Establishing Proposition 65 Safe Harbor Levels for Estragole and Methyleugenol in Cosmetics: A Mechanistic Read-Across from Safrole.

California's Proposition 65 requires warning labels for chemicals associated with carcinogenicity or reproductive toxicity, affecting global cosmetic and fragrance formulations. While safrole has an established safe harbor level (No Significant Risk Level, NSRL) of 3 µg/day, its structural analogues, estragole and methyleugenol, lack specific safe harbor thresholds. To address this data gap using non-animal methodologies, this study evaluated a Next-Generation Risk Assessment (NGRA) workflow combining read-across and in silico modeling. Structural, physicochemical, and skin-permeation profiling supported category homogeneity (Tanimoto Tc ≥ 0.82 and comparable log Kp values) in accordance with OECD and ECHA guidelines. Multi-platform QSAR consensus (ToxTree, Danish QSAR, and VEGA QSAR) indicated that while parent phenylpropenes lack direct bacterial mutagenicity, they share a hepatic bioactivation pathway leading to mammalian clastogenicity and carcinogenicity predictions. Molecular docking with human Cytochrome P450 1A2 (CYP1A2; PDB: 2H14) identified safrole as the category's conservative toxicological benchmark, exhibiting the highest binding affinity (-8.026 kcal/mol) and closest proximity to the catalytic HEME iron. Consequently, applying safrole's 3 µg/day NSRL to estragole and methyleugenol provides a quantitative threshold to evaluate consumer exposure in cosmetic products, ensuring public health protection and regulatory compliance.