[Background] Cetaceans, as apex predators in the marine ecosystem, accumulate persistent organic pollutants (POPs) at high concentrations. However, due to ethical constraints, toxicity testing using live animals is impossible, leaving a long-standing challenge of how to link the results of in vitro experiments (using cultured cells) to actual "in vivo risk assessments." Conventionally, the "nominal concentrations" in cell experiments were directly compared with the "measured total concentrations" in actual organisms. However, this method carries high uncertainty because it does not account for the binding of chemicals to proteins and lipids within the body.

[Methodology] Using the finless porpoise as a model species, this study introduced a quantitative approach to predict actual in vivo effects from cell experiment results by using the "freely dissolved concentration" (the concentration of free chemicals not bound to proteins or lipids), which is directly responsible for triggering toxicity, as a common metric. We measured the protein and lipid contents of finless porpoise blood and blubber, as well as the culture media used in the cell experiments, and constructed a mass distribution model for 15 types of POPs.

[Results] The mass distribution modeling revealed that the freely dissolved concentrations of POPs in the bioassay media, blood, and blubber were 2-3, 4-6, and 6-8 orders of magnitude lower than their nominal and measured total concentrations, respectively. Using this freely dissolved concentration as a baseline, we calculated the "QIVIVE ratio" (risk prediction ratio) to compare actual exposure concentrations in vivo with the effect concentrations in cell experiments. The results indicated that mixed exposure to POPs at current environmental levels poses a low risk of reducing cell viability or inducing apoptosis (cell death).

[Future Outlook] This predictive approach, based on mass distribution modeling and freely dissolved concentrations, enables a realistic and mechanistically grounded risk assessment of chemicals in cetaceans. In the future, by incorporating chronic endpoints such as endocrine disruption and immunotoxicity into this framework, it is expected to be applied to more comprehensive health risk assessments for marine mammals.