Developmental and Neurobehavioral Toxicity of Hexaphenoxycyclotriphosphazene: Implications for Its Safety as a Flame-Retardant Alternative to Triphenyl Phosphate

Hexaphenoxycyclotriphosphazene (HPCTP) has been introduced as a halogen-free replacement for the flame-retardant triphenyl phosphate (TPhP); however, its environmental behavior and toxicological safety remain poorly defined. Here, using zebrafish larvae with TPhP as a benchmark, we systematically evaluated the developmental and neurobehavioral toxicity of HPCTP at environmentally relevant concentrations (50–5000 ng/L) from 6 to 120 h postfertilization. HPCTP showed greater bioaccumulation potential than TPhP, suggesting the risk of long-term ecological persistence. Despite lower acute toxicity, HPCTP induced pronounced developmental defects, including reduced body length and cardiac malformations. Strikingly, HPCTP and TPhP produced divergent neurobehavioral outcomes. HPCTP caused dose-dependent depression-like behaviors (hypoactivity, light-zone avoidance) through suppression of serotonin 1A (5-HT₁A) signaling, supported by decreased htr1aa expression and serotonin levels and partially rescued by the 5-HT₁A receptor agonist 8-OH-DPAT. By contrast, TPhP elicited anxiety-like phenotypes (hyperactivity, thigmotaxis) via CD83-dependent neuroinflammation, evidenced by decreased cd83 expression and microglial activation, which were reversed by cd83 overexpression. Taken together, these findings demonstrate that HPCTP, though marketed as a safer alternative, poses distinct developmental and neurobehavioral hazards, underscoring the urgent need for comprehensive risk evaluation prior to its widespread application.