Development and Evaluation of Non-aqueous Microemulsions for Topical Use of Astaxanthin

Abstract

Background and aims: Skin is the largest organ of the human body, it is also a direct target of oxidative stress due to exposure to reactive oxygen species (ROS) from the surrounding environment. Ultra violet (UV) irradiation is one of the most potent environmental initiators of ROS produced in human skin, more than 80% of environmental ROS that damage the skin is due to UV irradiation. Supplementing the cutaneous antioxidant defense system using exogenous antioxidants is thought to be an important approach to maintaining skin health. Numerous antioxidants have been investigated for their potential to protect skin from UV damage, these include resveratrol, tocopherol, ascorbic acid, retinol and catechins. Recently, astaxanthin, an orange-red compound is of great interest due to its potent antioxidant activities against UV induced oxidative damages. However, topical application of this compound is challenging due to its lipophilic property and chemical instability that undergoes degradation in an aqueous environment. A number of drug delivery systems has been developed for topical delivery of antioxidants into the skin, however, majority of these systems contain aqueous components, which limit their use in delivery of hygroscopic compounds. Microemulsions (MEs) are liquid formulations that are transparent, optically isotropic systems and have been widely used in topical applications due to their penetration enhancing effects. MEs can be prepared without the use of water to form non-aqueous MEs and therefore can be used to deliver water labile compounds, which protect them from degradation and also facilitate penetration across the skin barrier. Therefore, the aim of this project was to develop non-aqueous MEs for delivery of astaxanthin into the skin to exert its protective effect against UV irradiation. Methods: A HPLC method was developed and validated for quantification and quantification of the compound; solubility studies were performed to aid material selection during formulation development. Non-aqueous MEs were developed and they were characterised by investigating their droplet size, drug loading capacity, zeta potential, rheological property and in vitro release profiles. The drug deposition and permeation studies were conducted using human skin on Franz diffusion cell. The cytotoxicity of pure drug, MEs and the protective effect of the drug loaded MEs were investigated using human skin fibroblast cells. Results and Discussion: An isocratic HPLC method was developed and validated for linearity, accuracy, precision and robustness according to the International Conference on Harmonisation (ICH) guidelines. A number of materials were screened and non-aqueous MEs were developed and pseudoternary diagrams were constructed. Three optimised MEs were selected to carry out further studies. The droplet size of the three MEs was around 20 nm and drug loading was 100 – 150 μg/ml. The studies with FTIR revealed that the drug was entrapped in the MEs as the characteristic peaks of the pure drug were masked. Rheological studies showed that MEs were Newtonian fluids. In vitro release studies suggested that the MEs could effectively achieve controlled drug release for a prolonged period. Besides, the MEs demonstrated both physical and chemical stability over a period of 90 days. Application of MEs on the skin resulted in enhanced drug penetration across stratum corneum (SC) and a significant enhancement of drug deposition was observed in epidermis and dermis layers. The in vitro study on human fibroblast cell suggested that the pure drug did not reduce cellular viability at the concentrations studied and the MEs were of less irritation potential compare to sodium lauryl sulfate. Furthermore, the drug-loaded MEs enhanced cell viability after exposure to UV irradiation. Conclusion: This project has demonstrated that the developed non-aqueous MEs were able to improve the physical and chemical stability of astaxanthin; enhance penetration across the skin barrier and significantly increase drug deposition in the deep layers of the skin and exert protective effect against oxidative stress such as UV irradiation on human skin fibroblast cells. Non-aqueous MEs are promising delivery systems for topical delivery of water labile antioxidants such as astaxanthin. The novel non-aqueous MEs may serve as effective tools for topical drug delivery and be used in both pharmaceutical and cosmetic products.