Dynamics of proteins and lipids in the stratum corneum: Effects of percutaneous permeation enhancers

Casting New Light on the Retinol and Retinyl Palmitate Functions as Chemical Enhancers for Transdermal/Topical Drug Delivery

Retinol and retinyl palmitate, with excellent whitening and antioxidant activities, are widely used as essential functional ingredients in transdermal preparations and skincare products. Herein, for the first time, the influence and underlying mechanisms of retinol and retinyl palmitate on the skin penetration of caffeine are investigated. It is found that the penetration of caffeine is very significantly enhanced by retinol and retinyl palmitate with 2.83 and 2.57 folds, respectively. Mechanistic studies showed that they can disrupt the lipids of intercellular orderly arrangement and reduce the content of orthorhombic lattices in the skin, making the skin loose and the barrier function decreased. Furthermore, due to the formation of chemical bonds with the skin keratin, they have a strong affinity with the skin, changing the dense secondary structure of keratin with a reduced skin barrier effect. This study provides new evidence for the potential of retinol and retinyl palmitate as novel skin penetration enhancers with high safety that possess great potential for application in the transdermal delivery of pharmaceutical and cosmetic active substances.

Use of lipolanthionine peptide, a toll-like receptor 2 inhibitor, enhances transdermal delivery efficiency

The transdermal delivery system (TDS) is able to obtain a systemic therapeutic effect by administration through the skin, which has low side effects and is able to maintain a sustained blood concentration. However, due to the barrier presented by the stratum corneum, numerous drugs have poor percutaneous permeability. Therefore, the improvement of skin permeability is key to TDS. The main method of promoting transdermal absorption is through the usage of penetration enhancers. Dimethyl sulfoxide (DMSO) is a commonly used penetration enhancer, which has anti-inflammatory analgesic effects and is able to penetrate the skin. Retinoic acid (RA) and lipolanthionine peptide (LP) may also benefit the permeation efficiency of TDS. Therefore, the present study examined the function of DMSO, RA and LP as penetration enhancers in TDS. Firstly, the optimum concentration of DMSO was confirmed by detecting the expression of the LacZ gene in vitro. Secondly, different combinations of LP, RA and DMSO were applied to mouse skin to analyze the penetration enhancer combination with the greatest efficacy. All the animals were divided into five groups: The RA + LP + DMSO + pORF-LacZ group, the RA + DMSO + pORF-LacZ group, the LP + DMSO + pORF-LacZ group, the DMSO + pORF-LacZ group and the control group. Skin was soaked in combinations of LP, RA and DMSO for seven days and then the pORF-LacZ plasmids were daubed onto the skin once daily three days. On the 11th day, all the animals were sacrificed by cervical dislocation and the skin and blood samples were collected. The blood samples were used to detect the expression of the LacZ gene by quantitative polymerase chain reaction and the skin samples were used to detect the expression of claudin-4 and zonula occluden-1 (ZO-1) proteins by immunohistochemistry and western blot analysis. The results demonstrated that the combination of LP, RA and DMSO exhibited the greatest transdermal delivery efficiency, which verified that RA and LP were able to increase the penetration effects. Following treatment with LP, the symptoms of dermal edema were relieved and the capillaries contracted, which suggested that LP was a safe and effective penetration enhancer able to reduce the side-effects caused by DMSO. The present study provides a guideline for the synthesis of novel penetration enhancers.

Glycerol and urea can be used to increase skin permeability in reduced hydration conditions

The natural moisturizing factor (NMF) is a group of hygroscopic molecules that is naturally present in skin and protects from severe drying. Glycerol and urea are two examples of NMF components that are also used in skin care applications. In the present study, we investigate the influence of glycerol and urea on the permeability of a model drug (metronidazole, Mz) across excised pig skin membranes at different hydrating conditions. The degree of skin hydration is regulated by the gradient in water activity across the membrane, which in turn depends on the water activity of the formulation in contact with the skin membrane. Here, we determine the water activity of all formulations employed using an isothermal calorimetric method. Thus, the gradient in water activity is controlled by a novel experimental set-up with well-defined boundary conditions on both sides of the skin membrane. The results demonstrate that glycerol and urea can retain high steady state flux of Mz across skin membranes at dehydrating conditions, which otherwise would decrease the permeability due to dehydration. X-ray diffraction measurements are performed to give insight into the effects of glycerol and urea on SC molecular organization. The novel steady state flux results can be related to the observation that water, glycerol, and urea all affect the structural features of the SC molecular components in a similar manner.

Electron paramagnetic resonance as a unique tool for skin and hair research

Electron paramagnetic resonance (EPR) spectroscopy and imaging (EPRI) are deeply rooted in the basic and quantum physics, but the spectrum of their applications in modern experimental and clinical dermatology and cosmetology is surprisingly wide. The main aim of this review was to show the physical foundation, technical limitations and versatility of this method in skin studies. Free radical and metal ion detection, EPR dosimetry, melanin study, spin trapping, spin labelling, oximetry and NO-metry, EPR imaging, new generation methods of EPR and EPR/NMR hybrid technology used under ex vivo and in vivo regime are portrayed in the context of clinical and experimental skin research to study problems such as oxidative and nitrosative stress generated by UV or inflammation, skin oxygenation, hydration of corneal layer of epidermis, transport and metabolism of drugs and cosmeceutics, skin carcinogenesis, skin tumors and many others. A part of the paper is devoted to hair and nail research. The review of dermatological applications of EPR is supplemented with a handful of advice concerning practical aspects of EPR experimentation and usage of EPR reagents.

Effects of ethanol/l-menthol on the dynamics and partitioning of spin-labeled lipids in the stratum corneum

The interaction of ethanol as well as ethanol/L-menthol mixtures with the uppermost layer of epidermis, the stratum corneum, was investigated by electron paramagnetic resonance (EPR) spectroscopy utilizing spin-labeled analogs of androstanol (ASL), stearic acid (5-DSA) and methyl stearate (5-DMS). The EPR spectra of these spin probes structured in stratum corneum tissue of neonatal rat are characterized by the coexistence of two spectral components indicating the presence of two classes of spin labels with very different states of mobility. Probably, one class of spin labels is H-bonded to the polar surface of the membrane and another class corresponds to spin labels more deeply inserted in the hydrophobic core. EPR results showed that in the ethanol range 0-70% neither fluidity in stratum corneum membranes nor the relative fractions of these two components changes were observed. Instead, ethanol only caused a selective extraction of spin labels. The removal of the steroid ASL began at 30% ethanol, reaching extraction levels over 50% at ethanol concentrations of 60-70%, whereas the more hydrophobic 5-DMS was partially removed only with 70% ethanol. Addition of 5% L-menthol to the solvent containing 20% ethanol increases both the mobility and the fraction of those spin labels situated in the hydrophobic core (more mobile spectral component). Altogether, these findings suggest that the L-menthol stabilizes mainly in the central region of stratum corneum membranes attracting the membrane lipids and causing hydrogen bond ruptures in the polar membrane interface.

Dynamics and partitioning of spin-labeled stearates into the lipid domain of stratum corneum

The EPR spectra of the positional isomers n-doxyl stearic acids (n-DSA), with n=5, 12 and 16, and 5-doxyl methyl stearate (5-DMS) structured in the lipid domain of intact stratum corneum (SC), are characterized by the thermodynamic equilibrium of two distinct spectral components provided by two different motional states of the spin-labeled chains. A two-component model used in the EPR spectra simulations provided the relative populations of the components, allowing for the calculation of the thermodynamic profile. Based on a detailed investigation, the more motionally restricted population of spin labels (component 1) is found to arise when the spin label is hydrogen-bonded to the polar surfaces of the membranes, while the less motionally restricted population (component 2) is generated by spin labels nonhydrogen-bonded and more deeply inserted in the hydrophobic core. The 5-DSA is bound tightly to the polar surfaces (DeltaG(o)2 --> 1=-1.75 kcal/mol and DeltaH(o)2 --> 1=-13.8 kcal/mol), whereas the more lipophilic 5-DMS has a major spin population stabilized in the hydrophobic core (DeltaG(o)2 --> 10.57 kcal/mol and DeltaH(o)2 --> 1=-9.1 kcal/mol). Upon lipid-depleting SC increases the interactions of the probe with the polar surfaces, thereby decreasing its rotational diffusion. In contrast, the treatment of SC with oleic acid, a permeation enhancer, drastically increases the mobility of the spin labels, particularly that of component 1, and the thermodynamic equilibrium shifts towards the formation of component 2. A mechanism for water permeation in SC is also proposed.