dehydroproline substitution for 7-proline in [Pen¹]OT increased antagonistic potency, whereas in [Pen¹,Thr⁴]OT and in [Pen¹,Phe²,Thr⁴]OT decreased antagonistic potency. Most significantly, analogs with O-alkyl-Tyr²,Orn⁸ and p-alkyl-Phe²,Orn⁸ substitutions were found to have prolonged action both in the isolated rat uterus assays and in the term pregnant rats. Generally, substitution of the alkyl groups resulted in a reduction in anti-OT potency, and increasing the size of the alkyl substituent from a methylene group to an ethyl group diminished antagonistic potencies markedly. The 2-ethyl,8-ornithine analogs, however, retained their prolonged action. This dissociation between potency and prolonged action gives further support to our hypothesis that potency (binding and transduction) and duration of action (reversibility) may each have different molecular requirements and that OT analogs can be designed rationally based on conformational considerations for potency, specificity and long action.