Wesam R. Kadhum

Prediction of skin permeation by chemical compounds using the artificial membrane, Strat-M™.

PURPOSE The usefulness of the synthetic membrane, Strat-M™ as an alternative to human and animal skins was evaluated by estimating the skin permeabilities of chemical compounds. METHOD Thirteen chemical compounds with molecular weights (M.W.) of 152-289 and lipophilicities (log Ko/w) of -0.9 to 3.5 were selected. Strat-M™, excised human skin, or hairless rat skin was set in a Franz-type diffusion cell and a saturated solution of each chemical compound was applied to determine membrane permeation profiles. The obtained permeability coefficients (log P) were compared among these membranes. RESULTS AND DISCUSSION Elevations were observed in log P for Strat-M™ with an increase in the log Ko/w of the applied compounds, and similar results were observed with the human and hairless rat skins. A correlation was obtained in log P values between Strat-M™ and human or hairless rat skin. Furthermore, the diffusion and partition parameters of chemicals in Strat-M™ were similar to those in the excised human and rat skins. These results suggest that Strat-M™ could be used as an alternative to animal or human skin in permeation studies.

Mathematical Model to Predict Skin Concentration after Topical Application of Drugs

Skin permeation experiments have been broadly done since 1970s to 1980s as an evaluation method for transdermal drug delivery systems. In topically applied drug and cosmetic formulations, skin concentration of chemical compounds is more important than their skin permeations, because primary target site of the chemical compounds is skin surface or skin tissues. Furthermore, the direct pharmacological reaction of a metabolically stable drug that binds with specific receptors of known expression levels in an organ can be determined by Hill’s equation. Nevertheless, little investigation was carried out on the test method of skin concentration after topically application of chemical compounds. Recently we investigated an estimating method of skin concentration of the chemical compounds from their skin permeation profiles. In the study, we took care of “3Rs” issues for animal experiments. We have proposed an equation which was capable to estimate animal skin concentration from permeation profile through the artificial membrane (silicone membrane) and animal skin. This new approach may allow the skin concentration of a drug to be predicted using Fick’s second law of diffusion. The silicone membrane was found to be useful as an alternative membrane to animal skin for predicting skin concentration of chemical compounds, because an extremely excellent extrapolation to animal skin concentration was attained by calculation using the silicone membrane permeation data. In this chapter, we aimed to establish an accurate and convenient method for predicting the concentration profiles of drugs in the skin based on the skin permeation parameters of topically active drugs derived from steady-state skin permeation experiments.

Estimation of skin concentrations of topically applied lidocaine at each depth profile

Skin concentrations of topically administered compounds need to be considered in order to evaluate their efficacies and toxicities. This study investigated the relationship between the skin permeation and concentrations of compounds, and also predicted the skin concentrations of these compounds using their permeation parameters. Full-thickness skin or stripped skin from pig ears was set on a vertical-type diffusion cell, and lidocaine (LID) solution was applied to the stratum corneum (SC) in order to determine in vitro skin permeability. Permeation parameters were obtained based on Ficks second law of diffusion. LID concentrations at each depth of the SC were measured using tape-stripping. Concentration-depth profiles were obtained from viable epidermis and dermis (VED) by analyzing horizontal sections. The corresponding skin concentration at each depth was calculated based on Ficks law using permeation parameters and then compared with the observed value. The steady state LID concentrations decreased linearly as the site became deeper in SC or VED. The calculated concentration-depth profiles of the SC and VED were almost identical to the observed profiles. The compound concentration at each depth could be easily predicted in the skin using diffusion equations and skin permeation data. Thus, this method was considered to be useful for promoting the efficient preparation of topically applied drugs and cosmetics.

In Silico Estimation of Skin Concentration Following the Dermal Exposure to Chemicals

PurposeTo develop an in silico method based on Fick’s law of diffusion to estimate the skin concentration following dermal exposure to chemicals with a wide range of lipophilicity.MethodsPermeation experiments of various chemicals were performed through rat and porcine skin. Permeation parameters, namely, permeability coefficient and partition coefficient, were obtained by the fitting of data to two-layered and one-layered diffusion models for whole and stripped skin. The mean skin concentration of chemicals during steady-state permeation was calculated using the permeation parameters and compared with the observed values.ResultsAll permeation profiles could be described by the diffusion models. The estimated skin concentrations of chemicals using permeation parameters were close to the observed levels and most data fell within the 95% confidence interval for complete prediction. The permeability coefficient and partition coefficient for stripped skin were almost constant, being independent of the permeant’s lipophilicity.ConclusionsSkin concentration following dermal exposure to various chemicals can be accurately estimated based on Fick’s law of diffusion. This method should become a useful tool to assess the efficacy of topically applied drugs and cosmetic ingredients, as well as the risk of chemicals likely to cause skin disorders and diseases.

Usefulness of liquid-crystal oral formulations to enhance the bioavailability and skin tissue targeting of p-amino benzoic acid as a model compound.

Topical formulations are not always suitable to deliver active ingredients to large areas of skin. Thus, in this study, we aimed to develop an oral formulation for skin tissue targeting with a high bioavailability using liquid crystal (LC) dispersions comprising cubosomes of a mal-absorptive model compound, p-amino benzoic acid (PABA), which is an active element in cosmeceuticals, dietary supplements and skin disorder medicines. The bioavailability and skin concentration of PABA were investigated after oral administration in rats. The effect of the remaining amount of the LC formulation in the stomach on the pharmacokinetic profiles of orally administered PABA was evaluated. The skin permeation and concentration of PABA were also investigated using an in vitro permeation experiment. As a result, the bioavailability of PABA was significantly improved by administration of PABA-LC formulations compared with PABA solution alone, although the effect was greatly influenced by the type of LC-forming lipids. The in vitro skin permeation study showed that the PABA concentration in the skin when applied from the dermis side was higher than when applied from the epidermis side. These findings suggested that oral administration advantageously supports skin targeting, and oral LC formulations could be a promising material in cosmeceutical, dietary and clinical fields.

Skin Permeation: Enhancing Ability of Liquid Crystal Formulations

Liquid crystals are intermediate states of matter or mesophases, a halfway between an isotropic liquid and a solid crystal. The penetration-enhancing ability of liquid crystals (LC) in topical formulations applied to the skin was evaluated by measuring in vitro, through excised hairless rat, the permeation profile of a model compound, calcein, entrapped in the LC formulation. Two physically stable formulations with LCs were prepared from a mixture of mono-, di-, and triesters (1) and monoesters (2) composed of erythritol and phytanylacetic acid. Cryo-transmission electron microscopy (cryo-TEM) and electron diffraction pattern observations of the LC nanodispersions showed that the structure of the LCs was reverse hexagonal (LC-A) and cubic (LC-B). The skin permeation of calcein was enhanced by its entrapping in LCs due to the increase in calcein partition from the LC dispersion into the skin, which was analyzed by the skin permeation-time profile. This chapter shows that LC dispersions can be used as topical carrier systems in drug formulations as well as in cosmetic formulations.

Design of a Topically Applied Gel Spray Formulation with Ivermectin Using a Novel Low Molecular Weight Gelling Agent, Palmitoyl-Glycine-Histidine, to Treat Scabies

Palmitoyl-glycine-histidine (Pal-GH) is a new low molecular weight gelling agent. It exhibits thixotropic behavior, low viscosity, and high dissolving properties for a wide range of hydrophilic to lipophilic drugs. Orally administered ivermectin (IVM) is used to treat scabies. However, this treatment is associated with well-known side effects, thus a study is awaited to search for alternative routes of administration. Although a topical formulation of IVM could be a candidate, it requires whole body application except the head and face for several hours on a daily basis. Therefore, in this study, we prepared a gel spray formulation containing IVM as an approach for application to large skin areas with a single spray application without further contact with the applied formulation. Pal-GH gel spray formulations were prepared from its aqueous solution by a heating and cooling method. Rheological behavior and physical appearance (spraying, spreading ability, volume of spraying, and homogeneity) of the prepared formulations were evaluated. Pal-GH gel with propylene glycol demonstrated impressive rheological properties (typical thixotropic behavior) with high hysteresis area among all the tested Pal-GH gels and spreading ability. The obtained IVM concentration in the skin after topical application of 0.1% IVM-containing Pal-GH formulation onto hairless rats was much higher than the reported therapeutic concentration obtained from oral administration in humans. These results suggested that topical application of IVM using a Pal-GH gel spray formulation could be an alternative to the conventional oral forms for the scabies treatment.

A Novel Chemical Enhancer Approach for Transdermal Drug Delivery with C17-Monoglycerol Ester Liquid Crystal-forming Lipid

Transdermal administration of drugs represents an excellent alternative to conventional pharmaceutical dosage forms. However, insufficient penetration of the active pharmaceutical substance through the skin is a common problem. Thus, in the present study we evaluated the skin permeation enhancing ability of liquid crystal (LC) topical formulations. A recently developed LC-forming lipid, C17- monoglycerol ester (MGE), was evaluated and compared with glycerol monoolate (GMO), which is considered as the gold standard for LC formulations. We initially prepared LC formulations containing drugs with different physiochemical properties (tranexamic acid [TXA], 4-methoxy-salicylic acid [4-MS], catechin [CC], and calcein [Cal]), and confirmed the LC phase structures in the prepared formulations using a polarizing light microscope and a small-angle X-ray scattering (SAXS). The physicochemical properties of these formulations were also assessed using a viscometer and a zetasizer. The release rate of the drugs from the LC formulations was determined using a dialysis release method. The skin penetration-enhancing ability of LC formulations was also investigated in an in vitro skin permeation study. The results showed that both MGE- and GMO-LC-forming lipids shared the same behavior in terms of their birefringence indexes, LC phase structures, particle sizes, and zeta potentials. Both the MGE- and GMO-LC formulations managed to improve the skin permeation for various drugs with a range of physiochemical properties. However, MGE formulations showed lower viscosity, faster drug release rate, and better skin penetration-enhancing ability than GMO formulations, strongly suggesting that the low viscosity of MGELC-forming lipids might influence drug diffusivity and permeability through the skin. The present MGELC formulation might be utilized as a promising new topical formulation for therapeutic drugs and cosmetic ingredients.

Establishment of an evaluation method to detect drug distribution in hair follicles

&NA; Development of an appropriate method to evaluate drug disposition or targeting ability in hair follicles (HFs) is urgently needed in order to develop useful pharmaceutical products with pharmacological effects in HFs. In the present study, a cyanoacrylate biopsy (CB) method was used to measure drug disposition in HFs using a model hydrophilic drug, caffeine (CAF), and a lipophilic drug, 4‐butylresorcinol (BR), in excised porcine skin. As a result, the height of HF replicas and the recovery ratio decreased with an increase in the application times of the CB method. HF replicas with a length of approximately 175 &mgr;m were obtained using a single application of the CB method. Drug distribution in the HF was detected even 5 min after topical application regardless of the lipophilicity of the drugs, although no drug disposition was observed in the deeper layers of the stratum corneum at the same time (5 min). Furthermore, significantly higher amounts of BR were observed in the stratum corneum and HF, compared with those of CAF. These results suggested that the CB method could be useful to evaluate the safety and efficacy as well as the disposition of topically applied chemicals, especially for HF‐targeting drugs.

The Synergistic Effect of Iontophoresis or Electroporation and Microneedles on the Skin Permeation of High Molecular Weight Compounds

Microneedle pretreatment has been introduced to increase skin permeability for several compounds by creating micron-scale pathways across the skin’s stratum corneum layer. In order to further enhance the skin permeability by microneedle pretreatment, several skin penetration-enhancing strategies have been developed such as a combination of microneedle system with iontophoresis or electroporation. The synergic effect of the combination of microneedle pretreatment and iontophoresis was evaluated for the potential to further increase skin permeation of drugs. Fluorescein isothiocyanate (FITC)-dextrans (FDs) (FD-4, FD-10, FD-40, FD-70, and FD-2000; average molecular weight of 4.3, 9.6, 42.0, 71.2, and 200.0 kDa) were used as models for high molecular weight compounds, and deuterium oxide (D2O) was used as a model for low molecular weight compounds in order to evaluate the effect of microneedle pretreatment and iontophoresis on their skin permeation in vitro using excised hairless rat skin. The combination of microneedle pretreatment and subsequent iontophoresis application significantly enhanced the cumulative amount and the flux of FDs compared to microneedle pretreatment alone or iontophoresis alone, whereas no synergistic effect was found in increasing the cumulative amount and the flux of D2O. On the other hand, the combination of microneedle pretreatment and electroporation was developed in order to enhance the delivery of macromolecular drugs to the deep skin tissues, the so-called in-skin electroporation (IN-SKIN EP). FITC-dextran (FD-4; average molecular weight, 4.3 kDa) was selected as a model high molecular weight compound. In vitro skin permeation experiments showed that the IN-SKIN EP provided a significantly higher skin penetration-enhancing effect for FD-4 than microneedles alone or ON-SKIN EP (conventional EP treatment). These results suggest that the combination of iontophoresis or electroporation with microneedle pretreatment may be a useful means to further increase skin permeation of high molecular weight compounds.