A population approach to eplerenone pharmacokinetics and saturable protein binding

Eplerenone deviates from linear pharmacokinetics at doses above the therapeutic dose range. In addition, saturable protein binding of eplerenone is observed in in vitro plasma protein binding studies. The purpose of the present study was to clarify the factors contributing to the nonlinear pharmacokinetics of eplerenone. Plasma concentration data for eplerenone and its metabolite SC-70303, to which eplerenone is reversibly converted, obtained from four phase I studies were analyzed using NONMEM. A population pharmacokinetic model incorporating protein binding and the reversible relationship between eplerenone and SC-70303 was developed. Models with linear and nonlinear protein binding were fitted to the observed concentration data. The observed concentration data of eplerenone and SC-70303 were best described by a model with nonlinear protein binding. The area under the plasma concentration-time curve of eplerenone simulated by the model increased less than proportionally with increasing dose, whereas that of SC-70303 increased proportionally with increasing dose, consistent with observations from the non-compartmental analysis. In conclusion, the nonlinear pharmacokinetics of eplerenone and the apparently linear pharmacokinetics of SC-70303 were described by applying a model with nonlinear protein binding to observed plasma eplerenone and SC-70303 concentrations, suggesting that nonlinear protein binding plays a role in the nonlinear kinetics.