Cutaneous delivery of [alpha]-tocopherol and lipoic acid using microemulsions: influence of composition and charge

Objectives To assess whether the composition and charge of microemulsions affect their ability to simultaneously deliver [alpha]-tocopherol and lipoic acid into viable skin layers. Methods [alpha]-Tocopherol and lipoic acid were added (1.1 and 0.5% w/w, respectively) to decylglucoside-based microemulsions containing mono-dicaprylin. Microemulsions containing surfactant:oil:water (w/w/w) at 60:30:10 (ME-O) and 46:23:31 (ME-W), as well as a cationic form of ME-W containing 1% phytosphingosine (ME-Wphy) were characterized, and their ability to disrupt the skin barrier and deliver the antioxidants in vitro in the skin was evaluated. Antioxidant activity in ME-Wphy-treated skin was assessed using the thiobarbituric acid-reactive substances (TBARS) assay. Key findings The internal phase diameters of microemulsions ranged between 42 and 55nm; phytosphingosine addition and pH adjustment to 5.0 increased zeta potential from -4.3 to +29.1mV. ME-O displayed w/o structure, whereas ME-W and ME-Wphy were consistent with o/w. Microemulsions affected skin electrical resistance and transepidermal water loss, but did not affect lipoic acid penetration. [alpha]-Tocopherol delivery increased following the order ME-O< ME-W< ME-Wphy. ME-Wphy presented suitable short-term stability. The antioxidants delivered by ME-Wphy decreased TBARS cutaneous levels. Conclusions Even though microemulsion structure only affected tocopherol penetration, delivered levels of both antioxidants were sufficient for a decrease in TBARS, supporting their use for enhanced protection. [PUBLICATION ABSTRACT]