A comprehensive review on invasomal carriers incorporating natural terpenes for augmented transdermal delivery

Background

Transdermal drug delivery is one of the most widely used drug administration routes, which offer several advantages over other routes of drug delivery. The apical layer of the skin called the stratum corneum is the most dominant obstacle in the transdermal drug delivery, which restricts the passage of drugs across the skin. Considerable strategies have been applied to enhance the rate of permeation across the epithelial cells; however, the most widely used strategy is the use of sorption boosters, also known as permeation enhancers.

Main body

Terpenes were considered as efficient skin permeation enhancers and are generally recognized as safe as per Food and Drug Administration. Terpenes improve the permeability of drugs either by destructing the stratum corneum’s tightly packed lipid framework, excessive diffusivity of drug in cell membrane or by rampant drug partitioning into epithelial cells. Various vesicular systems have been developed and utilized for the transdermal delivery of many drugs. Invasomes are one such novel vesicular system developed which are composed of phospholipids, ethanol and terpenes. The combined presence of ethanol and terpenes provides exceptional flexibility to the vesicles and improves the permeation across the barrier offered due to the stratum corneum as both ethanol and terpenes act as permeation enhancers. Therefore, utilization of invasomes as carriers to facilitate higher rate of drug permeation through the skin can be a very useful approach to improve transdermal drug delivery of a drug.

Conclusion

The paper focuses on a broad updated view of terpenes as effective permeation enhancers and invasomes along with their applications in the pharmaceutical formulations.

Assessing the Oxidative State of the Skin by Combining Classical Tape Stripping with ORAC Assay

The antioxidant barrier system of the skin acts as the main defence against environmental pro-oxidants. Impaired skin oxidative state is linked to unhealthy conditions such as skin autoimmune diseases and cancer. Thus, the evaluation of the overall oxidative state of the skin plays a key role in further understanding and prevention of these disorders. This study aims to present a novel ex vivo model to evaluate the skin oxidative state by the measurement of its antioxidant capacity (AOC). For this the ORAC assay was combined with classical tape stripping and infrared densitometry to evaluate the oxidative state of the stratum corneum (SC). Outcomes implied the suitability of the used model to determine the intrinsic antioxidant capacity (iAOC) of the skin. The average iAOC of untreated skin was determined as 140 ± 7.4 µM TE. Skin exposure to UV light for 1 h reduced the iAOC by about 17%, and exposure for 2 h decreased the iAOC by about 30%. Treatment with ascorbic acid (AA) increased the iAOC in a dose-dependent manner and reached an almost two-fold iAOC when 20% AA solution was applied on the skin. The application of coenzyme Q10 resulted in an increase in the iAOC at low doses but decreased the iAOC when doses > 1% were applied on the skin. The results show that the combination of classical tape stripping and ORAC assay is a cost-effective and versatile method to evaluate the skin oxidative state and the pro-oxidate and antioxidative effects of topical skin treatments on the iAOC of the skin. Therefore, the model can be considered to be a valuable tool in skin research.

Invasome: A Novel Nanocarrier for Transdermal Drug Delivery

Invasomes are novel vesicular systems that exhibit improved transdermal penetration compared to conventional liposomes. These vesicles contain phospholipids, ethanol, and terpene in their structures; these components confer suitable transdermal penetration properties to the soft vesicles. The main advantages of these nanovesicles lie in their ability to increase the permeability of the drug into the skin and decrease absorption into the systemic circulation, thus, limiting the activity of various drugs within the skin layer. In this paper, several features of invasomes, including their structure, mechanism of penetration, applications, characterization, and potential advantages in dermal drug delivery, are highlighted. Overall, this review suggests that enhanced transdermal penetration of drugs using invasomes provides an appropriate opportunity for the development of lipid vesicular carriers.

Investigation of TEMPO partitioning in different skin models as measured by EPR spectroscopy - Insight into the stratum corneum

Electron paramagnetic resonance (EPR) spectroscopy represents an established tool to study properties of microenvironments, e.g. to investigate the structure and dynamics of biological and artificial membranes. In this study, the partitioning of the spin probe 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) in ex vivo human abdominal and breast skin, ex vivo porcine abdominal and ear skin as well as normal and inflammatory in vitro skin equivalents was investigated by EPR spectroscopy. Furthermore, the stratum corneum (SC) lipid composition (as determined by high-performance thin-layer chromatography), SC lipid chain order (probed by infrared spectroscopy) and the SC thickness (investigated by histology) were determined in the skin models. X-band EPR measurements have shown that TEMPO partitions in the lipophilic and hydrophilic microenvironment in varying ratios in different ex vivo and in vitro skin models. Ex vivo human abdominal skin exhibited the highest amount of TEMPO in the lipophilic microenvironment. In contrast, the lowest amount of TEMPO in the lipophilic microenvironment was determined in ex vivo human breast skin and the inflammatory in vitro skin equivalents. Individual EPR spectra of epidermis including SC and dermis indicated that the lipophilic microenvironment of TEMPO mainly corresponds to the most lipophilic part of the epidermis, the SC. The amount of TEMPO in the lipophilic microenvironment was independent of the SC lipid composition and the SC lipid chain order but correlated with the SC thickness. In conclusion, EPR spectroscopy could be a novel technique to determine differences in the SC thickness, thus suitably complementing existing methods.

Investigation of the cutaneous penetration behavior of dexamethasone loaded to nano-sized lipid particles by EPR spectroscopy, and confocal Raman and laser scanning microscopy

An improvement of the penetration efficiency combined with the controlled release of actives in the skin can facilitate the medical treatment of skin diseases immensely. Dexamethasone (Dx), a synthetic glucocorticoid, is frequently used for the treatment of inflammatory skin diseases. To investigate the penetration of nano-sized lipid particles (NLP) loaded with Dx in comparison to a commercially available base cream, different techniques were applied. Electron paramagnetic resonance (EPR) spectroscopy was used to monitor the penetration of Dx, which was covalently labeled with the spin probe 3-(Carboxy)-2,2,5,5-tetramethyl-1-pyrrolidinyloxy (PCA). The penetration into hair follicles was studied using confocal laser scanning microscopy (CLSM) with curcumin-loaded NLP. The penetration of the vehicle was followed by confocal Raman microscopy (CRM). Penetration studies using excised porcine skin revealed a more than twofold higher penetration efficiency for DxPCA into the stratum corneum (SC) after 24h incubation compared to 4h incubation when loaded to the NLP, whereas when applied in the base cream, almost no further penetration was observed beyond 4h. The distribution of DxPCA within the SC was investigated by consecutive tape stripping. The release of DxPCA from the base cream after 24h in deeper SC layers and the viable epidermis was shown by EPR. For NLP, no release from the carrier was observed, although DxPCA was detectable in the skin after the complete SC was removed. This phenomenon can be explained by the penetration of the NLP into the hair follicles. However, penetration profiles measured by CRM indicate that NLP did not penetrate as deeply into the SC as the base cream formulation. In conclusion, NLP can improve the accumulation of Dx in the skin and provide a reservoir within the SC and in the follicular infundibula.

Cutaneous carotenoids: the mirror of lifestyle?

Carotenoids could serve as marker substances of the antioxidant status of the human skin. In the present study, an optical skin scanner was used to analyse the carotenoid concentration obtained from 4 volunteers over a period of 13 days. The measurements were taken daily at different time points. In addition, the volunteers were asked to keep track of their daily nutritional behaviour and stress situations in a diary. It was found that the carotenoid values reflect clearly the nutritional behaviour and stress situations of the volunteers. While a steady, increased intake of fruit and vegetables resulted in a gradual increase in the dermal carotenoid values for several days, stressful situations entailed an immediate decline in these values. Although the impact of healthy nutrition on the antioxidant protection system is generally known and notwithstanding the small number of cases, the present study clearly demonstrates that the avoidance and/or reduction of stress is similarly important in order not to counteract or nullify the results achieved by healthy nutrition.

Enhancement of skin radical scavenging activity and stratum corneum lipids after the application of a hyperforin-rich cream

Hyperforin is well-known for its anti-inflammatory, anti-tumor, anti-bacterial, and antioxidant properties. The application of a hyperforin-rich verum cream could strengthen the skin barrier function by reducing radical formation and stabilizing stratum corneum lipids. Here, it was investigated whether topical treatment with a hyperforin-rich cream increases the radical protection of the skin during VIS/NIR irradiation. Skin lipid profile was investigated applying HPTLC on skin lipid extracts. Furthermore, the absorption- and scattering coefficients, which influence radical formation, were determined. 11 volunteers were included in this study. After a single cream application, VIS/NIR-induced radical formation could be completely inhibited by both verum and placebo showing an immediate protection. After an application period of 4weeks, radical formation could be significantly reduced by 45% following placebo application and 78% after verum application showing a long-term protection. Furthermore, the skin lipids in both verum and placebo groups increased directly after a single cream application but only significantly for ceramide [AP], [NP1], and squalene. After long-term cream application, concentration of cholesterol and the ceramides increased, but no significance was observed. These results indicate that regular application of the hyperforin-rich cream can reduce radical formation and can stabilize skin lipids, which are responsible for the barrier function.

In vivo methods for the analysis of the penetration of topically applied substances in and through the skin barrier

The efficacy of a drug is characterized by its action mechanism and its ability to pass the skin barrier. In this article, different methods are discussed, which permit this penetration process to be analysed non-invasively. Providing qualitative and quantitative information, tape stripping is one of the oldest procedures for penetration studies. Although single cell layers of corneocytes are removed from the skin surface, this procedure is considered as non-invasive and is applicable exclusively to the stratum corneum. Recently, optical and spectroscopic methods have been used to investigate the penetration process. Fluorescence-labelled drugs can be easily detected in the skin by laser scanning microscopy. This method has the disadvantage that the dye labelling changes the molecular structures of the drug and consequently might influence the penetration properties. The penetration process of non-fluorescent substances can be analysed by Raman spectroscopy, electron paramagnetic resonance, CARS and multiphoton microscopic measurements. Using these methods, the concentration of the topically applied formulations in different depths of the stratum corneum can be detected by moving the laser focus from the skin surface deeper into the stratum corneum. The advantages and disadvantages of these methods will be discussed in this article.

Skin penetration enhancement of core-multishell nanotransporters and invasomes measured by electron paramagnetic resonance spectroscopy

In order to cross the skin barrier several techniques and carrier systems were developed to increase skin penetration of topical dermatics and to reduce systemic adverse effects by avoiding systemic application. Ultra-flexible vesicles, e.g. invasomes and core-multishell (CMS) nanotransporters are efficient drug delivery systems for dermatological applications. Electron paramagnetic resonance (EPR) spectroscopic techniques were used for the determination of localization and distribution of the spin label 3-carboxy-2,2,5,5-tetramethyl-1-pyrrolidinyloxy (PCA; logP=-1.7) within the carrier systems and the ability of the carriers to promote penetration of PCA into the skin. The results show an exclusive localization of PCA in the hydrophilic compartments of the invasome dispersion and the CMS nanotransporter solution. PCA penetration was enhanced 2.5 fold for CMS and 1.9 fold for invasomes compared to PCA solution. Investigation of penetration depth by step-wise removal of the stratum corneum by tape stripping revealed deepest PCA penetration for invasomes. UV-irradiation of PCA-exposed skin samples revealed that the spin label is still reactive. In conclusion novel polymer-based CMS nanotransporters and invasomes can favor the penetration of PCA or hydrophilic drugs. This offers possibilities for e.g. improved photodynamic therapy.