Prediction of time-dependent CYP3A4 drug–drug interactions by physiologically based pharmacokinetic modelling: Impact of inactivation parameters and enzyme turnover
Introduction
Many clinically relevant drug–drug interactions (DDIs) involving cytochrome P450 3A4 (CYP3A4), the most abundant P450 in the human liver and intestine, are due to time-dependent inhibition consequent to metabolism of the inhibitor to a reactive intermediate that binds irreversibly or quasi-irreversibly to the enzyme (Silverman, 1995, Venkatakrishnan and Obach, 2007). These DDIs are persistent since they require synthesis of new enzyme for recovery. Examples of the so-called ‘mechanism-based’ inhibitors of CYP3A4 include anti-retroviral agents such as ritonavir and saquinavir, the macrolides erythromycin, clarithromycin and azithromycin and the calcium channel blockers verapamil, diltiazem and mibefradil (Posicor®) (FDA, 2006). The withdrawal of the potent mechanism-based inhibitor mibefradil from the market a year after its introduction in 1997, as a consequence of reports of serious DDIs with CYP3A4 substrates, illustrates the importance of assessing the potential impact of mechanism based inhibitors in drug development (Huang and Lesko, 2004). Prediction of the magnitude of DDIs mediated by mechanism-based inhibition is now of significant interest, both academically and in an industrial context (Grimm et al., 2009), and there has been an evaluation of early decision making based on different in vitro methods and prediction methods (Grime et al., 2009).
Prediction of the magnitude of CYP3A4-based DDIs mediated by mechanism-based inhibition requires values of substrate-dependent, inhibitor-dependent and intrinsic parameters (Venkatakrishnan and Obach, 2007, Yang et al., 2007a) Substrate parameters include the fractions of the drug metabolised by the inhibited pathway (fm) in the gut wall and liver. Time-dependent inhibition is characterized by the maximal inactivation rate constant (kinact) and the concentration of inhibitor that produces half-maximal inactivation (KI). The significant impact the of experimental design on the values of these parameters has been discussed in detail (Ghanbari et al., 2006, Yang et al., 2007a). A key intrinsic factor is the turnover of the inhibited enzyme, characterized by a degradation rate constant (kdeg). This value cannot be determined directly in vivo in humans. Therefore, estimates of the degradation half-life of CYP3A4 have been derived from a variety of indirect sources including rats (in vivo) and Caco-2 cells (range 14–35 h; Mayhew et al., 2000), human hepatocytes and liver slices (range 26–44 h; Maurel, 1996, Pichard et al., 1992) and the time-course of recovery of baseline CYP3A4 activity as marked by plasma drug concentrations after mechanism-based inhibition or enzyme induction (range 43–140 h; collated by Yang et al. (2008). It should be noted that the higher values from the latter source do not correct for the residence time of the compound used to mark enzyme activity. Estimates of the half-life of intestinal CYP3A4 range from 12 to 33 h (Greenblatt et al., 2003, Lilja et al., 2000, Lundahl et al., 1995, Takanaga et al., 2000), largely reflecting the shorter turnover of enterocytes relative to that of the enzyme.
Attempts to predict the extent of DDIs involving CYP3A4-mediated time-dependent inhibition have been based on both static (Galetin et al., 2006, Mayhew et al., 2000, Wang et al., 2004) and dynamic models (Ito et al., 2003, Rowland Yeo et al., 2010, Zhang et al., 2009, Obach et al., 2007). The former evaluated simply the expected change in overall exposure of the victim drug, as indicated by a change in plasma AUC, and assume a constant concentration of inhibitor. The latter models account for temporal changes in inhibitor concentration, active CYP3A4 and the exposure to victim substrate. Although the algorithms used to model these changes were similar across the studies, values of kdeg ranged from 0.0077 h−1 (t1/2 – 90 h) to 0.03 h−1 (t1/2 – 23 h), and there has been considerable debate in the literature regarding the appropriate value to use for accurate predictions. Recently, Wang (2010) used the physiologically based pharmacokinetic (PBPK) model within the Simcyp Simulator (Version 9.3) to assess the impact of the value of kdeg on the accuracy of prediction of 54 DDIs. They concluded that there was a significant improvement in prediction accuracy when the value of kdeg was set at 0.03 h−1 (t1/2 – 23 h) rather than the value of 0.0077 h−1 (t1/2 – 90 h). Although the results appeared to be conclusive, the inactivation parameters were taken from a variety of literature sources and, in some cases, the predicted times-courses of plasma concentrations of substrates and inhibitors varied appreciably from the observed data.
The aim of this study was to use inactivation parameters determined anew in a single laboratory under standardised conditions together with data from substrate and inhibitor files within the Simcyp Simulator (Version 9.3) to determine a value of the hepatic kdeg appropriate for the prediction of DDIs involving time-dependent inhibition of CYP3A4. The conversion of midazolam to its 1′hydroxy metabolite (1’OH midazolam) by human liver microsomes was used as a marker of CYP3A4 activity. The inhibitors studied were azithromycin, clarithromycin, diltiazem and its N-demethylated metabolite (MA) and verapamil.
Section snippets
Chemicals and reagents
Midazolam and N-desmethyldiltiazem were obtained from Cerilliant Corporation (Round Rock, TX). Azithromycin, clarithromycin and erythromycin were purchased from Sequoia Research Products (Oxford, UK). Diltiazem, NADPH and verapamil were obtained from Sigma Chemical Company (St. Louis, MO). The metabolites 1′-hydroxymidazolam and [D4]1′-hydroxymidazolam were synthesised at Pfizer, Inc. (Groton, CT). Human liver microsomes were prepared from a pool of 50 donors provided by BD Biosciences (Woburn,
Time-dependent inhibition of midazolam metabolism
The effects of increasing pre-incubation time and concentration of azithromycin, clarithromycin, erythromycin, verapamil, diltiazem and its metabolite MA on the 1-hydroxylation of midazolam by human liver microsomes are shown in Fig. 1A–F. Inactivation parameters are shown in Table 5, in comparison with those previously reported in the literature. As indicated by the kinact/KI ratio, verapamil was the most potent inhibitor followed by MA, clarithromycin, diltiazem, erythromycin, and
Discussion
We have used inactivation parameters for azithromycin, clarithromycin, diltiazem, MA, erythromycin, and verapamil determined anew in a single laboratory under standardised conditions, together with data from substrate and inhibitor files within the Simcyp Population based ADME Simulator (Version 9.3), to predict DDIs involving time-dependent inhibition of CYP3A4, particularly with respect to the choice of kdeg value. While our findings are broadly in line with those of Wang (2010), who also
Acknowledgement
We thank James Kay for his assistance with the preparation of this manuscript.
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