Transdermal cellular membrane penetration of proteins with gold nanoparticles: a molecular dynamics study

Free-Energy Profiles for Membrane Permeation of Compounds Calculated Using Rare-Event Sampling Methods

The free-energy profile of a compound is an essential measurement in evaluating the membrane permeation process by means of theoretical methods. Computationally, molecular dynamics (MD) simulation allows the free-energy profile calculation. However, MD simulations frequently fail to sample membrane permeation because they are rare events induced in longer timescales than the accessible timescale of MD, leading to an insufficient conformational search to calculate an incorrect free-energy profile. To achieve a sufficient conformational search, several enhanced sampling methods have been developed and elucidated the membrane permeation process. In addition to these enhanced sampling methods, we proposed a simple yet powerful free-energy calculation of a compound for the membrane permeation process based on originally rare-event sampling methods developed by us. Our methods have a weak dependency on external biases and their optimizations to promote the membrane permeation process. Based on distributed computing, our methods only require the selection of initial structures and their conformational resampling, whereas the enhanced sampling methods may be required to adjust external biases. Furthermore, our methods efficiently search membrane permeation processes with simple scripts without modifying any MD program. As demonstrations, we calculated the free-energy profiles of seven linear compounds for their membrane permeation based on a hybrid conformational search using two rare-event sampling methods, that is, (1) parallel cascade selection MD (PaCS-MD) and (2) outlier flooding method (OFLOOD), combined with a Markov state model (MSM) construction. In the first step, PaCS-MD generated initial membrane permeation paths of a compound. In the second step, OFLOOD expanded the unsearched conformational area around the initial paths, allowing for a broad conformational search. Finally, the trajectories were employed to construct reliable MSMs, enabling correct free-energy profile calculations. Furthermore, we estimated the membrane permeability coefficients of all compounds by constructing the reliable MSMs for their membrane permeation. In conclusion, the calculated coefficients were qualitatively correlated with the experimental measurements (correlation coefficient (R²) = 0.8689), indicating that the hybrid conformational search successfully calculated the free-energy profiles and membrane permeability coefficients of the seven compounds.

Gold nanoparticles for skin drug delivery

Nanoparticle-based drug carriers are being pursued intensely to overcome the skin barrier and improve even hydrophilic or macromolecular drug delivery into or across the skin efficiently. Over the past few years, the application of gold nanoparticles as a novel kind of drug carrier for skin drug delivery has attracted increasing attention because of their unique properties and versatility. In this review, we summarized the possible factors contributing to the penetration behaviors of gold nanoparticles, including size, surface chemistry, and shape. Drug loading, release, and penetration patterns were captured towards implicating the design of gold nanoparticles for dermal or transdermal drug delivery. Physical methods applicable for future enhancing the delivery efficacy of GNPs were also presented, which mainly included microneedles and iontophoresis. As a promising "drug", the inherent activities of GNPs were finally discussed, especially regarding their application in the treatment of skin disease. Thus, this paper provided a comprehensive review of the use of gold nanoparticles for skin drug delivery, which would help the design of multifunctional systems for skin drug delivery based on gold nanoparticles.

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.

Boosting the anti-inflammatory effect of self-assembled hybrid lecithin-chitosan nanoparticles via hybridization with gold nanoparticles for the treatment of psoriasis: elemental mapping and in vivo modeling

Gold nanoparticles are a promising drug delivery system for treatment of inflammatory skin conditions, including psoriasis, due to their small size and anti-inflammatory properties. The aim of this study was to conjugate gold nanoparticles with anti-psoriatic formulations that previously showed successful results in the treatment of psoriasis (tacrolimus-loaded chitosan nanoparticles and lecithin-chitosan nanoparticles) by virtue of their surface charges, then examine whether the hybridization with gold nanoparticles would enhance the anti-psoriatic efficacy in vivo. Successful formation of gold nanoparticles was examined by elemental mapping and selected area electron diffraction (SAED). Hybrid conjugates were examined in terms of particle size and zeta potential by dynamic light scattering (DLS). Morphological features were captured by transmission electron microscope (TEM) and X-ray diffraction (XRD) analysis was conducted, as well. All characterization was conducted for the conjugated nanoparticles and compared with their bare counterparts. The in vivo results on imiquimod (IMQ)-induced mouse model showed promising anti-psoriatic effects upon application of gold conjugated tacrolimus-loaded lecithin-chitosan hybrid nanoparticles with a significant difference from the bare hybrid nanoparticles in some of the inflammatory markers. The anti-inflammatory effect of the gold conjugate was also evident by a lower spleen to body weight ratio and a better histopathological skin condition compared to other tested formulations.

The technology of transdermal delivery nanosystems: from design and development to preclinical studies

Transdermal administration has gained much attention due to the remarkable advantages such as patient compliance, drug escape from first-pass elimination, favorable pharmacokinetic profile and prolonged release properties. However, the major limitation of these systems is the limited skin penetration of the stratum corneum, the skin's most important barrier, which protects the body from the insertion of substances from the environment. Transdermal drug delivery systems are aiming to the disruption of the stratum corneum in order for the active pharmaceutical ingredients to enter successfully the circulation. Therefore, nanoparticles are holding a great promise because they can act as effective penetration enhancers due to their small size and other physicochemical properties that will be analyzed thoroughly in this report. Apart from the investigation of the physicochemical parameters, a comparison between the different types of nanoparticles will be performed. The complexity of skin anatomy and the unclear mechanisms of penetration should be taken into consideration to reach some realistic conclusions regarding the way that the described parameters affect the skin permeability. To the best of the authors knowledge, this is among the few reports on the literature describing the technology of transdermal delivery systems and how this technology affects the biological activity.

The interplay between surface-functionalized gold nanoparticles and negatively charged lipid vesicles

The comprehensive understanding of the interactions between gold nanoparticles (AuNPs) and phospholipid vesicles has important implications in various biomedical applications; however, this is not yet well understood. Here, coarse-grained molecular dynamics (CGMD) simulations were performed to study the interactions between functionalized AuNPs and negatively charged lipid vesicles, and the effects of the surface chemistry and surface charge density (SCD) of AuNPs were analyzed. It is revealed that AuNPs with different surface ligands adhere to the membrane surface (anionic AuNPs) or get into the vesicle bilayer (hydrophobic and cationic AuNPs). Due to the loose arrangement of lipid molecules, AuNPs penetrate curved vesicle membranes more easily than planar lipid bilayers. Cationic AuNPs present three different interaction modes with the vesicle, namely insertion, partial penetration and complete penetration, which are decided by the SCD difference. Both hydrophobic interaction and electrostatic interaction play crucial roles in the interplay between cationic AuNPs and lipid vesicles. For the cationic AuNP with a low SCD, it gets into the lipid bilayer without membrane damage through the hydrophobic interaction, and it is finally stabilized in the hydrophobic interior of the vesicle membrane in a thermodynamically stable "snorkeling" configuration. For the cationic AuNP with a high SCD, it crosses the vesicle membrane and gets into the vesicle core through a membrane pore induced by strong electrostatic interaction. In this process, the membrane structure is destroyed. These findings provide a molecular-level understanding of the interplay between AuNPs and lipid vesicles, which may further expand the application of functional AuNPs in modern biomedicine.

Topical drug delivery: History, percutaneous absorption, and product development

Topical products, widely used to manage skin conditions, have evolved from simple potions to sophisticated delivery systems. Their development has been facilitated by advances in percutaneous absorption and product design based on an increasingly mechanistic understanding of drug-product-skin interactions, associated experiments, and a quality-by-design framework. Topical drug delivery involves drug transport from a product on the skin to a local target site and then clearance by diffusion, metabolism, and the dermal circulation to the rest of the body and deeper tissues. Insights have been provided by Quantitative Structure Permeability Relationships (QSPR), molecular dynamics simulations, and dermal Physiologically Based PharmacoKinetics (PBPK). Currently, generic product equivalents of reference-listed products dominate the topical delivery market. There is an increasing regulatory interest in understanding topical product delivery behavior under 'in use' conditions and predicting in vivo response for population variations in skin barrier function and response using in silico and in vitro findings.

Synergistic Effect of Chemical Penetration Enhancers on Lidocaine Permeability Revealed by Coarse-Grained Molecular Dynamics Simulations

The search for new formulations for transdermal drug delivery (TDD) is an important field in medicine and cosmetology. Molecules with specific physicochemical properties which can increase the permeability of active ingredients across the stratum corneum (SC) are called chemical penetration enhancers (CPEs), and it was shown that some CPEs can act synergistically. In this study, we performed coarse-grained (CG) molecular dynamics (MD) simulations of the lidocaine delivery facilitated by two CPEs-linoleic acid (LA) and ethanol-through the SC model membrane containing cholesterol, N-Stearoylsphingosine (DCPE), and behenic acid. In our simulations, we probed the effects of individual CPEs as well as their combination on various properties of the SC membrane and the lidocaine penetration across it. We demonstrated that the addition of both CPEs decreases the membrane thickness and the order parameters of the DPCE hydrocarbon chains. Moreover, LA also enhances diffusion of the SC membrane components, especially cholesterol. The estimated potential of mean force (PMF) profiles for the lidocaine translocation across SC in the presence/absence of two individual CPEs and their combination demonstrated that while ethanol lowers the free energy barrier for lidocaine to enter SC, LA decreases the depth of the free energy minima for lidocaine inside SC. These two effects supposedly result in synergistic penetration enhancement of drugs. Altogether, the present simulations provide a detailed molecular picture of CPEs' action and their synergistic effect on the penetration of small molecular weight therapeutics that can be beneficial for the design of novel drug and cosmetics formulations.

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.

PrPC Aptamer Conjugated-Gold Nanoparticles for Targeted Delivery of Doxorubicin to Colorectal Cancer Cells

Anticancer drugs, such as fluorouracil (5-FU), oxaliplatin, and doxorubicin (Dox) are commonly used to treat colorectal cancer (CRC); however, owing to their low response rate and adverse effects, the development of efficient drug delivery systems (DDSs) is required. The cellular prion protein PrP^C, which is a cell surface glycoprotein, has been demonstrated to be overexpressed in CRC, however, there has been no research on the development of PrP^C-targeting DDSs for targeted drug delivery to CRC. In this study, PrP^C aptamer (Apt)-conjugated gold nanoparticles (AuNPs) were synthesized for targeted delivery of Dox to CRC. Thiol-terminated PrP^C-Apt was conjugated to AuNPs, followed by hybridization of its complementary DNA for drug loading. Finally, Dox was loaded onto the AuNPs to synthesize PrP^C-Apt-functionalized doxorubicin-oligomer-AuNPs (PrP^C-Apt DOA). The PrP^C-Apt DOA were spherical nanoparticles with an average diameter of 20 nm. Treatment of CRC cells with PrP^C-Apt DOA induced reactive oxygen species generation by decreasing catalase and superoxide dismutase activities. In addition, treatment with PrP^C-Apt DOA inhibited mitochondrial functions by decreasing the expression of peroxisome proliferator-activated receptor gamma coactivator 1-alpha, complex 4 activity, and oxygen consumption rates. Compared to free Dox, PrP^C-Apt DOA decreased proliferation and increased apoptosis of CRC cells to a greater degree. In this study, we demonstrated that PrP^C-Apt DOA targeting could effectively deliver Dox to CRC cells. PrP^C-Apt DOA can be used as a treatment for CRC, and have the potential to replace existing anticancer drugs, such as 5-FU, oxaliplatin, and Dox.

Mechanistic Understanding From Molecular Dynamics Simulation in Pharmaceutical Research 1: Drug Delivery

In this review, we outline the growing role that molecular dynamics simulation is able to play as a design tool in drug delivery. We cover both the pharmaceutical and computational backgrounds, in a pedagogical fashion, as this review is designed to be equally accessible to pharmaceutical researchers interested in what this new computational tool is capable of and experts in molecular modeling who wish to pursue pharmaceutical applications as a context for their research. The field has become too broad for us to concisely describe all work that has been carried out; many comprehensive reviews on subtopics of this area are cited. We discuss the insight molecular dynamics modeling has provided in dissolution and solubility, however, the majority of the discussion is focused on nanomedicine: the development of nanoscale drug delivery vehicles. Here we focus on three areas where molecular dynamics modeling has had a particularly strong impact: (1) behavior in the bloodstream and protective polymer corona, (2) Drug loading and controlled release, and (3) Nanoparticle interaction with both model and biological membranes. We conclude with some thoughts on the role that molecular dynamics simulation can grow to play in the development of new drug delivery systems.

Linalool-Loaded Glutathione-Modified Gold Nanoparticles Conjugated with CALNN Peptide as Apoptosis Inducer and NF-κB Translocation Inhibitor in SKOV-3 Cell Line

Background

Linalool is a monoterpene compound with various potential therapeutic applications in several medical fields. Previous studies have indicated the activity of linalool against cell lines; however, its high level of toxicity restricts its use. The aim of this study was to design and manufacture compounds with a novel structure that can be used for loading linalool, to reduce its toxicity and improve its reachable ability.

Methods

We synthesized and characterized a new molecule for loading linalool onto gold nanoparticles (GNPs) capped with glutathione and conjugated with a CALNN peptide. Linalool was loaded onto the GNPs via the reaction of the surface groups of both linalool and the GNPs. Moreover, the target peptide could be loaded onto the surface of the GNPs via a chemical reaction. The cytotoxic effects of linalool–GNP (LG) and linalool–GNP–CALNN peptide (LGC) conjugates against ovarian cancer cells were investigated, as were the possible mechanisms underlying the induction of apoptosis.

Results

Our findings illustrated the significant antiproliferative effect of LG and LGC on SKOV-3 cells. The cytotoxicity assay demonstrated that LG and LGC were selectively toxic in cancer cells and induced apoptosis by activating caspase-8, the p53 protein, and various proteins involved in apoptosis. The present data demonstrated that LG and LGC have a high therapeutic potential and should be given particular consideration as anticancer drug-delivery systems, as LG and LGC were remarkably more cytotoxic against a cancer cell line than were linalool and GNPs alone.

Conclusion

We concluded that LG and LGC are promising compounds that can be used for treating ovarian cancer (SKOV-3) cells via the induction of apoptosis through extrinsic and intrinsic pathways.

The effect of immobilized antioxidant enzymes on the oxidative stress in UV-irradiated rat skin

Aim: Superoxide dismutase (SOD) and catalase (CAT) immobilized on gold nanoparticles (AuNP) and silver nanoparticles (AgNP) nanoparticles were used to reduce UV radiation-induced oxidative stress in rat skin. Materials & methods: The antioxidant influence of the enzymes was investigated on level of malondialdehyde, 8-hydroksy-2'deoksyguanozine, myeloperoxidase, total antioxidant capacity, SOD2 and CAT activity and expression, and glutathione and glutathione peroxidase activity. Results: The application of immobilized SOD and CAT on UV-irradiated skin reduced malondialdehyde and 8-hydroksy-2'deoksyguanozine levels also SOD and CAT activity and expression increased. The tested enzymes influence glutathione peroxidase activity and level of total antioxidant capacity and glutathione. Conclusion: Immobilized enzymes increased the antioxidative potential of skin following UV irradiation.

Multiscale Modeling of Skin Electroporation

Human skin, the largest external organ of the body, provides a selective barrier to therapeutics applied topically. The molecules having specific chemical and physical properties can only penetrate the deeper layer of the skin. However, the lag time for reaching a steady state in the deeper layer is generally of the order of hours. In order to deliver higher-molecular-weight, charged, and hydrophilic therapeutics in the deeper layer, the skin barrier must be breached. Electroporation is one of the methods used to breach the skin barrier for enhancement of drug permeation and reduction of lag time. However, the underlying mechanism responsible for the enhancement of drug permeation is not well understood. In this study, a multiscale model of skin electroporation is developed by connecting molecular phenomena to a macroscopic model. At the atomic scale, molecular dynamics simulations of the lipid matrix of the human stratum corneum (SC) were performed under the influence of an external electric field. The pores get formed during the electroporation process and the transport properties (diffusivity) of drug molecules are computed. The diffusion coefficient obtained during electroporation was found to be higher than passive diffusion. However, this alone could not explain the multifold increase in the drug flux on application of an electric field as observed in the experiments. Hence, a finite element method (FEM) model of the skin SC is also developed. The release of fentanyl through this model is compared with the available experimental results. Both experimental and simulated results of pore formation on application of an electric field and many folds' increase in drug flux are comparable. Once validated, the framework was used for the design of skin electroporation experiments (in silico) by changing the electric pulse parameters such as voltage, pulse duration, and number of pulses. This multiscale modeling framework provides valuable insight at the molecular and macroscopic levels to design the electroporation experiments. The framework can be utilized as a design tool for skin electroporation applications.

The role of SP-B1–25 peptides in lung surfactant monolayers exposed to gold nanoparticles

Lung surfactant monolayer’s (acts as the first line barrier for inhaled nanoparticles) components (lipids and peptides) rearrange themselves by the influence of exposed gold nanoparticles at various stages of the breathing cycle.

Optical Studies of Nanodiamond-Tissue Interaction: Skin Penetration and Localization

In this work, several optical-spectroscopic methods have been used to visualize and investigate the penetration of diamond nanoparticles (NPs) of various sizes (3-150 nm), surface structures and fluorescence properties into the animal skin in vitro. Murine skin samples have been treated with nanodiamond (ND) water suspensions and studied using optical coherence tomography (OCT), confocal and two-photon fluorescence microscopy and fluorescence lifetime imaging (FLIM). An analysis of the optical properties of the used nanodiamonds (NDs) enables the selection of optimal optical methods or their combination for the study of nanodiamond-skin interaction. Among studied NDs, particles of 100 nm in nominal size were shown to be appropriate for multimodal imaging using all three methods. All the applied NDs were able to cross the skin barrier and penetrate the different layers of the epidermis to finally arrive in the hair follicle niches. The results suggest that NDs have the potential for multifunctional applications utilizing multimodal imaging.

Computer-Aided Design of Nanoparticles for Transdermal Drug Delivery

Human skin provides an excellent opportunity for drug delivery application. However, the delivery of hydrophilic drug and big protein molecules is challenging due to barrier provided by the top layer of skin known as stratum corneum (SC). The chemical permeation enhancers or specialized carriers such as nanoparticles (NPs) are needed which can deliver drug molecules into the deeper layer.Here, we describe the in silico design of nanoparticle carriers using molecular dynamics (MD) simulations for the transdermal drug delivery application. At first, setup of a skin lipid bilayer model is demonstrated. Further, nanoparticles are designed based on the Monte Carlo simulation technique. These nanoparticles are then tested on skin model using various MD simulation techniques.

Molecular mechanism of HIV-1 TAT peptide and its conjugated gold nanoparticles translocating across lipid membranes

The trans-acting activator of transcription (TAT) peptide, which is derived from human immunodeficiency virus-1 (HIV-1), has been widely used as an effective nanocarrier to transport extracellular substances into cells. However, the underlying translocation mechanism of TAT peptide across cell membranes still remains controversial. Besides, the molecular process of TAT peptide facilitating the transport of extracellular substances into cells is largely unknown. In this study, we explore the interactions of TAT peptides and their conjugated gold nanoparticles with lipid membranes by coarse-grained molecular dynamics simulations. It is found that the TAT peptides can hardly penetrate through the membrane at low peptide concentrations; after the concentration increases to a threshold value, they can cross the membrane through an induced nanopore due to the transmembrane electrostatic potential difference. The translocation of TAT peptides is mainly caused by the overall structural changes of membranes. Furthermore, we demonstrate that the translocation of gold nanoparticles (AuNPs) across the membrane is significantly affected by the number of grafted TAT peptides on the particle surface. The transmembrane efficiency of AuNPs may even be reduced when a small number of peptides modify them; whereas, when the number of grafted peptides increases to a certain value, the TAT-AuNP complex can translocate across the membrane in a pore-mediated way. Based on our findings, an effective strategy has been proposed to enhance the delivery efficiency of AuNPs. The present study can improve our understanding of the interactions between TAT peptides and cell membranes; it may also give some insightful suggestions on the design and development of nanocarriers with high efficiency for the delivery of nanoparticles and drugs.

Molecular Dynamics Simulations of Membrane Permeability

This Review illustrates the evaluation of permeability of lipid membranes from molecular dynamics (MD) simulation primarily using water and oxygen as examples. Membrane entrance, translocation, and exit of these simple permeants (one hydrophilic and one hydrophobic) can be simulated by conventional MD, and permeabilities can be evaluated directly by Fick's First Law, transition rates, and a global Bayesian analysis of the inhomogeneous solubility-diffusion model. The assorted results, many of which are applicable to simulations of nonbiological membranes, highlight the limitations of the homogeneous solubility diffusion model; support the utility of inhomogeneous solubility diffusion and compartmental models; underscore the need for comparison with experiment for both simple solvent systems (such as water/hexadecane) and well-characterized membranes; and demonstrate the need for microsecond simulations for even simple permeants like water and oxygen. Undulations, subdiffusion, fractional viscosity dependence, periodic boundary conditions, and recent developments in the field are also discussed. Last, while enhanced sampling methods and increasingly sophisticated treatments of diffusion add substantially to the repertoire of simulation-based approaches, they do not address directly the critical need for force fields with polarizability and multipoles, and constant pH methods.

Structural and barrier properties of the skin ceramide lipid bilayer: a molecular dynamics simulation study

Skin provides excellent protection against the harsh external environment and foreign substances. The lipid matrix of the stratum corneum, which contains various kinds of ceramides, plays a major role in the barrier function of the skin. Here we report a study of the effects of ceramide type on the structural and transport properties of ceramide bilayers using molecular dynamics (MD) simulations. Specifically, the effects of headgroup chemistry (number and positions of hydroxyl groups) and tail structure (unsaturation of the sphingoid moiety) on the structural and transport properties of various ceramide bilayers at 310 K were analyzed. Theoretical results for structural properties such as area per lipid, bilayer thickness, lateral arrangement, order parameter, and hydrogen bonding are reported here and compared with corresponding experimental data. Our study revealed that the presence of a double bond disrupts the bilayer packing, which leads to a low area compressibility modulus, a large area per lipid, and low bilayer thickness. Furthermore, the effect of structural changes on water permeation was studied using steered MD simulations. Water permeation was found to be influenced by headgroup polarity, chain packing, and the ability of the water to hydrogen bond with the ceramides. The molecular-level information obtained from the current study should aid the design of mixed bilayer systems with desired properties and provide the basis for the development of higher order coarse-grained models.

Effect of Chemical Permeation Enhancers on Skin Permeability: In silico screening using Molecular Dynamics simulations

Breaching of the skin barrier is essential for delivering active pharmaceutical ingredients (APIs) for pharmaceutical, dermatological and aesthetic applications. Chemical permeation enhancers (CPEs) are molecules that interact with the constituents of skin’s outermost and rate limiting layer stratum corneum (SC), and increase its permeability. Designing and testing of new CPEs is a resource intensive task, thus limiting the rate of discovery of new CPEs. In-silico screening of CPEs in a rigorous skin model could speed up the design of CPEs. In this study, we performed coarse grained (CG) molecule dynamics (MD) simulations of a multilayer skin lipid matrix in the presence of CPEs. The CPEs are chosen from different chemical functionalities including fatty acids, esters, and alcohols. A multi-layer in-silico skin model was developed. The CG parameters of permeation enhancers were also developed. Interactions of CPEs with SC lipids was studied in silico at three different CPE concentrations namely, 1% w/v, 3% w/v and 5% w/v. The partitioning and diffusion coefficients of CPEs in the SC lipids were found to be highly size- and structure-dependent and these dependencies are explained in terms of structural properties such as radial distribution function, area per lipid and order parameter. Finally, experimentally reported effects of CPEs on skin from the literature are compared with the simulation results. The trends obtained using simulations are in good agreement with the experimental measurements. The studies presented here validate the utility of in-silico models for designing, screening and testing of novel and effective CPEs.

Electroporation of Skin Stratum Corneum Lipid Bilayer and Molecular Mechanism of Drug Transport: A Molecular Dynamics Study

The electroporation technique has been used significantly to increase drug permeation through the skin. This technique relies on the application of short-timed (microseconds to millisecond) electric fields (generally, order of 50--300 V) on the skin to create microscopic pores. However, the molecular mechanism of pore formation, resulting in an enhanced flux of active molecules through the skin, remains poorly understood. In this study, extensive atomistic molecular dynamics simulation of skin lipids [made up of ceramide (CER), cholesterol (CHOL), and free fatty acid (FFA)] has been performed at various external electric fields. We show for the first time the pore formation in the skin lipid bilayer during electroporation. We show the effect of the applied external electrical field (0.6-1.0 V/nm) on the pore formation dynamics in the lipid bilayer of different sizes (154, 616, and 2464 lipids) and compositions (CER/CHOL/FFA, 1:0:0, 1:0:1, 1:1:0, 1:1:1). The pore formation and resealing kinetics were different and were found to be highly dependent on the composition of the skin lipid bilayer. The pore formation time decreased with increase in the bilayer size. The pore sustaining electric field was found to be in the range of 0.20-0.25 V/nm for equimolar CER, CHOL, and FFA lipid bilayers. The skin lipid bilayer (1:1:1) sealed itself within 20 ns after the removal of the external electric field. We also present the molecular mechanism of enhancement of drug permeation in the presence of external field as compared to the passive diffusion. The molecular-level understanding obtained here could help in optimizing/designing the electroporation experiments for effective drug delivery. For a given skin composition and size of the drug molecule, the combination of pore formation time and pore growth model can be used to know a priori the desired electric field and time for the application of the electric field.

In-silico design of nanoparticles for transdermal drug delivery application

Nanoparticles are used in the medical field for various applications like cell imaging, drug delivery, gene and si-RNA delivery, to name a few. Designing nanoparticles for a given application, purely based on the trial and error experimentation, requires a lot of time and effort. In this study we show that computer simulations could help in designing nanoparticles for drug delivery thus reducing the time and cost associated with their design, development and deployment. The permeation of nanoparticles, having various surface chemistries and patterns, through the skin lipid bilayer was studied using constrained and unconstrained molecular dynamics simulations. Interestingly, the permeation mechanism of nanoparticles having the same surface chemistry but different patterns was found to be completely different. Nanoparticles (NPs) were screened based on the free energy of permeation through the skin lipid bilayer. The behavior of the screened NPs was further validated with unconstrained simulations using the skin lipid bilayer. Nanoparticles thus screened through both of the techniques were further used for the co-delivery of a model protein into the skin lipid bilayer. It was observed that the nanoparticles having a 2 : 1 homogeneous ratio of hydrophobic to hydrophilic regions were the most promising in transdermal delivery of proteins. The obtained results are in line with the results of recent permeation experiments on cell and plasma membrane. Our study could help in in-silico design of nanoparticles for delivery of actives through skin. These in-silico experiments thus could help speed up the development process by guiding formulation chemists.

Effect of surface coverage and chemistry on self-assembly of monolayer protected gold nanoparticles: a molecular dynamics simulation study

The PMF between two nanoparticles (of any chemistry and coverage) can be used as a measure to predict their stability.

Understanding the Cellular Uptake of pH-Responsive Zwitterionic Gold Nanoparticles: A Computer Simulation Study

Surface functionalization of nanoparticles (NPs) with stealth polymers (e.g., hydrophilic and zwitterionic polymers) has become a common strategy to resist nonspecific protein adsorption recently. Understanding the role of surface decoration on NP-biomembrane interactions is of great significance to promote the application of NPs in biomedical fields. Herein, using coarse-grained molecular dynamics (CGMD) simulations, we investigate the interactions between stealth polymer-coated gold nanoparticles (AuNPs) and lipid membranes. The results show that AuNPs grafted with zwitterionic polymers can more easily approach the membrane surface than those coated with hydrophilic poly(ethylene glycol) (PEG), which can be explained by the weak dipole-dipole interaction between them. For zwitterionic AuNPs which can undergo pH-dependent charge conversion, different interaction modes which depend on the polymer protonation degree are found. When the protonation degree is low, the particles just adsorb on the membrane surface; at moderate protonation degrees, the particles can directly translocate across the lipid membrane through a transient hydrophilic pore formed on the membrane surface; the particles are fully wrapped by the curved lipid membrane at high protonation degrees, which may lead to endocytosis. Finally, the effect of polymer chain length on the cellular uptake of zwitterionic polymer-coated AuNPs is considered. The results demonstrate that longer polymer chain length will block the translocation of AuNPs across the lipid membrane when the protonation degree is not high; however, it can improve the transmembrane efficiency of AuNPs at high protonation degrees. We expect that these findings are of immediate interest to the design and synthesis of pH-responsive nanomaterials based on zwitterionic polymers and can prompt their further applications in the field of biomedicine.

In-Silico Skin Model: A Multiscale Simulation Study of Drug Transport

Accurate in-silico models are required to predict the release of drug molecules through skin in order to supplement the in-vivo experiments for faster development/testing of drugs. The upper most layer of the skin, stratum corneum (SC), offers the main resistance for permeation of actives. Most of the SC's molecular level models comprise cholesterol and phospholipids only, which is far from reality. In this study we have implemented a multiscale modeling framework to obtain the release profile of three drugs, namely, caffeine, fentanyl, and naphthol, through skin SC. We report for the first time diffusion of drugs through a realistic skin molecular model comprised of ceramides, cholesterol, and free fatty acid. The diffusion coefficients of drugs in the SC lipid matrix were determined from multiple constrained molecular dynamics simulations. The calculated diffusion coefficients were then used in the macroscopic models to predict the release profiles of drugs through the SC. The obtained release profiles were in good agreement with available experimental data. The partition coefficient exhibits a greater effect on the release profiles. The reported multiscale modeling framework would provide insight into the delivery mechanisms of the drugs through the skin and shall act as a guiding tool in performing targeted experiments to come up with a suitable delivery system.