(EN) Method of selecting a candidate drug compound comprising generating from a starting oligopeptide a plurality of oligopeptides having single site amino acid modifications which lead to a change in the ratio of binding affinity of said oligopeptide to a receptor to biological reponse of said binding, identification of components in the oligopeptide responsible for secondary structure and replacement of said components with non-natural amino acids, construction of combinatorial libraries based on the use of sets of amino acids for the modification sites identified and the construction of further libraries based upon oligopeptides produced in the first library having the desired binding activity. Drug candidates are selected from the identified members of the libraries.