Bradykinin antagonists with dehydrophenylalanine analogues at position 5

Continuing the studies on structural requirements of bradykinin antagonists, it has been found that analogues with dehydrophenylalanine (deltaPhe) or its ring-substituted analogues (deltaPhe(X)) at position 5 act as antagonists on guinea pig pulmonary artery, and on guinea pig ileum. Because both organs are considered to be bradykinin B2 receptor tissues, the analogues with deltaPhe or deltaPhe(X) at position 5, but without any replacement at position 7, seem to represent a new structural type of B2 receptor antagonist. All the analogues investigated act as partial antagonists; they inhibit the bradykinin-induced contraction at low concentrations and act as agonists at higher concentrations. Ring substitutions by methyl groups or iodine reduce both the agonistic and antagonistic activity. Only substitution by fluorine gives a high potency. Incorporation of deltaPhe into different representative antagonists with key modifications at position 7 does not enhance the antagonist activity of the basic structures, with one exception. Only the combination of deltaPhe at position 5 with DPhe at position 7 increases the antagonistic potency on guinea pig ileum by about one order of magnitude. Radioligand binding studies indicate the importance of position 5 for the discrimination of B2 receptor subtypes. The binding affinity to the low-affinity binding site (KL) was not significantly changed by replacement of Phe by deltaPhe. In contrast, ring-methylation of deltaPhe results in clearly reduced binding to KL. The affinity to the high-affinity binding site (KH) was almost unchanged by the replacement of Phe in position 5 by deltaPhe, whereas the analogue with 2-methyl-dehydrophenylalanine completely failed to detect the KH-site. The peptides were synthesized on the Wang-resin according to the Fmoc/Bu(t) strategy using Mtr protection for the side chain of Arg. The dehydrophenylalanine analogues were prepared by a strategy involving PyBop couplings of the dipeptide unit Fmoc-Gly-deltaPhe(X)-OH to resin-bound fragments.