A critical assessment of in vitro tissue models for ADME and drug delivery

Advances of dual-organ and multi-organ systems for gut, lung, skin and liver models in absorption and metabolism studies

Drug development is a costly and timely process with high risks of failure during clinical trials. Although in vitro tissue models have significantly advanced over the years, thus fostering a transition from animal-derived models towards human-derived models, failure rates still remain high. Current cell-based assays are still not able to provide an accurate prediction of the clinical success or failure of a drug candidate. To overcome the limitations of current methods, a variety of microfluidic systems have been developed as powerful tools that are capable of mimicking (micro)physiological conditions more closely by integrating physiological fluid flow conditions, mechanobiological cues and concentration gradients, to name only a few. One major advantage of these biochip-based tissue cultures, however, is their ability to seamlessly connect different organ models, thereby allowing the study of organ-crosstalk and metabolic byproduct effects. This is especially important when assessing absorption, distribution, metabolism, and excretion (ADME) processes of drug candidates, where an interplay between various organs is a prerequisite. In the current review, a number of in vitro models as well as microfluidic dual- and multi-organ systems are summarized with a focus on absorption (skin, lung, gut) and metabolism (liver). Additionally, the advantage of multi-organ chips in identifying a drug's on and off-target toxicity is discussed. Finally, the potential high-throughput implementation and modular chip design of multi-organ-on-a-chip systems within the pharmaceutical industry is highlighted, outlining the necessity of reducing handling complexity.

In vitro tear replenishment system: assessing drug delivery from contact lens biomaterials through corneal epithelial monolayer and multilayer under replenishment conditions

There is a need to develop improved in vitro ocular models for biocompatibility and drug delivery studies to assess the potential of in vivo performance of contact lenses. By using an in vitro corneal epithelial cell model combined with a tear replenishment method, this study aimed to investigate the delivery of the glaucoma drug latanoprost from contact lenses and compare the dynamic release results to no-replenishment (immersion) conditions. Corneal epithelial cells were grown as a monolayer or multilayer on curved cellulose cell culture inserts. Three contact lens materials (balafilcon A; senofilcon A; etafilcon A), soaked for 24 h in latanoprost, were placed on the curved cornea models (CCM) and drug concentration was determined on the basal (diffusion/transport) and apical (supernatant) sides after 1, 4, 8 and 12 h. The in vitro tear replenishment was achieved via intermittent flow of a tear solution over the CCM at a rate of 1 mL/hour. A zero-order kinetic was observed for basal drug concentration over the 12 h period. Similar basal and apical drug concentrations were observed with monolayer and multilayer CCM, except for the etafilcon A material. The apical release of latanoprost was significantly lower under replenishment compared to no-replenishment conditions. These results demonstrate the role that a dynamic release model will have in predicting the amount of drug that can be released from a contact lens into the tear film and the critical role of a cell monolayer in in vitro drug delivery studies.

Reactive Oxygen Species Responsive Supramolecular Prodrug Eyedrops for the Treatment of Choroidal Neovascularization

Choroidal neovascularization (CNV) represents a hallmark of neovascular fundus diseases, including age-related macular degeneration and diabetic retinopathy. Traditional eyedrops have encountered formidable challenges in treating CNV, primarily due to their extremely poor intraocular bioavailability and potential adverse off-target effects. Herein, an ocular-permeable supramolecular prodrug eyedrop (Di-DAS/P-PCD) has been developed for the on-demand delivery of antiangiogenic agents in the oxidative microenvironment of CNV. The eyedrop nanoformulation is composed of cell-penetrating peptide-modified PEGylated cyclodextrin (P-PCD) and reactive oxygen species (ROS)-sensitive antiangiogenic dasatinib prodrug Di-DAS. In a laser-induced CNV mouse model, daily instillation of Di-DAS/P-PCD has achieved remarkable penetration into the choroid and significantly suppressed CNV growth while exhibiting a good biocompatibility profile. Our results highlight the potential of the supramolecular prodrug eyedrops as a versatile approach for the targeted treatment of CNV and other neovascular eye disorders.

Mathematical Models of Ocular Drug Delivery

Drug delivery is an important factor for the success of ocular drug treatment. However, several physical, biochemical, and flow-related barriers limit drug exposure of anterior and posterior ocular target tissues during drug treatment via topical, subconjunctival, intravitreal, or systemic routes. Mathematical models encompass various barriers so that their joint influence on pharmacokinetics (PKs) can be simulated in an integrated fashion. The models are useful in predicting PKs and even pharmacodynamics (PDs) of administered drugs thereby fostering development of new drug molecules and drug delivery systems. Furthermore, the models are potentially useful in interspecies translation and probing of disease effects on PKs. In this review article, we introduce current modeling methods (noncompartmental analyses, compartmental and physiologically based PK models, and finite element models) in ocular PKs and related drug delivery. The roles of top-down models and bottom-up simulations are discussed. Furthermore, we present some future challenges, such as modeling of intra-tissue distribution, prediction of drug responses, quantitative systems pharmacology, and possibilities of artificial intelligence.

In situ gelling systems for ocular drug delivery

In situ gelling systems represent a burgeoning paradigm in ocular drug administration, addressing intrinsic challenges posed by extant ocular formulations, such as compromised bioavailability and constraints in traversing the corneal barrier. This systematic review endeavours to comprehensively examine the contemporary landscape of research in this domain, focusing on the nuanced capabilities of in situ gelling systems to optimize drug delivery and enhance therapeutic outcomes, without much technological complexity. Employing a meticulous search strategy across diverse databases for publications and patents spanning the years 2015 to 2023 a total of 26 research papers and 14 patents meeting stringent inclusion criteria were identified. Synthesizing the collective insights derived from these investigations, it becomes evident that in situ gelling systems confer an ability to protract the residence time of formulations or active pharmaceutical ingredients (APIs) within the ocular milieu. This sustained presence engenders extended drug release kinetics, thereby fostering improved patient compliance and mitigating the proclivity for side effects attendant to frequent dosing. These salutary effects extend to diminished systemic drug absorption, augmented ocular bioavailability, and the prospect of reduced dosing frequencies, thereby amplifying patient adherence to therapeutic regimens. Intriguingly, the protective attributes of in situ gelling systems extend to the establishment of an ocular surface barrier, thereby abating the susceptibility to infections and inflammatory responses. In summation, this review underscores the auspicious potential of in situ gelling systems as a transformative approach to advancing ocular drug delivery, warranting sustained research endeavours and developmental initiatives for the betterment of global patient outcomes.

Penetration pathways, influencing factors and predictive models for dermal absorption of exobiotic molecules: A critical review

This review provides a comprehensive summary of the skin penetration pathways of xenobiotics, including metals, organic pollutants, and nanoparticles (NPs), with a particular focus on the methodologies employed to elucidate these penetration routes. The impacts of the physicochemical properties of exogenous substances and the properties of solvent carriers on the penetration efficiencies were discussed. Furthermore, the review outlines the steady-state and transient models for predicting the skin permeability of xenobiotics, emphasizing the models which enable realistic visualization of pharmaco-kinetic phenomena via detailed geometric representations of the skin microstructure, such as stratum corneum (SC) (bricks and mortar) and skin appendages (hair follicles and sebaceous gland units). Limitations of published research, gaps in current knowledge, and recommendations for future research are highlighted, providing insight for a better understanding of the skin penetration behavior of xenobiotics and associated health risks in practical application contexts.

Corneal epithelium models for safety assessment in drug development: Present and future directions

The human corneal epithelial barrier plays a crucial role in drug testing studies, including drug absorption, distribution, metabolism, and excretion (ADME), as well as toxicity testing during the preclinical stages of drug development. However, despite the valuable insights gained from animal and current in vitro models, there remains a significant discrepancy between preclinical drug predictions and actual clinical outcomes. Additionally, there is a growing emphasis on adhering to the 3R principles (refine, reduce, replace) to minimize the use of animals in testing. To tackle these challenges, there is a rising demand for alternative in vitro models that closely mimic the human corneal epithelium. Recently, remarkable advancements have been made in two key areas: microphysiological systems (MPS) or organs-on-chips (OoCs), and stem cell-derived organoids. These cutting-edge platforms integrate four major disciplines: stem cells, microfluidics, bioprinting, and biosensing technologies. This integration holds great promise in developing powerful and biomimetic models of the human cornea.

In Vitro and Ex Vivo Models for Assessing Drug Permeation across the Cornea

Drug permeation across the cornea remains a major challenge due to its unique and complex anatomy and physiology. Static barriers such as the different layers of the cornea, as well as dynamic aspects such as the constant renewal of the tear film and the presence of the mucin layer together with efflux pumps, all present unique challenges for effective ophthalmic drug delivery. To overcome some of the current ophthalmic drug limitations, the identification and testing of novel drug formulations such as liposomes, nanoemulsions, and nanoparticles began to be considered and widely explored. In the early stages of corneal drug development reliable in vitro and ex vivo alternatives, are required, to be in line with the principles of the 3Rs (Replacement, Reduction, and Refinement), with such methods being in addition faster and more ethical alternatives to in vivo studies. The ocular field remains limited to a handful of predictive models for ophthalmic drug permeation. In vitro cell culture models are increasingly used when it comes to transcorneal permeation studies. Ex vivo models using excised animal tissue such as porcine eyes are the model of choice to study corneal permeation and promising advancements have been reported over the years. Interspecies characteristics must be considered in detail when using such models. This review updates the current knowledge about in vitro and ex vivo corneal permeability models and evaluates their advantages and limitations.

Comparison of barrier properties of outer blood-retinal barrier models - Human stem cell-based models as a novel tool for ocular drug discovery

The retinal pigment epithelial (RPE) cell monolayer forms the outer blood-retinal barrier and has a crucial role in ocular pharmacokinetics. Although several RPE cell models are available, there have been no systematic comparisons of their barrier properties with respect to drug permeability. We compared the barrier properties of RPE secondary cell lines (ARPE19, and ARPE19mel) and both primary (hfRPE) and stem-cell derived RPE (hESC-RPE) cells by investigating the permeability of nine drugs (aztreonam, ciprofloxacin, dexamethasone, fluconazole, ganciclovir, ketorolac, methotrexate, voriconazole, and quinidine) across cell monolayers. ARPE19, ARPE19mel, and hfRPE cells displayed a narrow P_app value range, with relatively high permeation rates (5.2-26 × 10^-6 cm/s). In contrast, hESC-RPE cells efficiently restricted the drug flux, and displayed even lower P_app values than those reported for bovine RPE-choroid, with the range of 0.4-32 cm^-6/s. Therefore, ARPE19, ARPE19mel, and hfRPE cells failed to form a tight barrier, whereas hESC-RPE cells restricted the drug flux to a similar extent as bovine RPE-choroid. Therefore, hESC-RPE cells are valuable tools in ocular drug discovery.

Mechanism-Driven and Clinically Focused Development of Botanical Foods as Multitarget Anticancer Medicine: Collective Perspectives and Insights from Preclinical Studies, IND Applications and Early-Phase Clinical Trials

Cancer progression and mortality remain challenging because of current obstacles and limitations in cancer treatment. Continuous efforts are being made to explore complementary and alternative approaches to alleviate the suffering of cancer patients. Epidemiological and nutritional studies have indicated that consuming botanical foods is linked to a lower risk of cancer incidence and/or improved cancer prognosis after diagnosis. From these observations, a variety of preclinical and clinical studies have been carried out to evaluate the potential of botanical food products as anticancer medicines. Unfortunately, many investigations have been poorly designed, and encouraging preclinical results have not been translated into clinical success. Botanical products contain a wide variety of chemicals, making them more difficult to study than traditional drugs. In this review, with the consideration of the regulatory framework of the USFDA, we share our collective experiences and lessons learned from 20 years of defining anticancer foods, focusing on the critical aspects of preclinical studies that are required for an IND application, as well as the checkpoints needed for early-phase clinical trials. We recommend a developmental pipeline that is based on mechanisms and clinical considerations.

Study on In Vitro Metabolism and In Vivo Pharmacokinetics of Beauvericin

Beauvericin (BEA) is a well-known mycotoxin produced by many fungi, including Beaveria bassiana. The purpose of this study was to evaluate the in vitro distribution and metabolism characteristics as well as the in vivo pharmacokinetic (PK) profile of BEA. The in vitro metabolism studies of BEA were performed using rat, dog, mouse, monkey and human liver microsomes, cryopreserved hepatocytes and plasma under conditions of linear kinetics to estimate the respective elimination rates. Additionally, LC-UV-MSn (n = 1~2) was used to identify metabolites in human, rat, mouse, dog and monkey liver microsomes. Furthermore, cytochrome P450 (CYP) reaction phenotyping was carried out. Finally, the absolute bioavailability of BEA was evaluated by intravenous and oral administration in rats. BEA was metabolically stable in the liver microsomes and hepatocytes of humans and rats; however, it was a strong inhibitor of midazolam 1′-hydroxylase (CYP3A4) and mephenytoin 4′-hydroxylase (CYP2C19) activities in human liver microsomes. The protein binding fraction values of BEA were >90% and the half-life (T1/2) values of BEA were approximately 5 h in the plasma of the five species. The absolute bioavailability was calculated to be 29.5%. Altogether, these data indicate that BEA has great potential for further development as a drug candidate. Metabolic studies of different species can provide important reference values for further safety evaluation.

Dual-acting therapeutic proteins for intraocular use

Intravitreally injected antibody-based medicines have revolutionised the treatment of retinal disease. Bispecific and dual-functional antibodies and therapeutic proteins have the potential to further increase the efficacy of intraocular medicines.

In vitro and computational modelling of drug delivery across the outer blood-retinal barrier

The ability to produce rapid, cost-effective and human-relevant data has the potential to accelerate the development of new drug delivery systems. Intraocular drug delivery is an area undergoing rapid expansion, due to the increase in sight-threatening diseases linked to increasing age and lifestyle factors. The outer blood-retinal barrier (OBRB) is important in this area of drug delivery, as it separates the eye from the systemic blood flow. This study reports the development of complementary in vitro and in silico models to study drug transport from silicone oil across the OBRB. Monolayer cultures of a human retinal pigmented epithelium cell line, ARPE-19, were added to chambers and exposed to a controlled flow to simulate drug clearance across the OBRB. Movement of dextran molecules and release of ibuprofen from silicone oil in this model were measured. Corresponding simulations were developed using COMSOL Multiphysics computational fluid dynamics software and validated using independent in vitro datasets. Computational simulations were able to predict dextran movement and ibuprofen release, with all of the features of the experimental release profiles being observed in the simulated data. Simulated values for peak concentrations of permeated dextran and ibuprofen released from silicone oil were within 18% of the in vitro results. This model could be used as a predictive tool for drug transport across this important tissue.

Quantitative Protein Expression in the Human Retinal Pigment Epithelium: Comparison Between Apical and Basolateral Plasma Membranes With Emphasis on Transporters

Purpose

Retinal pigment epithelium (RPE) limits the xenobiotic entry from the systemic blood stream to the eye. RPE surface transporters can be important in ocular drug distribution, but it has been unclear whether they are expressed on the apical, basal, or both cellular surfaces. In this paper, we provide quantitative comparison of apical and basolateral RPE surface proteomes.

Methods

We separated the apical and basolateral membranes of differentiated human fetal RPE (hfRPE) cells by combining apical membrane peeling and sucrose density gradient centrifugation. The membrane fractions were analyzed with quantitative targeted absolute proteomics (QTAP) and sequential window acquisition of all theoretical fragment ion spectra mass spectrometry (SWATH-MS) to reveal the membrane protein localization on the RPE cell surfaces. We quantitated 15 transporters in unfractionated RPE cells and scaled their expression to tissue level.

Results

Several proteins involved in visual cycle, cell adhesion, and ion and nutrient transport were expressed on the hfRPE plasma membranes. Most drug transporters showed similar abundance on both RPE surfaces, whereas large neutral amino acids transporter 1 (LAT1), p-glycoprotein (P-gp), and monocarboxylate transporter 1 (MCT1) showed modest apical enrichment. Many solute carriers (SLC) that are potential prodrug targets were present on both cellular surfaces, whereas putative sodium-coupled neutral amino acid transporter 7 (SNAT7) and riboflavin transporter (RFT3) were enriched on the basolateral and sodium- and chloride-dependent neutral and basic amino acid transporter (ATB0+) on the apical membrane.

Conclusions

Comprehensive quantitative information of the RPE surface proteomes was reported for the first time. The scientific community can use the data to further increase understanding of the RPE functions. In addition, we provide insights for transporter protein localization in the human RPE and the significance for ocular pharmacokinetics.

Drug Flux Across RPE Cell Models: The Hunt for An Appropriate Outer Blood-Retinal Barrier Model for Use in Early Drug Discovery

The retinal pigment epithelial (RPE) cell monolayer forms the outer blood–retinal barrier and has a crucial role in ocular pharmacokinetics. Although several RPE cell models are available, there have been no systematic comparisons of their barrier properties with respect to drug permeability. We compared the barrier properties of several RPE secondary cell lines (ARPE19, ARPE19mel, and LEPI) and both primary (hfRPE) and stem-cell derived RPE (hESC-RPE) cells by investigating the permeability of nine drugs (aztreonam, ciprofloxacin, dexamethasone, fluconazole, ganciclovir, ketorolac, methotrexate, voriconazole, and quinidine) across cell monolayers. ARPE19, ARPE19mel, and hfRPE cells displayed a narrow P_app value range, with relatively high permeation rates (5.2–26 × 10⁻⁶ cm/s. In contrast, hESC-RPE and LEPI cells efficiently restricted the drug flux, and displayed even lower P_app values than those reported for bovine RPE-choroid, with the range of 0.4–32 cm⁻⁶/s (hESC-RPE cells) and 0.4–29 × 10⁻⁶ cm/s, (LEPI cells). Therefore, ARPE19, ARPE19mel, and hfRPE cells failed to form a tight barrier, whereas hESC-RPE and LEPI cells restricted the drug flux to a similar extent as bovine RPE-choroid. Therefore, LEPI and hESC-RPE cells are valuable tools in ocular drug discovery.

Micro-respirometry of whole cells and isolated mitochondria

Oxygen consumption is a key metric of metabolism in aerobic organisms. Current respirometric methods led to seminal discoveries despite limitations such as high sample demand, exchange with atmospheric O₂, and cumulative titration protocols leading to limited choice of useable tissue, complex data interpretation, and restricted experimental design. We developed a sensitive and customizable method of measuring O₂ consumption rates by a variety of biological samples in microliter volumes without interference from the aerobic environment. We demonstrate that O₂ permeability of the photopolymer, VeroClear, is comparable to that of polyetheretherketone (0.125 vs. 0.143 barrer, respectively) providing an efficient barrier to oxygen ingress. Optical transparency of VeroClear, combined with high resolution 3D printing, allows for optode-based oxygen detection in enclosed samples. These properties yield a microrespirometer with over 100× dynamic range for O₂ consumption rates. Importantly, the enclosed respirometer configuration and very low oxygen permeability of materials makes it suitable, with resin pre-conditioning, for quantitative assessment of O₂ consumption rates at any desired [O₂], including hyperbaric, physiological or hypoxic conditions as necessary for each cell type. We characterized two configurations to study soluble enzymes, isolated mitochondria, cells in suspension, and adherent cells cultured on-chip. Improved sensitivity allows for routine quantitative detection of respiration by as few as several hundred cells. Specific activity of cell suspensions in the microrespirometer was in close agreement with that obtained by high-resolution polarographic respirometry. Adherent cell protocols allowed for physiologically relevant assessment of respiration in retinal pigment epithelial cells, ARPE-19, which displayed lower metabolic rates compared with those in suspension. By exchanging medium composition, we demonstrate that cells can be transiently inhibited by cyanide and that 99.6% of basal O₂ uptake is recovered upon its removal. This approach is amenable to new experimental designs and precision measurements on limited sample quantities across basic research and applied fields.

3D printed microfluidic respirometer allows for quantitative investigation of biological energy transduction in adherent and suspension samples.

A Model Mechanism-Based Explanation of an In Vitro-In Vivo Disconnect for Improving Extrapolation and Translation

An improved understanding of in vivo-to-in vitro hepatocyte changes is crucial to interpreting in vitro data correctly and further improving hepatocyte-based in vitro-to-in vivo extrapolations to human targets. We demonstrate using virtual experiments as a means of helping to untangle plausible causes of inaccurate extrapolations. We start with virtual mice that use biomimetic software livers. Previously, using these mice, we discovered model mechanisms that enabled achieving quantitative validation targets while also providing plausible causal explanations for temporal characteristics of acetaminophen hepatotoxicity. We isolated virtual hepatocytes, created a virtual culture, and then conducted dose-response experiments in both culture and mice. We expected to see differences between the two dose-response curves but were somewhat surprised that they crossed because it evidenced that simulated acetaminophen metabolism and toxicity are different for virtual culture and mouse contexts even though individual hepatocyte mechanisms were unchanged. Differences in dose-response curves provide a virtual example of an in vivo-to-in vitro disconnect. We use detailed results of experiments to explain this disconnect. Individual hepatocytes contribute differently to system-level phenomena. In liver, hepatocytes are exposed to acetaminophen sequentially. Relative production of the reactive acetaminophen metabolite is largest (smallest) in pericentral (periportal) hepatocytes. Because that sequential exposure is absent in culture, hepatocytes from different lobular locations do not respond the same. A virtual culture-to-mouse translation can stand as a scientifically challengeable hypothesis explaining an in vivo-to-in vitro disconnect. It provides a framework to develop more reliable interpretations of in vitro observations, which then may be used to improve extrapolations.

In vitro and ex vivo models to study drug delivery barriers in the posterior segment of the eye

Many ocular disorders leading to blindness could benefit from efficient delivery of therapeutics to the retina. However, despite extensive research into drug delivery vehicles and administration techniques, efficacy remains limited because of the many static and dynamic barriers present in the eye. Comprehension of the various barriers and especially how to overcome them can improve our ability to estimate the potential of existent drug delivery vectors and support the design of new ones. To this end, this review gives an overview of the most important ocular barriers for each administration route to the back of the eye. For each barrier, its biological composition and its role as an obstacle towards macromolecules, nanoparticles and viral vectors will be discussed; special attention will be paid to the influence of size, charge and lipophilicity of drug(s) (carrier) on their ability to overcome each barrier. Finally, the most significant available in vitro and ex vivo methods and models to test the potential of a therapeutic to cross each barrier are listed.

Principles of pharmacology in the eye

The eye is a highly specialized organ that is subject to a huge range of pathology. Both local and systemic disease may affect different anatomical regions of the eye. The least invasive routes for ocular drug administration are topical (e.g. eye drops) and systemic (e.g. tablets) formulations. Barriers that subserve as protection against pathogen entry also restrict drug permeation. Topically administered drugs often display limited bioavailability due to many physical and biochemical barriers including the pre-corneal tear film, the structure and biophysiological properties of the cornea, the limited volume that can be accommodated by the cul-de-sac, the lacrimal drainage system and reflex tearing. The tissue layers of the cornea and conjunctiva are further key factors that act to restrict drug delivery. Using carriers that enhance viscosity or bind to the ocular surface increases bioavailability. Matching the pH and polarity of drug molecules to the tissue layers allows greater penetration. Drug delivery to the posterior segment is a greater challenge and, currently, the standard route is via intravitreal injection, notwithstanding the risks of endophthalmitis and retinal detachment with frequent injections. Intraocular implants that allow sustained drug release are at different stages of development. Novel exciting therapeutic approaches include methods for promoting transscleral delivery, sustained release devices, nanotechnology and gene therapy.

Human corneal cell culture models for drug toxicity studies

In vivo toxicity and absorption studies of topical ocular drugs are problematic, because these studies involve invasive tissue sampling and toxic effects in animal models. Therefore, different human corneal models ranging from simple monolayer cultures to three-dimensional models have been developed for toxicological prediction with in vitro models. Each system has its own set of advantages and disadvantages. Use of non-corneal cells, inadequate characterization of gene-expression profiles, and accumulation of genomic aberrations in human corneal models are typical drawbacks that decrease their reliability and predictive power. In the future, further improvements are needed for verifying comparable expression profiles and cellular properties of human corneal models with their in vivo counterparts. A rapidly expanding stem cell technology combined with tissue engineering may give future opportunities to develop new tools in drug toxicity studies. One approach may be the production of artificial miniature corneas. In addition, there is also a need to use large-scale profiling approaches such as genomics, transcriptomics, proteomics, and metabolomics for understanding of the ocular toxicity.

Multifunctional organic-inorganic hybrid nanoparticles and nanosheets based on chitosan derivative and layered double hydroxide: cellular uptake mechanism and application for topical ocular drug delivery

To study the cellular uptake mechanism of multifunctional organic-inorganic hybrid nanoparticles and nanosheets, new chitosan-glutathione-valine-valine-layered double hydroxide (CG-VV-LDH) nanosheets with active targeting to peptide transporter-1 (PepT-1) were prepared, characterized and further compared with CG-VV-LDH nanoparticles. Both organic-inorganic hybrid nanoparticles and nanosheets showed a sustained release in vitro and prolonged precorneal retention time in vivo, but CG-VV-LDH nanoparticles showed superior permeability in the isolated cornea of rabbits than CG-VV-LDH nanosheets. Furthermore, results of cellular uptake on human corneal epithelial primary cells (HCEpiC) and retinal pigment epithelial (ARPE-19) cells indicated that both clathrin-mediated endocytosis and active transport of PepT-1 are involved in the internalization of CG-VV-LDH nanoparticles and CG-VV-LDH nanosheets. In summary, the CG-VV-LDH nanoparticle may be a promising carrier as a topical ocular drug delivery system for the treatment of ocular diseases of mid-posterior segments, while the CG-VV-LDH nanosheet may be suitable for the treatment of ocular surface diseases.

Stimuli-Responsive Polymeric Systems for Controlled Protein and Peptide Delivery: Future Implications for Ocular Delivery

Therapeutic proteins and peptides have become notable in the drug delivery arena for their compatibility with the human body as well as their high potency. However, their biocompatibility and high potency does not negate the existence of challenges resulting from physicochemical properties of proteins and peptides, including large size, short half-life, capability to provoke immune responses and susceptibility to degradation. Various delivery routes and delivery systems have been utilized to improve bioavailability, patient acceptability and reduce biodegradation. The ocular route remains of great interest, particularly for responsive delivery of macromolecules due to the anatomy and physiology of the eye that makes it a sensitive and complex environment. Research in this field is slowly gaining attention as this could be the breakthrough in ocular drug delivery of macromolecules. This work reviews stimuli-responsive polymeric delivery systems, their use in the delivery of therapeutic proteins and peptides as well as examples of proteins and peptides used in the treatment of ocular disorders. Stimuli reviewed include pH, temperature, enzymes, light, ultrasound and magnetic field. In addition, it discusses the current progress in responsive ocular drug delivery. Furthermore, it explores future prospects in the use of stimuli-responsive polymers for ocular delivery of proteins and peptides. Stimuli-responsive polymers offer great potential in improving the delivery of ocular therapeutics, therefore there is a need to consider them in order to guarantee a local, sustained and ideal delivery of ocular proteins and peptides, evading tissue invasion and systemic side-effects.

In vitro screening of nanomedicines through the blood brain barrier: A critical review

The blood-brain barrier accounts for the high attrition rate of the treatments of most brain disorders, which therefore remain one of the greatest health-care challenges of the twenty first century. Against this background of hindrance to brain delivery, nanomedicine takes advantage of the assembly at the nanoscale of available biomaterials to provide a delivery platform with potential to raising brain levels of either imaging or therapeutic agents. Nevertheless, to prevent later failure due to ineffective drug levels at the target site, researchers have been endeavoring to develop a battery of in vitro screening procedures that can predict earlier in the drug discovery process the ability of these cutting-edge drug delivery platforms to cross the blood-brain barrier for biomedical purposes. This review provides an in-depth analysis of the currently available in vitro blood-brain barrier models (both cell-based and non-cell-based) with the focus on their suitability for understanding the biological brain distribution of forthcoming nanomedicines. The relationship between experimental factors and underlying physiological assumptions that would ultimately lead to a more predictive capacity of their in vivo performance, and those methods already assayed for the evaluation of the brain distribution of nanomedicines are comprehensively discussed.

Pluripotent stem cell derived hepatocytes: using materials to define cellular differentiation and tissue engineering

Pluripotent stem cell derived liver cells (hepatocytes) represent a promising alternative to primary tissue for biological and clinical applications. To date, most hepatocyte maintenance and differentiation systems have relied upon the use of animal derived components. This serves as a significant barrier to large scale production and application of stem cell derived hepatocytes. Recently, the use of defined biologics has overcome those limitations in two-dimensional monolayer culture. In order to improve the cell phenotype further, three-dimensional culture systems have been employed to better mimic the in vivo situation, drawing upon materials chemistry, engineering and biology. In this review we discuss efforts in the field, to differentiate pluripotent stem cells towards hepatocytes under defined conditions.

Acetylated bacterial cellulose coated with urinary bladder matrix as a substrate for retinal pigment epithelium

This work evaluated the effect of acetylated bacterial cellulose (ABC) substrates coated with urinary bladder matrix (UBM) on the behavior of retinal pigment epithelium (RPE), as assessed by cell adhesion, proliferation and development of cell polarity exhibiting transepithelial resistance and polygonal shaped-cells with microvilli. Acetylation of bacterial cellulose (BC) generated a moderate hydrophobic surface (around 65°) while the adsorption of UBM onto these acetylated substrates did not affect significantly the surface hydrophobicity. The ABS substrates coated with UBM enabled the development of a cell phenotype closer to that of native RPE cells. These cells were able to express proteins essential for their cytoskeletal organization and metabolic function (ZO-1 and RPE65), while showing a polygonal shaped morphology with microvilli and a monolayer configuration. The coated ABC substrates were also characterized, exhibiting low swelling effect (between 1.5-2.0 swelling/mm(3)), high mechanical strength (2048MPa) and non-pyrogenicity (2.12EU/L). Therefore, the ABC substrates coated with UBM exhibit interesting features as potential cell carriers in RPE transplantation that ought to be further explored.

Comparison of the uptake of methacrylate-based nanoparticles in static and dynamic in vitro systems as well as in vivo

Polymer-based nanoparticles are promising drug delivery systems allowing the development of new drug and treatment strategies with reduced side effects. However, it remains a challenge to screen for new and effective nanoparticle-based systems in vitro. Important factors influencing the behavior of nanoparticles in vivo cannot be simulated in screening assays in vitro, which still represent the main tools in academic research and pharmaceutical industry. These systems have serious drawbacks in the development of nanoparticle-based drug delivery systems, since they do not consider the highly complex processes influencing nanoparticle clearance, distribution, and uptake in vivo. In particular, the transfer of in vitro nanoparticle performance to in vivo models often fails, demonstrating the urgent need for novel in vitro tools that can imitate aspects of the in vivo situation more accurate. Dynamic cell culture, where cells are cultured and incubated in the presence of shear stress has the potential to bridge this gap by mimicking key-features of organs and vessels. Our approach implements and compares a chip-based dynamic cell culture model to the common static cell culture and mouse model to assess its capability to predict the in vivo success more accurately, by using a well-defined poly((methyl methacrylate)-co-(methacrylic acid)) and poly((methyl methacrylate)-co-(2-dimethylamino ethylmethacrylate)) based nanoparticle library. After characterization in static and dynamic in vitro cell culture we were able to show that physiological conditions such as cell-cell communication of co-cultured endothelial cells and macrophages as well as mechanotransductive signaling through shear stress significantly alter cellular nanoparticle uptake. In addition, it could be demonstrated by using dynamic cell cultures that the in vivo situation is simulated more accurately and thereby can be applied as a novel system to investigate the performance of nanoparticle systems in vivo more reliable.

Rabbit as an animal model for intravitreal pharmacokinetics: Clinical predictability and quality of the published data

Intravitreal administration is the method of choice in drug delivery to the retina and/or choroid. Rabbit is the most commonly used animal species in intravitreal pharmacokinetics, but it has been criticized as being a poor model of human eye. The critique is based on some anatomical differences, properties of the vitreous humor, and observed differences in drug concentrations in the anterior chamber after intravitreal injections. We have systematically analyzed all published information on intravitreal pharmacokinetics in the rabbit and human eye. The analysis revealed major problems in the design of the pharmacokinetic studies. In this review we provide advice for study design. Overall, the pharmacokinetic parameters (clearance, volume of distribution, half-life) in the human and rabbit eye have good correlation and comparable absolute values. Therefore, reliable rabbit-to-man translation of intravitreal pharmacokinetics should be feasible. The relevant anatomical and physiological parameters in rabbit and man show only small differences. Furthermore, the claimed discrepancy between drug concentrations in the human and rabbit aqueous humor is not supported by the data analysis. Based on the available and properly conducted pharmacokinetic studies, the differences in the vitreous structure in rabbits and human patients do not lead to significant pharmacokinetic differences. This review is the first step towards inter-species translation of intravitreal pharmacokinetics. More information is still needed to dissect the roles of drug delivery systems, disease states, age and ocular manipulation on the intravitreal pharmacokinetics in rabbit and man. Anyway, the published data and the derived pharmacokinetic parameters indicate that the rabbit is a useful animal model in intravitreal pharmacokinetics.

Primary retinal cultures as a tool for modeling diabetic retinopathy: an overview

Experimental models of diabetic retinopathy (DR) have had a crucial role in the comprehension of the pathophysiology of the disease and the identification of new therapeutic strategies. Most of these studies have been conducted in vivo, in animal models. However, a significant contribution has also been provided by studies on retinal cultures, especially regarding the effects of the potentially toxic components of the diabetic milieu on retinal cell homeostasis, the characterization of the mechanisms on the basis of retinal damage, and the identification of potentially protective molecules. In this review, we highlight the contribution given by primary retinal cultures to the study of DR, focusing on early neuroglial impairment. We also speculate on possible themes into which studies based on retinal cell cultures could provide deeper insight.