ANALYSIS OF CYCLOSPORINE INTERACTIONS WITH ANTIBODIES AND CYCLOPHILINUSING THE BIACORE

The immunosuppressive cyclic undecapeptide cyclosporin A (CS) exists i n various conformers in water. Up to 1 h is needed to reach maximum co mplex formation after mixing the drug with its receptor, cyclophilin o r with a monoclonal antibody. Differences in the ability of CS and its analogs to bind to antibody or cyclophilin have been measured using t he BIAcore. These experiments suggest that the rate-limiting step of c omplex formation is determined by the interconversion between differen t CS conformers existing in solution. The contribution to antibody bin ding of individual atomic groups of CS was evaluated by measuring the equilibrium affinity constants of analogs with the BIAcore. When the b inding data were analyzed in terms of the known crystallographic struc ture 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 periph ery are more easily accommodated.