Beyond Sun Protection Factor: Astaxanthin Is a Multifunctional Marine-Derived Compound for Antioxidant and Anti-Aging Skin Protection

Ultraviolet (UV)-induced oxidative stress is a major cause of skin photoaging, prompting interest in multifunctional antioxidants of marine origin for topical photoprotection. Astaxanthin, a xanthophyll carotenoid predominantly obtained from the microalga Haematococcus pluvialis, exhibits antioxidant, anti-inflammatory, and matrix-metalloproteinase (MMP)-modulating activities relevant to skin protection. This study aimed to formulate and systematically characterize oil-in-water (O/W) sunscreen emulsions containing 1%, 2.5%, and 5% (w/w) natural astaxanthin-containing extract (AstaReal® P4AF), in combination with an inorganic titanium dioxide (TiO2) UV filter, and to evaluate the influence of astaxanthin concentration on formulation performance and biological tolerability. Formulations were characterized in terms of viscosity, antioxidant activity using the 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay, mechanical and adhesive performance using back-extrusion and adhesion tests, in vitro sun protection factor (SPF), UVA protection, water resistance (WR), and local tissue tolerability using the in ovo chorioallantoic membrane (CAM) model. Viscosity increased with AstaReal® P4AF concentration at both temperatures (from 3801.9 to 5704.6 mPa·s at 25 °C, and from 1931.8 to 5118.9 mPa s at 36 °C), with the 5% AstaReal® P4AF formulation showing the greatest thermal stability. DPPH radical scavenging activity increased markedly with AstaReal® P4AF concentration up to 2.5% (33.15% at 1% and 63.04% at 2.5%) and remained comparable at 5% (62.62%), versus 22.05% for the placebo formulation. Back-extrusion analysis revealed a non-linear, concentration-dependent effect on consistency and firmness, peaking at 2.5% AstaReal® P4AF, while adhesion parameters remained statistically unaffected across concentrations. SPF and UVA protection factor decreased with increasing AstaReal® P4AF concentration (74.37 to 35.02 and 37.62 to 19.78, respectively), whereas WR was improved at the highest concentration. CAM histopathological scoring indicated that AstaReal® P4AF incorporation did not increase tissue injury relative to the formulation base. These findings indicate that AstaReal® P4AF concentration critically influences the physicochemical and photoprotective balance of sunscreen emulsions, supporting further optimization toward a multifunctional, marine-derived ingredient.