Mixture design applied in compatibility studies of catechin and lipid compounds
The aim of this work was to evaluate the compatibility of (+)-catechin (CA) and excipients commonly used to prepare micro and nanoemulsions using thermal analysis along with complementary assays. Lipid compounds labrasol, plurol and ethyl oleate were combined with CA according to a simplex centroid mixture design and possible interactions between them were determined. Differential scanning calorimetry and thermogravimetric analyses were carried out together with Fourier transform infrared spectroscopy (FTIR) and morphologic characterization of the samples. A quantitative evaluation of thermal events involved in CA melting peak and initial sample decomposition temperature were performed. FTIR evaluation suggested an initial decomposition of CA mixtures exposed to a thermal aging depending on their composition corroborated by the darkening of these samples. The multiple regression analysis considering the thermal data revealed a thermal interaction compromising CA stability in multicomponent samples. Mixtures containing ethyl oleate exhibited a negative synergic action of this fatty acid with the others two lipid compounds (negative coefficients for two-factor and three-factor interaction terms). Indeed, samples decomposition was anticipated by at least 10 °C in the case of ternary and quaternary mixtures containing ethyl oleate. In conclusion, CA formulations produced with lipid components must have their stability closely monitored and production process involving heating should be avoided, especially in formulations containing ethyl oleate.
This study pointed out the importance of experimental design tools in preformulation studies. Indeed, the common stability pro- tocol involving binary mixtures of drug and excipient would not be able to show the deleterious influence of EO on multicomponent CA mixtures.
The synergism observed in this study among lipid components compromising CA stability occurs under a markedly influence of thermal stress and the presence of EO. Moreover, CA stability issues reinforce the need to monitor the stability of lipid formulations. In particular, CA delivery systems in which the production process involves heating, such as liposomes, should be avoided.