Avobenzone

Avobenzone exists predominantly in solution in its enolic forms due to intramolecular hydrogen bonding, which confers its UVA filtering activity. Upon UVA irradiation, avobenzone undergoes enol-to-keto tautomerization, converting to a diketo species that absorbs in the UVC range and serves as a photodegradation precursor leading to Norrish Type-I cleavage and loss of UVA protection. β-Cyclodextrin at concentrations 50 to 100 times that of avobenzone most effectively protected photostability by providing a hydrophobic cavity environment equivalent to 40-50 percent acetonitrile aqueous solution, which induces and stabilizes the diketo tautomer. The diketo form stored within the inclusion complex exhibited a hyperchromic effect at β-CD concentrations of 2 mM or higher due to complex aggregation. UV-vis spectral analysis confirmed that the β-CD cavity mitigates UVA1 reactivity toward avobenzone by stabilizing the diketo tautomer, thereby preventing further photodegradation pathways. A molar ratio of 50 to 100 of β-CD to avobenzone was identified as the optimal composition for maximum photostabilization.

Fig. 1 UV–vis spectra of 50 μM (a) AVB, (b) OXB, and (c) CUR irradiated with the UVA1 (365 nm) lamp. (Kuroda C.; et al. 2025)

Avobenzone remains the most widely used UVA filter despite its susceptibility to keto-enol tautomerism-induced photodegradation. Recent drug delivery strategies for avobenzone photostabilization include encapsulation in polymeric nanoparticles, lipid-based carriers (solid lipid nanoparticles and nanostructured lipid carriers), liposomes, cyclodextrin inclusion complexes, and Pickering emulsions. Encapsulation approaches physically separate avobenzone from direct UV exposure, reduce skin penetration, and limit tautomerization by restricting molecular mobility within the carrier matrix. Combination strategies incorporating photostabilizers, antioxidants, and quenchers have demonstrated synergistic effects.

Fig. 2 Schematic representation of Avobenzone-loaded micro/nanocarriers. (Gholap A D.; et al. 2023)