Phospholipid vesicle-based formulation to unlock the potential of a biosynthetic melanin: a strategy to overcome the insolubility barriers

Melanins, the main pigments in the skin of mammals, are known to protect from sun radiation damage thanks to their UV–visible absorption and reactive oxygen species scavenging properties. The exploitation of such properties for dermocosmetic or pharmaceutical purposes by using easily accessible synthetic melanins prepared from biosynthetic precursors, particularly 5,6-dihydroxyindole-2-carboxylic acid (DHICA), has so far been hampered by the insolubility of melanins in water and other polar solvents, which is not satisfactorily overcome, even when DHICA derivatives, such as its methyl ester (MeDHICA), are used as precursors. In this study, a melanin synthesized from MeDHICA via peroxidase/hydrogen peroxide oxidation was incorporated into phospholipid vesicles with the aim of increasing its solubility and producing an aqueous-based formulation that can be safely applied to the skin. The proposed nanoformulation contained glycerol, which was key to producing stable, small, homogeneous vesicles. Glycerol-liposomes exhibited an average diameter of 90 nm, a spherical shape with a unilamellar structure, good storage stability, and high entrapment efficiency (77 %), as determined by light scattering, cryogenic-Transmission Electron Microscopy (cryo-TEM) and UV–vis spectrophotometry measurements. As revealed by rheological analysis, the glycerol-liposomes showed pseudoplastic behavior characterized by less flow resistance when applied on a surface, while remaining thick enough to avoid drips and runs – desirable behavior for a topical formulation. The antioxidant activity of the MeDHICA melanin was marked and was not altered by the nanoformulation process, as demonstrated by 1,1-diphenyl-2-picrylhydrazyl (DPPH) and ferric ion reducing antioxidant power (FRAP) assays. The glycerol-liposomes were further tested in immortalized skin keratinocytes, and the results showed an absence of cytotoxicity, encouraging further investigation to confirm their effective and safe use on the skin.