Conformational and topographical considerations in designing agonist peptidomimetics from peptide leads

The design of peptidomimetic ligands with agonist biological activities in vitro and in vivo has been challenging. Lofty goals have been set for this research including high potency, high receptor type selectivity, high stability in vitro and in vivo, and high efficacy in vitro and in vivo for agonists. A systematic stepwise strategy has been developed to accomplish these goals. These include determining the primary amino acid side chain residues required for molecular recognition and, in the case of agonist activity, those required for information transduction. In addition to determining the preferred backbone conformation which can serve as a template for the bioactive conformation (an alpha-helix, beta-turn, beta-sheet, etc.), a strategy has been developed to examine and determine the preferred side chain conformations in chi space (chi1, chi2, etc.). These include specific covalent and non-covalent constraints which can place the constrained side chains at highly preferred gauche (-), or gauche (+), or trans conformations. Examples are provided that illustrate this methodology and provide insight into the topographical requirements for ligand receptor interactions. Often, at this juncture one can obtain a quite precise 3D pharmacophore for the ligand, as well as high stability to agonist biodegradation and good bioavailability including the ability to cross membrane barriers. If a non-peptide ligand is desired, efforts are in progress to develop templates, and aspects of conformational design that permit assembling of all components necessary for molecular recognition and transduction. Here the proper choice of template that can place the key side chain residue in 3D space is still difficult, and thus only partial success has been achieved in terms of potent and selective ligands. A few of these approaches are presented and discussed in some detail.