Cross-species comparisons of plasma binding and considerations for data evaluation

The US Environmental Protection Agency is increasingly employing new approach methods (NAMs), including in vitro plasma binding and hepatocyte clearance experiments to collect chemical-species specific data. This paper presents data from plasma binding experiments using rapid equilibrium dialysis (RED) devices and plasma from humans, rats, and rainbow trout with a 4-h incubation time. A total of 54 chemicals, utilizing two concentrations, were tested across the three species resulting in 238 chemical-species specific datasets. Mass balance controls for chemical plasma stability and dialysis system recovery were used to evaluate the datasets and almost 40 % of the datasets (92/238 datasets) produced quantitative measurements. Cross-species comparisons and evaluations of the impact of physicochemical properties on chemical-assay performance were also evaluated. Comparisons of human-rat plasma binding revealed rat plasma generally demonstrated higher f_up values for chemicals than human. While f_up values in trout plasma were frequently lower than rat or human plasma. A comparison with literature data was performed and correlations between plasma binding, expressed as fraction unbound in plasma (f_up), and log K_ow across all three species indicate that the strongest relationship occurs at log K_ow values between 1.5 and 4. The obtained datasets exhibited a wide range of behaviors, emphasizing the need for a robust approach to data quality assessment. The broader analysis of f_up values indicates that chemicals with log K_ow > 4.5 will be highly bound (f_up ≤ 0.0001), difficult to measure, and have low reproducibility across laboratories, suggesting that use of different methods may be needed across different physicochemical properties.