The Reliability of Estimating Ki Values for Direct, Reversible Inhibition of Cytochrome P450 Enzymes from Corresponding IC50 Values: A Retrospective Analysis of 343 Experiments

In the present study, we conducted a retrospective analysis of 343 in vitro experiments to ascertain whether observed (experimentally determined) values of K i for reversible cytochrome P450 (P450) inhibition could be reliably predicted by dividing the corresponding IC50 values by two, based on the relationship (for competitive inhibition) in which K i = IC50/2 when \[S\] (substrate concentration) = K m (Michaelis-Menten constant). Values of K i and IC50 were determined under the following conditions: 1) the concentration of P450 marker substrate, [S], was equal to K m (for IC50 determinations) and spanned K m (for K i determinations); 2) the substrate incubation time was short (5 minutes) to minimize metabolism-dependent inhibition and inhibitor depletion; and 3) the concentration of human liver microsomes was low (0.1 mg/ml or less) to maximize the unbound fraction of inhibitor. Under these conditions, predicted K i values, based on IC50/2, correlated strongly with experimentally observed K i determinations [r = 0.940; average fold error (AFE) = 1.10]. Of the 343 predicted K i values, 316 (92%) were within a factor of 2 of the experimentally determined K i values, and only one value fell outside a 3-fold range. In the case of noncompetitive inhibitors, K i values predicted from IC50/2 values were overestimated by a factor of nearly 2 (AFE = 1.85; n = 13), which is to be expected because, for noncompetitive inhibition, K i = IC50 (not IC50/2). The results suggest that, under appropriate experimental conditions with the substrate concentration equal to K m, values of K i for direct, reversible inhibition can be reliably estimated from values of IC50/2.