Differential modulation of the expression of important drug metabolising enzymes and transporters by endothelin-1 receptor antagonists ambrisentan and bosentan in vitro

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.

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.

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.

Homology modeling, molecular dynamics and virtual screening of endothelin-A receptor for the treatment of pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a progressive disease of pulmonary arteries, causing serious shortness of breath and right ventricular failure with high mortality. Numerous studies have verified that the symptoms of PAH could be attenuated effectively with endothelin-A receptor (ETAR) antagonists. Unfortunately, the 3D structure of ETAR has not been released, making it difficult to understand the interactions between ETAR and its antagonists. In this study, computational methods including homology modeling, molecular docking and molecular dynamics simulations were performed to build the structure of ETAR and predict the binding patterns of ETAR with its two antagonists. Based on these results, virtual screening study was implemented against Traditional Chinese Medicine (TCM) database to identify novel natural ETAR antagonists. Six compounds with best binding energies were screened out and two of them were found to bind steadily with ETAR validated through molecular dynamics simulations and MM-GBSA calculation, indicating that they were potential antagonists of ETAR. In a word, our research provided a deep exploration into the interaction between ETAR and its antagonists, which could promote the development of novel therapy against PAH.[Formula: see text]Communicated by Ramaswamy H. Sarma.

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.

Potential detoxification effect of active ingredients in liquorice by upregulating efflux transporter

BACKGROUND

As one of most widely used herbal medicine, liquorice exhibits diverse pharmacological activities, for instance, analgesic, antitussive, antiarrhythmic, anti-inflammatory, and immune regulation. Additionally, detoxification effects were observed in combination of liquorice with other herbal drugs. The mechanism of detoxification of liquorice has been extensively investigated through material basis and interference with CYPs though, investigations of its effect on transporters were very limited, according to the literature.

PURPOSE

The objective of this study was attempt to investigate the effect of active ingredients existing in liquorice on the efflux transporters as to clarify the potential mechanism of detoxification of liquorice.

METHODS

Multiple analytical approaches have been explored, including flow cytometry, fluorescent detection, RT-PCR, Western blot to measure the function, activity as well as mRNA/protein expression of efflux transporters on LS-180 cell model after treatments with active compounds of liquorice. Additionally, Caco-2 cell model was utilized to further investigate the potential impact of those ingredients on efflux transporter.

RESULTS

The resulting data indicated that those active ingredients, including flavonoids (liquiritin, liquiritigenin, isoliquiritin, isoliquiritigenin and licochalcone A) and pentacyclic triterpene saponin (glycyrrhetinic acid) were able to upregulate the expression of efflux transporters, for example P-gp, BCRP and MRP2. The gene expressions were approximately over 2.5 folds by comparison with that of control, and up to 13 folds and 16 folds for BCRP by isoliquiritin and isoliquiritigenin, and further confirmed by Western blot. The functional assay also supported up-regulation of efflux transporter by those ingredients. Flow cytometry study showed that the level of rhodamine123 as probe substrate in LS-180 cells decreased to approximately 50% after treatment with active ingredients of liquorice, compared with that of control. The fluorescent assay confirmed that change of rhodamine 123 was correlated with the concentrations of active ingredients given. The efflux transport of rhodamine 123 was enhanced in Caco-2 cell models as well.

CONCLUSION

The study clarified potential detoxification mechanism of liquorice by up-regulating efflux transporter as to reduce absorption of xenobiotics across small intestinal membrane, which provided a new insight into pharmacological function of liquorice.

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.

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.

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.

Clarithromycin substantially increases steady-state bosentan exposure in healthy volunteers

AIMS

The aim of this study was to assess the effect of the cytochrome P450 (CYP) 3A4 and organic anion-transporting polypeptide (OATP) 1B1 inhibitor clarithromycin on the pharmacokinetics of bosentan. We also aimed to evaluate the impact of CYP2C9 and SLCO1B1 (encoding for OATP1B1) genotypes and their combination.

METHODS

We assessed the effect of the OATP and CYP3A inhibitor clarithromycin on bosentan pharmacokinetics at steady state and concurrently quantified changes of CYP3A activity using midazolam as a probe drug. Sixteen healthy volunteers received therapeutic doses of bosentan (125 mg twice daily) for 14 days and clarithromycin (500 mg twice daily) concomitantly for the last 4 days, and bosentan pharmacokinetics was assessed on days 1, 10 and 14.

RESULTS

Clarithromycin significantly increased bosentan area under the plasma concentration-time curve of the dosing interval 3.7-fold and peak concentration 3.8-fold in all participants irrespective of the genotype. Clarithromycin also reduced CYP3A activity (midazolam clearance) in all participants; however, these changes were not correlated to the changes of bosentan clearance.

CONCLUSIONS

Clarithromycin substantially increases the exposure to bosentan, suggesting that dose reductions may be necessary.

Protective effects of endothelin receptor A and B inhibitors against doxorubicin-induced cardiomyopathy

The clinical efficiency of the highly potent antitumor agent doxorubicin is limited by cardiotoxic effects. In a murine doxorubicin cardiotoxicity model, increased endothelin-1 (ET-1) expression and cardioprotective effects of the dual ET-1 blocker bosentan were demonstrated. To date it is unclear if combined blocking of endothelin A/B receptors is necessary or whether selective inhibition of one of the ET-1 receptors is sufficient for the observed cardioprotection. Therefore, we investigated the impact of dual (bosentan) and single endothelin receptor antagonism through sitaxentan (receptor A blocker) or BQ788 (receptor B blocker) in a murine doxorubicin cardiotoxicity model (C57BL/6N). Simultaneous administration of each endothelin receptor antagonist (ERA) with doxorubicin resulted in a significantly improved hemodynamic performance in comparison to the impaired cardiac function in control mice with bosentan being most effective but closely followed by sitaxentan and also BQ788. This cardioprotection was not caused by diminished doxorubicin levels in heart since the doxorubicin content in cardiac tissue was not altered by ERAs significantly. However, whole transcript expression profiling showed partly different effects of the ERAs on doxorubicin-modulated cardiac gene expression of genes involved in signal transduction (e.g. Stat3, Pim1, Akt1, Plcb2), fibrosis (e.g. Myl4), energy production (e.g. Ant1) or oxidative stress (e.g. Aox1). Furthermore, doxorubicin-mediated gene regulations were verified in the murine cardiomyocyte model HL-1 showing partly reversed expression patterns after co-administration of the ERAs. In summary, our results demonstrate strong cardioprotective effects of blocking ET-1 receptors against the doxorubicin-related cardiomyopathy and provide evidence to potential underlying signaling pathways.

Ambrisentan for the treatment of adults with pulmonary arterial hypertension: a review

OBJECTIVE

To provide an overview of the drug profile of the orally active, selective endothelin A receptor antagonist ambrisentan, and its efficacy and safety in the treatment of patients with pulmonary arterial hypertension (PAH).

RESEARCH DESIGN AND METHODS

Medical literature on the use of ambrisentan in PAH was identified using MEDLINE and EMBASE. Additional references were identified from the reference lists of published articles and from the authors' own bibliographies.

RESULTS

Significant improvements in exercise capacity were observed with approved dosages of ambrisentan (5 or 10 mg once daily) in the AMB-220 dose-ranging study and the pivotal ARIES-1 and ARIES-2 trials, with sustained effects up to 2 years observed in ARIES-E. Improvements in cardiopulmonary hemodynamic variables were reported in AMB-220 and ARIES-E (subset analysis). Ambrisentan had little or no effect on hepatic transporters in in vitro studies and displayed a low risk of potential drug-drug interactions, including those with other PAH therapies. Results from the VOLT post-marketing program confirmed the safety profile of ambrisentan observed in the ARIES studies, including the low incidence of liver function test abnormalities. Peripheral edema and anemia were common side effects of endothelin receptor antagonist therapies, including ambrisentan. In the recently completed AMBITION study (ClinicalTrials.gov Identifier: NCT01178073), upfront initial combination therapy with ambrisentan and tadalafil significantly reduced the risk of clinical failure (primary endpoint) by 50% compared with the pooled monotherapy groups.

CONCLUSIONS

The long-term efficacy and safety profile of ambrisentan in patients with PAH is supported by data from a comprehensive clinical trial program and real-life, post-marketing observations.

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.

Pharmacologic treatments for pulmonary hypertension: exploring pharmacogenomics

Pulmonary hypertension (PH) is a disease with multiple etiologies and is categorized into five broad groups. Of these groups, pulmonary arterial hypertension (PAH) is the most studied and, therefore, all of the currently available drug classes (prostacyclin analogs, endothelin receptor antagonists and phosphodiesterase type 5 inhibitors) were developed to treat PAH. Thus, limited treatment data exist for the less-studied non-PAH forms of PH. Pharmacogenomics can be a tool to better understand the pathways involved in PH, as well as to improve personalization of therapy. However, little pharmacogenomic research has been carried out on this disease. New treatments for PH are on the horizon, deriving from both repurposed currently available drugs and novel therapeutics.

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.

In-vitro evaluation of chronic alcohol effects on expression of drug-metabolizing and drug-transporting proteins

Objectives

In alcoholics without alcoholic liver disease, boosted drug elimination has been reported. However, mechanistic explanations for this phenomenon remain uncertain. In particular, data on the potential role of drug transporters are sparse.

Methods

Using a well-established in-vitro model for induction of human drug-metabolizing and drug-transporting proteins, we evaluated the potency of ethanol and the major fermentation side-product isopentanol to alter expression and function of these proteins by quantitative real-time polymerase chain reaction, Western blotting and flow cytometry. P-glycoprotein (Pgp)-inhibiting properties of ethanol and isopentanol were investigated via calcein extrusion assay.

Key findings

Ethanol and isopentanol significantly changed expression levels of drug-metabolizing and drug-transporting proteins that normalized within 2 weeks upon withdrawal. Cytochrome P-450 2C19 and Pgp were most strongly induced. Ethanol-induced Pgp at the messenger RNA (mRNA) (twofold to eightfold) and protein level (twofold), but not at the functional level. Both compounds did not inhibit Pgp.

Conclusions

Ethanol is demonstrated to increase mRNA and protein expression of human drug transporters such as Pgp in vitro. Withdrawal of ethanol exposure causes return to non-induced conditions within weeks. Functional consequences of increased Pgp expression in alcoholics need to be evaluated by clinical trials applying selective Pgp substrates such as digoxin.

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.

Interaction profile of macitentan, a new non-selective endothelin-1 receptor antagonist, in vitro

Macitentan is a new non-selective endothelin-1 receptor antagonist under development for the treatment of pulmonary arterial hypertension. Information on the potential for macitentan to influence the pharmacokinetics of concomitantly administered drugs by inhibition or induction of drug metabolising enzymes or drug transporters is sparse. We therefore studied the potential of macitentan to inhibit and induce critical targets of drug metabolism and drug distribution (transporters) in vitro. Induction was quantified at the mRNA level by real-time RT-PCR in LS180 cells and revealed that macitentan significantly induced mRNA expression of cytochrome P450 3A4 (CYP3A4), P-glycoprotein (P-gp, ABCB1), solute carrier of organic anions 1B1 (SLCO1B1), and uridinediphosphate-glucuronosyltransferase 1A3 (UGT1A9). By means of a reporter gene assay our study establishes macitentan as a potent activator of pregnane X receptor (PXR). Inhibition of drug transporters was evaluated by using transporter over-expressing cell lines and fluorescent specific substrates of the respective transporters and revealed that macitentan is an inhibitor of P-gp, breast cancer resistance protein (BCRP), SLCO1B1, and SLCO1B3. Using commercial kits macitentan was demonstrated to be a moderate inhibitor of CYP3A4 and CYP2C19. In conclusion our data provide a comprehensive analysis of the interaction profile of macitentan with drug metabolising and transporting enzymes in vitro. Although macitentan has a similar or higher potency for induction and inhibition of drug metabolising enzymes and transporters than bosentan, its low plasma concentrations and minimal accumulation in the liver suggest that it will be markedly less prone to drug-drug interactions than bosentan.

Important Drug-Drug Interactions in HIV-Infected Persons on Antiretroviral Therapy: An Update on New Interactions Between HIV and Non-HIV Drugs

Advances in antiretroviral therapy have turned HIV into a chronic, manageable disease. Patients often require treatment for co-morbid conditions as well as HIV, and consequently, pharmacokinetic interactions between antiretrovirals (ARVs) and other drug classes are an increasing concern. Protease inhibitors and non-nucleoside reverse transcriptase inhibitors are involved in the CYP450 or other transporter systems, and may be associated with higher risk of clinically significant drug interactions. One reverse transcriptase inhibitor, abacavir, has demonstrated weak inhibition of CYP3A4, 2D6 and 2C9 in vitro, but is not associated with any clinically significant interactions involving the CYP450 system. The integrase inhibitor raltegravir is not involved in the CYP450 system, and may be a suitable option to use when trying to minimize interactions with other drug classes. This review summarizes recently published data on clinically significant drug interactions between ARVs and other drug classes including antineoplastics, immunosuppressant transplant drugs, directly acting antivirals for hepatitis C, antifungals, antimalarials, corticosteroids, psychotropics, hormonal contraceptives, anticoagulants, drugs for pulmonary hypertension, and herbal products. In situations of suspected or potential interactions, close monitoring is warranted, and dose adjustments or substitutions may be required.

Current understanding of hepatic and intestinal OATP-mediated drug-drug interactions

At present, many patients are medicated with various drugs, which are, at the same time, associated with an increased risk of drug-drug interactions (DDIs). Detailed analysis of mechanisms underlying DDIs is the basis of a better prediction of adverse drug events caused by drug interactions. In the last few decades, an involvement of transporters in such processes has been more and more recognized. Indeed, uptake transporters belonging to the organic anion-transporting polypeptide (OATP) family have been shown to interact with a variety of drugs in clinical use. Particularly, the subfamily of OATP1B transporters has been extensively studied, identifying several clinical significant DDIs based on those hepatic uptake transporters. By contrast, the role of OATP2B1 in this context is rather underestimated. Therefore, in addition to known interactions based on OATP1B transporters, we have focused on DDIs probably based on OATP2B1 inhibition in the liver and those possibly owing to the inhibition of OATP2B1-mediated drug absorption in the intestine.

Association of liver stiffness with hepatic expression of pharmacokinetically important genes in alcoholic liver disease

BACKGROUND

Enhanced drug elimination in alcoholics remains largely indefinable. In contrast, the reduced elimination of drugs in patients with advanced alcoholic liver disease (ALD) is normally owing to hepatic end-stage disease such as cirrhosis. We here study the mRNA expression of various hepatic drug metabolizing enzymes and transporters in association with liver stiffness (LS) being a novel noninvasive parameter for the assessment of cirrhosis to unravel the dynamic relationship between ALD and determinants of pharmacokinetics such as drug metabolizing enzymes and transporters.

METHODS

We quantified mRNA expression levels of various cytochrome P-450 isoenzymes (CYPs) and drug transporters in 26 liver specimens of chronic alcoholics and 5 controls by quantitative polymerase chain reaction. In addition, liver histology, clinical data, and LS evaluated by transient elastography (Fibroscan) were obtained.

RESULTS

Eighteen patients had a normal or moderate LS < 8 kPa (69.2%), while in the remaining 8 patients (30.7%) advanced F3 or F4 fibrosis could be established with an LS > 8 kPa. Overall, CYP3A4, CYP2E1, and solute carrier organic anion transporter 1B1 (SLCO1B1) were negatively correlated with increasing LS. CYPs and drug transporters tended to be up-regulated in alcoholics without advanced fibrosis (LS < 8.0 kPa) compared to healthy controls supporting data of boosted drug elimination in alcoholics without advanced ALD. However, in alcoholics with severely increased LS (>8 kPa), expression levels of CYP2E1, SLC22A2, and SLCO1B1 were significantly lower.

CONCLUSIONS

In conclusion, CYPs and drug transporters seem to be induced in chronic alcoholics without irreversible liver damage but decline in case of manifest cirrhosis. Our study also suggests that noninvasive measurements of LS could be useful for pharmacokinetic predictions and individualized pharmacotherapy.

Rifampicin alters the expression of reference genes used to normalize real-time quantitative RT-PCR data

Suitable reference genes for correct quantification of reverse transcription PCR (qRT-PCR) have to be constantly expressed in all samples under investigation. Thus, it is mandatory to determine expression stability of control genes before normalization. We aimed to establish optimum inducing concentrations for the prototypical enzyme and drug transporter inducer rifampicin in LS180 cells and concurrently assessed reference gene stability under rifampicin treatment. LS180 cells were treated with increasing concentrations of rifampicin (up to 200 μM), and expression of eight different reference genes and some target genes (CYP3A4, ABCB1, and ABCC1) was quantified using real-time qRT-PCR. To check whether the results can be generalized, HepG2 cells were also investigated. We demonstrated that higher concentrations of rifampicin (>50 μM) change the expression of reference genes and thus may complicate and adulterate normalization of qRT-PCR data. The results stress the need for proper validation of potential reference genes in respective cells, tissues, and particular experimental conditions. Programs like geNorm and NormFinder alone do not warrant an adequate choice of the most suitable reference gene. Scrutiny of the reference gene expression and plausibility of the data remain necessary and protect from erroneous quantification and misinterpretation of qRT-PCR data.

Interaction potential of the endothelin-A receptor antagonist atrasentan with drug transporters and drug-metabolising enzymes assessed in vitro

PURPOSE

Atrasentan is a highly potent and selective endothelin receptor A (ET(A)) antagonist under development for the treatment of prostate cancer. Only little data exist on its interaction with drug-metabolising enzymes and drug transporters possibly influencing its safety and effectiveness. Our study evaluated whether atrasentan can induce the expression of relevant human drug transporters and cytochrome P450 isozymes (CYPs), whether it retains its efficiency in multidrug resistant cell lines, and whether it inhibits P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP).

METHODS

Induction of transporters and enzymes was quantified at the mRNA level by real-time RT-PCR in LS180 cells and for P-gp also at the protein level by Western blot. P-gp inhibition was evaluated by calcein assay in P388/dx and L-MDR1 cells and BCRP inhibition in MDCKII-BCRP cells by pheophorbide A efflux. Substrate characteristics were evaluated by growth inhibition assays in MDCKII cells overexpressing particular ABC-transporters.

RESULTS

Atrasentan profoundly induced several CYPs and drug transporters (e.g. 12-fold induction of CYP3A4 at 50 μM). It was a moderate P-gp inhibitor (IC(50) in P388/dx cells = 15.1 ± 1.6 μM) and a weak BCRP inhibitor (IC(50) in MDCKII-BCRP cells = 59.8 ± 11 μM). BCRP or P-gp overexpressing cells were slightly more resistant towards antiproliferative effects of atrasentan.

CONCLUSIONS

Our data provide a comprehensive analysis of the induction profile of atrasentan and its interaction with P-gp and BCRP. The profound induction effects stress the need for thorough assessment of its interaction potential in vivo.