Influence of sildenafil and tadalafil on the enzyme- and transporter-inducing effects of bosentan and ambrisentan in LS180 cells

The differences in drug disposition gene induction by rifampicin and rifabutin are unlikely due to different effects on important pregnane X receptor (NR1I2) splice variants

Rifampicin and rifabutin can activate the pregnane X receptor (PXR, NR1I2), thereby inducing pharmacokinetically important genes/proteins and reducing exposure to co-administered drugs. Because induction effects vary considerably between these antibiotics, differences could be due to unequal rifamycin-induced activation or tissue expression of the three major NR1I2 splice variants, PXR.1 (NM_003889), PXR.2 (NM_022002), and PXR.3 (NM_033013). Consequently, PXR activation (PXR reporter gene assays) and mRNA expression levels of total NR1I2, PXR.1, PXR.2, and PXR.3 were investigated by polymerase chain reaction in colon and liver samples from eleven surgical patients, in LS180 cells, and primary human hepatocytes. Compared to the colon, total NR1I2 mRNA expression was higher in the liver. Both tissues showed similar expression levels of PXR.1 and PXR.3, respectively. PXR.2 was not quantifiable in the colon samples. Rifampicin and rifabutin similarly enhanced PXR.1 and PXR.2 activity when transfected into LS180 cells, while PXR.3 could not be activated. In LS180 cells, rifampicin (10 μM) reduced total NR1I2 and PXR.3 expression 2-fold after 24 h, while rifabutin (10 μM) increased total NR1I2, PXR.1, PXR.2, and PXR.3 mRNA by approx. 50% after 96-h exposure. In primary human hepatocytes, rifampicin (10 μM) suppressed total NR1I2, PXR.1, and PXR.3 after 48-h exposure, and rifabutin (10 μM) had no significant impact on total NR1I2 or any of the splice variants studied. In conclusion, both antibiotics activated the studied PXR splice variants similarly but modified their expression differently. While rifampicin can suppress mRNA of PXR forms, rifabutin rather increases their expression levels.

Rifabutin but not rifampicin can partly out-balance P-glycoprotein induction by concurrent P-glycoprotein inhibition through high affinity binding to the inhibitory site

Physiology-based pharmacokinetic modeling suggests that rifabutin can out-balance P-glycoprotein (P-gp) induction by concurrent P-gp inhibition. However, clinical or experimental evidence for this Janus-faced rifabutin effect is missing. Consequently, LS180 cells were exposed to a moderately (2 µM) and strongly (10 µM) P-gp-inducing concentration of rifampicin or rifabutin for 6 days. Cellular accumulation of the fluorescent P-gp substrate rhodamine 123 was evaluated using flow cytometry, either without (induction only) or with adding rifamycin drug to the cells during the rhodamine 123 efflux phase (induction + potential inhibition). Rhodamine 123 accumulation was decreased similarly by both drugs after 6-day exposure (2 µM: 55% residual fluorescence compared to non-induced cells, P < 0.01; 10 µM: 30% residual fluorescence compared to non-induced cells, P < 0.001), indicating P-gp induction. Rhodamine 123 influx transporters mRNA expressions were not affected, excluding off-target effects. Acute re-exposure to rifabutin, however, considerably re-increased rhodamine 123 accumulation (2 µM induction: re-increase by 55%, P < 0.01; 10 µM induction: 49% re-increase, P < 0.001), suggesting P-gp inhibition. In contrast, rifampicin only had weak effects (2 µM induction: no re-increase; 10 µM induction: 16% re-increase; P < 0.05). Molecular docking analysis eventually revealed that rifabutin has a higher binding affinity to the inhibitor binding site of P-gp than rifampicin (ΔG (kcal/mol) = -11.5 vs -5.3). Together, this study demonstrates that rifabutin can at least partly mask P-gp induction by P-gp inhibition, mediated by high affinity binding to the inhibitory site of P-gp.

Kids Mod PAH trial: A multicenter trial comparing mono- versus duo-therapy for initial treatment of pediatric pulmonary hypertension

Pulmonary hypertension (PH) is a significant health problem that contributes to high morbidity and mortality in diverse cardiac, pulmonary, and systemic diseases in children. Evidence-based advances in PH care have been challenged by a paucity of quality endpoints for assessing clinical course and the lack of robust clinical trial data to guide pharmacologic therapies in children. While the landmark adult AMBITION trial demonstrated the benefit of up-front combination PH therapy with ambrisentan and tadalafil, it remains unknown whether upfront combination therapy leads to more rapid and sustained clinical benefits in children with various categories of PH. In this article, we describe the inception of the Kids Mod PAH Trial, a multicenter Phase III trial, to address whether upfront combination therapy (sildenafil and bosentan vs. sildenafil alone) improves PH outcomes in children, recognizing that marked differences between the etiology and therapeutic response between adults and children exist. The primary endpoint of this study is WHO functional class (FC) 12 months after initiation of study drug therapy. In addition to the primary outcome, secondary endpoints are being assessed, including a composite measure of time to clinical worsening, WHO FC at 24 months, echocardiographic assessment of PH and quantitative assessment of right ventricular function, 6-min walk distance, and NT-proBNP levels. Exploratory endpoints include selected biomarkers, actigraphy, and assessments of quality of life. This study is designed to pave the way for additional clinical trials by establishing a robust infrastructure through the development of a PPHNet Clinical Trials Network.

Comprehensive in vitro analysis evaluating the variable drug-drug interaction risk of rifampicin compared to rifabutin

Compared to rifampicin (600 mg/day), standard doses of rifabutin (300 mg/day) have a lower risk of drug-drug interactions due to induction of cytochrome P450 3A4 (CYP3A4) or P-glycoprotein (Pgp/ABCB1) mediated by the pregnane X receptor (PXR). However, clinical comparisons with equal rifamycin doses or in vitro experiments respecting actual intracellular concentrations are lacking. Thus, the genuine pharmacological differences and the potential molecular mechanisms of the discordant perpetrator effects are unknown. Consequently, the cellular uptake kinetics (mass spectrometry), PXR activation (luciferase reporter gene assays), and impact on CYP3A4 and Pgp/ABCB1 expression and activity (polymerase chain reaction, enzymatic assays, flow cytometry) were evaluated in LS180 cells after treatment with different rifampicin or rifabutin concentrations for variable exposure times and eventually normalized to actual intracellular concentrations. In addition, inhibitory effects on CYP3A4 and Pgp activities were investigated. While rifampicin is poorly taken up by LS180 cells, it strongly activates PXR and leads to enhanced expression and activity of CYP3A4 and Pgp. In contrast, rifabutin is a significantly less potent and less efficient PXR activator and gene inducer, despite sixfold to eightfold higher intracellular accumulation. Finally, rifabutin is a potent inhibitor of Pgp (IC50 = 0.3 µM) compared to rifampicin (IC50 = 12.9 µM). Together, rifampicin and rifabutin significantly differ by their effects on the regulation and function of CYP3A4 and Pgp, even when controlled for intracellular concentrations. Rifabutin's concurrent Pgp inhibitory action might partly compensate the inducing effects, explaining its weaker clinical perpetrator characteristics.

Effects of atorvastatin on the Sirtuin/PXR signaling pathway in Mugilogobius chulae

Atorvastatin (ATV) is a hypolipidemic drug widely detected in the aquatic environment. Nevertheless, limited information is provided about the toxic effects of ATV on estuary or coastal species and the underlying mechanisms. In the present study, the responses of genes expression in pregnane X receptor (PXR) signaling pathway and enzymatic activities in the liver of the estuarine benthic fish (Mugilogobius chulae) were investigated under acute and sub-chronic ATV exposure. Results showed that PXR was significantly inhibited in the highest exposure concentration of ATV for a shorter time (24 h, 500 μg L^-1) but induced in a lower concentration (72 h, 5 μg L^-1). The downstream genes in PXR signaling pathway such as CYP3A, SULT, UGT, and GST showed similar trends to PXR. P-gp and MRP1 were repressed in most treatments. GCLC associated with GSH synthesis was mostly induced under ATV exposure for a long time (168 h), suggesting that reactive oxygen species (ROS) were generated under ATV exposure. Similarly, GST and SOD enzymatic activities significantly increased in most exposure treatments. Under ATV exposure, SIRT1 and SIRT2 displayed induction to some extent in most treatments, suggesting that SIRTs may affect PXR expression by regulating the acetylation levels of PXR. The investigation demonstrated that ATV exposure affected the expression of the Sirtuin/PXR signaling pathway, thus further interfered adaption of M. chulae to the environment.

How to avoid misinterpretation of dual reporter gene assay data affected by cell damage

The activity of nuclear receptors (e.g., pregnane x receptor, PXR) can be assessed by luminescence-based dual reporter gene assays. Under most conditions, receptor-activated firefly luminescence is normalized to Renilla luminescence, which is triggered by a constitutively active promoter. Simultaneous damage to the cells can however disrupt these signals and thus impair the interpretation of the data. Consequently, this study addressed three important aspects: First, idealized models were described, each highlighting crucial characteristics and important pitfalls of dual PXR reporter gene assays used to evaluate PXR activation or inhibition. Second, these models were supported by experimental data obtained with a strong PXR activator (rifampicin) with low cytotoxicity, a PXR activator with high cytotoxicity (dovitinib), a proposed PXR inhibitor that reportedly has no toxic effects (triptolide), and a cytotoxic control (oxaliplatin). Data were evaluated for relative PXR activity data, individual firefly or Renilla luminescence, and anti-proliferative effects of the compounds (assessed by crystal violet staining). Finally, a step-by-step guide is proposed to avoid misleading set-up of the assay or misinterpretation of the data obtained. Key considerations here include (1) omission of drug concentrations beyond 10–20% proliferation inhibition; (2) observation of Renilla luminescence, because this tends to indicate ‘false PXR activation’ when it inexplicably decreases; (3) parallel decrease of relative PXR activity and proliferation below baseline levels in conjunction with a sharp decrease in Renilla luminescence indicates ‘false PXR antagonism’; (4) non-sigmoidal relationships suggest the absence of concentration dependency.

Does the circulating ketoconazole metabolite N-deacetyl ketoconazole contribute to the drug-drug interaction potential of the parent compound?

Ketoconazole is a strong inhibitor of cytochrome P450 3A4 (CYP3A4) and of P-glycoprotein (P-gp) and is often used as an index inhibitor especially for CYP3A4-mediated drug metabolism. A preliminary physiologically based pharmacokinetic (PBPK) model for drug-drug interactions indicated possible involvement of a metabolite to the perpetrator potential of ketoconazole. Still unknown for humans, in rodents, N-deacetyl ketoconazole (DAK) has been identified as the major ketoconazole metabolite. We therefore investigated in vitro, whether DAK also inhibits the human CYPs and drug transporters targeted by ketoconazole and quantified DAK in human plasma from healthy volunteers after receiving a single oral dose of 400 mg ketoconazole. Our data demonstrated that DAK also inhibits CYP3A4 (2.4-fold less potent than ketoconazole), CYP2D6 (13-fold more potent than ketoconazole), CYP2C19 (equally potent), P-gp (3.4-fold less potent than ketoconazole), breast cancer resistance protein (more potent than ketoconazole) and organic anion transporting polypeptide 1B1 and 1B3 (7.8-fold and 2.6-fold less potent than ketoconazole). After a single oral dose of 400 mg ketoconazole, maximum concentrations of DAK in human plasma were only 3.1 ‰ of the parent compound. However, assuming that DAK also highly accumulates in the human liver as demonstrated for rodents, inhibition of the proteins investigated could also be conceivable in vivo. In conclusion, DAK inhibits several CYPs and drug transporters, which might contribute to the perpetrator potential of ketoconazole.

Time-Resolved Effect of Interferon-Alpha 2a on Activities of Nuclear Factor Kappa B, Pregnane X Receptor and on Drug Disposition Genes

Interferon-alpha (IFN-α) is suggested to cause pharmacokinetic drug interactions by lowering expression of drug disposition genes through affecting the activities of nuclear factor kappa B (NF-ĸB) and pregnane X receptor (PXR). The time-resolved impact of IFN-α 2a (1000 U/mL; 5000 U/mL; 2 h to 30 h) on the activities of NF-ĸB and PXR and mRNA expression (5000 U/mL; 24 h, 48 h) of selected drug disposition genes and on cytochrome P450 (CYP3A4) activity in LS180 cells (5000 U/mL; 24 h, 48 h) was evaluated using luciferase-based reporter gene assays, reverse transcription polymerase chain reaction, and luminescence-based CYP3A4 activity assays. The cross-talk between NF-ĸB activation and PXR suppression was evaluated by NF-ĸB blockage (10 µM parthenolide). IFN-α 2a initially (2 h, 6 h) enhanced NF-ĸB activity 2-fold and suppressed PXR activity by 30%. mRNA of CYP3A4 was halved, whereas UGT1A1 was increased (1.35-fold) after 24 h. After 48 h, ABCB1 expression was increased (1.76-fold). CYP3A4 activity remained unchanged after 24 h, but was enhanced after 48 h (1.35-fold). IFN-α 2a demonstrated short-term suppressive effects on PXR activity and CYP3A4 mRNA expression, likely mediated by activated NF-ĸB. Longer exposure enhanced CYP3A4 activity. Clinical trials should evaluate the relevance by investigating the temporal effects of IFN-α on CYP3A4 using a sensitive marker substrate.

In vitro evidence suggesting that the toll-like receptor 7 and 8 agonist resiquimod (R-848) unlikely affects drug levels of co-administered compounds

Resiquimod (R-848) is an immune response modifier activating toll-like receptor 7 and 8. Its potential to cause pharmacokinetic interactions with concurrently administered drugs is unknown. To study the time course of the effect of resiquimod in LS180 cells as a model for intestinal tissue, luciferase-based reporter gene assays and reverse transcription polymerase chain reaction were used to investigate whether resiquimod affects the activities of nuclear factor kappa B (NF-ĸB), pregnane x receptor (PXR) or the transcription of selected central genes for drug disposition (cytochrome P-450 isozyme 3A4 (CYP3A4), CYP1A1, UDP-glucuronosyltransferase 1A1 (UGT1A1), ATP-binding cassette transporters ABCC2, ABCB1). Its impact on the activities of organic anion transporting polypeptides 1 or 3 (OATP1B1/3), breast cancer resistance protein (BCRP), P-glycoprotein (P-gp) or CYP3A4 was evaluated using fluorescence- or luminescence-based activity assays. Resiquimod irrelevantly increased NF-ĸB activity after 2 h (1 µM: 1.07-fold, P = 0.0188; 10 µM: 1.09-fold, P = 0.0142), and diminished it after 24 h (1 µM: 0.64-fold, P < 0.0001; 10 µM: 0.68-fold, P < 0.0001) and 30 h (10 µM: 0.68-fold, P = 0.0003). Concurrently, PXR activity after 24 h was marginally increased by 10 µM (1.05-fold, P = 0.0019). Resiquimod did not alter mRNA expression levels, activities of uptake or efflux transporters, or CYP3A4 activity. Given the marginal effects on NF-ĸB, PXR, expression levels of selected PXR target genes, and activities of important drug transporters and CYP3A4 in vitro, resiquimod is not expected to cause major pharmacokinetic drug-drug interactions in vivo.

Low risk of the TMPRSS2 inhibitor camostat mesylate and its metabolite GBPA to act as perpetrators of drug-drug interactions

Camostat mesylate, a potent inhibitor of the human transmembrane protease, serine 2 (TMPRSS2), is currently under investigation for its effectiveness in COVID-19 patients. For its safe application, the risks of camostat mesylate to induce pharmacokinetic drug-drug interactions with co-administered drugs should be known. We therefore tested in vitro the potential inhibition of important efflux (P-glycoprotein (P-gp, ABCB1), breast cancer resistance protein (BCRP, ABCG2)), and uptake transporters (organic anion transporting polypeptides OATP1B1, OATP1B3, OATP2B1) by camostat mesylate and its active metabolite 4-(4-guanidinobenzoyloxy)phenylacetic acid (GBPA). Transporter inhibition was evaluated using fluorescent probe substrates in transporter over-expressing cell lines and compared to the respective parental cell lines. Moreover, possible mRNA induction of pharmacokinetically relevant genes regulated by the nuclear pregnane X receptor (PXR) and aryl hydrocarbon receptor (AhR) was analysed in LS180 cells by quantitative real-time PCR. The results of our study for the first time demonstrated that camostat mesylate and GBPA do not relevantly inhibit P-gp, BCRP, OATP1B1 or OATP1B3. Only OATP2B1 was profoundly inhibited by GBPA with an IC50 of 11 μM. Induction experiments in LS180 cells excluded induction of PXR-regulated genes such as cytochrome P450 3A4 (CYP3A4) and ABCB1 and AhR-regulated genes such as CYP1A1 and CYP1A2 by camostat mesylate and GBPA. Together with the summary of product characteristics of camostat mesylate indicating no inhibition of CYP1A2, 2C9, 2C19, 2D6, and 3A4 in vitro, our data suggest a low potential of camostat mesylate to act as a perpetrator in pharmacokinetic drug-drug interactions. Only inhibition of OATP2B1 by GBPA warrants further investigation.

Evaluation of the combination of endothelin receptor antagonists (ERA) and phosphodiesterase-5 inhibitors for the treatment of pulmonary arterial hypertension (PAH) in pathologic human pulmonary arteries in an ex-vivo organ bath model

PURPOSE

Medical combination therapy of pulmonary arterial hypertension (PAH) may alleviate the drawbacks of monotherapy by avoiding drug tolerance and by increasing effectiveness, as shown by the combination of ambrisentan and tadalafil (AMBITION trial). The present ex-vivo study evaluated the combination of the endothelin receptor antagonists (ERA) macitentan and bosentan with the phosphodiesterase-5 (PDE-5) inhibitor vardenafil in pulmonary arteries from patients suffering from terminal lung disease as a model of PAH.

METHODS

Segments of the pulmonary vessels were excised from resected lungs of patients requiring lung transplantation (LTX). Contraction of pulmonary arteries (PA) was elicited by consecutive dose-response curves of endothelin-1 (ET-1) followed by norepinephrine (NE) to allow inhibition by different pathways. Forces were measured isometrically in an organ bath in the presence and absence of ERA and PDE-5 inhibitors and their combination.

RESULTS

PA of 38 patients were examined between October 2016 and November 2019. Bosentan (1E-7 M) and macitentan (1E-8 M, 3E-8 M, 1E-7 M) inhibited ET-1 induced contractions, whereas vardenafil (1E-6 M, 3E-6 M, 1E-5 M) inhibited only the NE induced part of the contractions. Vardenafil enhanced bosentan-induced inhibition of vasoconstriction in a dose-dependent fashion. Combination effects exceeded single bosentan at 3E-6 M and 1E-5 M vardenafil, and they exceeded single vardenafil at the lower vardenafil concentrations. Macitentan showed a more pronounced inhibition than bosentan regardless of the lower concentrations. Accordingly, combination effects with vardenafil resembled those of macitentan alone.

CONCLUSIONS

Macitentan and bosentan were potent antagonists of vasoconstriction in PA of LTX patients. The benefit of drug combinations was demonstrated at selected concentrations only owing to a narrow therapeutic range of vardenafil in this ex-vivo model. These results suggest the utility of drug combinations other than the established pair of ambrisentan and tadalafil in PAH treatment but also make a case for a further assessment of vasodilator properties of drugs complementing ERA.

Potential Risk of Food-Drug Interactions: Citrus Polymethoxyflavones and Flavanones as Inhibitors of the Organic Anion Transporting Polypeptides (OATP) 1B1, 1B3, and 2B1

Background and Objectives

Citrus flavonoids are not only components of daily nutrition, they are also promoted as dietary supplements and are important ingredients in traditional medicines. Interactions of flavonoids with synthetic drugs represent an often neglected issue. We therefore investigated in vitro whether the polymethoxyflavones nobiletin, sinensetin, and tangeretin and the flavonoid rutinosides didymin, hesperidin, and narirutin can inhibit human organic anion transporting polypeptides (OATP) 1B1, 1B3, and 2B1, which are important transporters mediating drug-drug and food-drug interactions.

Methods

Inhibition was investigated by quantifying the decreased uptake of the fluorescent OATP1B1 and OATP1B3 substrate 8-fluorescein-cAMP in HEK293 cells overexpressing OATP1B1 or OATP1B3 and of the fluorescent OATP2B1 substrate 4′,5′-dibromofluorescein in HEK293 cells overexpressing OATP2B1.

Results

We demonstrate that all flavonoids investigated inhibit OATP2B1 in the lower micromolar range (IC₅₀ between 1.6 and 14.2 µM), but only the polymethoxyflavones also inhibit OATP1B1 and 1B3 (IC₅₀ between 2.1 and 21 µM).

Conclusions

All flavonoids investigated might contribute to the intestinal OATP2B1-based interactions with drugs observed with citrus juices or fruits. In contrast, the concentration of the polymethoxyflavones after consumption of citrus juices or fruits is most likely too low to reach relevant systemic concentrations and thus to inhibit hepatic OATP1B1 and OATP1B3, but there might be a risk when they are consumed as medicines or as dietary supplements.

Interaction of Hydroxychloroquine with Pharmacokinetically Important Drug Transporters

(1) Background: Hydroxychloroquine is used to treat malaria and autoimmune diseases, and its potential use against COVID-19 is currently under investigation. Thus far, information on interactions of hydroxychloroquine with drug transporters mediating drug-drug interactions is limited. We assessed the inhibition of important efflux (P-glycoprotein (P-gp), breast cancer resistance protein (BCRP)) and uptake transporters (organic anion transporting polypeptide (OATP)-1B1, OATP1B3, OATP2B1) by hydroxychloroquine, tested its P-gp and BCRP substrate characteristics, and evaluated the induction of pharmacokinetically relevant genes regulated by the nuclear pregnane X (PXR) (CYP3A4, ABCB1) and aryl hydrocarbon receptor (AhR) (CYP1A1, CYP1A2). (2) Methods: Transporter inhibition was evaluated in transporter over-expressing cell lines using fluorescent probe substrates. P-gp and BCRP substrate characteristics were assessed by comparing growth inhibition of over-expressing and parental cell lines. Possible mRNA induction was analysed in LS180 cells by quantitative real-time PCR. (3) Results: Hydroxychloroquine did not inhibit BCRP or the OATPs tested but inhibited P-gp at concentrations exceeding 10 µM. P-gp overexpressing cells were 5.2-fold more resistant to hydroxychloroquine than control cells stressing its substrate characteristics. Hydroxychloroquine did not induce genes regulated by PXR or AhR. (4) Conclusions: This is the first evidence that hydroxychloroquine’s interaction potential with drug transporters is low, albeit bioavailability of simultaneously orally administered P-gp substrates might be increased by hydroxychloroquine.

Drug-drug interaction potential of the HBV and HDV entry inhibitor myrcludex B assessed in vitro

BACKGROUND

Myrcludex B is a first-in-class virus entry inhibitor for patients with chronic hepatitis B or B/D infections. In patients it will be coadministered with drugs needed for the disease or comorbidities. We aimed to define the risk of drug-drug interactions by characterizing the influence of myrcludex B on relevant drug transporting and metabolizing enzymes in vitro.

METHODS

Inhibition of P-glycoprotein (P-gp; ABCB1), breast cancer resistance protein (BCRP/ABCG2), and the organic anion transporting polypeptides 1B1 and 1B3 (OATP1B1/SLCO1B1 and OATP1B3/SLCO1B3) was measured in cells over-expressing the respective transporter using fluorogenic substrates. Inhibition of cytochrome P450 enzymes (CYPs) was assessed with commercially available kits. mRNA induction of drug transporting and metabolizing enzymes was measured in LS180 cells after 4 days of treatment by quantitative real-time PCR. Pregnane X receptor (PXR) activation was assessed using a reporter-gene assay.

RESULTS

Whereas activities of P-gp and BCRP were not influenced by myrcludex B, OATP1B1 and OATP1B3 were specifically inhibited with a 50% inhibitory concentration (IC₅₀) of 0.5 and 8.7 µM, respectively. Myrcludex B weakly inhibited all CYPs tested at concentrations ≥10 µM except CYP2D6, which was not inhibited at concentrations up to 2 µM. Myrcludex B had no influence on mRNA expression of CYP1A1, CYP3A4, UGT1A3, ABCB1, ABCC2 and ABCG2, and on PXR activity.

CONCLUSIONS

Our in vitro study suggests that myrcludex B is not at major risk of acting as a perpetrator drug. A potential inhibition of the uptake transporters OATP1B1 and OATP1B3 and a previous clinical finding of potential CYP3A inhibition, requires further evaluation and should be carefully addressed in future trials.

ABC Transporters in Extrahepatic Tissues: Pharmacological Regulation in Heart and Intestine

ATP binding cassette (ABC) transporters are transmembrane proteins expressed in secretory epithelia like the liver, kidneys and intestine, in the epithelia exhibiting barrier function such as the blood-brain barrier and placenta, and to a much lesser extent, in tissues like reproductive organs, lungs, heart and pancreas, among others. They regulate internal distribution of endogenous metabolites and xenobiotics including drugs of therapeutic use and also participate in their elimination from the body. We here describe the function and regulation of ABC transporters in the heart and small intestine, as examples of extrahepatic tissues, in which ABC proteins play clearly different roles. In the heart, they are involved in tissue pathogenesis as well as in protecting this organ against toxic compounds and druginduced oxidative stress. The small intestine is highly exposed to therapeutic drugs taken orally and, consequently, ABC transporters localized on its surface strongly influence drug absorption and pharmacokinetics. Examples of the ABC proteins currently described are Multidrug Resistance-associated Proteins 1 and 2 (MRP1 and 2) for heart and small intestine, respectively, and P-glycoprotein (P-gp) and Breast Cancer Resistance Protein (BCRP) for both organs.

Herb⁻Drug Interaction Potential of Anti-Borreliae Effective Extracts from Uncaria tomentosa (Samento) and Otoba parvifolia (Banderol) Assessed In Vitro

Samento (extract from Uncaria tomentosa) and Banderol (extract from Otoba parvifolia) have been demonstrated to have anti-inflammatory and antimicrobial properties, e.g., against different morphological forms of Borrelia burgdorferi. However, there is hardly any data on the pharmacological safety of these two herbal medicines. This in vitro study aimed at scrutinizing their possible characteristics as perpetrators in pharmacokinetic herbal–drug interactions. Inhibition of cytochrome P450 enzymes (CYPs) was quantified by commercial kits and inhibition of drug transporters by use of fluorescent probe substrates. Induction was quantified by real-time RT-PCR and activation of pregnane x receptor (PXR) and aryl hydrocarbon receptor (AhR) by reporter gene assays. Organic anion transporting polypeptide 1B1 (OATP1B1) (IC₅₀ = 0.49 ± 0.28%) and OATP1B3 (IC₅₀ = 0.65 ± 0.29%) were potently inhibited by Banderol, but only weakly by Samento. CYP3A4 was inhibited about 40% at a Samento concentration of 1%. Samento significantly induced mRNA expression of CYP2J2, UGT1A3, UGT1A9, ABCB1, and SLCO1B1 and strongly activated PXR, but hardly AhR. In conclusion, the perpetrator profiles of Samento and Banderol for herb–drug interactions completely differ. Clinical studies are strongly recommended to clarify whether the effects observed in vitro are of clinical relevance.

From the Cover: MechanisticInsights in Cytotoxic and Cholestatic Potential of the Endothelial Receptor Antagonists Using HepaRG Cells

Several endothelin receptor antagonists (ERAs) have been developed for the treatment of pulmonary arterial hypertension (PAH). Some of them have been related to clinical cases of hepatocellular injury (sitaxentan [SIT]) and/or cholestasis (bosentan [BOS]). We aimed to determine if ambrisentan (AMB) and macitentan (MAC), in addition to BOS and SIT, could potentially cause liver damage in man by use of human HepaRG cells. Our results showed that like BOS, MAC-induced cytotoxicity and cholestatic disorders characterized by bile canaliculi dilatation and impairment of myosin light chain kinase signaling. Macitentan also strongly inhibited taurocholic acid and carboxy-2',7'-dichlorofluorescein efflux while it had a much lower inhibitory effect on influx activity compared to BOS and SIT. Moreover, these three drugs caused decreased intracellular accumulation and parallel increased levels of total bile acids (BAs) in serum-free culture media. In addition, all drugs except AMB variably deregulated gene expression of BA transporters. In contrast, SIT was hepatotoxic without causing cholestatic damage, likely via the formation of reactive metabolites and AMB was not hepatotoxic. Together, our results show that some ERAs can be hepatotoxic and that the recently marketed MAC, structurally similar to BOS, can also cause cholestatic alterations in HepaRG cells. The absence of currently known or suspected cases of cholestasis in patients suffering from PAH treated with MAC is rationalized by the lower therapeutic doses and Cmax, and longer receptor residence time compared to BOS.

Impact of enzalutamide and its main metabolite N-desmethyl enzalutamide on pharmacokinetically important drug metabolizing enzymes and drug transporters

Enzalutamide is a new drug against castration-resistant prostate cancer. Recent data indicate profound induction of drug metabolizing enzymes (e.g. cytochrome P450 isoenzyme (CYP) 3A4) but comprehensive in vitro data on other CYP enzymes, drug conjugating enzymes or drug transporters is scarce. Moreover, the mechanisms of induction are poorly investigated and the effects of the active metabolite N-desmethyl enzalutamide are unknown. Using LS180 cells as an induction model and quantitative real-time reverse transcription polymerase chain reaction, our study demonstrated a concentration-dependent induction of CYP1A1, CYP1A2, CYP3A5, CYP3A4, UGT1A3, UGT1A9, ABCB1, ABCC2 and ABCG2 mRNA. Induction of CYP3A4 and ABCB1 was confirmed by Western blot analysis and is likely mediated by activation of the nuclear receptor pregnane x receptor, elucidated by a luciferase-based reporter gene assay. Enzalutamide's main active metabolite N-desmethyl enzalutamide exhibited only weak induction properties. mRNA expression of UGT2B7 was suppressed by enzalutamide and its metabolite. Both compounds are apparently not transported by P-glycoprotein (P-gp) or breast cancer resistance protein (BCRP). N-desmethyl enzalutamide more potently inhibited important drug transporters (P-gp, BCRP, OATPs) than enzalutamide. Taken together, the pharmacokinetics of concurrently administered drugs is likely altered during enzalutamide therapy. Levels of metabolically (mainly CYP3A4) eliminated drugs are expected to be decreased, whereas the abundance of compounds with solely transporter-determined pharmacokinetics (P-gp, OATPs) is likely enhanced.

Bortezomib, carfilzomib and ixazomib do not mediate relevant transporter-based drug-drug interactions

In order to optimize the clinical application of an increasing number of proteasome inhibitors, investigations into the differences between their respective pharmacodynamic and pharmacokinetic profiles, including their ability to act as a perpetrator in drug-drug interactions, are warranted. Therefore, in the present in vitro study, it was investigated whether bortezomib, carfilzomib and ixazomib are able to alter the expression, and/or the activity, of specific drug transporters generally relevant for pharmacokinetic drug-drug interactions. Through induction experiments, the current study demonstrated that the aforementioned three proteasome inhibitors do not induce mRNA expression of the transporter genes ATP binding cassette (ABC)B1, C1, C2 and G2 in the LS180 cell line, which was used as a model for systemic induction. By contrast, in certain myeloma cell lines, ixazomib provoked minor alterations in individual transporter gene expression. None of the proteasome inhibitors tested relevantly inhibited drug transporters within the range of physiological plasma concentrations. Taken together, transporter-based drug-drug interactions are unlikely to be a primary concern in the clinical application of the tested compounds.

The long chain α-tocopherol metabolite α-13'-COOH and γ-tocotrienol induce P-glycoprotein expression and activity by activation of the pregnane X receptor in the intestinal cell line LS 180

SCOPE

Members of the vitamin E family or their metabolites may induce the xenobiotic transporter P-glycoprotein (P-gp), which can limit the bioavailability of drugs and phytochemicals. This study aimed to investigate if α- and γ-tocopherol, α- and γ-tocotrienol, the long chain metabolite α-tocopherol-13'-COOH, the short chain metabolites α- and γ-carboxyethylhydroxychromanol and plastochromanol-8 activate the pregnane X receptor (PXR) and thereby modulate P-gp expression and/or activity.

METHODS AND RESULTS

P-gp protein expression and activity were studied in LS 180 cells incubated with the respective test compound for 48 h. Furthermore, we determined if the compounds activate PXR in LS 180 cells, as PXR regulates P-gp expression. Neither P-gp protein expression and activity, nor PXR activity were influenced by α-tocopherol, γ-tocopherol and plastochromanol-8. α-Tocotrienol activated PXR in the reporter gene assay but did not induce protein expression or activity of P-gp. γ-Tocotrienol and α-13'-COOH activated PXR and induced protein expression and transporter activity of P-gp.

CONCLUSION

Because the induction of P-gp in the intestine may limit the systemic bioavailability of its substrates, the concurrent intake of drugs and γ-tocotrienol and, if ever applicable, α-13'-COOH should be avoided.

High-Energy Ball Milling as Green Process To Vitrify Tadalafil and Improve Bioavailability

In this study, the suitability of high-energy ball milling was investigated with the aim to vitrify tadalafil (TD) and improve its bioavailability. To achieve this goal, pure TD as well as binary mixtures composed of the drug and Soluplus (SL) were coprocessed by high-energy ball milling. Modulated differential scanning calorimetry (MDSC) and X-ray powder diffraction (XRD) demonstrated that after such coprocessing, the crystalline form of TD was transformed into an amorphous form. The presence of a single glass transition (T_g) for all the comilled formulations indicated that TD was dispersed into SL at the molecular level, forming amorphous molecular alloys, regardless of the drug concentration. The high values of T_g determined for amorphous formulations, ranging from 70 to 147 °C, foreshow their high stability during storage at room temperature, which was verified by XRD and MDSC studies. The stabilizing effect of SL on the amorphous form of TD in comilled formulations was confirmed. Dissolution tests showed immediate drug release with sustained supersaturation in either simulated gastric fluid of pH 1.2 or in phosphate buffer of pH 7.2. The beneficial effect of both amorphization and coamorphization on the bioavailability of TD was found. In comparison to aqueous suspension, the relative bioavailability of TD was only 11% for its crystalline form and 53% for the crystalline physical mixture, whereas the bioavailability of milled amorphous TD and the comilled solid dispersion was 128% and 289%, respectively. Thus, the results provide evidence that not only the presence of polymeric surfactant but also the vitrification of TD is necessary to improve bioavailability.

Venetoclax (ABT-199) Might Act as a Perpetrator in Pharmacokinetic Drug-Drug Interactions

Venetoclax (ABT-199) represents a specific B-cell lymphoma 2 (Bcl-2) inhibitor that is currently under development for the treatment of lymphoid malignancies. So far, there is no published information on its interaction potential with important drug metabolizing enzymes and drug transporters, or its efficacy in multidrug resistant (MDR) cells. We therefore scrutinized its drug-drug interaction potential in vitro. Inhibition of cytochrome P450 enzymes (CYPs) was quantified by commercial kits. Inhibition of drug transporters (P-glycoprotein (P-gp, ABCB1), breast cancer resistance protein (BCRP), and organic anion transporting polypeptides (OATPs)) was evaluated by the use of fluorescent probe substrates. Induction of drug transporters and drug metabolizing enzymes was quantified by real-time RT-PCR. The efficacy of venetoclax in MDR cells lines was evaluated with proliferation assays. Venetoclax moderately inhibited P-gp, BCRP, OATP1B1, OATP1B3, CYP3A4, and CYP2C19, whereas CYP2B6 activity was increased. Venetoclax induced the mRNA expression of CYP1A1, CYP1A2, UGT1A3, and UGT1A9. In contrast, expression of ABCB1 was suppressed, which might revert tumor resistance towards antineoplastic P-gp substrates. P-gp over-expression led to reduced antiproliferative effects of venetoclax. Effective concentrations for inhibition and induction lay in the range of maximum plasma concentrations of venetoclax, indicating that it might act as a perpetrator drug in pharmacokinetic drug-drug interactions.

Pharmacokinetic and Pharmacodynamic Comparison of Sildenafil-Bosentan and Sildenafil-Ambrisentan Combination Therapies for Pulmonary Hypertension

To elucidate whether the pharmacokinetics (PK) and pharmacodynamics (PD) of sildenafil are influenced differently when it is coadministered with bosentan (S+B) or with ambrisentan (S+A), we evaluated the PK and PD profiles of sildenafil before and after 4-5 weeks of S+A or S+B treatment in patients with pulmonary arterial hypertension. The area under the plasma concentration-time curve of sildenafil was significantly higher in S+A treatment than in S+B treatment (165.8 ng•h/mL vs. 396.8 ng•h/mL, P = 0.018) and the oral clearance of sildenafil was significantly lower after S+A treatment than after S+B treatment (120.6 L/h/kg vs. 50.4 L/h/kg, P = 0.018). In the PD study, incremental shuttle walking distance was superior during treatment with S+A than during treatment with S+B (S+B; 280 m vs. S+A; 340 m, P = 0.042). There were no concerns about safety with either combination therapy regime.

Obatoclax as a perpetrator in drug-drug interactions and its efficacy in multidrug resistance cell lines

OBJECTIVES

Obatoclax is a pan-Bcl-2 inhibitor with promising efficacy, especially when combined with other antineoplastic agents. Pharmacokinetic drug-drug interactions can occur systemically and at the level of the tumour cell. Thus, this study scrutinised the interaction potential of obatoclax in vitro.

METHODS

Obatoclax was screened for P-gp inhibition by calcein assay, for breast cancer resistance protein (BCRP) inhibition by pheophorbide A assay and for inhibition of cytochrome P450 isoenzymes (CYPs) by commercial kits. Induction of mRNA of drug-metabolising enzymes and drug transporters was quantified in LS180 cells via real-time polymerase chain reaction and involvement of nuclear receptors was assessed by reporter gene assays. Proliferation assays were used to assess whether obatoclax retains its efficacy in cell lines overexpressing BCRP, P-glycoprotein (P-gp) or multidrug resistance-associated protein 2 (MRP2).

KEY FINDINGS

Obatoclax induced the mRNA expression of several genes (e.g. CYP1A1, CYP1A2 and ABCG2 (five to seven-fold) through activation of the aryl hydrocarbon receptor in the nanomolar range. Obatoclax inhibits P-gp, BCRP and some CYPs at concentrations exceeding plasma levels. P-gp, MPR2 or BCRP overexpression did not influence the efficacy of obatoclax.

CONCLUSIONS

Obatoclax retains its efficacy in cells overexpressing P-gp, MRP2 or BCRP and might act as a perpetrator drug in interactions with drugs, for example being substrates of CYP1A2 or BCRP.

Effects of adrenolytic mitotane on drug elimination pathways assessed in vitro

Mitotane (1,1-dichloro-2-(o-chlorophenyl)-2-(p-chlorophenyl)ethane, o,p'-DDD) represents one of the most active drugs for the treatment of adrenocortical carcinoma. Its metabolites 1,1-(o,p'-dichlorodiphenyl) acetic acid (=o,p'-DDA) and 1,1-(o,p'-dichlorodiphenyl)-2,2 dichloroethene (=o,p'-DDE) partly contribute to its pharmacological effects. Because mitotane has a narrow therapeutic index and causes pharmacokinetic drug-drug interactions, knowledge about these compounds' effects on drug metabolizing and transporting proteins is crucial. Using quantitative real-time polymerase chain reaction, our study confirmed the strong inducing effects of o,p'-DDD on mRNA expression of cytochrome P450 3A4 (CYP3A4, 30-fold) and demonstrated that other enzymes and transporters are also induced (e.g., CYP1A2, 8.4-fold; ABCG2 (encoding breast resistance cancer protein, BCRP), 4.2-fold; ABCB1 (encoding P-glycoprotein, P-gp) 3.4-fold). P-gp induction was confirmed at the protein level. o,p'-DDE revealed a similar induction profile, however, with less potency and o,p'-DDA had only minor effects. Reporter gene assays clearly confirmed o,p'-DDD to be a PXR activator and for the first time demonstrated that o,p'-DDE and o,p'-DDA also activate PXR albeit with lower potency. Using isolated, recombinant CYP enzymes, o,p'-DDD and o,p'-DDE were shown to strongly inhibit CYP2C19 (IC50 = 0.05 and 0.09 µM). o,p'-DDA exhibited only minor inhibitory effects. In addition, o,p'-DDD, o,p'-DDE, and o,p'-DDA are demonstrated to be neither substrates nor inhibitors of BCRP or P-gp function. In summary, o,p'-DDD and o,p'-DDE might be potential perpetrators in pharmacokinetic drug-drug interactions through induction of drug-metabolizing enzymes or drug transporters and by potent inhibition of CYP2C19. In tumors over-expressing BCRP or P-gp, o,p'-DDD and its metabolites should retain their efficacy due to a lack of substrate characteristics.

Modulation of P-glycoprotein efflux pump: induction and activation as a therapeutic strategy

P-glycoprotein (P-gp) is an ATP-dependent efflux pump encoded by the MDR1 gene in humans, known to mediate multidrug resistance of neoplastic cells to cancer therapy. For several decades, P-gp inhibition has drawn many significant research efforts in an attempt to overcome this phenomenon. However, P-gp is also constitutively expressed in normal human epithelial tissues and, due to its broad substrate specificity, to its cellular polarized expression in many excretory and barrier tissues, and to its great efflux capacity, it can play a crucial role in limiting the absorption and distribution of harmful xenobiotics, by decreasing their intracellular accumulation. Such a defense mechanism can be of particular relevance at the intestinal level, by significantly reducing the intestinal absorption of the xenobiotic and, consequently, avoiding its access to the target organs. In this review, the current knowledge on this important efflux pump is summarized, and a new focus is brought on the therapeutic interest of inducing and/or activating P-gp for limiting the toxicity caused by its substrates. Several in vivo and in vitro studies validating the use of such a therapeutic strategy are discussed. An extensive literature search for reported P-gp inducers/activators and for the experimental models used in their characterization was conducted. Those studies demonstrate that effective antidotal pathways can be achieved by efficiently promoting the P-gp-mediated efflux of deleterious xenobiotics, resulting in a significant reduction in their intracellular levels and, consequently, in a significant reduction of their toxicity.

Interaction potential of the multitargeted receptor tyrosine kinase inhibitor dovitinib with drug transporters and drug metabolising enzymes assessed in vitro

Dovitinib (TKI-258) is under development for the treatment of diverse cancer entities. No published information on its pharmacokinetic drug interaction potential is available. Thus, we assessed its interaction with important drug metabolising enzymes and drug transporters and its efficacy in multidrug resistant cells in vitro. P-glycoprotein (P-gp, MDR1, ABCB1) inhibition was evaluated by calcein assay, inhibition of breast cancer resistance protein (BCRP, ABCG2) by pheophorbide A efflux, and inhibition of organic anion transporting polypeptides (OATPs) by 8-fluorescein-cAMP uptake. Inhibition of cytochrome P450 3A4, 2C19, and 2D6 was assessed by using commercial kits. Induction of transporters and enzymes was quantified by real-time RT-PCR. Possible aryl hydrocarbon receptor (AhR) activating properties were assessed by a reporter gene assay. Substrate characteristics were evaluated by growth inhibition assays in cells over-expressing P-gp or BCRP. Dovitinib weakly inhibited CYP2C19, CYP3A4, P-gp and OATPs. The strongest inhibition was observed for BCRP (IC50 = 10.3 ± 4.5 μM). Among the genes investigated, dovitinib only induced mRNA expression of CYP1A1, CYP1A2, ABCC3 (coding for multidrug resistance-associated protein 3), and ABCG2 and suppressed mRNA expression of some transporters and drug metabolising enzymes. AhR reporter gene assay demonstrated that dovitinib is an activator of this nuclear receptor. Dovitinib retained its efficacy in cell lines over-expressing P-gp or BCRP. Our analysis indicates that dovitinib will most likely retain its efficacy in tumours over-expressing P-gp or BCRP and gives first evidence that dovitinib might act as a perpetrator drug in pharmacokinetic drug-drug interactions.

Pharmacokinetic interaction profile of riociguat, a new soluble guanylate cyclase stimulator, in vitro

Riociguat is a new soluble guanylate cyclase stimulator under development for pulmonary arterial hypertension and chronic thromboembolic pulmonary hypertension. So far, the interaction potential of riociguat with other drugs is nearly unknown. Therefore, we assessed in vitro the potency of riociguat to inhibit important drug metabolising enzymes (cytochrome P450 (CYP) 3A4, CYP2C19, and CYP2D6) and drug transporters (P-glycoprotein (P-gp/ABCB1), breast cancer resistance protein (BCRP/ABCG2), and organic anion transporting polypeptides (OATP) 1B1 and 1B3). In addition we evaluated its substrate characteristics for P-gp, BCRP, and the multidrug resistance-associated protein 1 (MRP1/ABCC1). We also assessed riociguat's inducing properties on important drug metabolising enzymes and transporters and investigated its ability to activate the pregnane-X-receptor (PXR). Riociguat was identified as a weak to moderate inhibitor of P-gp (f2-value: 11.7 ± 4.8 μM), BCRP (IC50 = 46.2 ± 20.3 μM), OATP1B1 (IC50 = 34.1 ± 3.15 μM), OATP1B3 (IC50 = 50.3 ± 7.5 μM), CYP2D6 (IC50 = 12.4 ± 0.74 μM), and CYP2C19 (IC50 = 46.1 ± 7.14 μM). Furthermore, it induced mRNA expression of BCRP/ABCG2 (3-fold at 20 μM) and to a lesser extent of CYP3A4 (2.3-fold at 20 μM), UGT1A4, and ABCB11. The only weak inducing properties were confirmed by weak activation of PXR. Considering its systemic concentrations its interaction potential as a perpetrator drug seems to be low. In contrast, our data suggest that riociguat is a P-gp substrate and might therefore act as a victim drug when co-administered with strong P-gp inductors or inhibitors.

Telaprevir is a substrate and moderate inhibitor of P-glycoprotein, a strong inductor of ABCG2, but not an activator of PXR in vitro

Triple therapy combining the protease inhibitor telaprevir with interferon-α and ribavirin is a promising new option for long-term treatment of hepatitis C. The interaction potential of telaprevir has not yet been fully elucidated. The in vitro potency of telaprevir to inhibit P-glycoprotein (P-gp, ABCB1) and breast cancer resistance protein (BCRP, ABCG2) was assessed and its substrate characteristics for P-gp, BCRP and the multidrug resistance-associated proteins (MRPs, ABCCs) 1-3 were evaluated. The inducing properties of telaprevir on important drug-metabolising enzymes and transporters were also assessed and its ability to activate the pregnane X receptor (PXR) was investigated. Using growth inhibition assays, it was confirmed that telaprevir is a substrate of P-gp and it was demonstrated for the first time that it is not transported by BCRP and MRPs. Telaprevir only moderately inhibited P-gp in the calcein assay and did not inhibit BCRP in the pheophorbide A assay. In LS180 cells, telaprevir strongly induced mRNA expression of ABCG2 (4.3-fold at 30 μmol/L) and weakly induced ABCB11, CYP2C19 and UGT1A3. In contrast, telaprevir had no significant influence on mRNA expression of CYP3A4, UGT1A9, ABCB1, ABCC2 and SLCO1B1. In a reporter gene assay, telaprevir did not activate PXR. Thus, it appears unlikely that telaprevir induces CYP3A4 and P-gp in vivo in such a way as to provoke clinically relevant drug interactions. From the numerous perpetrator characteristics, telaprevir's inhibitor properties, especially of CYP3A4 and P-gp, appear to be the most relevant mechanism for drug interactions. The clinical relevance of the strong inducing effects on ABCG2 requires proper assessment.

Potential of the novel antiretroviral drug rilpivirine to modulate the expression and function of drug transporters and drug-metabolising enzymes in vitro

The objective of this study was to assess the drug-drug interaction potential of the new non-nucleoside reverse transcriptase inhibitor (NNRTI) rilpivirine in vitro. The following were evaluated: P-glycoprotein (P-gp/ABCB1) inhibition by calcein assay; breast cancer resistance protein (BCRP/ABCG2) inhibition by pheophorbide A efflux; and inhibition of organic anion transporting polypeptide (OATP) 1B1 and OATP1B3 by 8-fluorescein-cAMP uptake. Inhibition of cytochrome P450 enzymes was assessed using commercially available kits. Substrate characteristics were evaluated by growth inhibition assays in MDCKII cells overexpressing particular ABC transporters. Induction of drug-metabolising enzymes and transporters was quantified by real-time RT-PCR in LS180 cells, and activation of pregnane X receptor (PXR) by a reporter gene assay. Rilpivirine significantly inhibited P-gp (IC(50) = 13.1 ± 6.8 μmol/L), BCRP (IC(50) = 1.5 ± 0.3 μmol/L), OATP1B1 (IC(50) = 4.1 ± 1.8 μmol/L), OATP1B3 (IC(50) = 6.1 ± 0.9 μmol/L), CYP3A4 (IC(50) = 1.3 ± 0.6 μmol/L), CYP2C19 (IC(50) = 2.7 ± 0.3 μmol/L) and CYP2B6 (IC(50) = 4.2 ± 1.6 μmol/L). Growth inhibition assays indicate that rilpivirine is not a substrate of P-gp, BCRP, or multidrug resistance-associated proteins 1 and 2. In LS180 cells, rilpivirine induced mRNA expression of ABCB1, CYP3A4 and UGT1A3, whereas ABCC1, ABCC2, ABCG2, OATP1B1 and UGT1A9 were not induced. Moreover, rilpivirine was a PXR activator. In conclusion, rilpivirine inhibits and induces several relevant drug-metabolising enzymes and drug transporters, but owing to its low plasma concentrations it is most likely less prone to drug-drug interactions than older NNRTIs.