Abstract
Twelve analogues were synthesized, their structure derived from modifications of [(S)Pmp^1^, D‐Trp^2^, Pen^6^, Arg^8^]oxytocin, PA, in which (S)Pmp = β,β‐(3‐thiapentamethylene‐β‐mercaptopropionic acid). PA is a potent antagonist of the uterotonic effect of oxytocin in the rat (uterotonic test in vitro, pA~2~ = 8.86) and in the baboon. Truncated analogues of PA from the C‐terminus were systematically prepared ending in either the free acid or the amide, i.e. PA^1–9^ acid, PA^1–8^ acid, PA^1–7^ acid, PA^1–6^ acid, PA^1–8^ amide, PA^1–7^ amide and PA^1–6^ amide. PA^1–8^ amide was roughly as potent as PA in the rat uterotonic assay in vitro, and the shorter amides were only somewhat weaker antagonists. All four acid analogues were weaker antagonists than PA but still maintained rather high antagonistic potency. These findings suggest that, if these truncated acids form as metabolites in vivo, they may contribute to the overall biological effect of PA and their contribution should be taken into account. Furthermore, using these analogues, the radioimmunoassay measurements of PA may be standardized, as they may cross react with PA antibodies and interfere with the determination. In addition, five analogues were made by substituting Arg^8^ of PA with Lys^8^, Orn^8^, Dab^8^, Dap^8^ and Cit^8^. All of these analogues maintained high potency as OTAs in the uterotonic assay, although their activity was only about 1.5–3 times lower than PA. The most potent analogue in the uterotonic assay, [Dap^8^]PA, pA~2~ = 8.53, had weak pressor activity (pA~2~ = 6.90) and no antidiuretic effect. The pressor activity was lower for all tested acids, and for PA^1–6^ acid it was even below the detection limit. Additionally, PA^1–9^ acid, PA^1–7^ acid and PA^1–6^ acid showed no antidiuretic activity. Hence, the PA^1–6^ acid is a potent OTA with pA~2~ = 8.27 and no measurable effect in the pressor or antidiuretic tests and thus it is a pure oxytocin antagonist. This fact makes it an attractive candidate for further studies on inhibition of OT biological effects and on preterm labour. Copyright © 2003 European Peptide Society and John Wiley & Sons, Ltd.