(+)-Usnic Acid as a Promising Candidate for a Safe and Stable Topical Photoprotective Agent.

Agnieszka Galanty,Irma Podolak,Judyta Cielecka-Piontek,Elżbieta Pękala,Elżbieta Studzińska-Sroka,Paweł Paśko,Magdalena Paczkowska-Walendowska,Justyna Popiół

doi:10.3390/molecules26175224

Agnieszka Galanty, Irma Podolak + Show 6 more

Open Access

https://doi.org/10.3390/molecules26175224

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Abstract

The study aimed to examine whether usnic acid—a lichen compound with UV-absorbing properties—can be considered as a prospective photoprotective agent in cosmetic products. Moreover, a comparison of two usnic acid enantiomers was performed to preselect the more effective compound. To meet this aim, an in vitro model was created, comprising the determination of skin-penetrating properties via skin-PAMPA assay, safety assessment to normal human skin cells (keratinocytes, melanocytes, fibroblasts), and examination of photostability and photoprotective properties. Both enantiomers revealed comparable good skin-penetrating properties. Left-handed usnic acid was slightly more toxic to keratinocytes (IC50 80.82 and 40.12 µg/mL, after 48 and 72 h, respectively) than its right-handed counterpart. The latter enantiomer, in a cosmetic formulation, was characterized by good photoprotective properties and photostability, comparable to the UV filter octocrylene. Perhaps most interestingly, (+)-usnic acid combined with octocrylene in one formulation revealed enhanced photoprotection and photostability. Thus, the strategy can be considered for the potential use of (+)-usnic acid as a UV filter in cosmetic products. Moreover, the proposed model may be useful for the evaluation of candidates for UV filters.

Highlights

Usnic acid (2,6-diacetyl-7,9-dihydroxy-8,9b-dimethyl-1,3(2H,9bH)-dibenzofurandione) is a dibenzofuran derivative, synthesized exclusively by lichens
Right-handed usnic acid turned out to be a good candidate for a UV filter for topical use, in terms of its ability to penetrate the skin barrier, safety to skin cells, photoprotection, and photostability
It can be synthesized in order to avoid overexploitation of its natural sources

Summary

Introduction

Usnic acid (2,6-diacetyl-7,9-dihydroxy-8,9b-dimethyl-1,3(2H,9bH)-dibenzofurandione) is a dibenzofuran derivative, synthesized exclusively by lichens. Over the past several years, this compound has gained much attention due to the broad spectrum of its biological and pharmacological activities, combined with its high availability from natural sources (content up to 10%). Usnic acid reveals marked cytotoxic, anti-inflammatory, and antibacterial properties [2], some studies pointed to its hepatotoxicity [3,4], which may limit the internal administration of this compound. A few case studies have reported on contact allergy to usnic acid, but extracts with its compounds have been successfully used in antiperspirants [1]. Topical application of usnic acid seems to be a more promising approach as regards its future use

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