DEVELOPMENT AND IN-VIVO EVALUATION OF METABOLICALLY RESISTANT ANTAGONISTS OF B(1) RECEPTORS FOR KININS

The B1 receptors for kinins are selectively stimulated by bradykinin ( BK) and Lys-BK metabolites that do not have the C-terminal arginine, d es-Arg9-BK and Lys-des-Arg9-BK, respectively. B1 receptors mediate a d efinite subset of the cardiovascular effects of kinins in normal and s eptic animals. We have studied the metabolism of the best selective B1 antagonist, Lys[Leu8]des-Arg9-BK, in order to support the rational de sign of new antagonists that have increased metabolic stability. The a ffinity of the new compounds was evaluated using the pA2 scale and was based on the antagonism of the contractile effect of kinins in the ra bbit isolated aortic preparation. Acetylation of the a-amino group of the N-terminal Lys residue provided an excellent protection from the d egradation by rabbit blood plasma. This and the inhibitory effect of a mastatin on the metabolism of Lys[Leu8]des-Arg9-BK indicated that amin opeptidase M (AmM) is the major route of inactivation for this class o f peptides in plasma. Various other modifications afforded a more or l ess complete resistance to purified angiotensin I converting enzyme (A CE). One analog, Ac-Lys[MeAla6,Leu8]des-Arg9-BK, was found resistant t o the above-mentioned enzymes and to neutral endopeptidase-24.11 extra cted from rabbit kidney. This antagonist, although 100 times less pote nt than the parent peptide Lys[Leu8]des-Arg9-BK on the rabbit aortic p reparation, was equipotent in vivo against the hypotensive effect of a B1 agonist in lipopolysaccharide-treated rabbits, and, unlike the ori ginal compound, its effect was persistent after the end of the infusio n. Ac-Lys[MeAla6,Leu8]des-Arg9-BK does not antagonize B2 receptors eit her in vitro or in vivo. Comparative data on other antagonists with va rying degrees of potency and metabolic resistance and in vivo studies with peptidase inhibitors indicate the major roles of neutral endopept idase-24.11 and of ACE as in vivo inactivation pathways for B1 antagon ists.