Intracellular concentrations determine the cytotoxicity of adefovir, cidofovir and tenofovir

7-Hydroxycoumarin and its conjugated metabolites interact with organic anion transporters 1 and 3 in vitro and in vivo

7-Hydroxycoumarin (7-HC) is a natural coumarin compound rich in Chinese herbal medicines and has various pharmacological activities. After oral administration of 7-HC in rodents, its conjugated metabolites 7-hydroxycoumarin-β-D-glucuronide (7-HCG) and 7-hydroxycoumarin sulfate (7-HCS), exhibit high systemic exposure and urinary excretion. Organic anion transporters 1 and 3 (OAT1 and OAT3), mainly expressed in the proximal renal tubules, play an important role in drug-drug interactions and drug-induced kidney injury. We aimed to explore the mechanisms of OAT-mediated drug interactions and renal protective mechanisms of 7-HC and its conjugates. OAT-overexpressing cell models revealed that 7-HC was not a substrate for OAT1 and OAT3, while 7-HCG was specifically transported by OAT3. In contrast, 7-HCS can be transported by both OATs. Besides, 7-HC significantly inhibited the activity of OAT1 and OAT3, while 7-HCS had a strong inhibitory effect on OAT1 (IC50 < 10 μM). After co-administration of 100 mg/kg of 7-HC to mice, systemic exposure and clearance of furosemide (a clinical substrate of OATs) were significantly increased and decreased, respectively. In addition, 7-HC decreased OAT-mediated cytotoxicity and reduced the renal distribution of adefovir in mice. Together, these findings will provide support for OAT-mediated drug interactions and the renal protection of 7-HC.

Characterization of Organic Anion and Cation Transport in Three Human Renal Proximal Tubular Epithelial Models

The polarised expression of specific transporters in proximal tubular epithelial cells is important for the renal clearance of many endogenous and exogenous compounds. Thus, ideally, the in vitro tools utilised for predictions would have a similar expression of apical and basolateral xenobiotic transporters as in vivo. Here, we assessed the functionality of organic cation and anion transporters in proximal tubular-like cells (PTL) differentiated from human induced pluripotent stem cells (iPSC), primary human proximal tubular epithelial cells (PTEC), and telomerase-immortalised human renal proximal tubular epithelial cells (RPTEC/TERT1). Organic cation and anion transport were studied using the fluorescent substrates 4-(4-(dimethylamino)styryl)-N-methylpyridinium iodide (ASP) and 6-carboxyfluorescein (6-CF), respectively. The level and rate of intracellular ASP accumulation in PTL following basolateral application were slightly lower but within a 3-fold range compared to primary PTEC and RPTEC/TERT1 cells. The basolateral uptake of ASP and its subsequent apical efflux could be inhibited by basolateral exposure to quinidine in all models. Of the three models, only PTL showed a modest preferential basolateral-to-apical 6-CF transfer. These results show that organic cation transport could be demonstrated in all three models, but more research is needed to improve and optimise organic anion transporter expression and functionality.

CNS Viral Infections-What to Consider for Improving Drug Treatment: A Plea for Using Mathematical Modeling Approaches

Neurotropic viruses may cause meningitis, myelitis, encephalitis, or meningoencephalitis. These inflammatory conditions of the central nervous system (CNS) may have serious and devastating consequences if not treated adequately. In this review, we first summarize how neurotropic viruses can enter the CNS by (1) crossing the blood-brain barrier or blood-cerebrospinal fluid barrier; (2) invading the nose via the olfactory route; or (3) invading the peripheral nervous system. Neurotropic viruses may then enter the intracellular space of brain cells via endocytosis and/or membrane fusion. Antiviral drugs are currently used for different viral CNS infections, even though their use and dosing regimens within the CNS, with the exception of acyclovir, are minimally supported by clinical evidence. We therefore provide considerations to optimize drug treatment(s) for these neurotropic viruses. Antiviral drugs should cross the blood-brain barrier/blood cerebrospinal fluid barrier and pass the brain cellular membrane to inhibit these viruses inside the brain cells. Some antiviral drugs may also require intracellular conversion into their active metabolite(s). This illustrates the need to better understand these mechanisms because these processes dictate drug exposure within the CNS that ultimately determine the success of antiviral drugs for CNS infections. Finally, we discuss mathematical model-based approaches for optimizing antiviral treatments. Thereby emphasizing the potential of CNS physiologically based pharmacokinetic models because direct measurement of brain intracellular exposure in living humans faces ethical restrictions. Existing physiologically based pharmacokinetic models combined with in vitro pharmacokinetic/pharmacodynamic information can be used to predict drug exposure and evaluate efficacy of antiviral drugs within the CNS, to ultimately optimize the treatments of CNS viral infections.

The effect of ARVs on the MEKKK1 gene promoter, inflammatory cytokine expression and signalling in acute treated Jurkat T cells

ARVs alter the methylation status of the MEKKK1 gene promoter in acute treated Jurkat T cells with inflammatory outcomesInflammation is reduced in patients under going antiretroviral therapy; however the mechanism is not well understood. We investigated DNA methylation of the mitogen-activated protein kinase kinase kinase kinase 1 (MEKKK1) gene promoter in Jurkat T cells to determine whether the antiretroviral drugs, lamivudine, tenofovir disoproxil fumarate, dolutegravir, TLD (a combination of TDF, 3TC and DTG) and efavirenz modify the methylation status of the MEKKK1 gene - a known stimulus of inflammation.Acute antiretroviral treatments (24 h) were not cytotoxic to Jurkat T cells. MEKKK1 promoter hypomethylation occurred in cells treated with 5-aza-2'-deoxycytidine (Aza), TDF and 3TC, and MEKKK1 promoter hypermethylation occurred in cells treated with DTG; however, promoter DNA methylation of the MEKKK1 gene did not influence MEKKK1 gene expression; therefore, these drugs did not epigenetically regulate MEKKK1 and downstream signalling by promoter DNA methylation. Acute TLD and EFV treatments induced inflammation in Jurkat T cells by increasing MEKKK1, MAPK/ERK and NFκB expression, and activating tumour necrosis factor-α (TNF-α) expression. ARVs decreased IL-10 gene expression, showing no anti-inflammatory activity.The data shows that the inflammation caused by ARVs is not related to the methylation status of MEKKK1 gene promoter and suggests an alternative stimulus via post-transcriptional/post-translational modifications may activate the canonical MEKKK1/NFκB pathway that leads to inflammation. Finally, an increase in NFκB activity and pro-inflammatory cytokine activation seemed to occur via the MAPK/ERK pathway following ARV treatments in Jurkat T cells.

Mapping Adverse Outcome Pathways for Kidney Injury as a Basis for the Development of Mechanism-Based Animal-Sparing Approaches to Assessment of Nephrotoxicity

In line with recent OECD activities on the use of AOPs in developing Integrated Approaches to Testing and Assessment (IATAs), it is expected that systematic mapping of AOPs leading to systemic toxicity may provide a mechanistic framework for the development and implementation of mechanism-based in vitro endpoints. These may form part of an integrated testing strategy to reduce the need for repeated dose toxicity studies. Focusing on kidney and in particular the proximal tubule epithelium as a key target site of chemical-induced injury, the overall aim of this work is to contribute to building a network of AOPs leading to nephrotoxicity. Current mechanistic understanding of kidney injury initiated by 1) inhibition of mitochondrial DNA polymerase γ (mtDNA Polγ), 2) receptor mediated endocytosis and lysosomal overload, and 3) covalent protein binding, which all present fairly well established, common mechanisms by which certain chemicals or drugs may cause nephrotoxicity, is presented and systematically captured in a formal description of AOPs in line with the OECD AOP development programme and in accordance with the harmonized terminology provided by the Collaborative Adverse Outcome Pathway Wiki. The relative level of confidence in the established AOPs is assessed based on evolved Bradford-Hill weight of evidence considerations of biological plausibility, essentiality and empirical support (temporal and dose-response concordance).

Application of the Adverse Outcome Pathway Concept to In Vitro Nephrotoxicity Assessment: Kidney Injury due to Receptor-Mediated Endocytosis and Lysosomal Overload as a Case Study

Application of adverse outcome pathways (AOP) and integration of quantitative in vitro to in vivo extrapolation (QIVIVE) may support the paradigm shift in toxicity testing to move from apical endpoints in test animals to more mechanism-based in vitro assays. Here, we developed an AOP of proximal tubule injury linking a molecular initiating event (MIE) to a cascade of key events (KEs) leading to lysosomal overload and ultimately to cell death. This AOP was used as a case study to adopt the AOP concept for systemic toxicity testing and risk assessment based on in vitro data. In this AOP, nephrotoxicity is thought to result from receptor-mediated endocytosis (MIE) of the chemical stressor, disturbance of lysosomal function (KE1), and lysosomal disruption (KE2) associated with release of reactive oxygen species and cytotoxic lysosomal enzymes that induce cell death (KE3). Based on this mechanistic framework, in vitro readouts reflecting each KE were identified. Utilizing polymyxin antibiotics as chemical stressors for this AOP, the dose-response for each in vitro endpoint was recorded in proximal tubule cells from rat (NRK-52E) and human (RPTEC/TERT1) in order to (1) experimentally support the sequence of key events (KEs), to (2) establish quantitative relationships between KEs as a basis for prediction of downstream KEs based on in vitro data reflecting early KEs and to (3) derive suitable in vitro points of departure for human risk assessment. Time-resolved analysis was used to support the temporal sequence of events within this AOP. Quantitative response-response relationships between KEs established from in vitro data on polymyxin B were successfully used to predict in vitro toxicity of other polymyxin derivatives. Finally, a physiologically based kinetic (PBK) model was utilized to transform in vitro effect concentrations to a human equivalent dose for polymyxin B. The predicted in vivo effective doses were in the range of therapeutic doses known to be associated with a risk for nephrotoxicity. Taken together, these data provide proof-of-concept for the feasibility of in vitro based risk assessment through integration of mechanistic endpoints and reverse toxicokinetic modelling.

Quantitative Investigation of Irinotecan Metabolism, Transport, and Gut Microbiome Activation

The anticancer drug irinotecan shows serious dose-limiting gastrointestinal toxicity regardless of intravenous dosing. Although enzymes and transporters involved in irinotecan disposition are known, quantitative contributions of these mechanisms in complex in vivo disposition of irinotecan are poorly understood. We explained intestinal disposition and toxicity of irinotecan by integrating 1) in vitro metabolism and transport data of irinotecan and its metabolites, 2) ex vivo gut microbial activation of the toxic metabolite SN-38, and 3) the tissue protein abundance data of enzymes and transporters relevant to irinotecan and its metabolites. Integration of in vitro kinetics data with the tissue enzyme and transporter abundance predicted that carboxylesterase (CES)-mediated hydrolysis of irinotecan is the rate-limiting process in the liver, where the toxic metabolite formed is rapidly deactivated by glucuronidation. In contrast, the poor SN-38 glucuronidation rate as compared with its efficient formation by CES2 in the enterocytes is the key mechanism of the intestinal accumulation of the toxic metabolite. The biliary efflux and organic anion transporting polypeptide-2B1-mediated enterocyte uptake can also synergize buildup of SN-38 in the enterocytes, whereas intestinal P-glycoprotein likely facilitates SN-38 detoxification in the enterocytes. The higher SN-38 concentration in the intestine can be further nourished by β-d-glucuronidases. Understanding the quantitative significance of the key metabolism and transport processes of irinotecan and its metabolites can be leveraged to alleviate its intestinal side effects. Further, the proteomics-informed quantitative approach to determine intracellular disposition can be extended to determine susceptibility of cancer cells over normal cells for precision irinotecan therapy. SIGNIFICANCE STATEMENT: This work provides a deeper insight into the quantitative relevance of irinotecan hydrolysis (activation), conjugation (deactivation), and deconjugation (reactivation) by human or gut microbial enzymes or transporters. The results of this study explain the characteristic intestinal exposure and toxicity of irinotecan. The quantitative tissue-specific in vitro to in vivo extrapolation approach presented in this study can be extended to cancer cells.

Extracellular and intracellular zilpaterol and clenbuterol quantification in Hep G2 liver cells by UPLC-PDA and UPLC-MS/MS

Zilpaterol and Clenbuterol are β-adrenergic agonists that have been widely used to feed cattle. Although the use of Zilpaterol has been approved, Clenbuterol is still used illegally at unknown doses. However, the research of both substances has been based mainly on the evaluation of residues. To our knowledge, this is the first time that a cellular model using Hep G2 cells treated with Zilpaterol and Clenbuterol is presented as an alternative approach to quantify both drugs at the cellular level. Thus, a complete analytical methodology has been developed for the accurate quantitation of these β-adrenergic agonists in both cellular compartments. We propose the use of ultra-performance liquid chromatography with photodiode array detector (UPLC-PDA) for extracellular determinations while UPLC coupled to a tandem mass spectrometer (UPLC-MS/MS) for intracellular analysis. The methods were fully validated in terms of selectivity, linearity, accuracy, and precision, limits of detection and quantitation (LOD and LOQ, respectively), stability, carryover, and matrix effect. The method for intracellular content was linear ranging from 0.25 to 8 ng/mL while for extracellular content, the concentration of Zilpaterol and Clenbuterol ranged from 0.125 to 4 μg/mL, with correlation coefficients of R > 0.98 and >0.99, respectively. The combination of the two methodologies in the cellular model showed intracellular concentrations of 0.344 ± 0.06 μg/mL and 2.483 ± 0.36 μg/mL for Zilpaterol and Clenbuterol, respectively. Extracellular concentration was 0.728 ± 0.14 μg/mL and 0.822 ± 0.11 μg/mL for Zilpaterol and Clenbuterol, respectively. This work shows the potential applications of cellular modelling in the study of toxicity for the mentioned drugs.

Freshly isolated primary human proximal tubule cells as an in vitro model for the detection of renal tubular toxicity

Drug induced kidney injury (DIKI) is a common reason for compound attrition in drug development pipelines with proximal tubule epithelial cells (PTECs) most commonly associated with DIKI. Here, we investigated freshly isolated human (hPTECs) as an in vitro model for assessing renal tubular toxicity. The freshly isolated hPTECs were first characterized to confirm gene expression of important renal transporters involved in drug handling which was further corroborated by confirming the functional activity of organic cation transporter 2 and organic anion transporter 1 by using transporter specific inhibitors. Additionally, functionality of megalin/cubilin endocytic receptors was also confirmed. A training set of 36 compounds was used to test the ability of the model to classify them using six different endpoints which included three biomarkers (Kidney Injury Molecule-1, Neutrophil gelatinase-associated lipocalin, and Clusterin) and three non-specific injury endpoints (ATP depletion, LDH leakage, and barrier permeability via transepithelial electrical resistance) in a dose-dependent manner across two independent kidney donors. In general, biomarkers showed higher predictivity than non-specific endpoints, with Clusterin showing the highest predictivity (Sensitivity/Specificity - 65.0/93.8 %). By using the thresholds generated from the training set, nine candidate internal Takeda compounds were screened where PTEC toxicity was identified as one of the findings in preclinical animal studies. The model correctly classified four of six true positives and two of three true negatives, showing validation of the in vitro model for detection of tubular toxicants. This work thus shows the potential application of freshly isolated primary hPTECs using translational biomarkers in assessment of tubular toxicity within the drug discovery pipeline.

Intracellular Accumulation as an Indicator of Cytotoxicity to Screen Hepatotoxic Components of Chelidonium majus L. by LC-MS/MS

A novel strategy was developed to identify hepatotoxic compounds in traditional Chinese medicines (TCMs). It is based on the exposure of HL-7702 cells to a TCM extract, followed by the identification and further determination of potential hepatotoxic compounds accumulated in the cells by liquid chromatography–tandem mass spectrometry (LC–MS/MS). As a case study, potential hepatotoxic components in Chelidonium majus L. were screened out. Five alkaloids (sanguinarine, coptisine, chelerythrine, protopine, and chelidonine) were identified by LC–MS/MS within 10 min, and their intracellular concentrations were first simultaneously measured by LC–MS/MS with a run time of 4 min. A cell viability assay was performed to assess the cytotoxicity of each alkaloid. With their higher intracellular concentrations, sanguinarine, coptisine, and chelerythrine were identified as the main hepatotoxic constituents in Ch. majus. The study provides a powerful tool for the fast prediction of cytotoxic components in complex natural mixtures on a high-throughput basis.

Recellularized Native Kidney Scaffolds as a Novel Tool in Nephrotoxicity Screening

Drug-induced kidney injury in medicinal compound development accounts for over 20% of clinical trial failures and involves damage to different nephron segments, mostly the proximal tubule. Yet, currently applied cell models fail to reliably predict nephrotoxicity; neither are such models easy to establish. Here, we developed a novel three-dimensional (3D) nephrotoxicity platform on the basis of decellularized rat kidney scaffolds (DS) recellularized with conditionally immortalized human renal proximal tubule epithelial cells overexpressing the organic anion transporter 1 (ciPTEC-OAT1). A 5-day SDS-based decellularization protocol was used to generate DS, of which 100-μm slices were cut and used for cell seeding. After 8 days of culturing, recellularized scaffolds (RS) demonstrated 3D-tubule formation along with tubular epithelial characteristics, including drug transporter function. Exposure of RS to cisplatin (CDDP), tenofovir (TFV), or cyclosporin A (CsA) as prototypical nephrotoxic drugs revealed concentration-dependent reduction in cell viability, as assessed by PrestoBlue and Live/Dead staining assays. This was most probably attributable to specific uptake of CDDP by the organic cation transporter 2 (OCT2), TFV through organic anion transporter 1 (OAT1), and CsA competing for P-glycoprotein-mediated efflux. Compared with 2D cultures, RS showed an increased sensitivity to cisplatin and tenofovir toxicity after 24-hour exposure (9 and 2.2 fold, respectively). In conclusion, we developed a physiologically relevant 3D nephrotoxicity screening platform that could be a novel tool in drug development.

Involvement of multidrug resistance protein 4 in the hepatocyte efflux of lamivudine and entecavir

Multidrug resistance protein 4 (MRP4) is capable of transporting acyclic nucleotide phosphonates, but little is known about its role in lamivudine (LAM) and entecavir (ETV) transport. In the present study, the involvement of MRP4 in the transport of LAM and ETV was investigated through in vitro experiments. The cytotoxicity of three antiviral drugs and their activities against HBV as characterized in HepG2.4D14 [wild‑type hepatitis B virus (HBV)] and HepG2.A64 (ETV‑resistant HBV) cells. LAM, ETV and tenofovir (TFV) demonstrated a 50% effective concentration against HBV of 4.14±0.03, 0.13±0.02 and 3.24±0.01 µM in HepG2.4D14 cells and of 5.94±0.20, 6.28±0.07 and 11.43±0.09 µM in HepG2.A64 cells, respectively. After administering 3-([(3-(2-[7-chloro-2-quinolinyl]ethyl)phenyl]-[(3-dimethylamino-3-oxoporphyl)-thio)-methyl]-thio) propanoic acid (MK571), the intracellular concentrations of all three drugs were much lower than the extracellular drug concentrations in these two cell types, whereas the intracellular drug concentrations in wild‑type cells were higher than those in ETV‑resistant cells. Furthermore, the intracellular levels of LAM, ETV and TFV were enhanced and the extracellular concentrations were reduced by addition of MK571. Thus, MRP4 is mainly responsible for the efflux of LAM and ETV in hepatocyte cultures. These results may contribute to enhancing antiviral efficacy.

Apigenin, a novel candidate involving herb-drug interaction (HDI), interacts with organic anion transporter 1 (OAT1)

BACKGROUND

Apigenin is a flavonoid compound, widely distributed in natural plants. Various studies have suggested that apigenin has inhibitory effects towards several drug transporters, such as the organic anion transporting (OAT) polypeptides, 1B1 and 1B3 (OATP1B1 and OATP1B3). However, the mechanism by which apigenin interacts with OAT1 has not been well studied.

METHODS

MDCK cells stably-expressing OAT1 were used to examine the inhibitory effects of apigenin on OAT1. UPLC-MS/MS was used to evaluate the in vitro and in vivo effects of apigenin on the uptake of acyclovir by OAT1. Cytotoxicity was determined by the cell viability, MTT assays.

RESULTS

Apigenin effectively inhibited the activity of OAT1 in a dose-dependent manner with an IC50 value of 0.737μM. Pre-incubation of cells with apigenin caused a time-dependent inhibition (TDI) of OAT1. Additionally, we examined the interactions between apigenin and acyclovir or adefovir. Data showed that apigenin (1μM) significantly blocked the uptake of acyclovir by OAT1 in vitro with an inhibition rate of 55%. In vivo, apigenin could increase the concentration of acyclovir in plasma when co-administered with acyclovir. Importantly, the MTT assays showed that, at a dose of 50μM, apigenin significantly reduced the cytotoxicity of adefovir and substantially increased cell viability from 50.6% to 112.62%.

CONCLUSION

Our results demonstrate that apigenin regulates OAT1, and can cause TDI or herb-drug interaction (HDI) when used in combination with acyclovir or adefovir. Therefore, apigenin could be used as a nephroprotective agent when used in combination with the substrates of OAT1.

Establishment of HK-2 Cells as a Relevant Model to Study Tenofovir-Induced Cytotoxicity

Tenofovir (TFV) is an antiviral drug approved for treating Human Immunodeficiency Virus (HIV) and Hepatitis B. TFV is administered orally as the prodrug tenofovir disoproxil fumarate (TDF) which then is deesterified to the active drug TFV. TFV induces nephrotoxicity characterized by renal failure and Fanconi Syndrome. The mechanism of this toxicity remains unknown due to limited experimental models. This study investigated the cellular mechanism of cytotoxicity using a human renal proximal tubular epithelial cell line (HK-2). HK-2 cells were grown for 48 h followed by 24 to 72 h exposure to 0–28.8 μM TFV or vehicle, phosphate buffered saline (PBS). MTT (MTT, 3-(4,5-dimethylthiazol-2-yl)-2,5-Diphenyltetrazolium Bromide) and Trypan blue indicated that TFV diminished cell viability at 24–72 h. TFV decreased ATP levels at 72 h when compared to vehicle, reflecting mitochondrial dysfunction. TFV increased the oxidative stress biomarkers of protein carbonylation and 4-hydroxynonenol (4-HNE) adduct formation. Tumor necrosis factor alpha (TNFα) was released into the media following exposure to 14.5 and 28.8 μM TFV. Caspase 3 and 9 cleavage was induced by TFV compared to vehicle at 72 h. These studies show that HK-2 cells are a sensitive model for TFV cytotoxicity and suggest that mitochondrial stress and apoptosis occur in HK-2 cells treated with TFV.

Multidrug resistance protein 4 is a critical protein associated with the antiviral efficacy of nucleos(t)ide analogues

BACKGROUND & AIMS

Multidrug resistance protein 4 (MRP4) has been associated with nucleos(t)ide analogue (NA) antiretroviral therapy failure, though is unclear if MRP4 is also correlated with the failure of anti-hepatitis B virus (HBV) therapy.

METHODS

Multidrug resistance protein 4 expression in human peripheral blood mononuclear cells (PBMCs), liver tissues and human hepatoma cell lines was detected by real-time polymerase chain reaction (PCR), western blotting and immunohistochemistry assays. Supernatant and intracellular HBV DNA levels of MRP4-overexpressing or silenced HepG2.4D14 (wild-type) and HepG2.A64 (entecavir-resistant mutant) cells were measured by quantitative PCR. NA concentrations and HBV mutational analysis were assessed by liquid chromatography/mass spectrometry assays and DNA sequencing. Multivariate analysis was used to assess predictive factors for treatment failure.

RESULTS

High expression of MRP4 was found in hepatoma cell lines and PBMCs, and up- or down-regulation of MRP4 expression altered the susceptibility of cells to NAs. MRP inhibitors increased NA intracellular accumulation and decreased extracellular levels. Moreover, MRP4 expression in PBMCs was correlated with that in paired liver tissues. Furthermore, multivariate analysis showed MRP4 mRNA expression to be an independent predictor of NA treatment failure.

CONCLUSIONS

Multidrug resistance protein 4 is a critical protein associated with the antiviral efficacy of NAs, and combination therapy of NA and MRP inhibitors could reduce the dosage for long-term NA use. This is the first report to demonstrate that MRP4 expression is an important factor predicting treatment failure in chronic hepatitis B patients and will provide a potential therapeutic target against HBV.

A Human Renal Proximal Tubule Cell Line with Stable Organic Anion Transporter 1 and 3 Expression Predictive for Antiviral-Induced Toxicity

Drug-induced nephrotoxicity still hampers drug development, because current translation from in vitro or animal studies to human lacks high predictivity. Often, renal adverse effects are recognized only during clinical stages of drug development. The current study aimed to establish a robust and a more complete human cell model suitable for screening of drug-related interactions and nephrotoxicity. In addition to endogenously expressed renal organic cation transporters and efflux transporters, conditionally immortalized proximal tubule epithelial cells (ciPTEC) were completed by transduction of cells with the organic anion transporter (OAT) 1 or OAT3. Fluorescence-activated cell sorting upon exposure to the OAT substrate fluorescein successfully enriched transduced cells. A panel of organic anions was screened for drug-interactions in ciPTEC-OAT1 and ciPTEC-OAT3. The cytotoxic response to the drug-interactions with antivirals was further examined by cell viability assays. Upon subcloning, concentration-dependent fluorescein uptake was found with a higher affinity for ciPTEC-OAT1 (K_m = 0.8 ± 0.1 μM) than ciPTEC-OAT3 (K_m = 3.7 ± 0.5 μM). Co-exposure to known OAT1 and/or OAT3 substrates (viz. para-aminohippurate, estrone sulfate, probenecid, furosemide, diclofenac, and cimetidine) in cultures spanning 29 passage numbers revealed relevant inhibitory potencies, confirming the robustness of our model for drug-drug interactions studies. Functional OAT1 was directly responsible for cytotoxicity of adefovir, cidofovir, and tenofovir, while a drug interaction with zidovudine was not associated with decreased cell viability. Our data demonstrate that human-derived ciPTEC-OAT1 and ciPTEC-OAT3 are promising platforms for highly predictive drug screening during early phases of drug development.

Rapid Method To Determine Intracellular Drug Concentrations in Cellular Uptake Assays: Application to Metformin in Organic Cation Transporter 1-Transfected Human Embryonic Kidney 293 Cells

Because of the importance of intracellular unbound drug concentrations in the prediction of in vivo concentrations that are determinants of drug efficacy and toxicity, a number of assays have been developed to assess in vitro unbound concentrations of drugs. Here we present a rapid method to determine the intracellular unbound drug concentrations in cultured cells, and we apply the method along with a mechanistic model to predict concentrations of metformin in subcellular compartments of stably transfected human embryonic kidney 293 (HEK293) cells. Intracellular space (ICS) was calculated by subtracting the [(3)H]-inulin distribution volume (extracellular space, ECS) from the [(14)C]-urea distribution volume (total water space, TWS). Values obtained for intracellular space (mean ± S.E.M.; μl/10(6) cells) of monolayers of HEK cells (HEK-empty vector [EV]) and cells overexpressing human organic cation transporter 1 (HEK-OCT1), 1.21± 0.07 and 1.25±0.06, respectively, were used to determine the intracellular metformin concentrations. After incubation of the cells with 5 µM metformin, the intracellular concentrations were 26.4 ± 7.8 μM and 268 ± 11.0 μM, respectively, in HEK-EV and HEK-OCT1. In addition, intracellular metformin concentrations were lower in high K(+) buffer (140 mM KCl) compared with normal K(+) buffer (5.4 mM KCl) in HEK-OCT1 cells (54.8 ± 3.8 μM and 198.1 ± 11.2 μM, respectively; P < 0.05). Our mechanistic model suggests that, depending on the credible range of assumed physiologic values, the positively charged metformin accumulates to particularly high levels in endoplasmic reticulum and/or mitochondria. This method together with the computational model can be used to determine intracellular unbound concentrations and to predict subcellular accumulation of drugs in other complex systems such as primary cells.