The antiretroviral agent efavirenz enhances the systemic clearance of coadministered drugs that are cytochrome P450 (CYP) 3A4 substrates. The mechanism of the apparent increase in CYP3A4 activity by efavirenz and the magnitude of change relative to other known inducers are not known. The authors tested the hypothesis that increased enzymatic activity by efavirenz entails CYP3A4 induction and activation of the human pregnane X receptor (hPXR), a key transcriptional regulator of CYP3A4. Employing primary cultures of human hepatocytes, they compared the CYP3A4 inductive effects of efavirenz (1-10 microM) to rifampin (10 microM) and phenobarbital (2 mM). A cell-based reporter assay was employed to assess hPXR activation. The authors observed that efavirenz caused a concentration-dependent CYP3A4 induction and hPXR activation. Based on the CYP3A4 activity assay, the average magnitude of induction by efavirenz (5-10 microM) was approximately 3- to 4-fold. In comparison, phenobarbital (2 mM) and rifampin (10 microM) caused a 5- and 6-fold induction, respectively.

This article is available online at http://dmd.aspetjournals.org ABSTRACT:Tamoxifen is a widely utilized antiestrogen in the treatment and chemoprevention of breast cancer. Clinical studies document that tamoxifen administration markedly enhances the systemic elimination of other drugs. Additionally, tamoxifen enhances its own clearance following repeated dosing. The mechanisms that underlie these clinically important events remain unresolved. Here, we report that tamoxifen and its metabolite 4-hydroxytamoxifen markedly induce cytochrome P450 3A4, a drug-metabolizing enzyme of central importance, in primary cultures of human hepatocytes. Tamoxifen and 4-hydroxytamoxifen (1-10 M) significantly increased the CYP3A4 expression and activity (measured as the rate of testosterone 6␤-hydroxylation). Maximal induction was achieved at the 5 M level. At this level, tamoxifen and 4-hydroxytamoxifen caused a 1.5-to 3.3-fold (mean, 2.1-fold) and 3.4-to 17-fold (mean, 7.5-fold) increase in the CYP3A4 activity, respectively. In comparison, rifampicin treatment resulted in a 6-to 16-fold (mean, 10.5-fold) increase. We also observed corresponding increase in the CYP3A4 immunoreactive protein and mRNA levels. Furthermore, tamoxifen and 4-hydroxytamoxifen efficaciously activated the human pregnane X receptor (hPXR; also known as the steroid xenobiotic receptor), a key regulator of CYP3A4 expression. The efficacy of tamoxifen and 4-hydroxytamoxifen relative to rifampicin for hPXR activation was ϳ30 and 60%, respectively. Our results indicate that the mechanism of tamoxifen-mediated alteration in drug clearance pathways in humans may involve CYP3A4 induction by the parent drug and/or its metabolite. Furthermore, the CYP3A4 induction may be a result of hPXR activation. These findings have important implications for optimizing the use of tamoxifen and in the development of newer antiestrogens.Tamoxifen, a nonsteroidal triphenylethylene, is currently the endocrine therapeutic agent of choice for all stages of breast cancer. It was also recently approved for use as a chemopreventive agent in women with high risk of contracting this disease in the future. Despite its well documented beneficial effects, tamoxifen use is associated with several major problems including serious drug-drug interactions. Several clinical trials indicate that tamoxifen has the propensity to alter the drug elimination pathway(s), resulting in markedly reduced plasma levels of coadministered compounds. In this regard, recently completed clinical trials indicate that tamoxifen reduced the plasma levels of aromatase inhibitors letrozole and anastrozole by 37 and 27%, respectively ATAC Trialists' Group, 2001). Such drug-drug interactions are of special concern with tamoxifen since numerous women are required to take tamoxifen daily for an extended time period and as such are likely to be simultaneously exposed to many drugs and nutraceuticals. Another consequence of tamoxifen-induced changes in drug disposition is that tamoxifen pharmacokinetics exhibit time-depende...

ABSTRACT:Digoxin, an orally administered cardiac glycoside cardiovascular drug, has a narrow therapeutic window. Circulating digoxin levels (maximal concentration of ϳ1.5 ng/ml) require careful monitoring, and the potential for drug-drug interactions (DDI) is a concern. Increases in digoxin plasma exposure caused by inhibition of Pglycoprotein (P-gp) have been reported. Digoxin has also been described as a substrate of various organic anion-transporting polypeptide (OATP) transporters, posing a risk that inhibition of OATPs may result in a clinically relevant DDI similar to what has been observed for P-gp. Although studies in rats have shown that Oatps contribute to the disposition of digoxin, the role of OATPs in the disposition of digoxin in humans has not been clearly defined. Using two methods, Boehringer Ingelheim, GlaxoSmithKline, Pfizer, and Solvo observed that digoxin is not a substrate of OATP1A2, OATP1B1, OATP1B3, and OATP2B1. However, digoxin inhibited the uptake of probe substrates of OATP1B1 (IC 50 of 47 M), OATP1B3 (IC 50 > 8.1 M), and OATP2B1 (IC 50 > 300 M), but not OATP1A2 in transfected cell lines. It is interesting to note that digoxin is a substrate of a sodium-dependent transporter endogenously expressed in HEK293 cells because uptake of digoxin was significantly greater in cells incubated with sodium-fortified media compared with incubations conducted in media in which sodium was absent. Thus, although digoxin is not a substrate for the human OATP transporters evaluated in this study, in addition to P-gp-mediated efflux, its uptake and pharmacokinetic disposition may be partially facilitated by a sodium-dependent transporter.

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Quantitative prediction of the magnitude of transporter‐mediated clinical drug‐drug interactions (DDIs) solely from in vitro inhibition data remains challenging. The objective of the present work was to analyze the kinetic profile of an endogenous biomarker for organic anion‐transporting polypeptides 1B (OATP1B), coproporphyrin I (CPI), and to predict clinical DDIs with a probe OATP1B substrate (pravastatin) based on “in vivo” inhibition constants (Ki). The CPI kinetics in the presence and absence of strong and weak OATP1B inhibitors (rifampin and GDC‐0810) were described well with a one‐compartment model, and in vivo Ki were estimated. Clinical DDIs between pravastatin and these inhibitors were predicted using physiologically based pharmacokinetic (PBPK) models coupled with the estimated in vivo Ki and predicted magnitude matched well with the observed DDIs. In conclusion, model‐based analysis of the CPI profile has the potential to quantitatively predict liability of a new molecular entity (NME) as an OATP1B inhibitor early in drug development.

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Faldaprevir, an investigational agent for hepatitis C virus treatment, is well tolerated but associated with rapidly reversible, dosedependent, clinically benign, unconjugated hyperbilirubinemia. Multidisciplinary preclinical and clinical studies were used to characterize mechanisms underlying this hyperbilirubinemia. In vitro, faldaprevir inhibited key processes involved in bilirubin clearance: UDP glucuronosyltransferase (UGT) 1A1 (UGT1A1) (IC 50 0.45 mM), which conjugates bilirubin, and hepatic uptake and efflux transporters, organic anion-transporting polypeptide (OATP) 1B1 (IC 50 0.57 mM), OATP1B3 (IC 50 0.18 mM), and multidrug resistanceassociated protein (MRP) 2 (IC 50 6.2 mM), which transport bilirubin and its conjugates. In rat and human hepatocytes, uptake and biliary excretion of [ 3 H]bilirubin and/or its glucuronides decreased on coincubation with faldaprevir. In monkeys, faldaprevir ($20 mg/kg per day) caused reversible unconjugated hyperbilirubinemia, without hemolysis or hepatotoxicity. In clinical studies, faldaprevir-mediated hyperbilirubinemia was predominantly unconjugated, and levels of unconjugated bilirubin correlated with the UGT1A1*28 genotype. The reversible and dose-dependent nature of the clinical hyperbilirubinemia was consistent with competitive inhibition of bilirubin clearance by faldaprevir, and was not associated with liver toxicity or other adverse events. Overall, the reversible, unconjugated hyperbilirubinemia associated with faldaprevir may predominantly result from inhibition of bilirubin conjugation by UGT1A1, with inhibition of hepatic uptake of bilirubin also potentially playing a role. Since OATP1B1/1B3 are known to be involved in hepatic uptake of circulating bilirubin glucuronides, inhibition of OATP1B1/1B3 and MRP2 may underlie isolated increases in conjugated bilirubin. As such, faldaprevirmediated hyperbilirubinemia is not associated with any liver injury or toxicity, and is considered to result from decreased bilirubin elimination due to a drug-bilirubin interaction.

ABSTRACT:Previously we observed that the antiestrogens tamoxifen and 4-hydroxytamoxifen (4OHT) induce CYP3A4 in primary human hepatocytes and activate human pregnane X receptor (PXR) in cell-based reporter assays. Given the complex cross-talk between nuclear receptors, tissue-specific expression of CYP3A4, and the potential for tamoxifen and 4OHT to interact with a myriad of receptors, this study was undertaken to gain mechanistic insights into the inductive effects of tamoxifen and 4OHT. First, we observed that transfection of the primary cultures of human hepatocytes with PXRspecific small interfering RNA reduced the PXR mRNA expression and the extent of CYP3A4 induction by tamoxifen and 4OHT by 50%. Second, in LS174T colon carcinoma cells, which were observed to have significantly lower PXR expression relative to human hepatocytes, neither tamoxifen nor 4OHT induced CYP3A4.Third, N-desmethyltamoxifen, which did not induce CYP3A4 in human hepatocytes, also did not activate PXR in LS174T cells. We then used cell-based reporter assay to evaluate the effects of other receptors such as glucocorticoid receptor GR␣ and estrogen receptor ER␣ on the transcriptional activation of PXR. The cotransfection of GR␣ in LS174T cells augmented PXR activation by tamoxifen and 4OHT. On the other hand, the presence of ER␣ inhibited PXR-mediated basal activation of CYP3A4 promoter, possibly via competing for common cofactors such as steroid receptor coactivator 1 and glucocorticoid receptor interacting protein 1. Collectively, our findings suggest that the CYP3A4 induction by tamoxifen and 4OHT is primarily mediated by PXR but the overall stoichiometry of other nuclear receptors and transcription cofactors also contributes to the extent of the inductive effect.Tamoxifen is a prototypical selective estrogen receptor modulator by virtue of its tissue-specific estrogen/antiestrogen properties. It is a clinically effective endocrine agent for the treatment and chemoprevention of breast cancer, and despite the advent of new antiestrogens, such as the aromatase inhibitors anastrazole and letrozole, it remains the drug of choice, especially in premenopausal women. Although tamoxifen is generally well tolerated, its clinical use is associated with several unresolved problems. These include interindividual variability in its pharmacokinetics and the resulting variability in its safety and efficacy profile and drug-drug interactions. Because the systemic elimination of tamoxifen primarily entails hepatic metabolism, intersubject variability in its metabolism is the principal cause of the overall variability in its pharmacokinetics. Tamoxifen is biotransformed to a larger number of metabolites, including N-desmethyltamoxifen (NDMT) and 4-hydroxytamoxifen (4OHT), which are further converted to 4-hydroxy-N-desmethyltamoxifen (endoxifen) (Borges et al., 2006). Whereas NDMT is a weak antiestrogen, 4OHT and endoxifen are considerably more potent antiestrogens than tamoxifen and may contribute to the overall therapeutic properties of the latter. A...