Practical syntheses of D-erythro and L-threo-ceramide [NDS] and difference in contribution of each isomer in microstructure of stratum corneum intercellular lipids

Lipid Biomimetic Models as Simple Yet Complex Tools to Predict Skin Permeation and Drug-Membrane Biophysical Interactions

The barrier function of the skin is primarily determined by its outermost layer, the Stratum Corneum (SC). The SC consists of corneocytes embedded in a lipid matrix composed mainly of ceramides, cholesterol, and free fatty acids in equimolar proportions and is organised in a complex lamellar structure with different periodicities and lateral packings. This matrix provides a diffusion pathway across the SC for bioactive compounds that are administered to the skin. In this regard, and as the skin administration route has grown in popularity, there has been an increase in the use of lipid mixtures that closely resemble the SC lipid matrix, either for a deeper biophysical understanding or for pharmaceutical and cosmetic purposes. This review focuses on a systematic analysis of the main outcomes of using lipid mixtures as SC lipid matrix models for pharmaceutical and cosmetic purposes. Thus, a methodical evaluation of the main outcomes based on the SC structure is performed, as well as the main recent developments in finding suitable new in vitro tools for permeation testing based on lipid models.

Diesel Particulate Matter Permeation into Normal Human Skin and Intervention Using a Topical Ceramide Formulation

INTRODUCTION

Diesel particulate matter (DPM) emitted from diesel engines is a major source of air pollutants. DPM is composed of elemental carbon, which adsorbs organic compounds including toxic polycyclic aromatic hydrocarbons (PAHs). The skin, as well as airways, is directly exposed to DPM, and association of atopic dermatitis, psoriasis flares, and premature skin aging with air pollutant levels has been documented. In skin, the permeation of DPM and DPM-adsorbed compounds is primarily blocked by the epidermal permeability barrier deployed in the stratum corneum. Depending upon the integrity of this barrier, certain amounts of DPM and DPM-adsorbed compounds can permeate into the skin. However, this permeation into human skin has not been completely elucidated.

METHODS

We assessed the permeation of PAHs (adsorbed to DPM) into skin using ex vivo normal (barrier-competent) organ-cultured human skin after application of DPM. Two major PAHs, 2-methylnaphthalene and triphenylene, and a carcinogenic PAH, benzo(a)pyrene, all found in DPM, were measured in the epidermis and dermis using liquid chromatography electrospray ionization tandem mass spectrometry. In addition, we investigated whether a topical formulation can attenuate the permeation of DPM into skin.

RESULTS

2-Methylnaphthalene, triphenylene, and benzo(a)pyrene were recovered from the epidermis. Although these PAHs were also detected in the dermis after DPM application, these PAH levels were significantly lower than those found in the epidermis. We also demonstrated that a topical formulation that has the ability to form more uniform membrane structures can significantly suppress the permeation of PAHs adsorbed to DPM into the skin.

CONCLUSION

Toxic compounds adsorbed by DPM can permeate even barrier-competent skin. Hence, barrier-compromised skin, such as in atopic dermatitis, psoriasis, and xerosis, is even more vulnerable to air pollutants. A properly formulated topical mixture that forms certain membrane structures on the skin surface can effectively prevent permeation of exogenous substances, including DPM, into skin.

Synchrotron FTIR Micro-spectroscopy Studies Demonstrate the Release of Ceramide Molecules from a Ceramide-containing UV-curable Acrylic Adhesive Resin

A recent study has shown that the addition of ceramide to adhesive materials of medical sheets or tapes improves or maintains skin barrier functions despite the irritation and damage caused by the repeated removal of the sheet or tape. This fact may imply that ceramide molecules are released from the adhesive material and penetrate the skin. In this study, we investigated whether ceramide molecules are released from a UV-curable acrylic adhesive resin (acResin^®) sheet containing ceramide molecules attached to a cultured skin sample by estimating the local ceramide concentration from the data obtained by synchrotron Fourier transform infrared micro-spectroscopy analysis. Since section samples of uniform thickness could not be prepared, the intensity data of the amide I peak originating from ceramide were normalized by using the intensity data of the ester peak originating from the resin matrix. This analysis enabled the quantification of the change in local ceramide concentrations in the sheet samples. This result indicates that ceramide molecules were released from the acResin^® sheet attached to the cultured skin sheet.

Thermal-history-dependent Phase Behavior of Ceramide Molecular Assembly in a UV-curable Acrylic Adhesive Resin

The structure and thermal behavior of a synthetic D-erythro-ceramide [NDS], (2S,3R)-2-octadecanoylamino-octadecane-1,3-diol (CER), molecular assembly in a UV-curable acrylic adhesive resin (acResin^®) were investigated by differential scanning calorimetry (DSC), polarized-light microscopy, and X-ray diffraction (XRD). CER in the resin was found to exhibit a thermal-history-dependent polymorphic phase behavior that is similar but not identical to that observed for pure CER. The melting temperatures of the in-resin CER samples were lower than those of pure CER samples. Maintaining a melt-quenched in-resin CER sample at 60°C for 5-6 days induced a transformation from a metastable phase to a stable phase, where CER formed an ordered lamellar structure. The lamellar structure differed from that observed in the stable solid phase of pure CER samples. The findings of this study are expected to be useful for developing new medical tapes or sheets with ceramides added to the adhesives to protect skin.

Effects of Ceramide and Dihydroceramide Stereochemistry at C-3 on the Phase Behavior and Permeability of Skin Lipid Membranes

Ceramides (Cer) are key components of the skin permeability barrier. Sphingosine-based CerNS and dihydrosphingosine-based CerNdS (dihydroCer) have two chiral centers; however, the importance of the correct stereochemistry in the skin barrier Cer is unknown. We investigated the role of the configuration at C-3 of CerNS and CerNdS in the organization and permeability of model skin lipid membranes. Unnatural l-threo-CerNS and l-threo-CerNdS with 24-C acyl chains were synthesized and, along with their natural d-erythro-isomers, incorporated into membranes composed of major stratum corneum lipids (Cer, free fatty acids, cholesterol, and cholesteryl sulfate). The membrane microstructure was investigated by X-ray powder diffraction and infrared spectroscopy, including deuterated free fatty acids. Inversion of the C-3 configuration in CerNS and CerNdS increased phase transition temperatures, had no significant effects on lamellar phases, but also decreased the proportion of orthorhombic packing and decreased lipid mixing in the model membranes. These changes in membrane organization resulted in membrane permeabilities that ranged from unchanged to 5-fold higher (depending on the permeability markers, namely, water loss, electrical impedance, flux of theophylline, and flux of indomethacin) compared to membranes with natural CerNS/NdS isomers. Thus, the physiological d-erythro stereochemistry of skin Cer and dihydroCer appears to be essential for their correct barrier function.