skin permeability

Peptides are promising and attractive anti-wrinkle active ingredients, amongst which glycyl-histidyl-lysine peptide (GHK) is one of the most broadly promoted peptide for topical application. This simple sequence of amino acid residues not only has the capability of tissue regeneration and the enhancement of collagen and glycosaminoglycans synthesis but also is able to increase nerve outgrowth and angiogenesis. Consequently, GHK has several properties, from wound healing to prevention/reduction wrinkles. GHK-Cu and Pal-GHK are metal complex and palmitoylated derivatives of GHK, respectively. Although GHK-Cu and Pal-GHK are widely used in anti-wrinkle products available on the cosmetic market, the published information on their skin permeability, effectiveness, physicochemical properties and so on is insufficient.

This review aims to highlight whether GHK is sufficiently effective on wrinkle prevention/reduction. Apart from the effectiveness, another question that is tried to be answered is whether skin permeability of GHK allows it to act as an anti-wrinkle peptide at its site of action? Skin permeation enhancement methods employed so far are also reviewed.

Based on cellular studies, undoubtedly, GHK can be considered as an anti-wrinkle ingredient. Although GHK-Cu and Pal-GHK have been of interest as effective peptides to be incorporated in the anti-wrinkle products, there is a surprising absence of clinical studies using them. Metal complexation and chemical modification with a hydrophobic moiety increase permeability of this peptide. Besides, cell penetrating peptides seem promising to increase skin permeation of GHK and its derivatives. Skin pretreatment with microneedles also has the potential to be further studied for permeation enhancement of such peptides. As peptide ingredients, their formulation may encounter some challenges, mainly due to their hydrophilic (high aqueous solubility and low partition coefficient) and unstable nature.

Although GHK-Cu and Pal-GHK are effective and relatively skin permeable, their permeability could be successfully increased using permeation enhancement methodologies.

Cetirizine is a second-generation antihistamine with anti-allergy and anti-itching properties. The topical formulation of this medicine is used in androgenic alopecia treatment. Due to the hydrophilic nature of cetirizine, its skin absorption is negligible, so to increase its absorption, various enhancers were examined to see which can be used in the design of a topical formulation. First, the skin was exposed to enhancers, including eucalyptus, menthol, Tween 80, propylene glycol, and oleic acid, for 1 or 2 hours. Then, the permeability parameters of the cetirizine solution and the structural changes of the skin after exposure to enhancers were analyzed by differential scanning calorimetry (DSC) and Fourier-transform infrared spectroscopy (FTIR) techniques. The obtained results show that all used enhancers increased the permeability of the drug cetirizine compared to water. Various mechanisms, such as liquefaction of lipids, destruction of lipid structure, and irreversible denaturation of intracellular keratin, are involved in the increase in drug penetration caused by eucalyptus, mint, Tween 80, propylene glycol, and oleic acid. The results showed that among the studied absorption enhancers, eucalyptus and Tween 80 had the strongest, and propylene glycol had the weakest absorption enhancement effect after 2- and 1-hour pre-contact, respectively.

Caffeine is an edible chemical compound obtained from various plants, such as tea and coffee. Caffeine is an alkaloid that is highly hydrophilic and has limited skin permeability. The lipophilic nature of the stratum corneum is a major barrier to the passage of this substance through the skin. Topical drug delivery systems can effectively transfer caffeine to the skin.

This study investigated the effect of pretreatment time with chemical enhancers on caffeine’s skin permeation.

The skin was subjected to additives such as sodium lauryl sulfate, sodium lauryl ethyl sulfate, tynoline, nanoxinol, and lecithin for 5, 15, and 30 minutes. Then, the parameters of caffeine permeability and structural changes in the skin due to additive adsorption were studied using Fourier Transform Infrared (FT-IR) spectrometry.

The enhancers increased the permeation of caffeine through the skin. There are different mechanisms for penetration enhancers, including lipid liquefaction, disruption of lipid bilayers, and irreversible denaturation of intracellular keratin.

Sodium lauryl sulfate can affect the skin permeability of caffeine.

Octyl methoxycinnamate is one of the ingredients in sunscreen products. The main aim of this study was to investigate the effect of different enhancers of in vitro skin permeability of Octyl methoxycinnamate. Octyl methoxycinnamate permeability parameters were evaluated through the whole skin of the rat with and without chemical enhancers including eucalyptus oil, urea, menthol and olive oil by Franz cell diffusion. The effects of enhancers on skin structure were also studied using DSC and FT-IR techniques. The skin prevented the permeability of Octyl methoxycinnamate so that after 24 hours less than 3% of the substance passed through the skin. The results of this study showed that by increasing the time, it is possible to increase the skin permeation and the highest rate of skin absorption were corresponded to olive oil (ERflux=63.074), eucalyptus oil (ERflux=48.78) and menthol (ERflux=33.5), respectively while the least amount of skin absorption was related to urea (ERflux=29.53). Chemical penetration enhancers are substances that interfere with the complex structure of the skin and protein lipids. Two endothermic transitions were obtained at about 67 (Tm1) and 112 ° C (Tm2) in thermogram of the hydrated whole rat skin. Tm1 and Tm2 seems to be due to the melting of the lipids and the irreversible intracellular keratin or melting of the lipid-protein (keratin) complex, respectively. The amount of Tm1, ΔH1 and ΔH2 were decreased by all penetration enhancers compared to the hydrated skin. The FT-IR results suggested the mechanism of increasing absorption effect by lipid fluidization and lipid extraction. All of penetration enhancers used in this study significantly increased the skin permeability of Octyl methoxycinnamate.