ABSTRACT:Gemfibrozil more potently inhibits CYP2C9 than CYP2C8 in vitro, and yet the opposite inhibitory potency is observed in the clinic. To investigate this apparent paradox, we evaluated both gemfibrozil and its major metabolite, an acyl-glucuronide (gemfibrozil 1-O-␤-glucuronide) as direct-acting and metabolism-dependent inhibitors of the major drug-metabolizing cytochrome P450 enzymes (CYP1A2, 2B6, 2C8, 2C9, 2C19, 2D6, and 3A4) in human liver microsomes. Gemfibrozil most potently inhibited CYP2C9 (IC 50 of 30 M), whereas gemfibrozil glucuronide most potently inhibited CYP2C8 (IC 50 of 24 M). Unexpectedly, gemfibrozil glucuronide, but not gemfibrozil, was found to be a metabolism-dependent inhibitor of CYP2C8 only. The IC 50 for inhibition of CYP2C8 by gemfibrozil glucuronide decreased from 24 M to 1.8 M after a 30-min incubation with human liver microsomes and NADPH. Inactivation of CYP2C8 by gemfibrozil glucuronide required NADPH, and proceeded with a K I (inhibitor concentration that supports half the maximal rate of enzyme inactivation) of 20 to 52 M and a k inact (maximal rate of inactivation) of 0.21 min ؊1. Potent inhibition of CYP2C8 was also achieved by first incubating gemfibrozil with alamethicin-activated human liver microsomes and UDP-glucuronic acid (to form gemfibrozil glucuronide), followed by a second incubation with NADPH. Liquid chromatography-tandem mass spectrometry analysis established that human liver microsomes and recombinant CYP2C8 both convert gemfibrozil glucuronide to a hydroxylated metabolite, with oxidative metabolism occurring on the dimethylphenoxy moiety (the group furthest from the glucuronide moiety). The results described have important implications for the mechanism of the clinical interaction reported between gemfibrozil and CYP2C8 substrates such as cerivastatin, repaglinide, rosiglitazone, and pioglitazone.There have been several reports of clinical interactions between gemfibrozil (e.g., Lopid, Parke-Davis) and CYP2C8 substrates such as cerivastatin, repaglinide, rosiglitazone, and pioglitazone Niemi et al., 2003a,b;Jaakkola et al., 2005). Reports on the in vitro inhibitory potential of gemfibrozil demonstrated that this lipid-lowering drug is a more potent inhibitor of CYP2C9 than of CYP2C8 (Wen et al., 2001;Wang et al., 2002;Fujino et al., 2003). However, in the clinic, gemfibrozil is a more potent inhibitor of CYP2C8 than of CYP2C9. Coadministration of gemfibrozil with the CYP2C9 substrate warfarin does not increase the plasma concentrations of either R-or S-warfarin (in fact, it actually decreases them) (Lilja et al., 2005). An important step in providing a potential explanation for why gemfibrozil is a more potent inhibitor of CYP2C9 than CYP2C8 in vitro but is a more potent inhibitor of CYP2C8 than CYP2C9 in vivo was provided by Shitara et al. (2004), who demonstrated that gemfibrozil 1-O-␤-glucuronide is a more potent inhibitor than gemfibrozil of CYP2C8. These same authors demonstrated that gemfibrozil 1-O-␤-glucuronide inhibits in vitro the CYP2C8-mediated...

Coadministration with the human immunodeficiency virus (HIV) protease inhibitor ritonavir was investigated as a method for enhancing the levels of other peptidomimetic HIV protease inhibitors in plasma. In rat and human liver microsomes, ritonavir potently inhibited the cytochrome P450 (CYP)-mediated metabolism of saquinavir, indinavir, nelfinavir, and VX-478. The structural features of ritonavir responsible for CYP binding and inhibition were examined. Coadministration of other protease inhibitors with ritonavir in rats and dogs produced elevated and sustained plasma drug levels 8 to 12 h after a single dose. Drug exposure in rats was elevated by 8- to 46-fold. A > 50-fold enhancement of the concentrations of saquinavir in plasma was observed in humans following a single codose of ritonavir (600 mg) and saquinavir (200 mg). These results indicate that ritonavir can favorably alter the pharmacokinetic profiles of other protease inhibitors. Combination regimens of ritonavir and other protease inhibitors may thus play a role in the treatment of HIV infection. Because of potentially substantial drug level increases, however, such combinations require further investigation to establish safe regimens for clinical use.

To address the most appropriate endogenous biomarker for drug–drug interaction risk assessment, eight healthy subjects received an organic anion transporting polypeptide 1B (OATP1B) inhibitor (rifampicin, 150, 300, and 600 mg), and a probe drug cocktail (atorvastatin, pitavastatin, rosuvastatin, and valsartan). In addition to coproporphyrin I, a widely studied OATP1B biomarker, we identified at least 4 out of 28 compounds (direct bilirubin, glycochenodeoxycholate‐3‐glucuronide, glycochenodeoxycholate‐3‐sulfate, and hexadecanedioate) that presented good sensitivity and dynamic range in terms of the rifampicin dose‐dependent change in area under the plasma concentration‐time curve ratio (AUCR). Their suitability as OATP1B biomarkers was also supported by the good correlation of AUC0‐24h between the endogenous compounds and the probe drugs, and by nonlinear regression analysis (AUCR−1 vs. rifampicin plasma Cmax (maximum total concentration in plasma)) to yield an estimate of the inhibition constant of rifampicin. These endogenous substrates can complement existing OATP1B‐mediated drug–drug interaction risk assessment approaches based on agency guidelines in early clinical trials.

The aim of the present study was to establish a physiologically based pharmacokinetic (PBPK) model for coproporphyrin I (CP‐I), a biomarker supporting the prediction of drug‐drug interactions (DDIs) involving hepatic organic anion transporting polypeptide 1B (OATP1B), using clinical DDI data with an OATP1B inhibitor rifampicin (300 and 600 mg, orally). The in vivo inhibition constants of rifampicin used as initial input parameters for OATP1Bs (K i,u,OATP1Bs) and multidrug resistance‐associated protein two‐mediated biliary excretion were estimated as 0.23 and 0.87 μM, respectively, from previous reports. Sensitivity analysis demonstrated that the K i,u,OATP1Bs and biosynthesis rate of CP‐I affected the magnitude of the interaction. K i,u,OATP1Bs values optimized by nonlinear least‐squares fitting were ~0.5‐fold of the initial value. It was determined that the blood concentration‐time profiles of four statins were well‐predicted using corrected individual K i,u,OATP1B values (ratio of in vitro K i,u(statin)/in vitro K i,u(CP‐I)). In conclusion, PBPK modeling of CP‐I supports dynamic prediction of OATP1B‐mediated DDIs.

Organic anion-transporting polypeptides (OATP) 1B1, 1B3, and 2B1 can serve as the loci of drug-drug interactions (DDIs). In the present work, the cynomolgus monkey was evaluated as a potential model for studying OATP-mediated DDIs. Three cynomolgus monkey OATPs (cOATPs), with a high degree of amino acid sequence identity (91.9, 93.5, and 96.6% for OATP1B1, 1B3, and 2B1, respectively) to their human counterparts, were cloned, expressed, and characterized. The cOATPs were stably transfected in human embryonic kidney cells and were functionally similar to the corresponding human OATPs (hOATPs), as evident from the similar uptake rate of typical substrates (estradiol-17b-D-glucuronide, cholecystokinin octapeptide, and estrone-3-sulfate). Moreover, six known hOATP inhibitors exhibited similar IC 50 values against cOATPs. To further evaluate the appropriateness of the cynomolgus monkey as a model, a known hOATP substrate [rosuvastatin (RSV)]-inhibitor [rifampicin (RIF)] pair was examined in vitro; the monkey-derived parameters (RSV K m and RIF IC 50 ) were similar (within 3.5-fold) to those obtained with hOATPs and human primary hepatocytes. In vivo, the area under the plasma concentration-time curve of RSV (3 mg/kg, oral) given 1 hour after a single RIF dose (15 mg/kg, oral) was increased 2.9-fold in cynomolgus monkeys, consistent with the value (3.0-fold) reported in humans. A number of in vitro-in vivo extrapolation approaches, considering the fraction of the pathways affected and free versus total inhibitor concentrations, were also explored. It is concluded that the cynomolgus monkey has the potential to serve as a useful model for the assessment of OATP-mediated DDIs in a nonclinical setting. IntroductionDrug-drug interactions (DDIs) have often been attributed to cytochrome P450 (P450) enzymes because of their prominent role in the metabolic clearance of drugs (Vuppugalla et al., 2010). More recently, however, attention has turned to active transport processes in different organs and the close interplay between drug transport and metabolism at the cellular level. In particular, organic anion-transporting polypeptides (OATPs) are known to mediate the active uptake of numerous drugs into hepatocytes and hence govern their overall clearance, pharmacokinetic profile, and liver-toplasma ratio (Giacomini et al., 2010;Fenner et al., 2012;Yoshida et al., 2012).OATPs can also serve as the loci of important DDIs leading to changes in systemic and local drug concentrations, possibly resulting in altered efficacy and enhanced toxicity (Giacomini et al., 2010;Yoshida et al., 2012). For example, cyclosporine A (CsA) increases the area under the concentration-time curve (AUC) (∼15-fold) and C max (∼14-fold) of atorvastatin in s This article has supplemental material available at jpet.aspetjournals.org.ABBREVIATIONS: AUC, area under the concentration-time curve; CCK-8, cholecystokinin octapeptide; CI, confidence interval; cOATP, cynomolgus organic anion-transporting polypeptide; CsA, cyclosporine A; DDI, drug-drug inter...