Synergistic enhancement SPF from annatto seed oil and alkaline extracts in association with organic UV filters

Abstract

Objective: This study investigated the antioxidant activity of oil and alkaline aqueous extracts obtained from Bixa orellana seeds and evaluated their potential to modulate the in vitro sun protection factor (SPF) of a sunscreen formulation containing avobenzone (5% w/w) and ethylhexyl methoxycinnamate (10% w/w). Methods: Alkaline aqueous extracts from whole (AAE1) and ground seeds (AAE2) were prepared, freeze-dried and chemically characterised. Extracted oil (EO) and commercial oil (CO) were also evaluated. Phytochemical screening, chromatographic analyses (GC–MS and LC–MS), carotenoid quantification and in vitro antioxidant assays (DPPH, ABTS + and CUPRAC) were performed. Sunscreen formulations containing 5% (w/w) of each sample were developed, and in vitro SPF was determined before and after UV irradiation. Results: AAE1, AAE2 and EO exhibited relevant antioxidant activity (IC₅₀ < 50 μg/mL), whereas CO showed no detectable activity under the tested conditions. Formulations containing AAE1 and AAE2 demonstrated a marked increase in in vitro SPF prior to irradiation (86.7 and 86.3, respectively) compared to the base formulation (23.3). However, after irradiation, SPF values decreased substantially for AAE1 and AAE2 (11.51 and 21.36, respectively), indicating limited photostability. LC–MS analysis confirmed the presence of bixin and norbixin in aqueous extracts, while GC–MS revealed a more complex profile in EO. Notably, AAE2 exhibited greater SPF retention after irradiation, which may be associated with the presence of triterpenes. Conclusion: Although annatto-derived extracts enhanced in vitro SPF prior to irradiation, their limited photostability restricts their direct application as standalone photoprotective agents. These findings suggested that such extracts may be better positioned as auxiliary antioxidant ingredients in sunscreen formulations, particularly when combined with stabilisation strategies to mitigate photodegradation.