Tyrosine-derived nanospheres for enhanced topical skin penetration

The objective of this study was to investigate the passive skin penetration of lipophilic model agents encapsulated within tyrosine-derived nanospheres. The nanospheres were formed by the self-assembly of a biodegradable, non-cytotoxic ABA triblock copolymer. The A-blocks were poly(ethylene glycol) and the hydrophobic B-blocks were oligomers of suberic acid and desaminotyrosyl-tyrosine alkyl esters. These nanospheres had an average hydrodynamic diameter of about 50 nm and formed strong complexes with fluorescent dyes, 5-dodecanoylaminofluorescein (DAF, Log D = 7.54) and Nile Red (NR, Log D = 3.10). These dyes have been used here as models for lipophilic drugs. The distribution of topically applied nanosphere-dye formulations was studied in human cadaver skin using cryosectioning and fluorescence microscopy. Permeation analysis (quantified fluorescence) over a 24 h period revealed that the nanospheres delivered nine times more NR to the lower dermis than a control formulation using propylene glycol. For DAF, the nanosphere formulation was 2.5 times more effective than the propylene glycol based control formulation. We conclude that tyrosine-derived nanospheres facilitate the transport of lipophilic substances to deeper layers of the skin, and hence may be useful in topical delivery applications.