Modulation of Detoxification, Immune, and Epigenetic Systems by Two Aryl Organophosphorus Flame Retardants During Early Development in Zebrafish

Organophosphorus flame retardants (OPFRs) are emerging alternatives to halogenated compounds, yet their environmental toxicity remains underexplored. This study evaluated the developmental toxicity of two aryl-OPFRs, triphenyl phosphate (TPP) and tricresyl phosphate (TCP), in zebrafish (<i>Danio rerio</i>) from 2 h to 5 days post fertilization (hpf-dpf). Survival, hatching rate, and malformations were assessed across concentrations of 250-1000 µg/L, alongside with gene expression analysis at 5 dpf (250 and 500 µg/L) targeting detoxification (<i>ces2</i>), immune responses (<i>il1β</i>, <i>casp9</i>), and epigenetic markers (<i>dnmt1</i>, <i>dnmt3</i>). In vitro enzymatic assays evaluated interactions of both aryl-OPFRs with carboxylesterase (CE) and acetylcholinesterase (AChE) enzymes. While no significant morphological effects were observed, TPP showed higher toxicity than TCP. Notably, TCP (500 µg/L) downregulated genes linked to metabolism and immunity. CE activity and <i>ces2</i> modulation may suggest CE as a potential biomarker for aryl-OPFR exposure. These findings, although at concentrations above the environmental ones, may be valuable for mechanistic purposes and underscore the need for further investigation in developmental toxicity given their lipophilic nature and distinct molecular responses.