ABSTRACT:Because the expression of drug-metabolizing enzymes and drug efflux transporters has been shown in the intestine, the contribution of this tissue to the first-pass effect has become of significant interest. Consequently, a comprehensive understanding of the absorption barriers in key preclinical species would be useful for the precise characterization of drug candidates. In the present investigation, we evaluated the intestinal first-pass effect of midazolam (MDZ) and fexofenadine (FEX), typical substrates for CYP3A and P-glycoprotein (P-gp), respectively, with ketoconazole (KTZ) as a potent dual CYP3A/P-gp inhibitor in cynomolgus monkeys. When

ABSTRACT:Drugs with potential drug-drug interactions (DDIs) may have a limited scope of use and, at worst, may have to be withdrawn from the market. Therefore, during the drug discovery process it is important to select drug candidates with reduced potential for DDIs. In the present study, we evaluated the pharmacokinetics of simvastatin (SV), a typical substrate for cytochrome P450 (P450) 3A, and examined the DDI between SV and ketoconazole (KTZ), a P450 3A inhibitor, in monkeys. SV metabolism in monkey liver and intestinal microsomes was almost completely inhibited by addition of anti-P450 3A4 antiserum. A similar effect was seen in human microsomes, and the IC 50 values of KTZ for inhibition of SV metabolism were similar in monkey and human samples. In vivo, there were no significant differences in the pharmacokinetic parameters of SV and SVA after i.v. administration of SV in the presence of KTZ compared with those in controls, probably because of the limited systemic exposure to KTZ. In contrast, the pharmacokinetics of SV and SVA after p.o. administration of SV were significantly influenced by the presence of KTZ, and C max and area under the plasma concentration-time curve were approximately 5 to 10 times higher than those after p.o. dosing with SV alone. The increases in systemic SV exposure caused by a concomitant p.o. dose of KTZ in monkeys were similar to those observed in clinical studies, which suggests that monkeys might be a suitable animal model in which to predict DDIs involving P450 3A inhibition.

Predicting the pharmacokinetics of compounds in humans is an important part of the drug development process. In this study, the plasma concentration profiles of 10 marketed compounds exhibiting two-phase elimination after intravenous administration in humans were evaluated in terms of distribution volumes just after intravenous administration (V 1 ), at steady state (V ss ), and in the elimination phase (V b ) using physiologically based pharmacokinetic (PBPK) modeling implemented in a commercially available simulator (Simcyp). When developing human PBPK models, the insight gained from prior animal PBPK models based on nonclinical data informed the optimization of the lipophilicity input of the compounds and the selection of the appropriate mechanistic tissue partition methods. The accuracy of V 1 , V ss , and V b values predicted that using human PBPK models developed in accordance with prior animal PBPK models was superior to using those predicted using conventional approaches, such as allometric scaling, especially for V 1 and V b . By conventional approaches, the V 1 and V b values of 4-5 of 10 compounds were predicted within a 3-fold error of observed values, whereas V ss values for their majority were predicted as such. PBPK models predicted V 1 , V ss , and V b values for almost all compounds within 3-fold errors, resulting in better predictions of plasma concentration profiles than allometric scaling. The distribution volumes predicted using human PBPK models based on prior animal PBPK modeling were more accurate than those predicted without reference to animal models. This study demonstrated that human PBPK models developed with consideration of animal PBPK models could accurately predict distribution volumes in various elimination phases.

ABSTRACT:Irreversible inhibition, characterized as mechanism-based inhibition (MBI), of cytochrome P450 in drugs has to be avoided for their safe use. A comprehensive assessment of drug-drug interaction (DDI) potential is important during the drug discovery process. In the present study, we evaluated the effects of macrolide antibiotics, erythromycin (ERM), clarithromycin (CAM), and azithromycin (AZM), which are mechanism-based inhibitors of CYP3A, on biotransformation of midazolam (MDZ) in monkeys. These macrolides inhibited the formation of 1-hydroxymidazolam in monkey microsomes as functions of incubation time and macrolide concentration. Furthermore, the inactivation potentials of macrolides (k inact / K I : CAM Х ERM > AZM) were as effective as that observed in human samples. In in vivo studies, MDZ was administered orally (1 mg/kg) without or with multiple oral dosing of macrolides (15 mg/kg, twice a day on days 1-3). On day 3, the area under the plasma concentration-time curve (AUC) of MDZ increased 7.0-, 9.9-, and 2.0-fold with ERM, CAM, and AZM, respectively, compared with MDZ alone. Furthermore, the effects of ERM and CAM on the pharmacokinetics of MDZ were also observed on the day (day 4) after completion of macrolide treatments (AUC changes: 7.3-and 7.3-fold, respectively). Because the plasma concentrations of macrolides immediately before MDZ administration on day 4 were much lower than the IC 50 values for reversible CYP3A inhibition, the persistent effects may be predominantly caused by CYP3A inactivation. These results suggest that the monkey might be a suitable animal model to predict DDIs caused by MBI of CYP3A.