A P-gp vesicular transport inhibition assay – Optimization and validation for drug–drug interaction testing

Investigating how HIV-1 antiretrovirals differentially behave as substrates and inhibitors of P-glycoprotein via molecular dynamics simulations

HIV-1 can rapidly infect the brain upon initial infection, establishing latent reservoirs that induce neuronal damage and/or death, resulting in HIV-Associated Neurocognitive Disorder. Though anti-HIV-1 antiretrovirals (ARVs) suppress viral load, the blood-brain barrier limits drug access to the brain, largely because of highly expressed efflux proteins like P-glycoprotein (P-gp). While no FDA-approved P-gp inhibitor currently exists, HIV-1 protease inhibitors show promise as partial P-gp inhibitors, potentially enhancing drug delivery to the brain. Herein, we employed docking and molecular dynamics simulations to elucidate key differences in P-gp's interactions with several antiretrovirals, including protease inhibitors, with known inhibitory or substrate-like behaviors towards P-gp. Our results led us to hypothesize new mechanistic details of small-molecule efflux by and inhibition of P-gp, where the "Lower Pocket" in P-gp's transmembrane domain serves as the primary initial site for small-molecule binding. Subsequently, this pocket merges with the more traditionally studied drug binding site-the "Upper Pocket"-thus funneling small-molecule drugs, such as ARVs, towards the Upper Pocket for efflux. Furthermore, our results reinforce the understanding that both binding energetics and changes in protein dynamics are crucial in discerning small molecules as non-substrates, substrates, or inhibitors of P-gp. Our findings indicate that interactions between P-gp and inhibitory ARVs induce bridging of transmembrane domain helices, impeding P-gp conformational changes and contributing to the inhibitory behavior of these ARVs. Overall, insights gained in this study could serve to guide the design of future P-gp-targeting therapeutics for a wide range of pathological conditions and diseases, including HIV-1.

Inhibition of efflux transporters by poly ADP-ribose polymerase inhibitors

Poly ADP-ribose polymerase (PARP) inhibitors have been approved for the treatment of various cancers. They share a similar mechanism of action but have differences in pharmacokinetic characteristics and potential for drug-drug interactions (DDI). This study evaluated the potential ATP-binding cassette transporter-mediated interactions between PARP inhibitors (niraparib, olaparib and rucaparib) and statins (atorvastatin and rosuvastatin). We studied the inhibitory activity of PARP inhibitors on breast cancer resistance protein (BCRP), multidrug resistance-associated protein 3 (MRP3) and P-glycoprotein (P-gp) using vesicular transport assays and determined the concentrations required for 50% inhibition (IC50 ). Then, we predicted the increase of statin exposure followed by the administration of PARP inhibitors using a mechanistic static model. Rucaparib was the strongest inhibitor of BCRP-mediated rosuvastatin transport (IC50 13.7 μM), followed by niraparib (42.6 μM) and olaparib (216 μM). PARP inhibitors did not affect MRP3. While niraparib appeared to inhibit P-gp, the inhibition showed large variability. The inhibition of intestinal BCRP by rucaparib, niraparib and olaparib was predicted to elevate rosuvastatin exposure by 52%, 37% and 24%, respectively. The interactions between PARP inhibitors and rosuvastatin are probably of minor clinical significance alone, but combined with other predisposing factors, they may increase the risk of rosuvastatin-associated adverse effects.

Oral etoposide and zosuquidar bioavailability in rats: Effect of co-administration and in vitro-in vivo correlation of P-glycoprotein inhibition

P-glycoprotein inhibitors, like zosuquidar, have widely been used to study the role of P-glycoprotein in oral absorption. Still, systematic studies on the inhibitor dose-response relationship on intestinal drug permeation are lacking. In the present study, we investigated the effect of 0.79 nM-2.5 μM zosuquidar on etoposide permeability across Caco-2 cell monolayers. We also investigated etoposide pharmacokinetics after oral or IV administration to Sprague Dawley rats with co-administration of 0.063–63 mg/kg zosuquidar, as well as the pharmacokinetics of zosuquidar itself. Oral zosuquidar bioavailability was 2.6–4.2%, while oral etoposide bioavailability was 5.5 ± 0.9%, which increased with increasing zosuquidar doses to 35 ± 5%. The intestinal zosuquidar concentration required to induce a half-maximal increase in bioavailability was estimated to 180 μM. In contrast, the IC₅₀ of zosuquidar on etoposide permeability in vitro was only 5–10 nM, and a substantial in vitro-in vivo discrepancy of at least four orders of magnitude was thereby identified. Overall, the present study provides valuable insights for future formulation development that applies fixed dose combinations of P-glycoprotein inhibitors to increase the absorption of poorly permeable P-glycoprotein substrate drugs.

Evaluation of Ipatasertib Interactions with Itraconazole and Coproporphyrin I and III in a Single Drug Interaction Study in Healthy Subjects

Ipatasertib is a pan-AKT inhibitor in development for the treatment of cancer. Ipatasertib was metabolized by CYP3A4 to its major metabolite, M1 (G-037720), and was a P-gp substrate and OATP1B1/1B3 inhibitor in vitro. A phase I drug-drug interaction (DDI) study (n = 15) was conducted in healthy subjects to evaluate the effect of itraconazole (200-mg solution QD, 4 days), a strong CYP3A4 and P-gp inhibitor, on pharmacokinetics of ipatasertib (100-mg single dose). Itraconazole increased the Cmax and AUC0 -∞ of ipatasertib by 2.3- and 5.5-fold, respectively, increased the half-life by 53%, and delayed the tmax by 1 hour. The Cmax and AUC0-72h of its metabolite M1 (G-037720) reduced by 91% and 68%, respectively. This study confirmed that CYP3A4 plays a major role in ipatasertib clearance. Furthermore, the interaction of ipatasertib with coproporphyrin (CP) I and CPIII, the two endogenous substrates of OATP1B1/1B3, was evaluated in this study. CPI and CPIII plasma levels were unchanged in the presence of ipatasertib, both at exposures of 100 mg and at higher exposures in combination with itraconazole. This indicated no in vivo inhibition of OATP1B1/1B3 by ipatasertib. Additionally, it was shown that CPI and CPIII were not P-gp substrates in vitro, and itraconazole had no effect on CPI and CPIII concentrations in vivo. The latter is an important finding because it will simplify interpretation of future DDI studies using CPI/CPIII as OATP1B1/1B3 biomarkers. SIGNIFICANCE STATEMENT: This drug-drug interaction study in healthy volunteers demonstrated that CYP3A4 plays a major role in ipatasertib clearance, and that ipatasertib is not an organic anion transporting polypeptide 1B1/1B3 inhibitor. Furthermore, it was demonstrated that itraconazole, an inhibitor of CYP3A4 and several transporters, did not affect CPI/CPIII levels in vivo. This increases the understanding and application of these endogenous substrates as well as itraconazole in complex drug interaction studies.

Transporter-mediated drug-drug interactions: advancement in models, analytical tools, and regulatory perspective

Drug-drug interactions mediated by transporters are a serious clinical concern hence a tremendous amount of work has been done on the characterization of the transporter-mediated proteins in humans and animals. The underlying mechanism for the transporter-mediated drug-drug interaction is the induction or inhibition of the transporter which is involved in the cellular uptake and efflux of drugs. Transporter of the brain, liver, kidney, and intestine are major determinants that alter the absorption, distribution, metabolism, excretion profile of drugs, and considerably influence the pharmacokinetic profile of drugs. As a consequence, transporter proteins may affect the therapeutic activity and safety of drugs. However, mounting evidence suggests that many drugs change the activity and/or expression of the transporter protein. Accordingly, evaluation of drug interaction during the drug development process is an integral part of risk assessment and regulatory requirements. Therefore, this review will highlight the clinical significance of the transporter, their role in disease, possible cause underlying the drug-drug interactions using analytical tools, and update on the regulatory requirement. The recent in-silico approaches which emphasize the advancement in the discovery of drug-drug interactions are also highlighted in this review. Besides, we discuss several endogenous biomarkers that have shown to act as substrates for many transporters, which could be potent determinants to find the drug-drug interactions mediated by transporters. Transporter-mediated drug-drug interactions are taken into consideration in the drug approval process therefore we also provided the extrapolated decision trees from in-vitro to in-vivo, which may trigger the follow-up to clinical studies.

Interactions of Potential Anti-COVID-19 Compounds with Multispecific ABC and OATP Drug Transporters

During the COVID-19 pandemic, several repurposed drugs have been proposed to alleviate the major health effects of the disease. These drugs are often applied with analgesics or non-steroid anti-inflammatory compounds, and co-morbid patients may also be treated with anticancer, cholesterol-lowering, or antidiabetic agents. Since drug ADME-tox properties may be significantly affected by multispecific transporters, in this study, we examined the interactions of the repurposed drugs with the key human multidrug transporters present in the major tissue barriers and strongly affecting the pharmacokinetics. Our in vitro studies, using a variety of model systems, explored the interactions of the antimalarial agents chloroquine and hydroxychloroquine; the antihelmintic ivermectin; and the proposed antiviral compounds ritonavir, lopinavir, favipiravir, and remdesivir with the ABCB1/Pgp, ABCG2/BCRP, and ABCC1/MRP1 exporters, as well as the organic anion-transporting polypeptide (OATP)2B1 and OATP1A2 uptake transporters. The results presented here show numerous pharmacologically relevant transporter interactions and may provide a warning on the potential toxicities of these repurposed drugs, especially in drug combinations at the clinic.

Disruption of small molecule transporter systems by Transporter-Interfering Chemicals (TICs)

Small molecule transporters (SMTs) in the ABC and SLC families are important players in disposition of diverse endo- and xenobiotics. Interactions of environmental chemicals with these transporters were first postulated in the 1990s, and since validated in numerous in vitro and in vivo scenarios. Recent results on the co-crystal structure of ABCB1 with the flame-retardant BDE-100 demonstrate that a diverse range of man-made and natural toxic molecules, hereafter termed transporter-interfering chemicals (TICs), can directly bind to SMTs and interfere with their function. TIC-binding modes mimic those of substrates, inhibitors, modulators, inducers, and possibly stimulants through direct and allosteric mechanisms. Similarly, the effects could directly or indirectly agonize, antagonize or perhaps even prime the SMT system to alter transport function. Importantly, TICs are distinguished from drugs and pharmaceuticals that interact with transporters in that exposure is unintended and inherently variant. Here, we review the molecular mechanisms of environmental chemical interaction with SMTs, the methodological considerations for their evaluation, and the future directions for TIC discovery.

P-glycoprotein deficiency enhances metabolic activation of and platelet response to clopidogrel through marked up-regulation of Cyp3a11 in mice: Direct evidence for the interplay between P-glycoprotein and Cyp3a

Variability in P-glycoprotein (P-gp) efflux transporting activity was supposed to be involved in altered intestinal absorption and bioavailability of clopidogrel in patients; however, reliable evidence is still lacking. In this study, we sought to determine whether P-gp could play an important role in the metabolic activation of and platelet response to clopidogrel in mice. Abcb1a/1b knock-out (KO) and wild-type (WT) mice were used to evaluate differences in the intracellular accumulation of clopidogrel in the intestine, liver, and brain tissues and in systemic exposure of clopidogrel and its main metabolites as well as the mechanisms involved. Results indicated that, compared with WT mice, KO mice exhibited an 84% increase in systemic exposure of clopidogrel active thiol metabolite H4 and a 14.5% rise of suppression of ADP-induced platelet integrin αIIbβ3 activation, paralleled by a 41% decrease in systemic exposure of clopidogrel due to enhanced systemic clearance. Furthermore, KO mice displayed a 45% increase in Cyp3a11 but a 23% decrease in Ces1 at their protein levels compared with WT mice. Concurrently, intracellular clopidogrel concentrations in the tissues examined did not differ significantly between KO and WT mice. We conclude that although P-gp does not transport clopidogrel and its major metabolites in mice, P-gp-deficient mice exhibit elevated formation of the active metabolite H4 and enhanced antiplatelet effect of clopidogrel through up-regulation of Cyp3a11 and down-regulation of Ces1, suggesting that P-gp activity may correlate inversely with the formation of H4 and antiplatelet efficacy of clopidogrel in clinical settings due to P-gp and CYP3A4 interplay.

Inhibition of ABCG2/BCRP-mediated transport-correlation analysis of various expression systems and probe substrates

BCRP / ABCG2 is a key determinant of pharmacokinetics of substrate drugs. Several BCRP substrates and inhibitors are of low passive permeability, and the vesicular transport assay works well in this permeability space. Membranes were prepared from BCRP-HEK293, MCF-7/MX, and baculovirus-infected Sf9 cells with (BCRP-Sf9-HAM), and without (BCRP-Sf9) cholesterol loading. K_m values for three substrates - estrone-3-sulfate, sulfasalazine, topotecan - correlated well between the four expression systems. In contrast, a 10-20-fold range in V_max values was observed, with BCRP-HEK293 membranes possessing the largest dynamic range. IC₅₀ values of the different test systems were similar to each other, with 94.4% of pairwise comparisons being within 3-fold. Substrate dependent inhibition showed somewhat greater variation, as 81.4% of IC₅₀ values in the BCRP-HEK293 membranes were within 3-fold in pairwise comparisons. Overall, BCRP-HEK293 membranes demonstrated the highest activity. The IC₅₀ values showed good concordance but substrate dependent inhibition was observed for some drugs.

Human efflux transport of testosterone, epitestosterone and other androgen glucuronides

Several drug-metabolizing enzymes are known to control androgen homeostasis in humans. UDP-glucuronosyltransferases convert androgens to glucuronide conjugates in the liver and intestine, which enables subsequent elimination of these conjugated androgens via urine. The most important androgen is testosterone, while others are the testosterone metabolites androsterone and etiocholanolone, and the testosterone precursor dehydroepiandrosterone. Epitestosterone is another endogenous androgen, which is included as a crucial marker in urine doping tests. Since glucuronide conjugates are hydrophilic, efflux transporters mediate their excretion from tissues. In this study, we employed the membrane vesicle assay to identify the efflux transporters for glucuronides of androsterone, dehydroepiandrosterone, epitestosterone, etiocholanolone and testosterone. The human hepatic and intestinal transporters MRP2 (ABCC2), MRP3 (ABCC3), MRP4 (ABCC4), BCRP (ABCG2) and MDR1 (ABCB1) were studied in vitro. Of these transporters, only MRP2 and MRP3 transported the androgen glucuronides investigated. In kinetic analyses, MRP3 transported glucuronides of androsterone, epitestosterone and etiocholanolone at low K_m values, between 0.4 and 4 μM, while the K_m values for glucuronides of testosterone and dehydroepiandrosterone were 14 and 51 μM, respectively. MRP2 transported the glucuronides at lower affinity, as indicated by K_m values over 100 μM. Interestingly, the MRP2-mediated transport of androsterone and epitestosterone glucuronides was best described by sigmoidal kinetics. The inability of BCRP to transport any of the androgen glucuronides investigated is drastically different from its highly active transport of several estrogen conjugates. Our results explain the transporter-mediated disposition of androgen glucuronides in humans, and shed light on differences between the human efflux transporters MRP2, MRP3, MRP4, BCRP and MDR1.

Endogenous, cholesterol-activated ATP-dependent transport in membrane vesicles from Spodoptera frugiperda cells

Transport proteins of the ATP-binding cassette (ABC) family are found in all kingdoms of life. In humans, several ABC efflux transporters play a role in drug disposition and excretion. Therefore, in vitro methods have been developed to characterize the substrate and inhibitor properties of drugs with respect to these transporters. In the vesicular transport assay, transport is studied using inverted membrane vesicles produced from transporter overexpressing cell lines of both mammalian and insect origin. Insect cell expression systems benefit from a higher expression compared to background, but are not as well characterized as their mammalian counterparts regarding endogenous transport. Therefore, the contribution of this transport in the assay might be underappreciated. In this study, endogenous transport in membrane vesicles from Spodoptera frugiperda -derived Sf9 cells was characterized using four typical substrates of human ABC transporters: 5(6)-carboxy-2,'7'-dichlorofluorescein (CDCF), estradiol-17β-glucuronide, estrone sulfate and N-methyl-quinidine. Significant ATP-dependent transport was observed for three of the substrates with cholesterol-loading of the vesicles, which is sometimes used to improve the activity of human transporters expressed in Sf9 cells. The highest effect of cholesterol was on CDCF transport, and this transport in the cholesterol-loaded Sf9 vesicles was time and concentration dependent with a K_m of 8.06 ± 1.11 μM. The observed CDCF transport was inhibited by known inhibitors of human ABCC transporters, but not by ABCB1 and ABCG2 inhibitors verapamil and Ko143, respectively. Two candidate genes for ABCC-type transporters in the S. frugiperda genome (SfABCC2 and SfABCC3) were identified based on sequence analysis as a hypothesis to explain the observed endogenous ABCC-type transport in Sf9 vesicles. Although further studies are needed to verify the role of SfABCC2 and SfABCC3 in Sf9 vesicles, the findings of this study highlight the need to carefully characterize background transport in Sf9 derived membrane vesicles to avoid false positive substrate findings for human ABC transporters studied with this overexpression system.

ABCG2/BCRP: variants, transporter interaction profile of substrates and inhibitors

INTRODUCTION

ABCG2 has a broad substrate specificity and is one of the most important efflux proteins modulating pharmacokinetics of drugs, nutrients and toxicokinetics of toxicants. ABCG2 is an important player in transporter-mediated drug-drug interactions (tDDI). Areas covered: The aims of the review are i) to cover transporter interaction profile of substrates and inhibitors that can be utilized to test interaction of drug candidates with ABCG2, ii) to highlight main characteristics of in vitro testing and iii) to describe the structural basis of the broad substrate specificity of the protein. Preclinical data utilizing Abcg2/Bcrp1 knockouts and clinical studies showing effect of ABCG2 c.421C>A polymorphism on pharmacokinetics of drugs have provided evidence for a broad array of drug substrates and support drug - ABCG2 interaction testing. A consensus on using rosuvastatin and sulfasalazine as intestinal substrates for clinical studies is in the formation. Other substrates relevant to the therapeutic area can be considered. Monolayer efflux assays and vesicular transport assays have been extensively utilized in vitro. Expert opinion: Clinical substrates display complex pharmacokinetics due to broad interaction profiles with multiple transporters and metabolic enzymes. Substrate-dependent inhibition has been observed for several inhibitors. Harmonization of in vitro and in vivo testing makes sense. However, rosuvastatin and sulfasalazine are not efficiently transported in either MDCKII or LLC-PK1-based monolayers. Caco-2 monolayer assays and vesicular transport assays are potential alternatives.

Efflux transport of estrogen glucuronides by human MRP2, MRP3, MRP4 and BCRP

Estrone, estradiol and estriol are endogenous human estrogens that are rapidly conjugated with glucuronic acid in both intestinal and hepatic epithelial cells. The resulting glucuronides, estrone-3-glucuronide (E₁-G), estradiol-3- and 17-glucuronides (E₂-3G and E₂-17G), as well as estriol-3- and 16-glucuronides (E₃-3G and E₃-16G) are found in human plasma and urine. Unlike E₂-17G, the efflux transport of other estrogen glucuronides by human transporters has not yet been investigated comprehensively. We have studied the transport of E₁-G, E₂-3G, E₃-3G, E₃-16G and estrone-3-sulfate (E₁-S), another important estrogen conjugate, using the vesicular transport assay with recombinant human MRP2, MRP3, MRP4, MDR1 and BCRP that were expressed in insect cells. The transport screening assays revealed that whereas E₁-S was a good and specific substrate for BCRP, the less transporter-specific conjugates, E₁-G and E₂-3G, were still transported by BCRP at 10-fold higher rates than E₁-S. BCRP also transported E₃-16G at higher rates than the studied MRPs, while it transported E₃-3G at lower rates than MRP3. MRP2 exhibited lower or equal transport rates of E₁-G, E₂-3G, E₃-3G and E₃-16G in comparison to MRP3 and BCRP in the screening assays, mainly due to its high K_m values, between 180 and 790 μM. MRP3 transported all the tested glucuronides at rather similar rates, at K_m values below 20 μM, but lower V_max values than other transporters. In the case of E₃-3G, MRP3 was the most active transporter in the screening assay. MRP4 transported only E₃-16G at considerable rates, while none of the tested estrogen conjugates was transported by MDR1 at higher rates than control vesicles. These new results, in combination with previously reported in vivo human data, stimulate our understanding on the substrate specificity and role of efflux transporters in disposition of estrogen glucuronides in humans.

Substrate and method dependent inhibition of three ABC-transporters (MDR1, BCRP, and MRP2)

Drug transport and drug-drug interactions (DDI) with human ABC transporters are generally investigated in mammalian cell lines or inverted membrane vesicles from insect cells (Sf9) overexpressing the transporter of interest. In this study, we instead used membrane vesicles from human embryonic kidney cells (HEK293) overexpressing wild type MDR1/Pgp (ABCB1), BCRP (ABCG2), and MRP2 (ABCC2) with the aim to study the concentration dependent inhibition of shared and prototypic probe substrates. We first investigated 15 substrates and identified estrone-17-beta-glucorinide (E17G) as shared substrate. Nine specific and general inhibitors were then studied using E17G and prototypic probe substrates. The results were compared with those previously obtained in Sf9 vesicles and cell lines of canine (MDCKII) and human (Saos-2) origin. For the majority of inhibitors, K_i values differed <10-fold between E17G and probe substrates. Significant differences in K_i values were observed for about one third of the inhibitors. The transport inhibition potencies in HEK293 vesicles were in good agreement with those obtained in Sf9 vesicles. Large differences were found in the inhibition potencies observed in the vesicular systems compared to the cellular systems. Nevertheless, the rank order correlations between the different experimental systems were generally good. Our study provides further information on substrate dependent inhibition of ABC-transporters, and suggests that simple ranking of compounds can be used as a tier one approach to bridge results obtained in different experimental systems.

Management of Venetoclax-Posaconazole Interaction in Acute Myeloid Leukemia Patients: Evaluation of Dose Adjustments

PURPOSE

The effect of posaconazole, a strong cytochrome P450 3A (CYP3A) inhibitor and commonly used antifungal agent, on the pharmacokinetic properties of venetoclax, a CYP3A substrate, was evaluated in patients with acute myeloid leukemia to determine the dose adjustments needed to manage this potential interaction.

METHODS

Twelve patients received 20- to 200-mg ramp-up treatment with oral venetoclax and 20 mg/m2 of intravenous decitabine on days 1 through 5, followed by 400 mg of venetoclax alone on days 6 through 20. On days 21 through 28, patients received 300 mg of posaconazole plus reduced doses of venetoclax (50 or 100 mg) to account for expected increases in venetoclax plasma concentrations. Blood samples were collected before dosing and up to 24 hours after the venetoclax dose on days 20 and 28.

FINDINGS

Compared with a venetoclax dose of 400 mg when administered alone (day 20), coadministration of venetoclax at a 50-mg dose with multiple doses of posaconazole increased mean venetoclax Cmax and AUC0-24 by 53% and 76%, respectively, whereas coadministration of venetoclax at a 100-mg dose with posaconazole increased mean venetoclax Cmax and AUC0-24 by 93% and 155%, respectively. When adjusted for different doses and nonlinearity, posaconazole was estimated to increase venetoclax Cmax and AUC0-24 by 7.1- and 8.8-fold, respectively. Both the 50- and 100-mg venetoclax doses administered with posaconazole were well tolerated.

IMPLICATIONS

The results are consistent with inhibition of CYP3A-mediated metabolism of venetoclax. Posaconazole can be used for antifungal prophylaxis in patients with acute myeloid leukemia receiving venetoclax after reducing the venetoclax dose by at least 75%. ClinicalTrials.gov identifier: NCT02203773.

Inhibition of the Human ABC Efflux Transporters P-gp and BCRP by the BDE-47 Hydroxylated Metabolite 6-OH-BDE-47: Considerations for Human Exposure

High body burdens of polybrominated diphenyl ethers (PBDEs) in infants and young children have led to increased concern over their potential impact on human development. PBDE exposure can alter the expression of genes involved in thyroid homeostasis, including those of ATP-binding cassette (ABC) transporters, which mediate cellular xenobiotic efflux. However, little information exists on how PBDEs interact with ABC transporters such as P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP). The purpose of this study was to evaluate the interactions of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) and its hydroxylated metabolite 6-OH-BDE-47 with P-gp and BCRP, using human MDR1- and BCRP-expressing membrane vesicles and stably transfected NIH-3T3-MDR1 and MDCK-BCRP cells. In P-gp membranes, BDE-47 did not affect P-gp activity; however, 6-OH-BDE-47 inhibited P-gp activity at low µM concentrations (IC₅₀=11.7 µM). In BCRP membranes, BDE-47 inhibited BCRP activity; however, 6-OH-BDE-47 was a stronger inhibitor [IC₅₀=45.9 µM (BDE-47) vs. IC₅₀=9.4 µM (6-OH-BDE-47)]. Intracellular concentrations of known P-gp and BCRP substrates [(³H)-paclitaxel and (³H)-prazosin, respectively] were significantly higher (indicating less efflux) in NIH-3T3-MDR1 and MDCK-BCRP cells in the presence of 6-OH-BDE-47, but not BDE-47. Collectively, our results indicate that the BDE-47 metabolite 6-OH-BDE-47 is an inhibitor of both P-gp and BCRP efflux activity. These findings suggest that some effects previously attributed to BDE-47 in biological systems may actually be due to 6-OH-BDE-47. Considerations for human exposure are discussed.

Establishment and characterization of an MDCK cell line stably-transfected with chicken Abcb1 encoding P-glycoprotein

Chicken P-glycoprotein (chP-gp), encoded by Abcb1, determines the bioavailability because of its effect on pharmacokinetics of various drugs. However, comprehensive studies on chP-gp are still limited. In this study, the chicken full-length cDNA was first successfully cloned and then stably expressed in MDCK cell line. The open reading frame of chicken Abcb1 consists of 3864 nucleotides, encoding for a 1287-amino acid protein. Sequence alignments analysis showed that chicken P-gp had high identities with the homologues of turkey (95%), human (72%), pig (72%), rat (71%) and cattle (68%). The efflux ratio of rhodamine123 (Rho123, a human P-gp substrate) in chAbcb1 transfected MDCK cells was significantly higher than that in the wild type MDCK cell (6.24 vs 1.64, P<0.05), suggesting a good transporting function of chicken P-gp overexpressed in the transfected cell. Importantly, MDCK-chAbcb1 cells, unlike Caco-2 cells, exhibited biphasic saturation kinetics in transporting Rho123. In conclusion, an MDCK cell line stably expressing chAbcb1 was successfully established, which could provide a new cell model to screen its substrates and inhibitors and study the drug-drug interaction medicated via chicken P-gp.

Evaluation of Ketoconazole and Its Alternative Clinical CYP3A4/5 Inhibitors as Inhibitors of Drug Transporters: The In Vitro Effects of Ketoconazole, Ritonavir, Clarithromycin, and Itraconazole on 13 Clinically-Relevant Drug Transporters

Ketoconazole is a potent CYP3A4/5 inhibitor and, until recently, recommended by the Food and Drug Administration (FDA) and the European Medicines Agency as a strong CYP3A4/5 inhibitor in clinical drug-drug interaction (DDI) studies. Ketoconazole sporadically causes liver injury or adrenal insufficiency. Because of this, the FDA and European Medicines Agency recommended suspension of ketoconazole use in DDI studies in 2013. The FDA specifically recommended use of clarithromycin or itraconazole as alternative strong CYP3A4/5 inhibitors in clinical DDI studies, but many investigators have also used ritonavir as an alternative. Although the effects of these clinical CYP3A4/5 inhibitors on other CYPs are largely established, reports on the effects on the broad range of drug transporter activities are sparse. In this study, the inhibitory effects of ketoconazole, clarithromycin, ritonavir, and itraconazole (and its CYP3A4-inhibitory metabolites, hydroxy-, keto-, and N-desalkyl itraconazole) toward 13 drug transporters (OATP1B1, OATP1B3, OAT1, OAT3, OCT1, OCT2, MATE1, MATE2-K, P-gp, BCRP, MRP2, MRP3, and BSEP) were systematically assessed in transporter-expressing HEK-293 cell lines or membrane vesicles. In vitro findings were translated into clinical context with the basic static model approaches outlined by the FDA in its 2012 draft guidance on DDIs. The results indicate that, like ketoconazole, the alternative clinical CYP3A4/5 inhibitors ritonavir, clarithromycin, and itraconazole each have unique transporter inhibition profiles. None of the alternatives to ketoconazole provided a clean inhibition profile toward the 13 drug transporters evaluated. The results provide guidance for the selection of clinical CYP3A4/5 inhibitors when transporters are potentially involved in a victim drug's pharmacokinetics.

Classification of P-glycoprotein-interacting compounds using machine learning methods

P-glycoprotein (Pgp) is a drug transporter that plays important roles in multidrug resistance and drug pharmacokinetics. The inhibition of Pgp has become a notable strategy for combating multidrug-resistant cancers and improving therapeutic outcomes. However, the polyspecific nature of Pgp, together with inconsistent results in experimental assays, renders the determination of endpoints for Pgp-interacting compounds a great challenge. In this study, the classification of a large set of 2,477 Pgp-interacting compounds (i.e., 1341 inhibitors, 913 non-inhibitors, 197 substrates and 26 non-substrates) was performed using several machine learning methods (i.e., decision tree induction, artificial neural network modelling and support vector machine) as a function of their physicochemical properties. The models provided good predictive performance, producing MCC values in the range of 0.739-1 for internal cross-validation and 0.665-1 for external validation. The study provided simple and interpretable models for important properties that influence the activity of Pgp-interacting compounds, which are potentially beneficial for screening and rational design of Pgp inhibitors that are of clinical importance.