Rivera BN, Ghetu C, Rohlman D, Anderson K, Tilton S. A novel framework to develop sufficiently similar mixtures. Society of Toxicology webinar, Risk Assessment and Mixtures Joint Specialty Section Webinar, January 2022.
There is a growing need to establish alternative approaches for mixture safety assessment of polycyclic aromatic hydrocarbons (PAHs). Due to limitations with current component-based approaches, and lack of established methods for using whole mixtures, a promising alternative is to use sufficiently similar mixtures. Sufficiently similar mixtures are well characterized and representative of environmental exposures. However, there is a lack of an established framework to form sufficiently similar mixtures, from environmental sampling, to conduct a safety assessment. In this study, several approaches are explored to form sufficiently similar mixtures. A legacy creosote site impacted by wildfire smoke was used as our test site. Air sampling was conducted, and samples were analyzed for parent and alkylated PAHs, due to their high abundance in creosote and wildfire smoke. Of the 65 PAHs these samples were analyzed for, 32 were identified. A mixture of these PAHs was created and will be referred to as Creo-fire Mix (CF Mix). Existing toxicity values and chemical concentrations were incorporated to identify hazardous components in the CF Mix. Representative mixtures of the CF mix were formed based on relative abundance (Abun Mix), average toxicity values (Tox Mix), and a weighted approach incorporating toxicity and abundance (Weighted(W)-Tox Mix). When compared to the CF Mix, the W-Tox Mix most effectively captured both high abundance and high hazard chemicals and was hypothesized to be most sufficiently similar to the CF Mix. Hazard characterization of these mixtures was performed using high-throughput screening in primary normal human bronchial epithelium (NHBE) and zebrafish. In NHBE, oxidative stress, cell viability, and mitochondrial membrane potential (MMP) were assessed. In zebrafish, teratogenic endpoints were assessed. In NHBE, MMP was the most sensitive endpoint. CF Mix, W-Tox Mix and Abun Mix had similar EC50 values, and Tox Mix had the lowest EC50 value. Similarly, in zebrafish Tox Mix was the most bioactive mixture. These results highlight the importance of prioritizing chemicals not based on their relative abundance, but rather on their perceived hazard when identifying chemicals of concern from environmental exposures. Results from this study will help provide regulatory agencies with an experimental framework that can be used in whole mixture safety assessment.