Dermal Absorption and Quantitative Analysis of Amaranth, a Cosmetic Colorant, in Rat Skin Using an In Vitro Franz Diffusion Model

Amaranth (R2) is used as a color additive in cosmetics. In Korea, R2 is permitted only as a cosmetic colorant and is prohibited in products intended for infants and children under 13 years of age; in Europe, it is regulated solely as a cosmetic colorant rather than a hair dye ingredient. Despite its regulatory relevance, dermal absorption data for R2 are lacking. In this study, percutaneous absorption of R2 was evaluated using the Franz diffusion method with excised rat dorsal skin. Quantitative analysis of R2 was developed and validated using high-performance liquid chromatography (HPLC) in accordance with Korean Ministry of Food and Drug Safety guidelines, demonstrating acceptable linearity (r² = 0.9996–0.9999), accuracy (95.5–104.4%), and precision (0.3–5.8%). Two formulations (skin lotion and cream), each containing 1% R2, were applied at 113 mg/cm² for 24 h. Dermal absorption was assessed by analyzing receptor fluid, skin wash, stratum corneum, epidermis, and dermis. Total dermal absorption of R2 was 3.4 ± 2.7% for the lotion and 0% for the cream, corresponding to in vitro skin permeabilities of 34.5 ± 27.0 μg/cm² and 0 μg/cm², respectively. Total recovery ranged from 80.3 ± 8.2% to 91.4 ± 19.4%. These results provide essential data for cosmetic risk assessment of R2.