Principles and Experimental Considerations for In Vitro Transporter Interaction Assays.

Drug transporters are universally acknowledged as important determinants of the absorption, distribution, metabolism, and excretion of both endogenous and exogenous compounds. Altered transporter function, whether due to genetic polymorphism, DDIs, disease, or environmental factors such as dietary constituents, can result in changes in drug efficacy and/or toxicity due to changes in circulating or tissue levels of either drugs or endogenous substrates.Prediction of whether and to what extent the biological fate of a drug is influenced by drug transporters, therefore, requires in vitro test systems that can accurately predict the risk and magnitude of clinical DDIs. While these in vitro assessments appear simple in theory, practitioners recognize that there are multiple factors that can influence experimental outcomes. A better understanding of these variables, including test compound characteristics, test systems, assay formats, and experimental design, will enable clear, actionable steps and translatable outcomes that may avoid unnecessary downstream clinical engagement. This chapter will delineate the role of these variables in improving in vitro assay outcomes.

Case Study 7: Transporters Case Studies-In Vitro Solutions for Translatable Outcomes.

Assessing the interactions of a new drug candidate with transporters, either as a substrate, inhibitor, or inducer, is no simple matter. There are many clinically relevant transporters, as many as nine to be evaluated for an FDA submission and up to 11 for the EMA as of 2020. Additionally, it is likely that if a compound is a substrate or inhibitor of one transporter, it will be so for other transporters as well. There are practically no specific substrates or inhibitors, presumably because the specificities of drug transporters are so broad and overlapping, and even fewer clinically relevant probes that can be used to evaluate transporter function in humans. In the case of some transporters, it is advisable to evaluate an NCE with more than one test system and/or more than one probe substrate in order to convince oneself (and regulatory authorities) that a clinical drug interaction study is not warranted. Finally, each test system has its own unique set of advantages and disadvantages. One has to appreciate the nuances of the available tools (test systems, probe substrates, etc.) to select the most relevant tools for the study and design the optimal in vitro experiment. In this chapter, several examples are used to illustrate the successful interpretation of in vitro data for both efflux and uptake transporters. Some data presented in this chapter are unpublished at the time of the compilation of this book. It has been included in this chapter to provide a sense of the complexities in transporter kinetics to the reader.

A differential equation based modelling approach to predict supersaturation and in vivo absorption from in vitro dissolution-absorption system (idas2) data.

In vitro dissolution tests are widely used to monitor the quality and consistency of oral solid dosage forms, but to increase the physiological relevance of in vitro dissolution tests, newer systems combine dissolution and permeation measurements. Some of these use artificial membranes while others (e.g., in the in vitro dissolution absorption system 2; IDAS2), utilize cell monolayers to assess drug permeation. We determined the effect of the precipitation inhibitor Hypromellose Acetate Succinate (HPMCAS) on the supersaturation/permeation of Ketoconazole and Dipyridamole in IDAS2 and its effect on their absorption in rats. Thus the main objectives of this study were to determine: (1) whether dissolution and permeation data from IDAS2 could be used to predict rat plasma concentration using an absorption model and (2) whether the effect of the precipitation inhibitor HPMCAS on supersaturation and permeation in IDAS2 was correlated with its effect on systemic absorption in the rat. Predicted drug concentrations in rat plasma, generated using parameters estimated from IDAS2 dissolution/permeation data and a mathematical absorption model, showed good agreement with measured concentrations. While in IDAS2, the prolongation of Ketoconazole's supersaturation caused by HPMCAS led to higher permeation, which paralleled the higher systemic absorption in rats, Dipyridamole showed no supersaturation and, thus, no effect of HPMCAS in dissolution or permeation in IDAS2 and no effect on Dipyridamole absorption in rats. The ability of IDAS2 to detect supersaturation following a pH-shift supports the potential value of this system for studying approaches to enhance intestinal absorption through supersaturation and the accuracy of plasma concentration predictions in rats suggest the possibility of combining IDAS2 with absorption models to predict plasma concentration in different species.

Evaluation of the In-Vitro Dissolution Permeation Systems 1 (IDAS1) as a potential tool to monitor for unexpected changes in generic medicaments in poorly regulated markets.

Panama, like most Latin American countries, has insufficient regulatory safeguards to ensure the safety and efficacy of all pharmaceutical products in the market, a situation that results in a two-tier system, where affluent citizens can afford innovator products while poor citizens must consume 'generics' of uncertain quality. Given that one lot of each drug product is analyzed every five years during registration while commercial lots are not, and since most products are not bioequivalent but simply copies or similars, there is a concern that commercial and registration lots of these 'generics' may not be of the same quality. The objective of this study was to assess the ability of various in vitro quality control tests to detect difference among five amlodipine products available in the Panamanian market: four 'generics', made in various countries, and the innovator, made in Germany and used as reference listed drug in Panama (Pan-RLD). The innovator manufactured in the United States (US-RLD) was used to compare the two RLDs. The Content Uniformity test, 30-min Dissolution test and multiple-pH Dissolution Profiles did not show any difference among the products. However, the in vitro dissolution absorption system 1 (IDAS1) showed a statistically significant difference in the amount dissolved between Pan-RLD and three out of the four 'generics', and significantly lower permeated amount for all the 'generics' compared with Pan-RLD; only US-RLD was similar to Pan-RLD. Thus, IDAS1 showed promise as a potential tool that authorities in weakly regulated markets can use to monitor for possible lot-to-lot product changes, which can help improve the quality of pharmaceutical products available to their entire populations. The significance of the similarity between the innovators made in Germany and the United States and their difference from the 'generics' (manufactured in other countries) is not known but deserves investigation.

Development of Gene Therapy Vectors: Remaining Challenges.

Almost 20 years after the tragic death of a young patient due to an experimental gene therapy trial to treat Ornithine Transcarboxylase deficiency, the FDA approved its first landmark gene therapy drug i.e. Luxturna® to treat inherited blindness, and dozens of gene therapy studies are underway. Whether it is replacing the mutant copies of the gene with the wild type one or editing the mutant one in or ex-vivo to elicit the production of functional proteins, numerous viral and non-viral vectors for delivering the gene payload are being evaluated. While, non-viral vectors avoid or mitigate limiting factors such as immunogenicity and the presence of neutralizing antibodies (NAbs), viral vectors such as recombinant adeno-associated viruses (AAVs) have shown early success as a delivery vehicle, because of the overall safety, target specificity, and long-term stability profile. Nonetheless, multiple challenges during the AAV product development and approval process are still looming. AAV serotypes are continuously being engineered which requires multiple cell-based assays to not only assess the neutralizing antibodies (NAb) seroprevalence but also to develop the in-vitro bio potency assays. Hence, we focus on some critical aspects of the AAVs that determine the path forward for pre-clinical and clinical product development.

In vitro dissolution absorption system (IDAS2): Use for the prediction of food viscosity effects on drug dissolution and absorption from oral solid dosage forms.

Existing in vitro dissolution or permeation models to predict food effect are mainly based on Pharmacopeias' compendial media, which specify such variables as pH, bile salts, lipolytic enzymes, and phospholipids content. However, the viscosity of food in the gastrointestinal (GI) tract is not taken into account, although it can affect both the dissolution of the oral solid dosage form and absorption of the released drug. Here, a new in vitro dissolution absorption system (IDAS2) is utilized, which comprises a dissolution apparatus USP2 (DISTEK) equipped with specially constructed permeability chambers containing Caco-2 monolayers, thereby allowing dissolution and transepithelial absorption to be ascertained simultaneously. The IDAS2 was used to evaluate the effect of medium viscosity on both the dissolution of oral solid dosage forms and absorption of released drugs. Such information, which is not ordinarily determined in dissolution and permeation studies, will be helpful to the formulators developing robust oral dosage forms. Commercially available solid dosage forms of ten model drugs from across all BCS classifications were used in this evaluation: metoprolol, minoxidil, and propranolol from BCS class 1; carbamazepine, ketoprofen, and simvastatin from BCS class 2; atenolol and ranitidine from BCS class 3; and acetazolamide and saquinavir from BCS class 4. The study revealed the applicability of IDAS2 as a tool for in vitro screening of dissolution and absorption of intact oral solid products to predict food viscosity effect. The most profound viscosity effect on dissolution and absorption was observed of solid dosage forms for the BCS class 2 compounds carbamazepine and simvastatin. A higher medium viscosity significantly slowed down the dissolution rate of tested BSC class 4 compounds acetazolamide and saquinavir, without significant effect on their absorption. The solid dosage forms least affected by the viscosity of the medium tested were the BCS class 1 compounds minoxidil and propranolol.

Near infrared (NIR)-spectroscopy and in-vitro dissolution absorption system 2 (IDAS2) can help detect changes in the quality of generic drugs.

While Health authorities in Panama strive to increase generic drug use to contain the rising costs of medicines, there is still hesitation to embrace generic drugs. Thus, regulators and drug companies need to ensure the quality, safety and efficacy of generic drugs. One prevailing concern is the absence of control over lot-to-lot changes, which may impact consistent therapeutic performance. The objective of this work was to determine whether near-infrared spectroscopy (NIR) could detect product changes. Calibration models were built using reference (standard) tablets of two products: Virax® (200 mg acyclovir) and Amlopin® (5 mg amlodipine). Then, to assess the sensitivity of NIR to product changes we compared reference versus deliberately-modified formulations of these products. Comparisons were made using principal component analysis (PCA) and partial least squares-discriminant analysis (PLS-DA) of NIR spectra. Several modified lots were different from reference lots, and 3D score plots showed greater discrimination by PLS-DA than PCA. The Kth nearest neighbor scores (KNN) of the modified batches were used to classify formulations as identical or not identical versus the reference. In addition, the differences detected by NIR were correlated with different in vitro dissolution and/or permeation in the in vitro dissolution absorption system 2 (IDAS2): NIR and IDAS2 yielded the same rank-order of difference for the modified lots tested. This study suggests that NIR and IDAS2 can help detect lots of generic drugs that differ from the reference lots. This strategy may help regulatory agencies in developing countries to safeguard patients against lot-to-lot changes in generic products.

Simultaneous Analysis of Dissolution and Permeation Profiles of Nanosized and Microsized Formulations of Indomethacin Using the In Vitro Dissolution Absorption System 2.

The in vitro dissolution absorption system 2 (IDAS2), a recent invention comprised a conventional dissolution vessel containing 2 permeation chambers with Caco-2 cell monolayers mounted with their apical side facing the dissolution media, permits simultaneous measurement of dissolution and permeation of drugs from intact clinical dosage forms. The objectives of this study were (1) to assess the utility of IDAS2 in the determination of the effect of particle size on in vitro performance of indomethacin and (2) to find out whether the behavior in IDAS2 of 2 indomethacin products differing in particle size is correlated with their in vivo behavior. Indomethacin dissolution and permeation across Caco-2 cell monolayers were simultaneously measured in IDAS2; the dissolution and permeation profiles were simultaneously modeled using a simple two-compartment model. Compared to microsized indomethacin, the nanosized formulation increased the dissolution rate constant by fivefold, whereas moderately increasing the permeation rate constant and the kinetic solubility. As a result, the drug amount permeated across the Caco-2 cell monolayers doubled in the nanosized versus microsized formulation. The in vitro results showed a good correlation with in vivo human oral pharmacokinetic parameters, thus emphasizing the physiological relevance of IDAS2 data in predicting in vivo absorption.

Innovative in vitro methodologies for establishing therapeutic equivalence.

To improve the quality of pharmaceutical products in their markets, several Latin American countries have begun to require that new generic products demonstrate bioequivalence against innovator or reference products. However, given the number of products involved, it is not feasible to rely on clinical studies to comply with this requirement. Instead, it makes sense to adopt or develop strategies that are appropriate to the characteristics of the region. To streamline drug development and accelerate patients' access to quality drug products, 15 years ago the United States Food and Drug Administration (FDA) decided to grant exemptions from clinical bioequivalence studies (i.e., biowaivers) for certain types of drug products based on the Biopharmaceutics Classification System (BCS). Biowaivers can significantly reduce development time and cost and can also prevent unnecessary human exposure to potentially dangerous drugs while providing a robust, consistent standard for therapeutic equivalence of generic drug products. In addition, the limited success of translating in vitro dissolution data into in vivo performance can be enhanced using innovative tools such as the in vitro dissolution and absorption systems (IDAS). By integrating in vitro dissolution and permeability tests, these systems can provide useful insights for formulation development. A thorough assessment of the potential of in vitro techniques, along with formalization of their use through regulatory science initiatives when appropriate, may lead to cost-effective tools to help address some of the quality and regulatory challenges faced in the Latin American and Caribbean region.

Innovative in vitro methodologies for establishing therapeutic equivalence / Métodos in vitro innovadores para determinar la equivalencia terapéutica

To improve the quality of pharmaceutical products in their markets, several Latin American countries have begun to require that new generic products demonstrate bioequivalence against innovator or reference products. However, given the number of products involved, it is not feasible to rely on clinical studies to comply with this requirement. Instead, it makes sense to adopt or develop strategies that are appropriate to the characteristics of the region. To streamline drug development and accelerate patients’ access to quality drug products, 15 years ago the United States Food and Drug Administration (FDA) decided to grant exemptions from clinical bioequivalence studies (i.e., biowaivers) for certain types of drug products based on the Biopharmaceutics Classification System (BCS). Biowaivers can significantly reduce development time and cost and can also prevent unnecessary human exposure to potentially dangerous drugs while providing a robust, consistent standard for therapeutic equivalence of generic drug products. In addition, the limited success of translating in vitro dissolution data into in vivo performance can be enhanced using innovative tools such as the in vitro dissolution and absorption systems (IDAS). By integrating in vitro dissolution and permeability tests, these systems can provide useful insights for formulation development. A thorough assessment of the potential of in vitro techniques, along with formalization of their use through regulatory science initiatives when appropriate, may lead to cost-effective tools to help address some of the quality and regulatory challenges faced in the Latin American and Caribbean region.

Para mejorar la calidad de los productos farmacéuticos comercializados en su mercado, varios países latinoamericanos han empezado a exigir que se demuestre la bioequivalencia de los nuevos medicamentos genéricos frente a los medicamentos innovadores o de referencia. Sin embargo, dado el gran número de medicamentos, resulta, poco factible realizar estudios clínicos para cumplir con este requisito pero tiene sentido incorporar o elaborar estrategias que sean acordes a las características de la región. Para simplificar el desarrollo de fármacos y optimizar el acceso de los pacientes a medicamentos de buena calidad, hace 15 años la Administración de Alimentos y Medicamentos de los Estados Unidos de América (FDA) decidió conceder exenciones a la realización de estudios clínicos de bioequivalencia (es decir, bioexenciones) a algunos tipos de medicamentos conforme al Sistema de Clasificación Biofarmacéutica. Las bioexenciones reducen significativamente el tiempo y el costo de desarrollo, y también evitan la exposición innecesaria de seres humanos a medicamentos que podrían ser nocivos, a la vez que constituyen una norma robusta y uniforme que garantiza la equivalencia terapéutica de los medicamentos genéricos. Por otra parte, los métodos innovadores, como los sistemas de disolución y absorción in vitro, permiten ampliar los resultados limitados obtenidos al aplicar los datos de disolución in vitro para simular los efectos in vivo. Dado que combinan las pruebas de disolución in vitro con las de permeabilidad, estos sistemas brindan conocimientos útiles para el desarrollo galénico. Es probable que la evaluación meticulosa del potencial de las técnicas in vitro, junto con su formalización mediante iniciativas de normalización científica cuando corresponda, permita concebir métodos eficaces en función de los costos que ayuden a encarar algunos de los retos relativos a la calidad y la regulación de los medicamentos que enfrentan América Latina y el Caribe.

The Effect of Excipients on the Permeability of BCS Class III Compounds and Implications for Biowaivers.

PURPOSE: Currently, the FDA allows biowaivers for Class I (high solubility and high permeability) and Class III (high solubility and low permeability) compounds of the Biopharmaceutics Classification System (BCS). Scientific evidence should be provided to support biowaivers for BCS Class I and Class III (high solubility and low permeability) compounds. METHODS: Data on the effects of excipients on drug permeability are needed to demonstrate that commonly used excipients do not affect the permeability of BCS Class III compounds, which would support the application of biowaivers to Class III compounds. This study was designed to generate such data by assessing the permeability of four BCS Class III compounds and one Class I compound in the presence and absence of five commonly used excipients. RESULTS: The permeability of each of the compounds was assessed, at three to five concentrations, with each excipient in two different models: Caco-2 cell monolayers, and in situ rat intestinal perfusion. No substantial increases in the permeability of any of the compounds were observed in the presence of any of the tested excipients in either of the models, with the exception of disruption of Caco-2 cell monolayer integrity by sodium lauryl sulfate at 0.1 mg/ml and higher. CONCLUSION: The results suggest that the absorption of these four BCS Class III compounds would not be greatly affected by the tested excipients. This may have implications in supporting biowaivers for BCS Class III compounds in general.

Optimization of PEGylated nanoemulsions for improved pharmacokinetics of BCS class II compounds.

The objective of the study was the optimization of nanoemulsion formulations to prevent their rapid systemic clearance after intravenous administration. An amphiphilic PEG derivative DSPE-PEG (1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-methoxy-poly(polyethylene glycol) with different chain lengths and concentration was used as a nanoemulsion droplet surface modifier. The danazol loading in all nanoemulsions was kept on the same level of ∼2 mg/mL. In the present investigation, PEGylated and non-PEGylated nanoemulsions were compared in vitro phagocytosis by incubating with lung macrophages and in vivo after intravenous administration in rats. Danazol-containing nanoemulsions (NE) modified with various PEG chain lengths (2000-10 000) and concentrations (3-12 mg/mL) were prepared and characterized. Nanoemulsion droplets were reproducibly obtained in the size range of 213-340 nm. The non-PEGylated NE had the surface charge of -25.4 mV. This absolute charge value decreased with increasing chain length and concentration. With increase in chain length and density the macrophage uptake decreased which could be due to decrease in surface charge and hydrophilicity of droplets. The greatest shielding of the NE droplets was reached with DSPE-PEG5000 at the concentration of 6 mg/mL where the surface charge changed to -1.27 mV. Following intravenous administration a maximum danazol exposure (401 ± 68.2 h ng/mL) was observed with the lowest clearance rate (5.06 ± 0.95 L/h/kg) from 6 mg/mL DSPE-PEG5000 nanoemulsion. PEG5000 and PEG10000 altered the pharmacokinetic of danazol by decreasing clearance and volume of distribution which is likely explained by the presence of hydrophilic shields around the droplets that prevent their rapid systemic clearance and tissue partitioning.

Principles and experimental considerations for in vitro transporter interaction assays.

Drug transporters are now universally acknowledged as important determinants of the absorption, distribution, metabolism and excretion of both endogenous and exogenous compounds. Altered transporter function, whether due to genetic polymorphism, DDIs, disease, or environmental factors such as dietary constituents, can result in changes in drug efficacy and/or toxicity due to changes in circulating or tissue levels of either drugs or endogenous substrates.Prediction of whether and to what extent the biological fate of a drug is influenced by drug transporters, therefore, requires in vitro test systems that can accurately predict the risk and magnitude of clinical DDIs. While these in vitro assessments appear simple in theory, practitioners recognize that there are multiple factors that can influence experimental outcomes. A better understanding of these variables, including test compound characteristics, test systems, assay formats, and experimental design will enable clear, actionable steps and translatable outcomes that may avoid unnecessary downstream clinical engagement. This chapter will delineate the role of these variables in improving in vitro assay outcomes.

Case study 6. Transporter case studies: in vitro solutions for translatable outcomes.

Assessing the interactions of a new drug candidate with transporters, either as a substrate or as an inhibitor, is no simple matter. There are many clinically relevant transporters, as many as nine to be evaluated for an FDA submission and up to eleven for the EMA as of 2013. Additionally, it is likely that if a compound is a substrate or inhibitor of one transporter, it will be so for other transporters as well. There are practically no specific substrates or inhibitors, presumably because the specificities of drug transporters are so broad and overlapping, and even fewer clinically relevant probes that can be used to evaluate transporter function in humans. In the case of some transporters, it is advisable to evaluate an NCE with more than one test system and/or more than one probe substrate in order to convince oneself (and regulatory authorities) that a clinical drug interaction study is not warranted. Finally, each test system has its own unique set of advantages and disadvantages. One has to really appreciate the nuances of the available tools (test systems, probe substrates, etc.) to select the best tools for the job and design the optimal in vitro experiment. In this chapter, several examples are used to illustrate the successful interpretation of in vitro data for both efflux and uptake transporters. Some data presented in this chapter is unpublished at the time of compilation of this book. It has been incorporated in this chapter to provide a sense of complexities in transporter kinetics to the reader.

Compartmental models for apical efflux by P-glycoprotein--part 1: evaluation of model complexity.

PURPOSE: With the goal of quantifying P-gp transport kinetics, Part 1 of these manuscripts evaluates different compartmental models and Part 2 applies these models to kinetic data. METHODS: Models were developed to simulate the effect of apical efflux transporters on intracellular concentrations of six drugs. The effect of experimental variability on model predictions was evaluated. Several models were evaluated, and characteristics including membrane configuration, lipid content, and apical surface area (asa) were varied. RESULTS: Passive permeabilities from MDCK-MDR1 cells in the presence of cyclosporine gave lower model errors than from MDCK control cells. Consistent with the results in Part 2, model configuration had little impact on calculated model errors. The 5-compartment model was the simplest model that reproduced experimental lag times. Lipid content and asa had minimal effect on model errors, predicted lag times, and intracellular concentrations. Including endogenous basolateral uptake activity can decrease model errors. Models with and without explicit membrane barriers differed markedly in their predicted intracellular concentrations for basolateral drug exposure. Single point data resulted in clearances similar to time course data. CONCLUSIONS: Compartmental models are useful to evaluate the impact of efflux transporters on intracellular concentrations. Whereas a 3-compartment model may be sufficient to predict the impact of transporters that efflux drugs from the cell, a 5-compartment model with explicit membranes may be required to predict intracellular concentrations when efflux occurs from the membrane. More complex models including additional compartments may be unnecessary.

Transport inhibition of digoxin using several common P-gp expressing cell lines is not necessarily reporting only on inhibitor binding to P-gp.

We have reported that the P-gp substrate digoxin required basolateral and apical uptake transport in excess of that allowed by digoxin passive permeability (as measured in the presence of GF120918) to achieve the observed efflux kinetics across MDCK-MDR1-NKI (The Netherlands Cancer Institute) confluent cell monolayers. That is, GF120918 inhibitable uptake transport was kinetically required. Therefore, IC50 measurements using digoxin as a probe substrate in this cell line could be due to inhibition of P-gp, of digoxin uptake transport, or both. This kinetic analysis is now extended to include three additional cell lines: MDCK-MDR1-NIH (National Institute of Health), Caco-2 and CPT-B2 (Caco-2 cells with BCRP knockdown). These cells similarly exhibit GF120918 inhibitable uptake transport of digoxin. We demonstrate that inhibition of digoxin transport across these cell lines by GF120918, cyclosporine, ketoconazole and verapamil is greater than can be explained by inhibition of P-gp alone. We examined three hypotheses for this non-P-gp inhibition. The inhibitors can: (1) bind to a basolateral digoxin uptake transporter, thereby inhibiting digoxin's cellular uptake; (2) partition into the basolateral membrane and directly reduce membrane permeability; (3) aggregate with digoxin in the donor chamber, thereby reducing the free concentration of digoxin, with concomitant reduction in digoxin uptake. Data and simulations show that hypothesis 1 was found to be uniformly acceptable. Hypothesis 2 was found to be uniformly unlikely. Hypothesis 3 was unlikely for GF120918 and cyclosporine, but further studies are needed to completely adjudicate whether hetero-dimerization contributes to the non-P-gp inhibition for ketoconazole and verapamil. We also find that P-gp substrates with relatively low passive permeability such as digoxin, loperamide and vinblastine kinetically require basolateral uptake transport over that allowed by +GF120918 passive permeability, while highly permeable P-gp substrates such as amprenavir, quinidine, ketoconazole and verapamil do not, regardless of whether they actually use the basolateral transporter.

Evaluation of a nanoemulsion formulation strategy for oral bioavailability enhancement of danazol in rats and dogs.

The objective of this study was to determine whether nanoemulsion formulations constitute a viable strategy to improve the oral bioavailability of danazol, a compound whose poor aqueous solubility limits its oral bioavailability. Danazol-containing oil-in-water nanoemulsions (NE) with and without cosurfactants stearylamine (SA) and deoxycholic acid (DCA) were prepared and characterized. Nanoemulsion droplets size ranging from 238 to 344 nm and with surface charges of -24.8 mV (NE), -26.5 mV (NE-DCA), and +27.8 mV (NE-SA) were reproducibly obtained. Oral bioavailability of danazol in nanoemulsions was compared with other vehicles such as PEG400, 1% methylcellulose (MC) in water (1% MC), Labrafil, and a Labrafil/Tween 80 (9:1) mixture, after intragastric administration to rats and after oral administration of NE-SA, a Labrafil solution, or a Danocrine® tablet to dogs. The absolute bioavailability of danazol was 0.6% (PEG400), 1.2% (1% MC), 6.0% (Labrafil), 7.5% (Labrafil/Tween80), 8.1% (NE-DCA), 14.8% (NE), and 17.4% (NE-SA) in rats, and 0.24% (Danocrine), 6.2% (Labrafil), and 58.7% (NE-SA) in dogs. Overall, danazol bioavailability in any nanoemulsion was higher than any other formulation. Danazol bioavailability from NE and NE-SA was 1.8- to 2.2-fold higher than NE-DCA nanoemulsion and could be due to significant difference in droplet size.

Application of exogenous mixture of glutathione and stable isotope labeled glutathione for trapping reactive metabolites in cryopreserved human hepatocytes. Detection of the glutathione conjugates using high resolution accurate mass spectrometry.

Metabolites (including reactive metabolites) of troglitazone were generated by incubation with cryopreserved human hepatocytes and trapped in the presence of an exogenous mixture of unlabeled and stable isotope labeled (SIL: [1,2-(13)C, (15)N]-glycine) glutathione (GSH/SIL-GSH). The incubation samples were analyzed using liquid chromatography-high resolution accurate mass spectrometry (LC-HRAMS) implemented on a LTQ Orbitrap mass spectrometer. The GSH conjugates of the reactive metabolites were detected via a characteristic mono-isotopic pattern (peaks separated by 3.0037u). Analysis of the incubation samples led to detection of a number of previously described GSH conjugates, as well as two novel methylated GSH conjugates, which were partially characterized based on accurate mass measurements and MS/MS data. The addition of exogenous GSH led to an increase in the apparent level of detected GSH conjugates. Kinetic isotopic measurements showed that the rates of incorporation of exogenous GSH are conjugate-specific. In conclusion, this approach, based on the use of a mixture of GSH/SIL-GSH, allows facile capture and detection of reactive metabolites in human hepatocytes. Moreover, the data suggest that routine addition of glutathione to the assay medium may be advisable for experiments with cryopreserved hepatocytes.

Kinetic analysis of human and canine P-glycoprotein-mediated drug transport in MDR1-MDCK cell model: approaches to reduce false-negative substrate classification.

Madin-Darby canine kidney (MDCK) cells transfected with the multidrug resistance 1 (MDR1) gene, MDR1-MDCK, are widely used as an in vitro model to classify compounds as human P-glycoprotein (hPgp) substrates or nonsubstrates. Because MDCK cells express endogenous canine Pgp (cPgp), which is prone to downregulation after transfection with hPgp, this situation could lead to false-negative classification of hPgp substrates. The aim of this study was to investigate factors that influence hPgp substrate classification in MDR1-MDCK model and to seek ways to reduce false classification. Three-compartment models were used to derive flux equations describing the drug transport processes; factors influencing hPgp substrate classification were evaluated by simulations. Pgp functionality was assessed by determining the bidirectional permeability of a series of test compounds. Expressions of hPgp and cPgp were measured by quantitative polymerase chain reaction (qPCR). Kinetic model analysis revealed that the current net flux ratio calculation for hPgp substrate classification is influenced by endogenous cPgp expression as well as hPgp-cPgp expression ratio; the effect was more pronounced in low hPgp-cPgp region and diminished in high ratio region. On the basis of kinetic considerations, this study provides a rational experimental approach and appropriate mathematical corrections to minimize the potential occurrence of false-negative classification of new molecular entities.

The role of a basolateral transporter in rosuvastatin transport and its interplay with apical breast cancer resistance protein in polarized cell monolayer systems.

Membrane transporters can play a clinically important role in drug absorption and disposition; Caco-2 and Madin-Darby canine kidney (MDCK) cells are the most widely used in vitro models for studying the functions of these transporters and associated drug interactions. Transport studies using these cell models are mostly focused on apical transporters, whereas basolateral drug transport processes are largely ignored. However, for some hydrophilic drugs, a basolateral uptake transporter may be required for drugs to enter cells before they can interact with apical efflux transporters. The objective of this study was to evaluate potential differences in drug transport across Caco-2 and MDCK basolateral membrane that could cause discrepancy in the identification of efflux transporter substrates and to elucidate the underlying factors that may cause such differences, using rosuvastatin as a model substrate. Bidirectional transport results in Caco-2 and breast cancer resistance protein-MDCK cells demonstrated the necessity of an uptake transporter at the basolateral membrane for rosuvastatin. Kinetic study revealed saturable and nonsaturable processes for rosuvastatin uptake across the Caco-2 basolateral membrane, with the saturable process encompassing >75% of overall rosuvastatin basolateral uptake at concentrations below the K(m) (4.2 µM). Furthermore, rosuvastatin basolateral transport exhibited cis-inhibition and trans-stimulation phenomena, indicating a facilitated diffusion mechanism. This basolateral transporter appeared to be a prerequisite for rosuvastatin and perhaps for other hydrophilic substrates to interact with apical efflux transporters. Deficit of such a basolateral transporter in certain cell models may lead to false-negative results when screening drug interactions with apical efflux transporters.