Tacrolimus loaded biocompatible lecithin-based microemulsions with improved skin penetration: Structure characterization and in vitro/in vivo performances

In order to improve skin penetration of tacrolimus we aimed to develop potentially non-irritant, lecithin-based microemulsions containing ethanol, isopropanol and/or propylene glycol as cosurfactants, varying caprylic/capric triglycerides and propylene glycol monocaprylate as oil phase. The influence of excipients on the size of microemulsion region in pseudo-ternary phase diagrams and their ability to form different types of microemulsions was evaluated. The comprehensive physicochemical characterization of microemulsions and the evaluation of their structure was performed, while the localization of tacrolimus in microemulsions was further investigated using electron paramagnetic resonance spectroscopy. Moreover, stability studies proved no change in tacrolimus content during one year of storage at room temperature. In addition, in vivo skin performance indicated no skin irritation potential of blank microemulsions, whereas in vitro release testing using Franz diffusion cells showed superior release rate of tacrolimus from microemulsions (0.98 +/- 0.10 and 0.92 +/- 0.11 mu g/cm(2)/h for two bicontinuous and 1.00 +/- 0.24 mu g/cm(2)/h for oil-in-water microemulsion) compared to referent Protopic ointment (0.15 +/- 0.08 mu g/cm(2)/h). Furthermore, ex vivo penetration assessed through porcine ear skin using tape stripping, confirmed superiority of two microemulsions related to the reference, implying developed microemulsions as promising carriers for dermal delivery of tacrolimus