<i>In vitro</i> skin permeation of flavonoid esters enzymatically derived from natural oils: release mechanism from gel emulsion, stability, and dermatological compatibility

Due to their broad spectrum of biological activities and attractive pharmacological properties, flavonoids are very promising molecules for application in skin care products. In this study, phloridzin and naringin medium- and long-chain fatty acid esters were enzymatically synthesized in reaction with natural oils (coconut and linseed oil) and <i>in vitro</i> transdermal delivery of synthesized esters through artificial Strat-M^® membrane was investigated. Experimental results were succesfully fitted using Peppas and Sahlin model which includes the <i>lag</i> phase. Release kinetics of all examined flavonoid esters from gel emulsions through the membrane depended on both diffusion and polymer relaxation effect (0.5<<i>n</i> < 1). The estimated effective diffusion coefficients ranged from 0.168·10^-8 to 6.149·10^-8 cm² s^-1 for phloridzin esters and from 0.116·10^-8 to 4.210·10^-8 cm² s^-1 for naringin esters. The effective diffusion coefficients decreased with the increase in ester molecular weight indicating the size-dependent diffusion. All formulation showed good stability, excellent hydration effect, and excellent dermatological compatibility without irritating effect. It can be concluded that gel emulsions with a mixture of flavonoid esters enzymatically synthesized in reaction with vegetable oils can be effectively topically applied as a skin care products.