3-(3,4,5-Trimethoxybenzylidene)-2,4-pentanedione: Design of a novel photostabilizer with in vivo SPF boosting properties and its use in developing broad-spectrum sunscreen formulations.

Abstract

The study concerned the synthesis of a novel photostabilizer based on benzylidenepentanedione chemistry and the evaluation of its potential in developing a broad-spectrum sunscreen formulation containing avobenzone. 3-(3,4,5-Trimethoxybenzylidene)-2-4-pentanedione (TMBP) was synthesized through a condensation reaction and incorporated into a sunscreen formulation containing, inter alia, avobenzone. The SPF, critical wavelength and in vitro photostability of the product were measured. The photostability was compared with that afforded by current avobenzone photostabilizers, namely octocrylene, ethylhexylmethoxycrylene and diethylhexylsyringylidenemalonate. The photostability of TMBP either alone or in the presence of avobenzone in a methanolic solution was also evaluated by UV spectrophotometric and HPLC analyses. The optical properties of TMBP were estimated experimentally and supported by time-dependent density functional theory (TD-DFT) calculations. The ability of TMBP to stabilize avobenzone under ultraviolet (UV) light exposure was shown both in formulated products and in solution. A comparative stability study incorporating various combinations of avobenzone, TMBP (vs. three commercial photostabilizers) and UVB sunscreens clearly showed TMBP to be a very effective stabilizer. The photostabilizing effect of TMBP arises from triplet-state energy transfer from avobenzone to TMBP and through light-induced reactions that preserve the main chromophores. Interestingly, a 50% in vivo SPF boosting was observed when TMBP was used with organic and inorganic sunscreens when alone it has no contribution to SPF. TMBP-containing sunscreen formulations clearly showed a critical wavelength of well over 370 nm and can thus be categorized as broad-spectrum sunscreens. We were able to design a very effective photostabilizer, trimethoxybenzylidene pentanedione (INCI name), based on benzylidenepentanedione chemistry. TMBP is very efficient in stabilizing avobenzone in formulated products and boosts in vivo SPF by >50% for organic and inorganic sunscreens, and the formulations have critical wavelengths of >370 nm. These efficacious properties make it a promising additive for inclusion in broad-spectrum photoprotective products.