Integration of in vitro binding mechanism into the semiphysiologically based pharmacokinetic interaction model between ketoconazole and midazolam

Abstract

In vitro screening for drug-drug interactions is an integral component of drug development, with larger emphasis now placed on the use of in vitro parameters to predict clinical inhibition. However, large variability exists in Ki reported for ketoconazole with midazolam, a model inhibitor-substrate pair for CYP3A. We reviewed the literature and extracted Ki for ketoconazole as measured by the inhibition of hydroxymidazolam formation in human liver microsomes. The superset of data collected was analyzed for the impact of microsomal binding, using Langmuir and phase equilibrium binding models, and fitted to various inhibition models: competitive, noncompetitive, and mixed. A mixed inhibition model with binding corrected by an independent binding model was best able to fit the data (Kic = 19.2 nmol/l and Kin = 39.8 nmol/l) and to predict clinical effect of ketoconazole on midazolam area under the concentration-time curve. The variability of reported Ki may partially be explained by microsomal binding and choice of inhibition model.

Figure 1

Nonspecific binding of (a) midazolam and (b) ketoconazole. Fraction unbound, f_u, is the ratio of the free concentration, S_f, to the total concentration, S_t. Circles, triangles, and plus marks represent concentrations of 1, 2, and 7.5 µmol/l of midazolam. Circles and triangles represent observed data at 1 and 100 µmol/l of ketoconazole. Lines indicate predicted binding at the different concentrations of midazolam (solid line: 1 µmol/l; dashed line: 2 µmol/l; dotted line: 7.5 µmol/l) or ketoconazole (solid line: 1 µmol/l; dashed line: 100 µmol/l).

Figure 2

Model (M-BIM) fits of the data from studies A-F (a–f, respectively) for the inhibition of α-hydroxymidazolam formation by ketoconazole in human liver microsomes. Data are normalized by the parameter, α, which adjusts the rate, v, in each study according to the ratio of the observed study V_max and the average V_max for all studies. Midazolam concentration is the total concentration, whereas the rate is based on the measured (free) concentration of the metabolite. Each curve represents a different total concentration level of inhibitor as reported in each study.

Figure 3

Predicted and observed concentration vs. time plots of (a) i.v. midazolam alone and (b) in the presence of 200 mg ketoconazole. (a) Gray lines indicate observed concentrations of midazolam, blue line represents model prediction for midazolam alone. (b) Gray lines indicate observed concentrations of midazolam, blue line indicates predicted concentrations based on the competitive inhibition model, red line indicates predicted concentrations based on the mixed inhibition model, and green line indicates predicted concentrations based on the noncompetitive inhibition model.