Structural features of the bradykinin receptor as determined by computer simulations, mutagenesis experiments, and conformationally constrained ligands: establishing the framework for the design of new antagonists.

1. In recent years, two classes of second generation bradykinin receptor antagonists have been reported. Both are of the general sequence D-Arg0-Arg1-Pro2-W3-Gly4-X5-Ser6-Y7-Z8+ ++-Arg9, where W is either Pro or Hyp, and X is an aromatic or aliphatic side chain-containing amino acid. Y and Z are unnatural amino acids, presumed to enforce a beta-turn structure. The de novo design of a non-peptide receptor antagonist (or the optimization of a lead discovered by random screening) will ultimately require knowledge about the receptor topology. In the absence of an experimentally determined structure of the bradykinin-bradykinin receptor complex, we have attempted to gain insights from other sources. 2. We have synthesized conformationally constrained ligands and completed extensive computer modeling on the bradykinin receptor. Moreover, using systematic synthetic modifications, we have explored the relative importances of selected amide bonds and side chains in second generation peptides and have made a series of C alpha- and/or N-methyl substitutions at positions four and five which led to the discovery of two new cyclic peptide antagonists. 3. Computational simulations led to a proposed model of bradykinin bound to its receptor which was found to be in good agreement with mutagenesis results. This model led ultimately to the design and synthesis of D-Arg0-Arg1-(12-aminododecanoyl)2-Ser3-D-Tic4-Oic5+ ++-Arg6. Consideration of this new lead compound, together with the extensive structure-activity relationship (SAR) which has been developed for peptide ligands and the receptor, represents a tangible framework for the design of more potent and longer-lasting antagonists of the bradykinin receptor.