Differentiating Pathway-Specific From Nonspecific Effects in High-Throughput Toxicity Data: A Foundation for...

Area of Science:

• Environmental Toxicology
• Computational Chemistry
• Biomolecular Assays

Background:

• The U.S. Environmental Protection Agency's ToxCast program screens chemicals using high-throughput in vitro bioassays.
• Adverse outcome pathways (AOPs) link biological activity to apical effects for risk assessment.
• A "cytotoxic burst" (CTB) phenomenon occurs when many ToxCast assays respond near cytotoxic concentrations.

Purpose of the Study:

• To focus AOP development on molecular targets identified by ToxCast assays.
• To identify ToxCast assays that frequently respond at concentrations below the CTB.
• To prioritize assays for AOP development based on response patterns relative to CTB.

Main Methods:

• Conducted a meta-analysis of ToxCast assay data to identify assays responding below the CTB.
• Ranked assays by the fraction of chemical hits below the CTB relative to chemicals tested.
• Employed a diagnostic-odds-ratio approach to prioritize assays with high specificity and low responsivity.

Main Results:

• Identified novel targets like peripheral benzodiazepine and progesterone receptors for AOP development.
• Affirmed the importance of existing AOPs linked to ToxCast targets such as thyroperoxidase and estrogen receptor.
• Prioritization methods were robust and not influenced by inter-assay differences in chemical bioavailability.

Conclusions:

• Developed and validated methods to prioritize ToxCast assays for targeted AOP development.
• The findings aid in linking high-throughput screening data to adverse health outcomes.
• This approach enhances chemical risk assessment by focusing on relevant molecular initiating events.