Insulin aggregation and asymmetric transport across human bronchial epithelial cell monolayers (Calu-3)

Nicotine transport across calu-3 cell monolayer: effect of nicotine salts and flavored e-liquids

OBJECTIVE

This study aimed to investigate the transport capability of nicotine across Calu-3 cell monolayer in various nicotine forms, including nicotine freebase, nicotine salts, and flavored e-liquids with nicotine benzoate.

SIGNIFICANCE

Nicotine is rapidly absorbed from the respiratory system into systemic circulation during e-cigarettes use. However, the mechanism of nicotine transport in the lung has not been well understood yet. This study may offer critical biological evidence and have implications for the use and regulation of e-cigarettes.

METHODS

The viability of Calu-3 cells after administration of nicotine freebase, nicotine salts and representative e-liquid were evaluated using the MTT assay, and the integrity of the Calu-3 cell monolayer was evaluated by transepithelial electrical resistance measurement and morphological analysis. Further, the nicotine transport capacity across the Calu-3 cell monolayer in various formulations of nicotine was investigated by analysis of nicotine transport amount.

RESULTS

The findings indicated that nicotine transport occurred passively and was time-dependent across the Calu-3cell monolayer. In addition, the nicotine transport was influenced by the type of nicotine salts and their respective pH value. The nicotine benzoate exhibited the highest apparent permeability coefficient (Papp), and higher nicotine-to-benzoic acid ratios led to higher Papp values. The addition of flavors to e-liquid resulted in increased Papp values, with the most significant increment being observed in tobacco-flavored e-liquid.

CONCLUSIONS

In summary, the transport capability of nicotine across the Calu-3 cell monolayer was influenced by the pH values of nicotine salts and flavor additives in e-liquids.

A viral pan-end RNA element and host complex define a SARS-CoV-2 regulon

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of COVID-19, generates multiple protein-coding, subgenomic RNAs (sgRNAs) from a longer genomic RNA, all bearing identical termini with poorly understood roles in regulating viral gene expression. Insulin and interferon-gamma, two host-derived, stress-related agents, and virus spike protein, induce binding of glutamyl-prolyl-tRNA synthetase (EPRS1), within an unconventional, tetra-aminoacyl-tRNA synthetase complex, to the sgRNA 3'-end thereby enhancing sgRNA expression. We identify an EPRS1-binding sarbecoviral pan-end activating RNA (SPEAR) element in the 3'-end of viral RNAs driving agonist-induction. Translation of another co-terminal 3'-end feature, ORF10, is necessary for SPEAR-mediated induction, independent of Orf10 protein expression. The SPEAR element enhances viral programmed ribosomal frameshifting, thereby expanding its functionality. By co-opting noncanonical activities of a family of essential host proteins, the virus establishes a post-transcriptional regulon stimulating global viral RNA translation. A SPEAR-targeting strategy markedly reduces SARS-CoV-2 titer, suggesting a pan-sarbecoviral therapeutic modality.

From Static to Dynamic: A Review on the Role of Mucus Heterogeneity in Particle and Microbial Transport

Mucus layers (McLs) are on the front line of the human defense system that protect us from foreign abiotic/biotic particles (e.g., airborne virus SARS-CoV-2) and lubricates our organs. Recently, the impact of McLs on human health (e.g., nutrient absorption and drug delivery) and diseases (e.g., infections and cancers) has been studied extensively, yet their mechanisms are still not fully understood due to their high variety among organs and individuals. We characterize these variances as the heterogeneity of McLs, which lies in the thickness, composition, and physiology, making the systematic research on the roles of McLs in human health and diseases very challenging. To advance mucosal organoids and develop effective drug delivery systems, a comprehensive understanding of McLs' heterogeneity and how it impacts mucus physiology is urgently needed. When the role of airway mucus in the penetration and transmission of coronavirus (CoV) is considered, this understanding may also enable a better explanation and prediction of the CoV's behavior. Hence, in this Review, we summarize the variances of McLs among organs, health conditions, and experimental settings as well as recent advances in experimental measurements, data analysis, and model development for simulations.

Biological Models of the Lower Human Airways-Challenges and Special Requirements of Human 3D Barrier Models for Biomedical Research

In our review, we want to summarize the current status of the development of airway models and their application in biomedical research. We start with the very well characterized models composed of cell lines and end with the use of organoids. An important aspect is the function of the mucus as a component of the barrier, especially for infection research. Finally, we will explain the need for a nondestructive characterization of the barrier models using TEER measurements and live cell imaging. Here, organ-on-a-chip technology offers a great opportunity for the culture of complex airway models.

Nebulised surface-active hybrid nanoparticles of voriconazole for pulmonary Aspergillosis demonstrate clathrin-mediated cellular uptake, improved antifungal efficacy and lung retention

BACKGROUND

Incidence of pulmonary aspergillosis is rising worldwide, owing to an increased population of immunocompromised patients. Notable potential of the pulmonary route has been witnessed in antifungal delivery due to distinct advantages of direct lung targeting and first-pass evasion. The current research reports biomimetic surface-active lipid-polymer hybrid (LPH) nanoparticles (NPs) of voriconazole, employing lung-specific lipid, i.e., dipalmitoylphosphatidylcholine and natural biodegradable polymer, i.e., chitosan, to augment its pulmonary deposition and retention, following nebulization.

RESULTS

The developed nanosystem exhibited a particle size in the range of 228-255 nm and drug entrapment of 45-54.8%. Nebulized microdroplet characterization of NPs dispersion revealed a mean diameter of  ≤ 5 μm, corroborating its deep lung deposition potential as determined by next-generation impactor studies. Biophysical interaction of LPH NPs with lipid-monolayers indicated their surface-active potential and ease of intercalation into the pulmonary surfactant membrane at the air-lung interface. Cellular viability and uptake studies demonstrated their cytocompatibility and time-and concentration-dependent uptake in lung-epithelial A549 and Calu-3 cells with clathrin-mediated internalization. Transepithelial electrical resistance experiments established their ability to penetrate tight airway Calu-3 monolayers. Antifungal studies on laboratory strains and clinical isolates depicted their superior efficacy against Aspergillus species. Pharmacokinetic studies revealed nearly 5-, 4- and threefolds enhancement in lung AUC, Tmax, and MRT values, construing significant drug access and retention in lungs.

CONCLUSIONS

Nebulized LPH NPs were observed as a promising solution to provide effective and safe therapy for the management of pulmonary aspergillosis infection with improved patient compliance and avoidance of systemic side-effects.

Mucus models to evaluate the diffusion of drugs and particles

Mucus is a complex hydrogel that acts as a natural barrier to drug delivery at different mucosal surfaces including the respiratory, gastrointestinal, and vaginal tracts. To elucidate the role mucus plays in drug delivery, different in vitro, in vivo, and ex vivo mucus models and techniques have been utilized. Drug and drug carrier diffusion can be studied using various techniques in either isolated mucus gels or mucus present on cell cultures and tissues. The species, age, and potential disease state of the animal from which mucus is derived can all impact mucus composition and structure, and therefore impact drug and drug carrier diffusion. This review provides an overview of the techniques used to characterize drug and drug carrier diffusion, and discusses the advantages and disadvantages of the different models available to highlight the information they can afford.

Polarized Airway Epithelial Models for Immunological Co-Culture Studies

Epithelial cells line all cavities and surfaces throughout the body and play a substantial role in maintaining tissue homeostasis. Asthma and other atopic diseases are increasing worldwide and allergic disorders are hypothesized to be a consequence of a combination of dysregulation of the epithelial response towards environmental antigens and genetic susceptibility, resulting in inflammation and T cell-derived immune responses. In vivo animal models have long been used to study immune homeostasis of the airways but are limited by species restriction and lack of exposure to a natural environment of both potential allergens and microflora. Limitations of these models prompt a need to develop new human cell-based in vitro models. A variety of co-culture systems for modelling the respiratory epithelium exist and are available to the scientific community. The models have become increasingly sophisticated and specific care needs to be taken with regard to cell types, culture medium and culture models, depending on the aim of the study. Although great strides have been made, there is still a need for further optimization, and optimally also for standardization, in order for in vitro co-culture models to become powerful tools in the discovery of key molecules dictating immunity and/or tolerance, and for understanding the complex interplay that takes place between mucosa, airway epithelium and resident or infiltrating immune cells. This review focuses on current knowledge and the advantages and limitations of the different cell types and culture methods used in co-culture models of the human airways.

Permeation of Therapeutic Drugs in Different Formulations across the Airway Epithelium In Vitro

BACKGROUND

Pulmonary drug delivery is characterized by short onset times of the effects and an increased therapeutic ratio compared to oral drug delivery. This delivery route can be used for local as well as for systemic absorption applying drugs as single substance or as a fixed dose combination. Drugs can be delivered as nebulized aerosols or as dry powders. A screening system able to mimic delivery by the different devices might help to assess the drug effect in the different formulations and to identify potential interference between drugs in fixed dose combinations. The present study evaluates manual devices used in animal studies for their suitability for cellular studies.

METHODS

Calu-3 cells were cultured submersed and in air-liquid interface culture and characterized regarding mucus production and transepithelial electrical resistance. The influence of pore size and material of the transwell membranes and of the duration of air-liquid interface culture was assessed. Compounds were applied in solution and as aerosols generated by MicroSprayer IA-1C Aerosolizer or by DP-4 Dry Powder Insufflator using fluorescein and rhodamine 123 as model compounds. Budesonide and formoterol, singly and in combination, served as examples for drugs relevant in pulmonary delivery.

RESULTS AND CONCLUSIONS

Membrane material and duration of air-liquid interface culture had no marked effect on mucus production and tightness of the cell monolayer. Co-application of budesonide and formoterol, applied in solution or as aerosol, increased permeation of formoterol across cells in air-liquid interface culture. Problems with the DP-4 Dry Powder Insufflator included compound-specific delivery rates and influence on the tightness of the cell monolayer. These problems were not encountered with the MicroSprayer IA-1C Aerosolizer. The combination of Calu-3 cells and manual aerosol generation devices appears suitable to identify interactions of drugs in fixed drug combination products on permeation.

A microdevice for parallelized pulmonary permeability studies

We describe a compartmentalized microdevice specifically designed to perform permeability studies across a model of lung barrier. Epithelial cell barriers were reproduced by culturing Calu-3 cells at the air-liquid interface (AIC) in 1 mm² microwells made from a perforated glass slide with an embedded porous membrane. We created a single basolateral reservoir for all microwells which eliminated the need to renew the growth medium during the culture growth phase. To perform drug permeability studies on confluent cell layers, the cell culture slide was aligned and joined to a collection platform consisting in 35 μL collection reservoirs connected at the top and bottom with microchannels. The integrity and functionality of the cell barriers were demonstrated by measurement of trans-epithelial electrical resistance (TEER), confocal imaging and permeability assays of ¹⁴C-sucrose. Micro-cell barriers were able to form confluent layers in 1 week, demonstrating a similar bioelectrical evolution as the Transwell systems used as controls. Tight junctions were observed throughout the cell-cell interfaces, and the low permeability coefficients of ¹⁴C-sucrose confirmed their functional presence, creating a primary barrier to the diffusion of solutes. This microdevice could facilitate the monitoring of biomolecule transport and the screening of formulations promoting their passage across the pulmonary barrier, in order to select candidates for pulmonary administration to patients.

Protein comparability assessments and potential applicability of high throughput biophysical methods and data visualization tools to compare physical stability profiles

In this review, some of the challenges and opportunities encountered during protein comparability assessments are summarized with an emphasis on developing new analytical approaches to better monitor higher-order protein structures. Several case studies are presented using high throughput biophysical methods to collect protein physical stability data as function of temperature, agitation, ionic strength and/or solution pH. These large data sets were then used to construct empirical phase diagrams (EPDs), radar charts, and comparative signature diagrams (CSDs) for data visualization and structural comparisons between the different proteins. Protein samples with different sizes, post-translational modifications, and inherent stability are presented: acidic fibroblast growth factor (FGF-1) mutants, different glycoforms of an IgG1 mAb prepared by deglycosylation, as well as comparisons of different formulations of an IgG1 mAb and granulocyte colony stimulating factor (GCSF). Using this approach, differences in structural integrity and conformational stability profiles were detected under stress conditions that could not be resolved by using the same techniques under ambient conditions (i.e., no stress). Thus, an evaluation of conformational stability differences may serve as an effective surrogate to monitor differences in higher-order structure between protein samples. These case studies are discussed in the context of potential utility in protein comparability studies.

Pharmaceutical applications of the Calu-3 lung epithelia cell line

INTRODUCTION

The Calu-3 lung cell line has been shown to be a promising in vitro model of airway epithelia due to its similarity to in vivo physiology. Hence, over the past decade, it has found increasing applications in the pharmaceutical industry.

AREAS COVERED

This review focuses on the pharmaceutical applications of the Calu-3 cell line in areas such as mechanisms of drug transport, studying aerosol deposition, controlled release studies and identification of possible drug-drug interactions. The main findings of various studies, as well as the predictive potential of this model, are presented and discussed in this review.

EXPERT OPINION

There is still a lack of mechanistic knowledge regarding transport of inhaled therapeutics across the lungs. Cell culture models such as Calu-3 provide a simple and reproducible system to study the underlying mechanisms by which inhaled therapeutics interact with the lungs. However, more complex systems that integrate particle deposition onto different cell culture systems may be useful in addressing some fundamental questions to generate a better understanding of determinants that influences pulmonary drug dissolution, absorption, metabolism and efficacy. Ultimately the use of the Calu-3 cell line provides a basic research tool that enables the development of safer and more effective inhaled therapeutics.

Multimerization and aggregation of native-state insulin: effect of zinc

The aggregation of insulin is complicated by the coexistence of various multimers, especially in the presence of Zn(2+). Most investigations of insulin multimerization tend to overlook aggregation kinetics, while studies of insulin aggregation generally pay little attention to multimerization. A clear understanding of the starting multimer state of insulin is necessary for the elucidation of its aggregation mechanism. In this work, the native-state aggregation of insulin as either the Zn-insulin hexamer or the Zn-free dimer was studied by turbidimetry and dynamic light scattering, at low ionic strength and pH near pI. The two states were achieved by varying the Zn(2+) content of insulin at low concentrations, in accordance with size-exclusion chromatography results and literature findings (Tantipolphan, R.; Romeijn, S.; Engelsman, J. d.; Torosantucci, R.; Rasmussen, T.; Jiskoot, W. J. Pharm. Biomed. 2010, 52, 195). The much greater aggregation rate and limiting turbidity (τ(∞)) for the Zn-insulin hexamer relative to the Zn-free dimer was explained by their different aggregation mechanisms. Sequential first-order kinetic regimes and the concentration dependence of τ(∞) for the Zn-insulin hexamer indicate a nucleation and growth mechanism, as proposed by Wang and Kurganov (Wang, K.; Kurganov, B. I. Biophys. Chem. 2003, 106, 97). The pure second-order process for the Zn-free dimer suggests isodesmic aggregation, consistent with the literature. The aggregation behavior at an intermediate Zn(2+) concentration appears to be the sum of the two processes.

Nanocarriers for pulmonary administration of peptides and therapeutic proteins

Peptides and therapeutic proteins have been the target of intense research and development in recent years by the pharmaceutical and biotechnology industry. Preferably, they are administered through the parenteral route, which is associated with reduced patient compliance. Formulations for noninvasive administration of peptides and therapeutic proteins are currently being developed. Among them, inhalation appears as a promising alternative for the administration of such products. Several formulations for pulmonary delivery are in various stages of development. Despite positive results, conventional formulations have some limitations such as reduced bioavailability and side effects. Nanocarriers may be an alternative way to overcome the problems of conventional formulations. Some nanocarrier-based formulations of peptides and therapeutic proteins are currently under development. The results obtained are promising, revealing the usefulness of these systems in the delivery of such drugs.

Calu-3 cells grown under AIC and LCC conditions: implications for dipeptide uptake and transepithelial transport of substances

The aim of the present study was to investigate whether Calu-3 cell culture conditions influence drug and nutrient transport known to occur via carriers or transporters. Calu-3 cell layers, an in vitro model of the lung epithelium, were cultured using air interfaced culture (AIC) or liquid covered culture (LCC) on either polycarbonate or polyester as filter support material. We found that the development of the Calu-3 cell layer barrier function did not depend on the filter material but rather on the culture conditions as follows: (i) the apical uptake of Gly-Sar was significantly larger for cells grown in AIC compared to LCC, (ii) the TEER values for cells grown in LCC were approximately three times larger than for cells grown in AIC, (iii) the transepithelial transport in both AIC and LCC Calu-3 cells was polarized in the apical-basolateral direction of proline, glycine, α-methyl-d-glucoside, glipizide, taurocholic acid and estrone-3-sulfate, whereas inulin, mannitol and Gly-Sar showed no polarized transport. Etoposide showed polarized efflux (basolateral to apical transport) in AIC and LCC Calu-3 layers. These findings provide information about nutrient and drug transport in Calu-3 cells, and this may have implications for selecting culture conditions for transport studies in this in vitro model of the lung epithelium.

Insulin-induced electrophysiology changes in human pleura are mediated via its receptor

Background. Insulin directly changes the sheep pleural electrophysiology. The aim of this study was to investigate whether insulin induces similar effects in human pleura, to clarify insulin receptor's involvement, and to demonstrate if glibenclamide (hypoglycemic agent) reverses this effect. Methods. Human parietal pleural specimens were mounted in Ussing chambers. Solutions containing insulin or glibenclamide and insulin with anti-insulin antibody, anti-insulin receptor antibody, and glibenclamide were used. The transmesothelial resistance (R_TM) was determined. Immunohistochemistry for the presence of Insulin Receptors (IRa, IRb) was also performed. Results. Insulin increased R_TM within 1st min (P = .016), when added mesothelially which was inhibited by the anti-insulin and anti-insulin receptor antibodies. Glibenclamide also eliminated the insulin-induced changes. Immunohistochemistry verified the presence of IRa and IRb. Conclusion. Insulin induces electrochemical changes in humans as in sheep via interaction with its receptor. This effect is abolished by glibenclamide.

Transcellular passage of Neisseria meningitidis across a polarized respiratory epithelium

Neisseria meningitidis is a major cause of sepsis and meningitis but is also a common commensal, present in the nasopharynx of between 8 and 20% of healthy individuals. During carriage, the bacterium is found on the surface of the nasopharyngeal epithelium and in deeper tissues, while to develop disease the meningococcus must spread across the respiratory epithelium and enter the systemic circulation. Therefore, investigating the pathways by which N. meningitidis crosses the epithelial barrier is relevant for understanding carriage and disease but has been hindered by the lack of appropriate models. Here, we have established a physiologically relevant model of the upper respiratory epithelial cell barrier to investigate the mechanisms responsible for traversal of N. meningitidis. Calu-3 human respiratory epithelial cells were grown on permeable cell culture membranes to form polarized monolayers of cells joined by tight junctions. We show that the meningococcus crosses the epithelial cell barrier by a transcellular route; traversal of the layer did not disrupt its integrity, and bacteria were detected within the cells of the monolayer. We demonstrate that successful traversal of the epithelial cell barrier by N. meningitidis requires expression of its type 4 pili (Tfp) and capsule and is dependent on the host cell microtubule network. The Calu-3 model should be suitable for dissecting the pathogenesis of infections caused by other respiratory pathogens, as well as the meningococcus.

Passive asymmetric transport of hesperetin across isolated rabbit cornea

Hesperetin, an aglycone of the flavanone hesperidin, is a potential candidate for the treatment of diabetic retinopathy and macular edema. The purpose of this investigation was to determine solubility, stability and in vitro permeability characteristics of hesperetin across excised rabbit corneas. Aqueous and pH dependent solubility was determined using standard shake flask method. Solution stability was evaluated as a function of pH (1.2-9) and temperature (25 and 40 degrees C). Permeability of hesperetin was determined across the isolated rabbit cornea utilizing a side-bi-side diffusion apparatus, in the apical to basolateral (A-B) and basolateral to apical (B-A) directions. Hesperetin displayed asymmetrical transcorneal transport with a 2.3-fold higher apparent permeability in the B-A direction compared to the A-B direction. The transport process was observed to be pH dependent. Surprisingly, however, the involvement of efflux transporters or proton-coupled carrier-systems was not evident in this asymmetric transcorneal diffusion process. The passive and pH dependent corneal transport of hesperetin could probably be attributable to corneal ultrastructure, physicochemical characteristics of hesperetin and the role of transport buffer components.

Technosphere insulin: defining the role of Technosphere particles at the cellular level

BACKGROUND

Technosphere Insulin (TI) is a novel inhalation powder for the treatment of diabetes mellitus. Technosphere Insulin delivers insulin with an ultra rapid pharmacokinetic profile that is distinctly different from all other insulin products but similar to natural insulin release. Such rapid absorption is often associated with penetration enhancers that disrupt cellular integrity.

METHODS

Technosphere Insulin was compared to a panel of known penetration enhancers in vitro using the Calu-3 lung cell line to investigate the effects of TI on insulin transport.

RESULTS

Measures of tight junction integrity such as transepithelial electrical resistance, Lucifer yellow permeability, and F-actin staining patterns were all unaffected by TI. Cell viability and plasma membrane integrity were also not affected by TI. In contrast, cells treated with comparable (or lower) concentrations of penetration enhancers showed elevated Lucifer yellow permeability, disruption of the F-actin network, reduced cell viability, and compromised plasma membranes.

CONCLUSIONS

These results demonstrate that TI is not cytotoxic in an in vitro human lung cell model and does not function as a penetration enhancer. Furthermore, TI does not appear to affect the transport of insulin across cellular barriers.

ENaC α-subunit variants are expressed in lung epithelial cells and are suppressed by oxidative stress

Amiloride-sensitive epithelial sodium channel (ENaC) is a major sodium channel in the lung facilitating fluid absorption. ENaC is composed of α-, β-, and γ-subunits, and the α-subunit is indispensable for ENaC function in the lung. In human lungs, the α-subunit is expressed as various splice variants. Among them, α1- and α2-subunits are two major variants with different upstream regulatory sequences that possess similar channel characteristics when tested in Xenopus oocytes. Despite the importance of α-ENaC, little was known about the relative abundance of its variants in lung epithelial cells. Furthermore, lung infection and inflammation are often accompanied by reduced α-ENaC expression, oxidative stress, and pulmonary edema. However, it was not clear how oxidative stress affects expression of α-ENaC variants. In this study, we examined relative expression levels of α-subunit variants in four human lung epithelial cell lines. We also tested the hypothesis that oxidative stress inhibits α-ENaC expression. Our results show that both α1- and α2-ENaC variants are expressed in the cells we tested, but relative abundance varies. In the two monolayer-forming cell lines, H441 and Calu-3, α2-ENaC is the predominant variant. We also show that H2O2 specifically suppresses α1- and α2-ENaC variant expression in H441 and Calu-3 cells in a dose-dependent fashion. This suppression is achieved by inhibition of their promoters and is attenuated by dexamethasone. These data demonstrate the importance of the α2-subunit variant and suggest that glucocorticoids and antioxidants may be useful in correcting infection/inflammation-induced lung fluid imbalance.

Indication of transcytotic movement of insulin across human bronchial epithelial cells

This study was performed to evaluate insulin permeability across human bronchial epithelial cell lines and investigate if insulin is transported via the paracellular or transcellular pathway. The movement of insulin across two bronchial epithelial cells, 16HBE14o- and Calu-3, was studied in the presence or absence of octylmaltoside. Mannitol and propanolol have been used as paracellular and transcellular marker, respectively, and transepithelial electrical resistance (TEER) was determined to investigate the tight junctional integrity of the monolayers. The possible endocytotic mechanism of insulin across these two cell lines was studied by confocal laser scanning microscopy after incubating the cells with fluorescent-labeled insulin. The TEER values for both cell monolayers were >400 Omega cm2 at confluency. There was a decrease in the TEER values when octylmaltoside was added to the apical side of transwells. Similarly, the apparent permeability coefficient (P(app)) values of insulin, mannitol and propanolol, showed an increase with the rise in the concentration of octylmaltoside. In the absence of octylmaltoside, the P(app) values for insulin and the markers were in the following order: propanolol > mannitol > insulin. Confocal microscopic studies revealed that the uptake of insulin by the bronchial epithelial cells perhaps occurs via translocation across the cell. The data presented in this study demonstrate that insulin perhaps moves across the bronchial cells via both paracellular and transcellular pathways.

In vivo, in vitro and ex vivo models to assess pulmonary absorption and disposition of inhaled therapeutics for systemic delivery

Despite the interest in systemic delivery of therapeutic molecules including macromolecular proteins and peptides via the lung, the accurate assessment of their pulmonary biopharmaceutics is a challenging experimental task. This article reviews in vivo, in vitro and ex vivo models currently available for studying lung absorption and disposition for inhaled therapeutic molecules. The general methodologies are discussed with recent advances, current challenges and perspectives, especially in the context of their use in systemic pulmonary delivery research. In vivo approaches in small rodents continue to be the mainstay of assessment by virtue of the acquisition of direct pharmacokinetic data, more meaningful when attention is given to reproducible dosing and control of lung-regional distribution through use of more sophisticated lung-dosing methods, such as forced instillation, microspray, nebulization and aerosol puff. A variety of in vitro lung epithelial cell lines models and primary cultured alveolar epithelial (AE) cells when grown to monolayer status offer new opportunity to clarify the more detailed kinetics and mechanisms of transepithelial drug transport. While continuous cell lines, Calu-3 and 16HBE14o-, show potential, primary cultured AE cell models from rat and human origins may be of greater use, by virtue of their universally tight intercellular junctions that discriminate the transport kinetics of different therapeutic entities. Nevertheless, the relevance of using these reconstructed barriers to represent complex disposition of intact lung may still be debatable. Meanwhile, the intermediate ex vivo model of the isolated perfused lung (IPL) appears to resolve deficiencies of these in vivo and in vitro models. While controlling lung-regional distributions, the preparation alongside a novel kinetic modeling analysis enables separate determinations of kinetic descriptors for lung absorption and non-absorptive clearances, i.e., mucociliary clearance, phagocytosis and/or metabolism. This ex vivo model has been shown to be kinetically predictive of in vivo, with respect to macromolecular disposition, despite limitations concerning short viable periods of 2-3 h and likely absence of tracheobronchial circulation. Given the advantages and disadvantages of each model, scientists must make appropriate selection and timely exploitation of the best model at each stage of the research and development program, affording efficient progress toward clinical trials for future inhaled therapeutic entities for systemic delivery.

Culture of Calu-3 cells at the air interface provides a representative model of the airway epithelial barrier

PURPOSE

The aim of this study was to compare the effect of liquid-covered culture (LCC) and air-interfaced culture (AIC) on Calu-3 cell layer morphology and permeability, thus assessing the fitness of these culture systems as models of airway epithelium barrier function.

METHODS

Cell layers were grown on 0.33 cm2 Transwell polyester cell culture supports. Cell layers grown using LCC and AIC were evaluated by using light and electron microscopy, transepithelial electrical resistance (TER), and permeability to the transepithelial flux of fluorescein sodium (flu-Na), and by varying molecular weight dextrans labeled with fluorescein isothiocyanate (FITC-dex). The tight junction protein, zona occludens protein-1 (ZO-1), was visualized by confocal microscopy and apical glycoprotein secretions were identified by using alcian blue.

RESULTS

Cells grown via AIC produced a more columnar epithelium with a more rugged apical topography and greater glycoprotein secretion compared to cells grown via LCC. Apical protrusions appearing to be cilia-like structures were observed on occasional cells using AIC, but typical airway ciliated cell phenotypes were not produced under either condition. Secretory granules were observed in cells cultured under both conditions. Cells cultured using LCC exhibited higher levels of ZO-1 protein than the AIC counterpart. The maximal TER of cells using LCC, 1,086 +/- 113 ohms cm2 at 11-16 days, was significantly greater than the TER of cells cultured using AIC, 306 +/- 53 ohms cm2 at 11-13 days. Apparent permeability (P(app)) values for the transport of flu-Na using LCC and AIC were 1.48 +/- 0.19x10(-7) and 3.36 +/- 0.47x10(-7) cm s(-1), respectively. Transport rates of flu-Na and FITC-dex were inversely proportional to molecular weight, and were significantly lower (p < 0.05) in cell layers grown using LCC than AIC. Renkin analysis fitted the data to single pore populations of radii 7.7 and 11.0 nm for LCC and AIC, respectively.

CONCLUSION

Distinct differences in morphology and permeability result when Calu-3 cells are grown using AIC or LCC. Cells cultured using AIC generate a model more morphologically representative of the airway epithelium than cells cultured using LCC.

Assessment of transport rates of proteins and peptides across primary human alveolar epithelial cell monolayers

In this study, we investigated bi-directional fluxes (i.e., in absorptive and secretive directions) of human serum proteins [albumin (HSA), transferrin (TF), and immunoglobulin G (IgG)] and peptides/proteins of potential therapeutic relevance [insulin (INS), glucagon-like peptide-1 (GLP-1), growth hormone (GH), and parathyroid hormone (PTH)] across tight monolayers of human alveolar epithelial cells (hAEpC) in primary culture. Apparent permeability coefficients (P(app); x10(-7)cm/s, mean+/-S.D.) for GLP-1 (6.13+/-0.87 (absorptive) versus 1.91+/-0.51 (secretive)), HSA (2.45+/-1.02 versus 0.21+/-0.31), TF (0.88+/-0.15 versus 0.30+/-0.03), and IgG (0.36+/-0.22 versus 0.15+/-0.16) were all strongly direction-dependent, i.e., net absorptive, while PTH (2.20+/-0.30 versus 1.80+/-0.77), GH (8.33+/-1.24 versus 9.02+/-3.43), and INS (0.77+/-0.15 versus 0.72+/-0.36) showed no directionality. Trichloroacetic acid precipitation analysis of tested molecules collected from donor and receiver fluids exhibited very little degradation. This is the first study on permeability data for a range of peptides and proteins across an in vitro model of the human alveolar epithelial barrier. These data indicate that there is no apparent size-dependent transport conforming to passive restricted diffusion for the tested substances across human alveolar barrier, in part confirming net absorptive transcytosis. The obtained data differ significantly from previously published reports utilising monolayers from different species. It can be concluded that the use of homologous tissue should be preferred to avoid species differences.

Human respiratory epithelial cell culture for drug delivery applications

Recent developments in delivering drugs to the lung are driving the need for in vitro methods to evaluate the fate of inhaled medicines. Constraints on experimentation using animals have promoted the use of human respiratory epithelial cell cultures to model the absorption barrier of the lung; with two airway cell lines, 16HBE14o- and Calu-3, and primary cultured human alveolar type I-like cells (hAEpC) gaining prominence. These in vitro models develop permeability properties which are comparable to those reported for native lung epithelia. This is in contrast to the high permeability of the A549 human alveolar cell line, which is unsuitable for use in drug permeability experiments. Tabulation of apparent permeability coefficients (Papp) of compounds measured in 'absorptive' and 'secretory' directions reveals that fewer compounds (< 15) have been evaluated in 16HBE14o- cells and hAEpC compared to Calu-3 cells (> 50). Vectorial (asymmetric) transport of compounds is reported in the three cell types with P-glycoprotein, the most studied transport mechanism, being reported in all. Progress is being made towards in vitro-in vivo-correlation for pulmonary absorption and in the use of cultured respiratory cells to evaluate drug metabolism, toxicity and targeting strategies. In summary, methods for the culture of human respiratory epithelial cell layers have been established and data regarding their permeability characteristics and suitability to model the lung is becoming available. Discerning the circumstances under which the use of human respiratory cell models will be essential, or offers advantages over non-organ, non-species specific cell models, is the next challenge.

Evidence for LHRH-receptor expression in human airway epithelial (Calu-3) cells and its role in the transport of an LHRH agonist

PURPOSE

To determine whether LHRH-receptor is expressed in Calu-3, a human bronchial epithelial cell line, and to further determine whether this receptor plays a role in the transport of deslorelin, an LHRH agonist.

METHODS

Using cultured monolayers of Calu-3 grown at air-interface, the presence and localization of LHRH-receptors in Calu-3 cells was determined using immunochemical methods. To determine the mechanisms of deslorelin transport, the directionality [apical-basolateral (A-B) and basolateral-apical (B-A)] of deslorelin transport across Calu-3 monolayers and the effects of temperature (37 degrees C and 4 degrees C) and an energy depletor (2,4-dinitrophenol) were investigated. To determine the role of LHRH-receptor in deslorelin transport across Calu-3 monolayers, the influence of an LHRH-receptor antisense oligonucleotide on the LHRH-receptor expression and deslorelin transport was studied. Also, the effect of a competing LHRH agonist, buserelin, on deslorelin transport was determined.

RESULTS

Immunofluorescence studies indicated the predominance of LHRH-receptor in Calu-3 cells at the apical and lateral surfaces. Western blot and RT-PCR studies further confirmed the expression of LHRH-receptor in Calu-3 cells. Deslorelin transport across Calu-3 monolayers was vectorial, with the cumulative A-B transport (1.79 +/- 0.29%) at the end of 240 min being higher than the B-A transport (0.34 +/- 0.11%). Low temperature as well as 2,4-dinitrophenol abolished this directionality. LHRH-receptor antisense oligonucleotide decreased the receptor expression at the mRNA and protein level and reduced the A-B deslorelin transport by 55 +/- 4%, without affecting the B-A transport, suggesting a role for LHRH-receptor in the vectorial transport of deslorelin. In addition, buserelin reduced the A-B deslorelin transport by 56 +/- 5% without affecting the B-A transport.

CONCLUSIONS

Taken together, our results provide evidence that deslorelin is transported across the respiratory epithelium via the LHRH-receptor.

Development of a size-dependent aerosol deposition model utilising human airway epithelial cells for evaluating aerosol drug delivery

Aerosol delivery to the airways of the human respiratory tract, followed by absorption, constitutes an alternative route of administration for compounds unsuitable for delivery by conventional oral and parenteral routes. The target for aerosol drug delivery is the airways epithelium, i.e. tracheal, bronchial, bronchiolar and alveolar cells, which become the site of drug deposition. These epithelial layers also serve as a barrier to the penetration of inhaled material. An in vitro model for aerosol deposition and transport across epithelia in the human airways may be a good predictor of in vivo disposition. The present preliminary studies begin an investigation that blends the dynamics of aerosol delivery and the basis of an in vitro simulated lung model to evaluate the transport properties of a series of molecular weight marker compounds across human-derived bronchiolar epithelial cell monolayers. An Andersen viable cascade impactor was used as a delivery apparatus for the deposition of size-segregated particles onto monolayers of small airway epithelial cells and Calu-3 cells. It was shown that these cell layers can withstand placement in the impactor, and that permeability can be tested subsequent to removal from the impactor.

Drug transport and metabolism characteristics of the human airway epithelial cell line Calu-3

Pulmonary drug delivery serves two purposes, namely the application of locally active compounds for treatment of diseases afflicting the lung, and the utilization of the pulmonary epithelia as absorption sites for macromolecular drugs. To elucidate the mechanisms involved in the pulmonary absorption and metabolism of compounds on a cellular level, cell culture models have shown to be, though limited, rather useful in predicting in vivo conditions. The Calu-3 cell line has been employed recently as a model for the airway epithelium in a number of drug transport and metabolism studies. The results of these studies, as well as an evaluation of the predictive potency of the model, are presented here.

Transport of HIV-protease inhibitors across 1 alpha,25di-hydroxy vitamin D3-treated Calu-3 cell monolayers: modulation of P-glycoprotein activity

PURPOSE

The presence of P-glycoprotein (P-gp) within the lipid bi-layers of the absorptive cells greatly influences drug entry into the HIV-infected sanctuary sites. The objective of this study was to acces the potential role of pulmonary cells expressing high levels of P-gp in the efflux of potent anti-HIV drugs such as protease inhibitors.

METHODS

Human airway epithelium-derived Calu-3 cells grown in the presence of 0.025 mM 1alpha,25di-hydroxy Vitamin D3 (di-OH vit D3) were used as a model to evaluate the effects of p-glycoprotein efflux of HIV protease inhibitors. Cells used as controls were not treated with di-OH vit D3. The anti-HIV agents 3H Ritonavir and Saquinavir (50 microM) were used as model compounds for influx and efflux studies

RESULTS

Di-OH vit D3 treatment enhanced the differentiation c Calu-3 cells indicated by more cilia and mucus secretion. It also caused elevated P-gp expression as demonstrated by Western Bla analysis and enhanced basal to apical transport of cyclosporine as compared with untreated cells. The amount of Saquinavir transported, after 3 h, across untreated Calu-3 cells (A-B) was 3-fold higher (1.62 microg; Papp = 2A (+/- 0.79) x 10(-6) cm/s) than di-OH vi D3-treated cells (0.57 microg with the Papp = 5.02 (+/- 0.62) x 10(-7) cm/s) Similar transport profiles were obtained for 3H ritonavir and a significant increase (p < 0.05) in the A-B transport (2.5-fold) of 3H ritonavir was observed when the cell monolayers were preincubated with testosterone prior to transport studies. However, transport of AZT remained unaltered in di-OH vit D3 treated monolayers.

CONCLUSION

Modulation of P-gp activity may be necessary to increase the therapeutic efficacy of protease inhibitors against HIV-1 reservoirs across alveolar lining cells and fluids.