In vitro release and stability of rooibos tea extract in topical formulations

Abstract

Aspalathus linearis (rooibos tea) is an indigenous plant that is commonly used to prepare a tea-based health beverage and aspalathin is the major flavonoid uniquely found in rooibos extract (RE). The use of RE in aqueous solutions is challenging, since aspalathin is chemically unstable at increasing temperatures and pH. There is limited research data available regarding the chemical stability improvement of aspalathin in RE topical formulations. Furthermore, the release of aspalathin from topical formulations has not yet been investigated. Aspalathin has previously been shown to exhibit poor absorption across biological membranes. The purpose of this study was to develop different topical gel formulations containing aspalathin-enriched RE with the addition of selected anti-oxidants in order to improve the chemical stability of aspalathin over time. Eight experimental gel formulations containing aspalathin-enriched RE were manufactured, each containing selected anti-oxidants and a control gel without anti-oxidants. Stability studies were performed where gel formulations were subjected to long-term and accelerated storage conditions for three months. Chemical and physical properties were regularly evaluated and included aspalathin content assays, pH, viscosity (rheology) and visual inspection. Furthermore, in vitro diffusion studies were performed where the rate and extent of aspalathin release from all RE gel formulations were evaluated using synthetic polyvinylidene fluoride (PVDF) membranes mounted in a Sweetana-Grass diffusion chamber apparatus. The stability studies demonstrated chemical degradation of aspalathin in all RE gel formulations at the specific storage conditions. Since no aspalathin could be quantified in the control gel formulation (containing no anti-oxidants) at the end of stability testing, while aspalathin was present in the experimental gel formulations, it was evident that the addition of anti-oxidants have improved the chemical stability of aspalathin in RE gel formulations. The gel formulation, which contained all three anti-oxidants (ascorbic acid, citric acid and sodium metabisulfite) exhibited the best chemical stability of aspalathin. With regards to physical stability evaluation, the pH remained stable within the pre-determined pH range (4.7 ± 0.3) in all gel formulations over time and was considered safe for topical application. No pronounced changes were observed in the viscosity of most of the gel formulations, except for the appearance of syneresis in gels, which contained ascorbic acid. The rheograms showed a pseudoplastic flow behaviour in all gel formulations that remained unchanged throughout the study. Pronounced colour changes were observed in most gel formulations at the end of stability studies. Gel formulations containing sodium metabisulfite exhibited the least colour change and it was suggested that the darker colour that occurred in the control and other gel formulations, could be attributed to flavonoid oxidation in the RE. In vitro studies revealed that aspalathin release could be achieved in all gel formulations with a maximum cumulative amount of aspalathin release of 11.4%. The results showed that the extent of aspalathin release was inversely related to the viscosity of the formulations. Different aspalathin-enriched RE topical gel formulations were prepared and evaluated with regards to chemical and physical stability, and in vitro release. Based on the literature review it is likely that these experimental results are the first to be reported regarding the chemical stability enhancement of aspalathin in RE topical gel formulations. Information regarding in vitro release studies of aspalathin from RE topical formulations also provides novel research reporting that can contribute to RE topical product development.