Prediction of the Time to Reach Equilibrium for Improved Estimation of the Unbound Fraction of Compounds in...

Equilibrium dialysis (ED) is widely used in pharmacokinetics to determine the fraction of unbound (f_u) compounds in plasma; however, the kinetics of drugs in the ED system with respect to their permeation across semi-permeable membranes has not been systemically studied. Here, the kinetics of the ED system, including the binding of drugs to plasma proteins, non-specific binding, and permeation across the membrane, was described to enable verification of the equilibrium, prediction of the time to reach equilibrium, and estimations of f_u with data obtained during pre-equilibrium. Using data obtained during pre-equilibrium, the time to reach 90% equilibrium (t_90%) and f_u were estimated with reasonable accuracy. Notably, f_u could be estimated reasonably well using one-time-point data for the calculation. Furthermore, the current modeling approach allowed concurrent estimations of f_u and the decomposition rate of compounds that were metabolically unstable in the plasma. Reasonable metabolic rate constants were determined for cefadroxil and diltiazem, demonstrating the practicality of this method for determining kinetics related to f_u characterization. Because the determination of f_u of compounds with 'unfavorable' physicochemical properties is known to be experimentally challenging, the current method may be useful in determining the f_u of compounds in vitro.