Nanocrystals for dermal penetration enhancement - Effect of concentration and underlying mechanisms using curcumin as model.

Nanocrystals have received considerable attention in dermal application due to their ability to enhance delivery to the skin and overcome bioavailability issues caused by poor water and oil drug solubility. The objective of this study was to investigate the effect of nanocrystals on the mechanism of penetration behavior of curcumin as a model drug. Curcumin nanocrystals were produced by the smartCrystals® process, i.e. bead milling followed by high pressure homogenization. The mean particle size of the curcumin crystals was about 200nm. Stabilization was performed with alkyl polyglycoside surfactants. The distribution of curcumin within the skin was determined in vitro on cross-sections of porcine skin and visualized by fluorescent microscopy. The skin penetration profile was analyzed for the curcumin nanosuspension with decreasing concentrations (2%, 0.2%, 0.02% and 0.002% by weight) and compared to nanocrystals in a viscous hydroxypropylcellulose (HPC) gel. This study demonstrated there was minor difference between low viscous nanosuspension and the gel, but low viscosity seemed to favor skin penetration. Localization of curcumin was observed in the hair follicles, also contributing to skin uptake. Looking at the penetration of curcumin from formulations with decreasing nanocrystal concentration, formulations with 2%, 0.2% and 0.02% showed a similar penetration profile, whereas a significantly weaker fluorescence was observed in the case of a formulation containing 0.002% of curcumin nanocrystals. In this study we have shown that curcumin nanocrystals prepared by the smartCrystal® process are promising carriers in dermal application and furthermore, we identified the ideal concentration of 0.02% nanocrystals in dermal formulations. The comprehensive study of decreasing curcumin concentration in formulations revealed that the saturation solubility (Cs) is not the only determining factor for the penetration. A new mechanism based also on the concentration of the nanocrystals on skin surface was proposed.