Phospholipid Vesicle-Based Permeation Assay and EpiSkin® in Assessment of Drug Therapies Destined for Skin Administration

Development of a novel human stratum corneum mimetic phospholipid -vesicle-based permeation assay models for in vitro permeation studies

OBJECTIVES

To develop and evaluate a novel human stratum corneum (SC) mimetic phospholipid vesicle-based permeation assay (PVPASC) model for in vitro permeation studies.

SIGNIFICANCE

Due to the increasing restrictions on the use of human and animal skins, artificial skin models have attracted substantial interest in pharmaceuticals and cosmetic industries. In this study, a modified PVPASC model containing both SC lipids and proteins was developed.

METHODS

The PVPASC model was optimized by altering the lipid composition and adding keratin in the formulation of large liposomes. The barrier properties were monitored by measuring the electrical resistance (ER) and permeability of Rhodamine B (RB). The modified PVPASC model was characterized in terms of the surface topography, solvent influence and storage stability. The permeation studies of the active components in Compound Nanxing Zhitong Plaster (CNZP) were performed to examine the capability of PVPASC in the application of skin penetration.

RESULTS

The ER and Papp values of RB obtained from the optimized PVPASC model indicated a similar barrier property to porcine ear skin. Scanning electron microscope analysis demonstrated a mimic 'brick-and-mortar' structure. The PVPASC model can be stored for three weeks at -20 °C, and withstand the presence of different receptor medium for 24 h. The permeation studies of the active components demonstrated a good correlation (r2 = 0.9136) of Papp values between the drugs' permeation through the PVPASC model and porcine ear skin.

CONCLUSION

Keratin contained composite phospholipid vesicle-based permeation assay models have been proven to be potential skin tools in topical/transdermal permeation studies.

-Omics potential of in vitro skin models for radiation exposure

There is a growing need to uncover biomarkers of ionizing radiation exposure that leads to a better understanding of how exposures take place, including dose type, rate, and time since exposure. As one of the first organs to be exposed to external sources of ionizing radiation, skin is uniquely positioned in terms of model systems for radiation exposure study. The simultaneous evolution of both MS-based -omics studies, as well as in vitro 3D skin models, has created the ability to develop a far more holistic understanding of how ionizing radiation affects the many interconnected biomolecular processes that occur in human skin. However, there are a limited number of studies describing the biomolecular consequences of low-dose ionizing radiation to the skin. This review will seek to explore the current state-of-the-art technology in terms of in vitro 3D skin models, as well as track the trajectory of MS-based -omics techniques and their application to ionizing radiation research, specifically, the search for biomarkers within the low-dose range.

Skin models for dermal exposure assessment of phthalates

Phthalates are a class of compounds that have found widespread use in industrial applications, in particular in the polymer, cosmetics and pharmaceutical industries. While ingestion, and to a lesser degree inhalation, have been considered as the major exposure routes, especially for higher molecular weight phthalates, dermal exposure is an important route for lower weight phthalates such as diethyl phthalate (DEP). Assessing the dermal permeability of such compounds is of great importance for evaluating the impact and toxicity of such compounds in humans. While human skin is still the best model for studying dermal permeation, availability, cost and ethical concerns may preclude or restrict its use. A range of alternative models has been developed over time to substitute for human skin, especially in the early phases of research. These include ex vivo animal skin, human reconstructed skin and artificial skin models. While the results obtained using such alternative models correlate to a lesser or greater degree with those from in vivo human studies, the use of such models is nevertheless vital in dermal permeation research. This review discusses the alternative skin models that are available, their use in phthalate permeation studies and possible new avenues of phthalate research using skin models that have not been used so far.

Evaluation of Two Novel Hydantoin Derivatives Using Reconstructed Human Skin Model EpiskinTM: Perspectives for Application as Potential Sunscreen Agents

This study aimed to assess two novel 5-arylideneimidazolidine-2,4-dione (hydantoin) derivatives (JH3 and JH10) demonstrating photoprotective activity using the reconstructed human skin model Episkin^TM. The skin permeability, irritation, and phototoxicity of the compounds was evaluated in vitro. Moreover, the in vitro genotoxicity and human metabolism of both compounds was studied. For skin permeation and irritation experiments, the test compounds were incorporated into a formulation. It was shown that JH3 and JH10 display no skin irritation and no phototoxicity. Both compounds did not markedly enhance the frequency of micronuclei in CHO-K1 cells in the micronucleus assay. Preliminary in vitro studies with liver microsomes demonstrated that hydrolysis appears to constitute their important metabolic pathway. Episkin^TM permeability experiments showed that JH3 permeability was lower than or close to currently used UV filters, whereas JH10 had the potential to permeate the skin. Therefore, a restriction of this compound permeability should be obtained by choosing the right vehicle or by optimizing it, which should be addressed in future studies.

Harnessing of Doxylamine Succinate/Pyridoxine Hydrochloride-Dual Laden Bilosomes as a Novel Combinatorial Nanoparadigm for Intranasal Delivery: In Vitro Optimization and In Vivo Pharmacokinetic Appraisal

The present work is concerned with tailoring and appraisal of a novel nano-cargo; bilosomes (BLS) dual laded with doxylamine succinate (DAS) and pyridoxine hydrochloride (PDH), the first treatment option against gestational nausea and vomiting, for intranasal delivery. This bifunctional horizon could surmount constraints of orally-commercialized platforms both in dosage regimen and pharmacokinetic profile. For accomplishing this purpose, DAS/PDH-BLS were elaborated integrating phospholipid, sodium cholate and cholesterol applying thin-film hydration method based on Box-Behnken design. Utilizing Design-Expert® software, the effect of formulation variables on BLS physicochemical features alongside the optimal formulation selection were investigated. Then, the optimum DAS/PDH-BLS formulation was incorporated into a thermally-triggered in situ gelling base. The in vivo pharmacokinetic studies were explored in rats for intranasal DAS/PDH-BLS in situ gel compared with analogous intranasal free in situ gel and oral solution. The optimized BLS disclosed vesicle size of 243.23 nm, ζ potential of -31.33 mV, entrapment efficiency of 59.18 and 41.63%, accumulative % release within 8 h of 63.30 and 85.52% and accumulative permeated amount over 24 h of 347.92 and 195.4 µg/cm² for DAS/PDH, respectively. Following intranasal administration of the inspected BLS in situ gel, pharmacokinetic studies revealed a 1.64- and 2.3-fold increment in the relative bioavailability of DAS and a 1.7- and 3.73-fold increase for PDH compared to the intranasal free in situ gel and oral solution, respectively besides significantly extended mean residence times for both drugs. Thus, the intranasally exploited DAS/PDH-BLS could be deemed as a promising hybrid nanoplatform with fruitful pharmacokinetics and tolerability traits.

Design and development of topical liposomal formulations in a regulatory perspective

The skin is the absorption site for drug substances intended to treat loco-regional diseases, although its barrier properties limit the permeation of drug molecules. The growing knowledge of the skin structure and its physiology have supported the design of innovative nanosystems (e.g. liposomal systems) to improve the absorption of poorly skin-permeable drugs. However, despite the dozens of clinical trials started, few topically applied liposomal systems have been authorized both in the EU and the USA. Indeed, the intrinsic complexity of the topically applied liposomal systems, the higher production costs, the lack of standardized methods and the more stringent guidelines for assessing their benefit/risk balance can be seen as causes of such inefficient translation. The present work aimed to provide an overview of the physicochemical and biopharmaceutical characterization methods that can be applied to topical liposomal systems intended to be marketed as medicinal products, and the current regulatory provisions. The discussion highlights how such methodologies can be relevant for defining the critical quality attributes of the final product, and they can be usefully applied based on the phase of the life cycle of a liposomal product: to guide the formulation studies in the early stages of development, to rationally design preclinical and clinical trials, to support the pharmaceutical quality control system and to sustain post-marketing variations. The provided information can help define harmonized quality standards able to overcome the case-by-case approach currently applied by regulatory agencies in assessing the benefit/risk of the topically applied liposomal systems.

Protocol for the Isolation of Stratum Corneum from Pig Ear Skin: Evaluation of the Trypsin Digestion Conditions

Stratum corneum (SC) represents the outermost layer of the skin, being an effective barrier against the entry of molecules and pathogens. Skin research has given particular focus to SC as it hampers effective drug delivery for cosmetical and therapeutical purposes. Following recommendations to develop alternative models to animals, the SC isolated from skin obtained from medical procedures or from pigs has gained extensive attention. Yet, there is still missing a standard and simple procedure accepted within the scientific community to avoid application of different isolated SC methodologies, a fact that may hamper progress in skin research. Considering this challenge, the present study evaluated different experimental conditions aiming to establish a useful and sustainable solvent-free procedure for the obtention of a realistic SC model. The studied trypsin digestion parameters included concentration, incubation period and temperature. Isolated SC was characterized using histological analysis and calcein’s permeability, after the procedure and during a 6-week storage period. Data recommend trypsin digestion at 4 °C for 20 h as the most effective procedure to isolate SC from pig ear skin. This work contributes to standardize the SC isolation procedure, and to obtain a valuable and reliable SC mimetic model for skin drug development.

Ultra-deformable liposomes containing terpenes (terpesomes) loaded fenticonazole nitrate for treatment of vaginal candidiasis: Box-Behnken design optimization, comparative ex vivo and in vivo studies

Fenticonazole nitrate (FTN) is a potent antifungal drug adopted in the treatment of vaginal candidiasis. It has inadequate aqueous solubility hence, novel ultra-deformable liposomes 'Terpesomes' (TPs) were developed that might prevail over FTN poor solubility besides TPs might abstain the obstacles of mucus invasion. TPs were assembled by thin-film hydration then optimized by Box Behnken design utilizing terpenes ratio (X₁), sodium deoxycholate amount (X₂), and ethanol concentration (X₃) as independent variable, whereas their impact was inspected for entrapment efficiency (Y₁), particle size (Y₂), and polydispersity index (Y₃). Design Expert^® was bestowed to select the optimal TP for more studies. The optimal TP had entrapment efficiency of 62.18 ± 1.39%, particle size of 310.00 ± 8.16 nm, polydispersity index of 0.20 ± 0.10, and zeta potential of -10.19 ± 0.2.00 mV. Elasticity results were greater in the optimal TP related to classical bilosomes. Further, ex vivo permeation illustrated tremendous permeability from the optimal TP correlated to classical bilosomes, and FTN suspension. Besides, in vivo assessment displayed significant inhibition effect in rats from FTN-TPs gel compared to FTN gel. The antifungal potency with undermost histopathological variation was detected in rats treated with FTN-TPs gel. Overall, the acquired findings verified the potency of utilizing FTN-TPs gel for treatment of vaginal candidiasis.

Self-Assembled Palmitoyl-Glycine-Histidine as a Permeation Enhancer for Transdermal Delivery

The stratum corneum (SC) covers the outer surface of the skin and prevents the permeation of unwanted materials; however, the SC barrier also inhibits the desired permeation of active pharmaceutical ingredients (APIs). Therefore, the development of a novel method to enhance the permeation of APIs through the skin has been the focus of significant attention. Palmitoyl-glycine-histidine (Pal-GH)-comprising palmitic acid, glycine, and histidine-can be co-assembled with various additives to form a thixotropic hydrogel. Self-assembled Pal-GH enhances the permeation of ivermectin through the skin; however, the permeation mechanism is unclear and has not yet been discussed in detail. In the present study, the self-assembled structure of Pal-GH was analyzed using X-rays and infrared, and its permeation enhancement effect was verified. There was a correlation between the amount of Pal-GH in the skin and permeation enhancement, suggesting the involvement of the Pal-GH molecule. The presence of Pal-GH in the skin was confirmed by liquid chromatography-mass spectrometry and fluorescence labeling (labeling with Thioflavin T, a fluorescent dye that responds to β-sheets). The self-assembled Pal-GH permeated the SC without disrupting its organization. However, the structure of the Pal-GH caused changes to the lipid organization of the SC. The findings indicated that self-assembled Pal-GH is an effective permeation enhancer for transdermal delivery and does not induce skin irritation.

Application of the human stratum corneum lipid-based mimetic model in assessment of drug-loaded nanoparticles for skin administration

A lipid-based permeation assay (PVPA_SC) with a lipid composition similar to Human stratum corneum layer has been previously reported. The aim of this study was to further characterize the PVPA_SC model in the presence of co-solvents and to determine its applicability to evaluate drug permeability with drug-loaded nanoparticles. Data obtained from PVPA_SC model were compared with results from isolated SC from pig ear skin. The characterization revealed that the PVPA_SC barriers retain integrity and calcein permeability when stored up to 12 weeks at -20 °C, in the presence of different co-solvents, and under a skin environment pH range. The permeation profile of calcein in the lipid-based barrier correlated well with data obtained for the isolated SC model and revealed higher reproducibility. Cyclosporine A (CsA) was selected as a model drug, given its relevance for skin-inflammatory diseases and two types of lipid nanoparticles were used to assess the permeability of the PVPA_SC model. It was possible to distinguish the permeability between free and nanoparticles' loaded cyclosporine. Data obtained with CsA-loaded nanoformulations indicated a higher permeation rate than the obtained for the solid lipid nanoparticles or the free drug. The PVPA_SC model could be applied as a cost-effective alternative for skin early drug development.

Human skin models: From healthy to disease-mimetic systems; characteristics and applications

Skin drug delivery is an emerging route in drug development, leading to an urgent need to understand the behaviour of active pharmaceutical ingredients within the skin. Given, As one of the body's first natural defences, the barrier properties of skin provide an obstacle to the successful outcome of any skin drug therapy. To elucidate the mechanisms underlying this barrier, reductionist strategies have designed several models with different levels of complexity, using non-biological and biological components. Besides the detail of information and resemblance to human skin in vivo, offered by each in vitro model, the technical and economic efforts involved must also be considered when selecting the most suitable model. This review provides an outline of the commonly used skin models, including healthy and diseased conditions, in-house developed and commercialized models, their advantages and limitations, and an overview of the new trends in skin-engineered models.

Development of a novel human stratum corneum model, as a tool in the optimization of drug formulations

Transdermal delivery represents a very attractive administration route that provides various advantages over other methods of administration, including enhanced patient compliance via non-invasive, painless and simple application and reduced side effects. Thereby, the research on suitable drugs for this route continues to increase. However, most of drug candidates face the challenges of low drug permeability across the skin's biologically active barrier - the stratum corneum (SC). In this context, a low cost, simple screening tool to evaluate penetration of drug candidates in a human SC barrier model was developed. The in vitro model is based on a modified phospholipid vesicle-based permeation assay (PVPA) with a lipid composition close to human SC layer. The new SC PVPA model can be stored up to 2 weeks at -20 °C, withstand a pH range from 2.0 to 8.0 and the presence of co-solvents (DMSO, oleic acid and cremophor®) without losing their integrity. The human mimicking SC PVPA model was able to detect calcein permeability differences when different drugs, applied in the therapy of skin-related diseases, were present. The obtained data correlated well with the well accepted pig ear model, which highlights the potential of this new human SC model.

FoxO1 enhances differentiation and apoptosis in human primary keratinocytes

Forkhead box-O1 (FoxO1) is a key nutrient- and growth factor-dependent regulator of metabolism, but its functional role in human primary keratinocytes (HPKs) is less known. To investigate the role of FoxO1 in HPKs and effect of insulin-like growth factor 1 (IGF-1) and isotretinoin on FoxO1 expression, HPKs were treated with 1.2 mmol/L calcium chloride, 1-20 ng/mL IGF-1 and 0.1-10 μmol/L isotretinoin. Recombinant adenovirus expressing FoxO1 or FKHR shRNA lentivirus transfection was introduced to upregulate or silence FoxO1 expression. Epidermal FoxO1 immunostaining was lower in acne lesion than in normal skin. FoxO1 overexpression induced involucrin expression, G2/M arrest and apoptosis but suppressed proliferation, while FoxO1 silencing decreased involucrin expression but increased proliferation, S phase and viable cells in HPKs. IGF-1 downregulated FoxO1 and involucrin but upregulated p-Akt expression in HPKs, which was blocked by pretreatment with LY294002. Isotretinoin enhanced FoxO1, p53 and p21 but inhibited p-FoxO1 and involucrin expression in HPKs. These results demonstrate that FoxO1 promotes differentiation and apoptosis in HPKs. IGF-1 may reduce keratinocyte differentiation through PI3K/Akt/FoxO1 pathway, while isotretinoin can reinforce FoxO1 expression. FoxO1 may be involved in acne pathogenesis and could serve as a potential therapeutic target.

Skin Tissue Substitutes and Biomaterial Risk Assessment and Testing

Tremendous progress has been made over the past few decades to develop skin substitutes for the management of acute and chronic wounds. With the advent of tissue engineering and the ability to combine advanced manufacturing technologies with biomaterials and cell culture systems, more biomimetic tissue constructs have been emerged. Synthetic and natural biomaterials are the main constituents of these skin-like constructs, which play a significant role in tissue grafting, the body's immune response, and the healing process. The act of implanting biomaterials into the human body is subject to the body's immune response, and the complex nature of the immune system involves many different cell types and biological processes that will ultimately determine the success of a skin graft. As such, a large body of recent studies has been focused on the evaluation of the performance and risk assessment of these substitutes. This review summarizes the past and present advances in in vitro, in vivo and clinical applications of tissue-engineered skins. We discuss the role of immunomodulatory biomaterials and biomaterials risk assessment in skin tissue engineering. We will finally offer a roadmap for regulating tissue engineered skin substitutes.

Drug permeability profiling using cell-free permeation tools: Overview and applications

Cell-free permeation systems are gaining interest in drug discovery and development as tools to obtain a reliable prediction of passive intestinal absorption without the disadvantages associated with cell- or tissue-based permeability profiling. Depending on the composition of the barrier, cell-free permeation systems are classified into two classes including (i) biomimetic barriers which are constructed from (phospho)lipids and (ii) non-biomimetic barriers containing dialysis membranes. This review provides an overview of the currently available cell-free permeation systems including Parallel Artificial Membrane Permeability Assay (PAMPA), Phospholipid Vesicle-based Permeation Assay (PVPA), Permeapad®, and artificial membrane based systems (e.g. the artificial membrane insert system (AMI-system)) in terms of their barrier composition as well as their predictive capacity in relation to well-characterized intestinal permeation systems. Given the potential loss of integrity of cell-based permeation barriers in the presence of food components or pharmaceutical excipients, the superior robustness of cell-free barriers makes them suitable for the combined dissolution/permeation evaluation of formulations. While cell-free permeation systems are mostly applied for exploring intestinal absorption, they can also be used to evaluate non-oral drug delivery by adjusting the composition of the membrane.

Epidermal Penetration of Gold Nanoparticles and Its Underlying Mechanism Based on Human Reconstructed 3D Episkin Model

Nanomaterials are widely used in diverse aspects, and their translocation behavior through the skin would be helpful in understanding the corresponding exposure risks. To reveal how surface functionalization of nanoparticles influences the skin penetration, three kinds of gold nanoparticles (GNPs) with negatively, neutrally, and positively charged surfaces, that is, cetyltrimethylammonium bromide-coated GNPs (CTAB@GNP), polyvinylpyrrolidone-coated GNPs (PVP@GNP), and citrate-coated GNPs (Citrate@GNP), were studied using human reconstructed 3D Episkin model. The measurement of Au distribution in diverse layers of the Episkin model indicated that all three GNPs could penetrate through the epidermis, wherein CTAB@GNP with positive surface charges exhibited the highest efficiency in skin penetration. The upward osmosis of the medium proteins confirmed the occurrence of skin permeation induced by GNP treatment, and the lipid network in the stratum corneum was also altered as the consequence of GNP exposure. When compared to Citrate@GNP and PVP@GNP, CTAB@GNP significantly compromised the tight junction of keratinocytes, causing paracellular penetration of nanoparticles. The existence of cytoplasmic gold showed the transcytosis pathway through endocytosis and exocytosis processes was the main epidermic penetration behavior of the tested GNPs. The study on GNP penetration process through the 3D Episkin model has, on one hand, offered a promising approach to evaluate the translocation process of nanoparticles across the skin, and, on the other hand, provided mechanism explanation for diverse penetration behaviors of GNPs with different surface charges. The findings herein would be of great help in nanotechnology improvement and nanosafety evaluation.

In vitro models to estimate drug penetration through the compromised stratum corneum barrier

OBJECTIVES

The phospholipid vesicle-based permeation assay (PVPA) is a recently established in vitro stratum corneum model to estimate the permeability of intact and healthy skin. The aim here was to further evolve this model to mimic the stratum corneum in a compromised skin barrier by reducing the barrier functions in a controlled manner.

METHODS

To mimic compromised skin barriers, PVPA barriers were prepared with explicitly defined reduced barrier function and compared with literature data from both human and animal skin with compromised barrier properties. Caffeine, diclofenac sodium, chloramphenicol and the hydrophilic marker calcein were tested to compare the PVPA models with established models.

RESULTS AND DISCUSSIONS

The established PVPA models mimicking the stratum corneum in healthy skin showed good correlation with biological barriers by ranking drugs similar to those ranked by the pig ear skin model and were comparable to literature data on permeation through healthy human skin. The PVPA models provided reproducible and consistent results with a distinction between the barriers mimicking compromised and healthy skin. The trends in increasing drug permeation with an increasing degree of compromised barriers for the model drugs were similar to the literature data from other in vivo and in vitro models.

CONCLUSIONS

The PVPA models have the potential to provide permeation predictions when investigating drugs or cosmeceuticals intended for various compromised skin conditions and can thus possibly reduce the time and cost of testing as well as the use of animal testing in the early development of drug candidates, drugs and cosmeceuticals.

Biomimetic PVPA in vitro model for estimation of the intestinal drug permeability using fasted and fed state simulated intestinal fluids

A prerequisite for successful oral drug therapy is the drug's ability to cross the gastrointestinal barrier. Considering the increasing number of new chemical entities in modern drug discovery, reliable and fast in vitro models are required for early and efficient prediction of intestinal permeability. To mimic the intestinal environment, use of biorelevant media may provide valuable information on in vivo drug permeation. The present study aims at improving the novel biomimetic phospholipid vesicle-based permeation assay's (PVPAbiomimetic) biorelevance by investigating the applicability of the biorelevant media; fasted state simulated intestinal fluid (FaSSIF) and fed state simulated intestinal fluid (FeSSIF). The FaSSIF and FeSSIF's influence on the permeability of the model drugs acyclovir, indomethacin, griseofulvin and nadolol was then assessed. The barriers' robustness in terms of storage stability was also evaluated. The barriers were found to maintain their integrity in presence of FaSSIF and FeSSIF. The model drugs showed changes in permeability in presence of the different simulated intestinal fluids that were in agreement with previous reports. Moreover, the barrier showed improved storage stability by maintaining its integrity for 6months. Altogether, this study moves the PVPAbiomimetic an important step towards a better in vitro permeability model for use in drug development.

In vitro skin models as a tool in optimization of drug formulation

(Trans)dermal drug therapy is gaining increasing importance in the modern drug development. To fully utilize the potential of this route, it is important to optimize the delivery of active ingredient/drug into/through the skin. The optimal carrier/vehicle can enhance the desired outcome of the therapy therefore the optimization of skin formulations is often included in the early stages of the product development. A rational approach in designing and optimizing skin formulations requires well-defined skin models, able to identify and evaluate the intrinsic properties of the formulation. Most of the current optimization relies on the use of suitable ex vivo animal/human models. However, increasing restrictions in use and handling of animals and human skin stimulated the search for suitable artificial skin models. This review attempts to provide an unbiased overview of the most commonly used models, with emphasis on their limitations and advantages. The choice of the most applicable in vitro model for the particular purpose should be based on the interplay between the availability, easiness of the use, cost and the respective limitations.