Citation:

Nelms, M., P. Pradeep, AND G. Patlewicz. Evaluating potential refinements to existing Thresholds of Toxicological Concern (TTC) values for environmentally-relevant compounds. REGULATORY TOXICOLOGY AND PHARMACOLOGY. Elsevier Science Ltd, New York, NY, 109:104505, (2019). https://doi.org/10.1016/j.yrtph.2019.104505

Impact/Purpose:

Evaluation of current established TTC values and whether these need to be refined in light of substances of interest to the Agency.

Description:

The Toxic Substances Control Act (TSCA) mandates the US EPA perform risk-based prioritisation of chemicals in commerce and then, for high-priority substances, develop risk evaluations that integrate toxicity data with exposure information. One approach being considered for chemicals with limited chemical-specific toxicity data is a Threshold of Toxicological Concern (TTC)-to-Exposure ratio. Here, TTC values derived using oral (sub)chronic No Observable (Adverse) Effect Level (NO(A)EL) data from the EPA’s Toxicity Values database (ToxValDB) were compared with published TTC values from Munro et al. (1996). 4554 chemicals with structures present in ToxValDB were assigned into their respective TTC categories using the Toxtree software tool. Chemicals were assigned into the five TTC classes (Cramer structural class I, II, III, containing alerts for genotoxicity and acetylcholinesterase inhibitors). 114 (2.5%) chemicals were determined to be not appropriate for TTC. The TTC values derived from the ToxValDB were similar, but not identical to the Munro TTC values: Cramer I (37.3 compared to 30 ug/kg-day), Cramer II (34.6 compared to 9 ug/kg-day) and Cramer III (3.9 compared to 1.5 ug/kg-day). The 5th percentile values of Cramer classes I and II for the ToxValDB and Munro datasets were not statistically different whereas the class III 5th percentile values were different. Chemical features of the two class III datasets were evaluated to account for the differences in TTC values. The revised Kroes workflow was then applied to a large set of chemicals (~45,000). TTC values derived from this expanded dataset of toxicity values substantiated the original TTC values derived by Munro et al. (1996), reaffirming the utility of TTC as a promising tool in a risk-based prioritisation approach.

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