Study of FK-binding protein : FK506-metabolite complexes by electrospray mass spectrometry: Correlation to immunosuppressive activity

Electrospray ionization mass spectrometry was used to study several noncova lent FK-binding protein (FKBP) immunosuppressant complexes in the gas phase . Relative FKBP binding affinities were determined from the signal ratio fo r the 7(+) charge states of bound and unbound complexes as a function of ca pillary exit voltage. All complexes displayed a 1:1 binding stoichiometry. The relative gas-phase binding affinities were found to be well correlated with in vitro FKBP binding and in vitro immunosuppression (rapamycin > FK50 6 greater than or equal to 31-demethyl FK506 > 13-demethyl FK506 much great er than Cyclosporin A; CsA). The method demonstrates potential as a simple, rapid, and automatable technique for prediction of the immunosuppressive a ctivity of FKBP:drug complexes.