The potential of plant polyphenols as natural photostable sunscreen active ingredients.

Abstract

Protection from the sun dates to prehistoric times as a measure against the deleterious effects of solar ultraviolet (UV) radiation. Overexposure to this harmful UV radiation is a leading cause of pathological changes of the skin, such as erythema and the worst being skin cancer. Commercial sunscreen products contain UV filter substances that are designed to protect human skin from erythema. These sunscreen products incorporate chemical UV-absorbing compounds and physical blockers that allow UV radiation to be reflected, scattered, or absorbed. This research discusses the use of certain organic UV absorbers and their importance in protection against the effects of UV radiation. The issues faced with certain UV absorbers are that they are susceptible to photodegradation over time, their degradation products can be phototoxic, and they provide protection only over a limited wavelength region. This causes a significant problem in sunscreen products as their efficacy is reduced. This has led to research on finding an alternative solution that can provide beneficial protection and maintain photostability. Interest was drawn to the plant kingdom for a more natural source of sunscreen absorbers since plants have developed various photoprotection mechanisms to counter enhanced levels of UV radiation. Polyphenols in plants have strong UV-absorbing properties that cover a broad spectrum, and they also possess excellent antioxidant activity. Therefore, they can play a dual role by acting as both natural absorbers of harmful radiation and quenchers of free radical damage also caused by sunlight. This project highlights the limitations in photostability of four commonly used chemical UV filters found in commercial sunscreen products, namely, avobenzone, benzophenone-9, 2-ethylhexyl-p-methoxycinnamate and 4-methylbenzylidene camphor, and involves testing a South African based plant extract to increase the current efficacy of sunscreens due to its photochemoprotective properties. Attempts at photostabilising the selected UV filters were made by incorporating extracts from Sutherlandia frutescens, also known as the Cancer Bush plant. Photostability studies were conducted on the extracts alone and when mixed with each of the sunscreen filters by means of UV-visible spectrophotometry. Phenolic compounds, namely, flavonoids and phenolic acids, were extracted, identified and quantified. From an industrial perspective, a cheaper alternative to the Cancer Bush plant is extracting polyphenols from commercial tea. Phenolic acids were extracted from Rooibos and green teas by means of a simple ethanol-water extraction technique. For comparison, the tea extracts were

compared with the Cancer Bush extract. Green tea was found to provide excellent photostability. The phenolic acids that were extracted from the Cancer Bush plant and teas were analysed by high-performance liquid chromatography (HPLC). The identification and quantitation of six phenolic acids was achieved by reversed-phased HPLC. Identification of phenolic acids was achieved by matching the retention times and spectra of the extract components with those of phenolic acid standards. The identified phenolic acids were gallic acid, p-hydroxybenzoic acid, vanillic acid, caffeic acid, syringic acid and p-coumaric acid. The six phenolic acids were identified in the Cancer Bush ethanol-water extract and p-coumaric acid showed the highest concentration. Only three phenolic acids were successfully identified in the green tea extract and four in the Rooibos tea extract with the highest concentration being p-coumaric acid. A general sunscreen formulation was employed to assess the possible inclusion of polyphenols in sunscreens. Sunscreens were made with and without UV filters and photostability tests were conducted. A small amount of sunscreen was smeared onto a quartz plate and exposed to sunlight for a total of six hours. UV-visible spectrophotometric analysis was conducted at 30-minute intervals before and after exposure to determine their photostability. Sunscreen formulations containing plant extracts showed noteworthy photostability when compared with products without plant extracts by increasing the absorbance values. This shows that plant extracts may contribute synergistically to improve the efficacy of sunscreen formulations. The stability of the sunscreens was tested to determine formulation stability and safety. Furthermore, in vitro testing was applied to test sunscreen formulation parameters, such as, sun protection factor (SPF) and occlusion factor. Results show that the addition of the Cancer Bush plant improved the SPF of the sunscreen preparation. Therefore, this research has shown that the addition of the Cancer Bush extract can improve photostability and provide additional UV protection.