Component-specific effects shape shifts between synergism and antagonism in organophosphate ester mixtures

Organophosphate esters (OPEs) frequently co-occur in human exposure, yet their combined hazards cannot be reliably inferred from single-compound toxicity. How individual OPEs reshape non-additive effects across compositions and effect levels remains unclear. Here, we investigated interaction patterns and associated biological responses of representative OPE mixtures, including tributyl phosphate (TNBP), triphenyl phosphate (TPHP), and tris(1,3-dichloro-2-propyl) phosphate (TDCIPP), using human keratinocytes as a dermal exposure model. Combination index (CI) analysis revealed composition- and effect-level-dependent interactions. Ternary mixtures exhibited distinct composition-dependent CI-Fa trajectories, with some compositions trending toward additivity at higher effect levels, whereas TPHP-rich mixtures showed progressively stronger synergism. Binary mixtures showed pair-dependent interactions, with TPHP-TDCIPP and TNBP-TDCIPP exhibiting contrasting interaction modes. At total nominal molar concentrations of 0.1, 1, and 10 μM for the 1:1:1 ternary mixture, co-exposure modestly increased HaCaT monolayer permeability, reaching statistical significance at 10 μM, and altered differentiation-related gene expression, revealing barrier-associated effects. Untargeted metabolomics of individual and mixture exposures further revealed distinct lipid remodeling patterns, including interaction-associated differences in glycerophospholipid responses. Collectively, OPE mixture hazards depend not only on individual toxicity but also on the capacity of specific components to modify combined effects, supporting improved mixture hazard characterization and prioritization of interaction-modifying contaminants.