The reliability of estimating Ki values for direct, reversible inhibition of cytochrome P450 enzymes from corresponding IC50 values: A retrospective analysis of 343 experiments
Lois J. Haupt, Faraz Kazmi, Brian W. Ogilvie, David B. Buckley, Brian D. Smith, Sarah Leatherman, Brandy Paris, Oliver Parkinson, and Andrew Parkinson
In the present study we conducted a retrospective analysis of 343 in vitro experiments to ascertain whether observed (experimentally determined) values of Ki for reversible P450 inhibition could be reliably predicted by dividing the corresponding IC50 values by two, based on the relationship that, for competitive inhibition, Ki = IC50/2 when [S] = Km. Values of Ki and IC50 were determined under the following conditions: (1) the concentration of P450 marker substrate, [S], was equal to Km (for IC50 determinations) and spanned Km (for Ki determinations); (2) the substrate incubation time was short (5 min) 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 Ki values, based on IC50/2, correlated strongly with experimentally observed Ki determinations (r = 0.940; average fold error [AFE] = 1.10). Of the 343 predicted Ki values, 316 (92%) were within a factor of 2 of the experimentally determined Ki values and only one value fell outside a threefold range. In the case of noncompetitive inhibitors, Ki values predicted from IC50/2 values were overestimated by a factor of nearly two (AFE = 1.85; n = 13), which is to be expected because, for noncompetitive inhibition, Ki = IC50 (not IC50/2). The results suggest that, under appropriate experimental conditions with the substrate concentration equal to Km, values of Ki for direct, reversible inhibition can be reliably estimated from values of IC50/2.