Analysis of cyclosporin interactions with antibodies and cyclophilin using the BIAcore

The immunosuppressive cyclic undecapeptide cyclosporin A (CS) exists in various conformers in water. Up to 1 h is needed to reach maximum complex formation after mixing the drug with its receptor, cyclophilin or with a monoclonal antibody. Differences in the ability of CS and its analogs to bind to antibody or cyclophilin have been measured using the BIAcore. These experiments suggest that the rate-limiting step of complex formation is determined by the interconversion between different CS conformers existing in solution. The contribution to antibody binding of individual atomic groups of CS was evaluated by measuring the equilibrium affinity constants of analogs with the BIAcore. When the binding data were analyzed in terms of the known crystallographic structure of the CS/Fab complex, it could be shown that modifications of CS residues located in the central part of the binding site drastically affect affinity, while modifications of residues located at the periphery are more easily accommodated.