Models for the Stratum Corneum Lipid Matrix: Effects of Ceramide Concentration, Ceramide Hydroxylation, and Free Fatty Acid Protonation

An Investigation into the Transdermal Behavior of Active Ingredients by Combination of Experiments and Multiscale Simulations

Transdermal behavior is a critical aspect of studying delivery systems and evaluating the efficacy of cosmetics. However, existing methods face challenges such as lengthy experiments, high cost, and limited model accuracy. Therefore, developing accurate transdermal models is essential for formulation development and effectiveness assessment. In this study, we developed a multiscale model to describe the transdermal behavior of active ingredients in the stratum corneum. Molecular dynamics simulations were used to construct lipid bilayers and determine the diffusion coefficients of active ingredients in different regions of these bilayers. These diffusion coefficients were integrated into a multilayer lipid pathway model using finite element simulations. The simulation results were in close agreement with our experimental results for three active ingredients (mandelic acid (MAN), nicotinamide (NIC), and pyruvic acid (PYR)), demonstrating the effectiveness of our multiscale model. This research provides valuable insights for advancing transdermal delivery methods.

Mechanisms of the Drug Penetration Enhancer Propylene Glycol Interacting with Skin Lipid Membranes

Very few drugs have the necessary physicochemical properties to cross the skin's main permeability barrier, the stratum corneum (SC), in sufficient amounts. Propylene glycol (PG) is a chemical penetration enhancer that could be included in topical formulations in order to overcome the barrier properties of the skin and facilitate the transport of drugs across it. Experiments have demonstrated that PG increases the mobility and disorder of SC lipids and may extract cholesterol from the SC, but little is known about the molecular mechanisms of drug permeation enhancement by PG. In this work, we have performed molecular dynamics (MD) simulations to investigate the molecular-level effects of PG on the structure and properties of model SC lipid bilayers. The model bilayers were simulated in the presence of PG concentrations over the range of 0-100% w/w PG, using both an all-atom and a united atom force field. PG was found to localize in the hydrophilic headgroup regions at the bilayer interface, to occupy the lipid-water hydrogen-bonding sites, and to slightly increase lipid tail disorder in a concentration-dependent manner. We showed with MD simulation that PG enhances the permeation of small molecules such as water by interacting with the bilayer interface; the results of our study may be used to guide the design of formulations for transdermal drug delivery with enhanced skin permeation, as well as topical formulations and cosmetic products.

Permeability of Dermatological Solutes through the Short Periodicity Phase of Human Stratum Corneum Lipid Bilayers

Determining the permeability of drug-like solutes through the densely packed and heterogeneous stratum corneum lipid layer presents a significant challenge. In this study, we employed umbrella sampling with a periodic weighing function applied to the center of mass of the lipid bilayers. Precise umbrella sampling was conducted with an interframe distance of 0.5 Å, spanning from the bilayer center to the water phase, and each frame was simulated for at least 20 ns. Autocorrelation functions, potential of mean force (PMF), and diffusivity profiles were analyzed for six solutes (testosterone, benzene, caffeine, ethanol, mannitol, and histidine). The results revealed that autocorrelations were dependent solely on the medium, whether water or lipid phase. Diffusivity and PMF profiles along the reaction coordinate were influenced by the hydrophilicity of the solute rather than its size. For hydrophobic solutes, the PMF curves exhibited a minimum at the bilayer center, while for hydrophilic solutes, the PMFs peaked at the bilayer center and lipid tails (where the lipid tails are not interacting with the cholesterol). Diffusivity curves were low at the bilayer center and water phase, with peaks observed at the headgroup or the boundary between fatty acid and cholesterol (1 nm from the bilayer center). The quantitative findings presented in this work hold significance for pharmacists and drug designers.

Nanostructured Lipid Carrier-Based Gel for Repurposing Simvastatin in Localized Treatment of Breast Cancer: Formulation Design, Development, and In Vitro and In Vivo Characterization

Simvastatin (SMV) is noticed as a repurposed candidate to be effective against breast cancer (BC). However, poor solubility, dose-limiting toxicities, and side effects are critical hurdles in its use against BC. The above drawbacks necessitate the site-specific (localized) delivery of SMV via suitable nanocarriers. Therefore, the present study intended to develop SMV nanostructured lipid carrier (NLC)-based gel using carbopol-934 as a gelling agent to achieve local delivery and improve patient compliance while combating BC. The SMV NLCs were fabricated by melt-emulsification ultrasonication technique using stearic acid as solid lipid, olive oil (OO) as liquid lipid, tween 20 as a surfactant, and PEG-200 as a co-surfactant, and optimized by Box-Behnken design. The optimized SMV-loaded NLCs displayed % entrapment efficiency of 91.66 ± 5.2% and particle size of 182 ± 11.9 nm. The pH of NLC-based gels prepared using a 2.0% w/v of carbopol-934 was found in the range of 5.3-5.6 while the viscosity was in the range of 5.1-6.6 Pa.S. Besides, NLC-based gels exhibited higher and controlled SMV release (71-76%) at pH 6.8 and (78-84%) at pH 5.5 after 48 h than SMV conventional gel (37%) at both pH 6.8 and 5.5 after 48 h. The ex vivo permeation of SMV from NLC-based gel was 3.8 to 4.5 times more than conventional gel. Notably, SMV-loaded NLCs displayed ameliorated cytotoxicity than plain SMV against MCF-7 and MDA-MB-231 BC cells. No substantial difference was noticed in the cytotoxicity of NLC-based gels and pure SMV against both cell lines. The SMV NLC-based gel exhibited the absence of skin irritation in vivo in the mice following topical application. In addition, the histopathological study revealed no alteration in the mice skin anatomy. Furthermore, the SMV-loaded NLCs and NLC-based gels were stable for 6 months at refrigerator conditions (4°C ± 2°C). Thus, the present research confirms that NLC-based gel can be a safe, efficacious, and novel alternative to treat BC.

Quality by Design Assisted Optimization and Risk Assessment of Black Cohosh Loaded Ethosomal Gel for Menopause: Investigating Different Formulation and Process Variables

Black cohosh (Cimicifuga racemosa) (CR) is a popular herb and is medically lauded for ameliorating myriad symptoms associated with menopause. However, its pharmaceutical limitations and non-availability of a patient-compliant drug delivery approach have precluded its prevalent use. Henceforth, the current research premise is aimed at developing an ethosomal gel incorporating triterpene enriched fraction (TEF) obtained from CR and evaluating its effectiveness through the transdermal application. TEF-loaded ethosomes were formulated using solvent injection, optimized and characterised. The optimized ethosomes were then dispersed into a polymeric gel base to form ethosomal gel which was further compared with the conventional gel by in-vitro and ex-vivo experiments. Here, the quality by design (QbD) approach was exploited for the optimization and development of ethosomal gel. The elements of QbD comprising initial risk assessment, design of experimentation (DoE), and model validation for the development of formulation have all been described in detail. The optimized ethosomes (F03) showed a nanometric size range, negative zeta potential and good entrapment. The in vitro release profile of gel revealed a burst release pattern following the Korsmeyer Peppas model having Fickian diffusion. The transdermal flux of ethosomal gel was observed to be more than that of conventional gel. Texture analysis and rheological characterization of the gel, revealed good strength showing shear thinning and pseudoplastic behaviour. The confocal microscope investigation revealed the deeper skin permeation of ethosomal gel than conventional gel. This result was further strengthened by DSC, IR and histological assessment of the animal skin (Wistar rat), treated with the optimized formulation. Conclusively, the implementation of QbD in the formulation resulted in a better understanding of the process and the product. It aids in the reduction of product variability and defects, hence improving product development efficiencies. Additionally, the ethosomal gel was found to be a more effective and successful carrier for TEF than the conventional gel through the transdermal route. Moreover, this demands an appropriate animal study, which is underway, for a stronger outcome.

Biomimetic Stratum Corneum Liposome Models: Lamellar Organization and Permeability Studies

The stratum corneum (SC), the outer layer of the skin, plays a crucial role as a barrier protecting the underlying cells from external stress. The SC comprises three key components: ceramide (CER), free fatty acid (FFA), and cholesterol, along with small fractions of cholesterol sulfate and cholesterol ester. In order to gain a deeper understanding about the interdependence of the two major components, CER and FFA, on the organizational, structural, and functional properties of the SC layer, a library of SC lipid liposome (SCLL) models was developed by mixing CER (phytosphingosine or sphingosine), FFA (oleic acid, palmitic acid, or stearic acid), cholesterol, and cholesterol sulfate. Self-assembly of the SC lipids into lamellar phases was first confirmed by small-angle X-ray scattering. Short periodicity and long periodicity phases were identified for SCLLs containing phytosphingosines and sphingosine CERs, respectively. Furthermore, unsaturation in the CER acyl and FFA chains reduced the lipid conformational ordering and packing density of the liposomal bilayer, which were measured by differential scanning calorimetry and Fourier transform infrared spectroscopy. The introduction of unsaturation in the CER and/or FFA chains also impacted the lamellar integrity and permeability. This extensive library of SCLL models exhibiting physiologically relevant lamellar phases with defined structural and functional properties may potentially be used as a model system for screening pharmaceuticals or cosmetic agents.

Investigating the nanostructure of a CER[NP]/CER[AP]-based stratum corneum lipid matrix model: A combined neutron diffraction & molecular dynamics simulations approach

The human skin provides a physiochemical and biological protective barrier due to the unique structure of its outermost layer known as the Stratum corneum. This layer consists of corneocytes and a multi-lamellar lipid matrix forming a composite, which is a major determining factor for the barrier function of the Stratum corneum. A substantiated understanding of this barrier is necessary, as controlled breaching or modulation of the same is also essential for various health and personal care applications such as topical drug delivery and cosmetics to a name few. In this study, we discuss the state-of-the-art of neutron diffraction techniques, using specifically deuterated lipids, combined with the information obtained from molecular models using molecular dynamics simulations, to understand the structure and barrier function of the Stratum corneum lipid matrix. As an example, the effect of ceramide concentration on a lipid lamella system consisting of CER[NP]/CER[AP]/Cholesterol/free fatty acid (deprotonated) is studied. This study demonstrates the usefulness of the combined approach of neutron diffraction and molecular dynamics simulations for effective analysis of the model systems created for the Stratum corneum lipid matrix. The optimization of force fields by comparison with experimental data is furthermore an important step in the direction of providing a predictive quality.

Using molecular simulation to understand the skin barrier

Skin's effectiveness as a barrier to permeation of water and other chemicals rests almost entirely in the outermost layer of the epidermis, the stratum corneum (SC), which consists of layers of corneocytes surrounded by highly organized lipid lamellae. As the only continuous path through the SC, transdermal permeation necessarily involves diffusion through these lipid layers. The role of the SC as a protective barrier is supported by its exceptional lipid composition consisting of ceramides (CERs), cholesterol (CHOL), and free fatty acids (FFAs) and the complete absence of phospholipids, which are present in most biological membranes. Molecular simulation, which provides molecular level detail of lipid configurations that can be connected with barrier function, has become a popular tool for studying SC lipid systems. We review this ever-increasing body of literature with the goals of (1) enabling the experimental skin community to understand, interpret and use the information generated from the simulations, (2) providing simulation experts with a solid background in the chemistry of SC lipids including the composition, structure and organization, and barrier function, and (3) presenting a state of the art picture of the field of SC lipid simulations, highlighting the difficulties and best practices for studying these systems, to encourage the generation of robust reproducible studies in the future. This review describes molecular simulation methodology and then critically examines results derived from simulations using atomistic and then coarse-grained models.

Implications of surfactant hydrophobic chain architecture on the Surfactant-Skin lipid model interaction

Although surfactants have been widely used in skin care and other related applications, our knowledge about how surfactants interact with stratum corneum (SC) lipids remains limited. This work reports how surfactants interact with a lipid SC model by neutron diffraction and molecular dynamics (MD) simulations, focusing on examining the impact of surfactant molecular architecture. The surfactant-SC mixed membrane was constructed by an equimolar mixture of ceramide/cholesterol/fatty acids and surfactant at 1% molar ratio of total lipids. The arrangements of water and surfactant molecules in the membrane were obtained through neutron scattering length density (NSLD) profiles via contrast variation method, meanwhile, MD simulation clearly demonstrated the mechanism of hydration change in the surfactant-model SC mixed membrane. No drastic difference was detected in the repeating distance of the short periodicity phase (SPP) upon adding surfactants, however, it significantly enhanced the membrane hydration and reduced the amount of phase separated crystalline cholesterol, showing a strong dependence on surfactant chain length, branching and double bond. This work clearly demonstrates how surfactant architecture affects its interaction with the SC membrane, providing useful guidance for either choosing an existing surfactant or designing a new one for surfactant-based transdermal application.

Quantitative Structure-Activity Relationship of Enhancers of Licochalcone A and Glabridin Release and Permeation Enhancement from Carbomer Hydrogel

This study aimed to systematically compare licochalcone A (LicA) and glabridin (Gla) (whitening agents) release and permeation from Carbomer 940 (CP) hydrogels with different enhancers, and evaluate the relationship between the quantitative enhancement efficacy and structures of the enhancers. An in vitro release study and an in vitro permeation experiment in solution and hydrogels using porcine skin were performed. We found that the Gla–CP hydrogel showed a higher drug release and skin retention amount than LicA–CP due to the higher solubility in medium and better miscibility with the skin of Gla than that of LicA. Enhancers with a higher molecular weight (MW) and lower polarizability showed a higher release enhancement effect (ERrelease) for both LicA and Gla. The Van der Waals forces in the drug–enhancers–CP system were negatively correlated with the drug release percent. Moreover, enhancers with a higher log P and polarizability displayed a higher retention enhancement effect in solution (ERsolution retention) for LicA and Gla. Enhancers decreased the whole intermolecular forces indrug–enhancers-skin system, which had a linear inhibitory effect on the drug retention. Moreover, C=O of ceramide acted asthe enhancement site for drug permeation. Consequently, Transcutol® P (TP) and propylene glycol (PG), seven enhancers showed a higher retention enhancement effect in hydrogel (ERhydrogel retention) for LicA and Gla. Taken together, the conclusions provide a strategy for reasonable utilization of enhancers and formulation optimization in topical hydrogel whitening.

All-Atom Modeling of Complex Cellular Membranes

Cell membranes are composed of a variety of lipids and proteins where they interact with each other to fulfill their roles. The first step in modeling these interactions in molecular simulations is to have reliable mimetics of the membrane's lipid environment. This Feature Article presents our recent efforts to model complex cellular membranes using all-atom force fields. A short review of the CHARMM36 (C36) lipid force field and its recent update to incorporate the long-range dispersion is presented. Key examples of model membranes mimicking various species and organelles are given. These include single-celled organisms such as bacteria (E. coli., chlamydia, and P. aeruginosa) and yeast (plasma membrane, endoplasmic reticulum, and trans-Golgi network) and more advanced ones such as plants (soybean and Arabidopsis thaliana) and mammals (ocular lens, stratum corneum, and peripheral nerve myelin). Leaflet asymmetry in composition has also been applied to some of these models. With the increased lipid diversity in the C36 lipid FF, these complex models can better reflect the structural, mechanical, and dynamic properties of realistic membranes and open an opportunity to study biological processes involving other molecules.

Glycyrrhiza acid micelles loaded with licochalcone A for topical delivery: Co-penetration and anti-melanogenic effect

The co-penetration of micellar vehicles and the encapsulated drugs into the skin layers, as well as the mechanisms underlying the penetration enhancement have not been clearly elucidated. We developed licochalcone A (LA)-loaded glycyrrhiza acid (GA) (GA+LA) micelles for topical delivery of LA into the epidermis. The in vitro co-penetration, penetration pathways, mechanism of interaction between skin and the micelles, and the in vitro and in vivo whitening effect of GA+LA micelles were evaluated. Co-penetration and penetration pathways were visualized on the abdominal skin of rats model with confocal laser scanning microscopy (CLSM) using a nile blue A-labeled GA (GA-NB). We found that GA significantly increased the transport of LA into the skin predominantly via the hair follicles and GA mainly accumulated in the SC and epidermis, while LA was localized in the epidermis and dermis. Moreover, 73.4% of the LA deposited into the epidermis within 12 h and approximately 9.32% of the LA permeated across the SC in the form of entire micelles within 24 h. GA-NB+LA micelles disaggregated and accumulated in the specific skin layers, and the LA released from the carrier penetrated into deeper layers. Moreover, the GA+LA micelles promoted drug penetration via intracellular or intercellular routes by loosening the skin surface and enhancing fluidization through lipid distortion and keratin denaturation. Furthermore, GA+LA micelles exhibited synergistic whitening effect on B16 cells and UVB-exposed C57BL/6 mice. Collectively, GA micelles can enhance penetration of LA to the epidermis mainly via the hair follicles following topical application, and reduce skin pigmentation.

The Epidemiology and Global Burden of Atopic Dermatitis: A Narrative Review

The global epidemiology of atopic dermatitis (AD) in the current decade (2009-2019) has not been extensively reported. Epidemiological studies play an important role in presenting the risk factors of AD, as detailed prevalence and incidence data could demonstrate the burden of disease in the population of adults, adolescents, and children in different geographical regions. Thus, the primary objective of this review was to assess and summarize the epidemiological studies of the prevalence and incidence of AD in different age groups, focusing on data from studies published for 2009 to 2019. After a thorough literature search, six countries were identified from African, Asian, and European regions respectively, who published studies on AD. In contrast, only two studies were identified from Australia and New Zealand, three countries from North America and two from South America published AD studies, respectively. The highest prevalence of AD from included studies was noted among Swedish children with 34%, while the lowest prevalence was in Tunisian children with 0.65%; studies reporting incidence data were far less numerous. A common trend in the prevalence of AD was that children would have a higher prevalence as compared to adolescents and adults. The severity and morbidity of the disease showed variance with age, sex, socioeconomic characteristics, geographical location, and ethnicity. Environmental factors played an important role as causative agents in AD. The risk factors that were proven to cause and induce AD were skin barrier impairments due to FLG mutation, changes in the environment, and diet. FLG mutation may impair the skin barrier function by disruption of pH and hydration maintenance of the skin. Lastly, there were only a few studies on the incidence of AD in the 21st century. Therefore, epidemiological studies on childhood and adulthood AD in different continents are still needed, especially on the incidence of AD during adulthood.

How to control interactions of cellulose-based biomaterials with skin: the role of acidity in the contact area

Being able to control the interactions of biomaterials with living tissues and skin is highly desirable for many biomedical applications. This is particularly the case for cellulose-based materials which provide one of the most versatile platforms for tissue engineering due to their strength, biocompatibility and abundance. Achieving such control, however, requires detailed molecular-level knowledge of the dominant interaction mechanisms. Here, we employed both biased and unbiased atomic-scale molecular dynamics simulations to explore how cellulose crystals interact with model stratum corneum bilayers, ternary mixtures of ceramides, cholesterol, and free fatty acids. Our findings show that acidity in the contact area directly affects binding between cellulose and the stratum corneum bilayer: Protonation of free fatty acids in the bilayer promotes attractive cellulose-bilayer interactions. We identified two major factors that control the cellulose-skin interactions: (i) the electrostatic repulsion between a cellulose crystal and the charged (anionic due to deprotonated fatty acids) surface of a stratum corneum bilayer and (ii) the cellulose-stratum corneum hydrogen bonding. When less than half of the fatty acids in the bilayer are protonated, the first factor dominates and there is no binding to skin. At a larger degree of fatty acid protonation the cellulose-stratum corneum hydrogen bonding prevails yielding a tight binding. Remarkably, we found that ceramide molecules are the key component in hydrogen bonding with cellulose. Overall, our findings highlight the critical role of fatty acid protonation in biomaterial-stratum corneum interactions and can be used for optimizing the surface properties of cellulose-based materials aimed at biomedical applications such as wound dressings.

Not only in silico drug discovery: Molecular modeling towards in silico drug delivery formulations

The use of methods at molecular scale for the discovery of new potential active ligands, as well as previously unknown binding sites for target proteins, is now an established reality. Literature offers many successful stories of active compounds developed starting from insights obtained in silico and approved by Food and Drug Administration (FDA). One of the most famous examples is raltegravir, a HIV integrase inhibitor, which was developed after the discovery of a previously unknown transient binding area thanks to molecular dynamics simulations. Molecular simulations have the potential to also improve the design and engineering of drug delivery devices, which are still largely based on fundamental conservation equations. Although they can highlight the dominant release mechanism and quantitatively link the release rate to design parameters (size, drug loading, et cetera), their spatial resolution does not allow to fully capture how phenomena at molecular scale influence system behavior. In this scenario, the "computational microscope" offered by simulations at atomic scale can shed light on the impact of molecular interactions on crucial parameters such as release rate and the response of the drug delivery device to external stimuli, providing insights that are difficult or impossible to obtain experimentally. Moreover, the new paradigm brought by nanomedicine further underlined the importance of such computational microscope to study the interactions between nanoparticles and biological components with an unprecedented level of detail. Such knowledge is a fundamental pillar to perform device engineering and to achieve efficient and safe formulations. After a brief theoretical background, this review aims at discussing the potential of molecular simulations for the rational design of drug delivery systems.

Examining Tail and Headgroup Effects on Binary and Ternary Gel-Phase Lipid Bilayer Structure

The structural properties of two- and three-component gel-phase bilayers were studied using molecular dynamics simulations. The bilayers contain distearoylphosphatidylcholine (DSPC) phospholipids mixed with alcohols and/or fatty acids of varying tail lengths, with carbon chain lengths of 12, 16, and 24 studied. Changes in both headgroup chemistry and tail length are found to affect the balance between steric repulsion and van der Waals attraction within the bilayers, manifesting in different bilayer structural properties. Lipid components are found to be located at different depths within the bilayer depending on both chain length and headgroup chemistry. The highest bilayer ordering and lowest area per tail are found in systems with medium-length tails. While longer tails can enhance van der Waals attractions, the increased tail-length asymmetry is found to induce disorder and reduce tail packing. Bulkier headgroups further increase steric repulsion, as reflected in increased component offsets and reduced tail packing. These findings help explain how bilayer composition affects the structure of gel-phase bilayers.