Background: Melanins are high molecular weight pigments responsible for the mammalian skin and hair colour and play a key role in skin protection from UV radiation; however, their overproduction and excessive accumulation lead to pigmentation problems including melasma, freckles, uneven colouring, and age spots. Therefore, the modulation of melanin synthesis represents a critical issue in medicine and cosmetology. In the present study, an innovative polymeric antioxidant to be used as skin whitening agent is developed by the conjugation of dextran with rosmarinic acid.

Methods: Dextran-rosmarinic acid conjugates (DEX-RA) were synthesized in a one-pot method starting from Origanum vulgare aqueous leaf extract and dextran. The total polyphenol content and the antioxidant activity were assessed by Folin-Ciocalteau assay and 2,2-diphenyl-1-picrylhydrazyl radical (DPPH) and bleaching tests, respectively. The efficacy of DEX-RA was evaluated by inhibition of tyrosinase activity, in vitro diffusion and stability studies and in vivo studies. The biocompatibility of the conjugates was investigated by 3-[4,5-Dimethylthiaoly]-2,5-diphenyltetrazoliumbromide (MTT) and EPISKIN™ model.

Conclusion: The polymeric conjugates, comparing to the free antioxidant, show a long-lasting efficacy combined to an enhanced stability resulting in an improved performance of the cosmetic formulations prepared using this innovative whitening agent as a bioactive ingredient.

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