Targetomics for the Exposome: Decoding the Molecular First Domino for Next-Generation Environmental Toxicology

The rapid expansion of environmental chemicals has intensified the demand for mechanism-informed risk assessment. However, for most environmental chemicals, molecular initiating events (MIEs)─the "first domino" in adverse outcome pathways─remain poorly defined, limiting mechanistic resolution of toxicity. This gap stems from several limitations in current approaches: the narrow focus on predefined protein targets, the difficulty in distinguishing direct molecular triggers from downstream biological noise, and the lack of relevance to real-world exposures. To overcome these bottlenecks, we propose targetomics, an unbiased framework that leverages proteome-wide mapping of pollutant-protein interactions and integrates with systems biology to identify MIEs that define the mode of action. We outline three strategic pillars to operationalize this concept: (i) expanding target coverage through integration of global chemoproteomics; (ii) prioritizing key MIEs through engagement specificity, cross-layer convergence across multiomics and phenotypes, functional control within biological networks, and causal validation; and (iii) enhancing real-world relevance by resolving mixture effect complexity, temporal dynamics, and organ-specific target engagement. By deciphering where and how chemicals engage biological systems, targetomics facilitates bottom-up causal reconstruction of toxicity pathways, helping resolve the mechanistic "black box" of environmental toxicology and informing regulatory decisions under increasingly complex exposure scenarios.