Towards a unified model of passive drug permeation I: Origins of the unstirred water layer with applications to ionic permeation

Cyclodextrins and drug membrane permeation: Thermodynamic considerations

Cyclodextrins are hydrophilic oligosaccharides that can increase aqueous solubility of lipophilic drugs through formation of water-soluble drug/cyclodextrin complexes. Although the complexes are hydrophilic, and as such do not permeate biological membranes, the complexes are known to enhance drug permeation through lipophilic membranes and improve drug bioavailability after, for example, oral administration. However, it is not clear how cyclodextrins enhance the permeation. An artificial biomembrane (PermeaPad®) was used to study the effect of donor medium composition on drug permeation. It was observed that in aqueous solutions the hydrophilic cyclodextrins behave not like disperse systems but rather like organic cosolvents such as ethanol, increasing the solubility without having significant effect on the molecular mobility and ability of lipophilic drug molecules to partition into the lipophilic membrane. Also, that partition of dissolved drug molecules from the aqueous exterior into the membrane is at its maximum when their thermodynamic activity is at its maximum. In other words, that drug flux from aqueous cyclodextrin solutions through lipophilic membranes depends on both the concentration and the thermodynamic activity of dissolved drug. Maximum flux is obtained when both the drug concentration and thermodynamic activity of the dissolved drug molecules are at their maximum value.

The Whole Is Bigger than the Sum of Its Parts: Drug Transport in the Context of Two Membranes with Active Efflux

Cell envelope plays a dual role in the life of bacteria by simultaneously protecting it from a hostile environment and facilitating access to beneficial molecules. At the heart of this ability lie the restrictive properties of the cellular membrane augmented by efflux transporters, which preclude intracellular penetration of most molecules except with the help of specialized uptake mediators. Recently, kinetic properties of the cell envelope came into focus driven on one hand by the urgent need in new antibiotics and, on the other hand, by experimental and theoretical advances in studies of transmembrane transport. A notable result from these studies is the development of a kinetic formalism that integrates the Michaelis-Menten behavior of individual transporters with transmembrane diffusion and offers a quantitative basis for the analysis of intracellular penetration of bioactive compounds. This review surveys key experimental and computational approaches to the investigation of transport by individual translocators and in whole cells, summarizes key findings from these studies and outlines implications for antibiotic discovery. Special emphasis is placed on Gram-negative bacteria, whose envelope contains two separate membranes. This feature sets these organisms apart from Gram-positive bacteria and eukaryotic cells by providing them with full benefits of the synergy between slow transmembrane diffusion and active efflux.

Consideration of the Unbound Drug Concentration in Enzyme Kinetics

The study of enzyme kinetics in drug metabolism involves assessment of rates of metabolism and inhibitory potencies over a suitable concentration range. In all but the very simplest in vitro system, these drug concentrations can be influenced by a variety of nonspecific binding reservoirs that can reduce the available concentration to the enzyme system(s) under investigation. As a consequence, the apparent kinetic parameters, such as K_m or K_i, that are derived can deviate from the true values. There are a number of sources of these nonspecific binding depots or barriers, including membrane permeation and partitioning, plasma or serum protein binding, and incubational binding. In the latter case, this includes binding to the assay apparatus as well as biological depots, depending on the characteristics of the in vitro matrix being used. Given the wide array of subcellular, cellular, and recombinant enzyme systems utilized in drug metabolism, each of these has different components which can influence the free drug concentration. The physicochemical properties of the test compound are also paramount in determining the influential factors in any deviation between true and apparent kinetic behavior. This chapter describes the underlying mechanisms determining the free drug concentration in vitro and how these factors can be accounted for in drug metabolism studies, illustrated with case studies from the literature.

A Novel Physiologically Based Model of Creatinine Renal Disposition to Integrate Current Knowledge of Systems Parameters and Clinical Observations

Creatinine is the most common clinical biomarker of renal function. As a substrate for renal transporters, its secretion is susceptible to inhibition by drugs, resulting in transient increase in serum creatinine and false impression of damage to kidney. Novel physiologically based models for creatinine were developed here and (dis)qualified in a stepwise manner until consistency with clinical data. Data from a matrix of studies were integrated, including systems data (common to all models), proteomics-informed in vitro-in vivo extrapolation of all relevant transporter clearances, exogenous administration of creatinine (to estimate endogenous synthesis rate), and inhibition of different renal transporters (11 perpetrator drugs considered for qualification during creatinine model development and verification on independent data sets). The proteomics-informed bottom-up approach resulted in the underprediction of creatinine renal secretion. Subsequently, creatinine-trimethoprim clinical data were used to inform key model parameters in a reverse translation manner, highlighting best practices and challenges for middle-out optimization of mechanistic models.

VIVD: Virtual in vitro distribution model for the mechanistic prediction of intracellular concentrations of chemicals in in vitro toxicity assays

In vitro toxicity testing routinely uses nominal treatment concentrations as the driver for measured toxicity endpoints. However, test compounds can bind to the plastic of culture vessels or interact with culture media components, such as lipids and albumin. Additionally, volatile compounds may partition into the air above culture media. These processes reduce the free concentrations of compound to which cells are exposed. Models predicting the freely dissolved concentrations by accounting for these interactions have been published. However, these have only been applied to neutral compounds or assume no differential ionisation of test compounds between the media and cell cytoplasm. Herein, we describe an in vitro distribution model, based on the Fick-Nernst Planck equation accounting for differential compound ionisation in culture medium and intracellular water. The model considers permeability of ionised and unionised species and accounts for membrane potential in the partitioning of ionised moieties. By accounting for lipid and protein binding in culture medium, binding to cell culture plastic, air-partitioning, and lipid binding in the cell, the model can predict chemical concentrations (free and total) in medium and cells. The model can improve in vitro in vivo extrapolation of toxicity endpoint by determining intracellular concentrations for translation to in vivo.

Derivation of a System-Independent Ki for P-glycoprotein Mediated Digoxin Transport from System-Dependent IC50 Data

It has been previously demonstrated that IC₅₀ values for inhibition of digoxin transport across confluent polarized cell monolayers are system-dependent. Digoxin IC₅₀ data from five laboratories participating in the P-glycoprotein (P-gp) IC₅₀ Initiative, using Caco-2, MDCKII-hMDR1 or LLC-PK1-hMDR1 cells, were fitted by the structural mass action kinetic model for P-gp-mediated transport across confluent cell monolayers. We determined their efflux-active P-gp concentration [T(0)], inhibitor elementary dissociation rate constant from P-gp (k_rQ), digoxin basolateral uptake clearance (k_B), and inhibitor binding affinity to the digoxin basolateral uptake transporter (K_QB). We also fitted the IC₅₀ data for inhibition of digoxin transport through monolayers of primary human proximal tubule cells (HPTCs). All cell systems kinetically required a basolateral uptake transporter for digoxin, which also bound to all inhibitors. The inhibitor k_rQ was cell system-independent, thereby allowing calculation of a system-independent Ki. The variability in efflux-active P-gp concentrations and basolateral uptake clearances in the five laboratories was about an order of magnitude. These laboratory-to-laboratory variabilities can explain more than 60% of the IC₅₀ variability found in the principal component analysis plot in a previous study, supporting the hypothesis that the observed IC₅₀ variability is primarily due to differences in expression levels of efflux-active P-gp and the basolateral digoxin uptake transporter. HPTCs had 10- to 100-fold lower efflux-active P-gp concentrations than the overexpressing cell lines, whereas their digoxin basolateral uptake clearances were similar. HPTC basolateral uptake of digoxin was inhibited 50% by 10 μM ouabain, suggesting involvement of OATP4C1.

Understanding the Interplay Between Uptake and Efflux Transporters Within In Vitro Systems in Defining Hepatocellular Drug Concentrations

One of the most holistic in vitro systems for prediction of intracellular drug concentrations is sandwich-cultured hepatocytes (SCH); however, a comprehensive evaluation of the utility of SCH to estimate uptake and biliary clearances and the need for additional kinetic parameters has yet to be carried out. Toward this end, we have selected 9 compounds (rosuvastatin, valsartan, fexofenadine, pravastatin, repaglinide, telmisartan, atorvastatin, saquinavir, and quinidine) to provide a range of physicochemical and hepatic disposition properties. Uptake clearances were determined in SCH and compared with conventional monolayer and suspension hepatocyte systems, previously reported by our laboratory. CL_uptake ranged from 1 to 41 μL/min/10⁶ cells in SCH which were significantly lower (1%-10%) compared with the other hepatocyte models. The hepatocyte-to-media unbound concentration ratio (Kp_u) has been assessed and ranged 0.7-59, lower compared with other hepatocyte systems (8-280). Estimates of in vitro biliary clearance (CL_bile) for 4 drugs were determined and were scaled to predict in vivo values using both intracellular concentration and media drug concentrations. These studies demonstrate that reduced uptake in rat SCH may limit drug access to canalicular efflux transport proteins and highlight the importance of elucidating the interplay between these proteins for accurate prediction of hepatic clearance.

In vitro oral drug permeation models: the importance of taking physiological and physico-chemical factors into consideration

INTRODUCTION

The assessment of intestinal membrane permeability properties of new chemical entities is a crucial step in the drug discovery and development process and a variety of in vitro models, methods and techniques are available to estimate the extent of oral drug absorption in humans. However, variations in certain physiological and physico-chemical factors are often not reflected in the results and the complex dynamic interplay between these factors is sometimes oversimplified with in vitro models. Areas covered: In vitro models to evaluate drug pharmacokinetics are briefly outlined, while both physiological and physico-chemical factors that may have an influence on these techniques are critically reviewed. The shortcomings identified for some of the in vitro techniques are discussed in conjunction with novel ways to improve and thereby overcome some challenges. Expert opinion: Although conventional in vitro methods and theories are used as basic guidelines to predict drug absorption, critical evaluations have identified some shortcomings. Advancements in technology have made it possible to investigate and understand the role of physiological and physico-chemical factors in drug delivery more clearly, which can be used to improve and refine the techniques to more closely mimic the in vivo environment.

Key to Opening Kidney for In Vitro-In Vivo Extrapolation Entrance in Health and Disease: Part I: In Vitro Systems and Physiological Data

The programme for the 2015 AAPS Annual Meeting and Exhibition (Orlando, FL; 25-29 October 2015) included a sunrise session presenting an overview of the state-of-the-art tools for in vitro-in vivo extrapolation (IVIVE) and mechanistic prediction of renal drug disposition. These concepts are based on approaches developed for prediction of hepatic clearance, with consideration of scaling factors physiologically relevant to kidney and the unique and complex structural organisation of this organ. Physiologically relevant kidney models require a number of parameters for mechanistic description of processes, supported by quantitative information on renal physiology (system parameters) and in vitro/in silico drug-related data. This review expands upon the themes raised during the session and highlights the importance of high quality in vitro drug data generated in appropriate experimental setup and robust system-related information for successful IVIVE of renal drug disposition. The different in vitro systems available for studying renal drug metabolism and transport are summarised and recent developments involving state-of-the-art technologies highlighted. Current gaps and uncertainties associated with system parameters related to human kidney for the development of physiologically based pharmacokinetic (PBPK) model and quantitative prediction of renal drug disposition, excretion, and/or metabolism are identified.

Structural perturbation of a dipalmitoylphosphatidylcholine (DPPC) bilayer by warfarin and its bolaamphiphilic analogue: A molecular dynamics study

Compounds with nominally similar biological activity may exhibit differential toxicity due to differences in their interactions with cell membranes. Many pharmaceutical compounds are amphiphilic and can be taken up by phospholipid bilayers, interacting strongly with the lipid-aqueous interface whether or not subsequent permeation through the bilayer is possible. Bolaamphiphilic compounds, which possess two hydrophilic ends and a hydrophobic linker, can likewise undergo spontaneous uptake by bilayers. While membrane-spanning bolaamphiphiles can stabilize membranes, small molecules with this characteristic have the potential to create membrane defects via disruption of bilayer structure and dynamics. When compared to the amphiphilic therapeutic anticoagulant, warfarin, the bolaamphiphilic analogue, brodifacoum, exhibits heightened toxicity that goes beyond superior inhibition of the pharmacological target enzyme. We explore, herein, the consequences of anticoagulant accumulation in a dipalmitoylphosphatidylcholine (DPPC) bilayer. Coarse-grained molecular dynamics simulations reveal that permeation of phospholipid bilayers by brodifacoum causes a disruption of membrane barrier function that is driven by the bolaamphiphilic nature and size of this molecule. We find that brodifacoum partitioning into bilayers causes membrane thinning and permeabilization and promotes lipid flip-flop - phenomena that are suspected to play a role in triggering cell death. These phenomena are either absent or less pronounced in the case of the less toxic, amphiphilic compound, warfarin.

Direct In Vivo Human Intestinal Permeability (Peff ) Determined with Different Clinical Perfusion and Intubation Methods

Regional in vivo human intestinal effective permeability (Peff ) is calculated by measuring the disappearance rate of substances during intestinal perfusion. Peff is the most relevant parameter in the prediction of rate and extent of drug absorption from all parts of the intestine. Today, human intestinal perfusions are not performed on a routine basis in drug development. Therefore, it would be beneficial to increase the accuracy of the in vitro and in silico tools used to evaluate the intestinal Peff of novel drugs. This review compiles historical Peff data from 273 individual measurements of 80 substances from 61 studies performed in all parts of the human intestinal tract. These substances include: drugs, monosaccharaides, amino acids, dipeptides, vitamins, steroids, bile acids, ions, fatty acids, and water. The review also discusses the determination and prediction of Peff using in vitro and in silico methods such as quantitative structure-activity relationship, Caco-2, Ussing chamber, animal intestinal perfusion, and physiologically based pharmacokinetic (PBPK) modeling. Finally, we briefly outline how to acquire accurate human intestinal Peff data by deconvolution of plasma concentration-time profiles following regional intestinal bolus dosing.

How drugs get into cells: tested and testable predictions to help discriminate between transporter-mediated uptake and lipoidal bilayer diffusion

One approach to experimental science involves creating hypotheses, then testing them by varying one or more independent variables, and assessing the effects of this variation on the processes of interest. We use this strategy to compare the intellectual status and available evidence for two models or views of mechanisms of transmembrane drug transport into intact biological cells. One (BDII) asserts that lipoidal phospholipid Bilayer Diffusion Is Important, while a second (PBIN) proposes that in normal intact cells Phospholipid Bilayer diffusion Is Negligible (i.e., may be neglected quantitatively), because evolution selected against it, and with transmembrane drug transport being effected by genetically encoded proteinaceous carriers or pores, whose “natural” biological roles, and substrates are based in intermediary metabolism. Despite a recent review elsewhere, we can find no evidence able to support BDII as we can find no experiments in intact cells in which phospholipid bilayer diffusion was either varied independently or measured directly (although there are many papers where it was inferred by seeing a covariation of other dependent variables). By contrast, we find an abundance of evidence showing cases in which changes in the activities of named and genetically identified transporters led to measurable changes in the rate or extent of drug uptake. PBIN also has considerable predictive power, and accounts readily for the large differences in drug uptake between tissues, cells and species, in accounting for the metabolite-likeness of marketed drugs, in pharmacogenomics, and in providing a straightforward explanation for the late-stage appearance of toxicity and of lack of efficacy during drug discovery programmes despite macroscopically adequate pharmacokinetics. Consequently, the view that Phospholipid Bilayer diffusion Is Negligible (PBIN) provides a starting hypothesis for assessing cellular drug uptake that is much better supported by the available evidence, and is both more productive and more predictive.

Permeability comparison between hepatocyte and low efflux MDCKII cell monolayer

Determination of passive permeability is not only important for predicting oral absorption and brain penetration, but also for accurately predicting hepatic clearance. High throughput (HT) measurement of passive permeability across hepatocyte cell membrane is technically more challenging than using monolayer cell-based permeability assays. In this study, we evaluated if the HT Madin-Darby canine kidney II-low efflux (MDCKII-LE) cell monolayer permeability assay can be used as a surrogate to predict the passive permeability of hepatocytes. Apparent passive permeability of MDCKII-LE is well correlated to passive diffusion clearance of human and rat hepatocytes, suggesting that the HT MDCKII-LE assay can be used as a surrogate to estimate the passive permeability of hepatocytes. In addition, lipophilicity (Log D determined at pH 7.4) was also found to be well correlated with both MDCKII-LE and hepatocyte permeability for most compounds, hence it may serve as another permeability surrogate.