A tiered next-generation risk assessment framework integrating toxicokinetics and NAM-based toxicodynamics: "proof of concept" case study using pyrethroids

Abstract

New Approach Methodologies (NAMs) in Next-Generation Risk Assessment (NGRA), integrating toxicokinetics (TK) with toxicodynamics (TD), provides an accurate evaluation of combined chemical exposures. This study assesses pyrethroids, which pose regulatory challenges due to their widespread use and cumulative exposure risks. A tiered NGRA framework was compared with conventional risk assessment (RA) to evaluate regulatory applicability. In Tier 1, ToxCast data established gene and tissue bioactivity indicators, facilitating hypothesis-driven hazard identification. Tier 2 examined combined risk assessments, rejecting the hypothesis of the same mode of action and highlighting inconsistencies in in vitro data and NOAEL/ADI correlations. Tier 3 applied Margin of Exposure (MoE) analysis and TK modeling to realistic exposure estimations for risk assessment screening based on internal doses, identifying tissue-specific pathways as critical risk drivers. Tier 4 refined bioactivity indicators using TK approaches to improve the NAM-based effect assessment, including an in vitro vs. in vivo comparison, with coherent results based on interstitial concentrations, though intracellular estimations remained uncertain. Tier 5 confirmed that dietary exposure in healthy adults is close to but below levels of concern, with bioactivity MoE values remaining below concern thresholds, and in vivo MoEs within the standard safety factors. Nevertheless, the MoEs are insufficient for addressing the additional non-dietary exposure expected from other pyrethroid uses such as biocides or pharmaceuticals. Results demonstrate that NGRA with TK-NAM-based TD offers a nuanced, regulatory-relevant framework for risk assessment. The proposed approach integrates the information on individual pyrethroids using bioactivity indicators; and the re-assessment of regulatory toxicity studies to select organ-relevant NOAELs allowed an improved in vitro-in vivo comparison, demonstrating the capacity of bioactivity-based MoEs for combined exposure assessments. This tiered approach provides key insights for regulatory decision-making, establishing a robust model for evaluating pyrethroids and similar chemical classes.

Keywords: New Alternative Methods (NAMs); Next-generation risk assessment (NGRA); Pyrethroids; Toxicodynamics; Toxicokinetics.

Fig. 1

Next-generation risk assessment framework—proof-of-concept case study design. Graphical abstract of methods and tiers used for this novel NGRA increasing complexity

Fig. 2

Radial chart of pyrethroid bioactivity by relative potency of the average AC₅₀. Tissue category comparison

Fig. 3

Radial chart of pyrethroid bioactivity by relative potency of the average AC₅₀. Gene category comparison

Fig. 4

Correlation of ToxCast relative potencies with Acceptable Daily Intakes (ADI) relative potencies. Correlation between relative potencies (ratio vs. the most toxic substance for the endpoint) for apical effects and ToxCast bioactivities. The black line indicates equipotency

Fig. 5

Correlation of ToxCast relative potencies with NOAEL relative potencies. Correlation between relative potencies (ratio vs. the most toxic substance for the endpoint) for apical effects and ToxCast bioactivities. The black line indicates equipotency

Fig. 6

Graph comparing AC₅₀ (uM) vs. C_interstitial (umol/L). Graph represented in logarithmic scale. The equipotency, the 10-fold and 100-fold lines have been added

Fig. 7

Graph comparing AC₅₀ (uM) vs. C_{intracellular} (umol/L). Graph represented in logarithmic scale. The equipotency, the 10-fold and 100-fold lines have been added

Fig. 8

Graph comparing C_free (umol/L) vs. C_interstitial (umol/L). Graph represented in logarithmic scale. The equipotency, the 10-fold lines have been added. No 100-fold line was required

Fig. 9

Graph comparing C_cell (umol/L) vs. C_{intracellular} (umol/L) - vascular and immune concentrations adapted. Graph represented in logarithmic scale. The equipotency, the 10-fold and 100-fold lines have been added