38 Biotransformation-driven pulmonary fate of iron oxide nanoparticles after repeated inhalation exposure

Abstract As engineered nanomaterials are increasingly utilized in various industrial and biomedical applications, their biological fate and potential toxicity have become crucial considerations for risk assessment. Biotransformation is a key determinant of nanoparticle behaviour, as it alters their physicochemical characteristics after deposition in biological tissues, thereby affecting clearance and persistence. In this study, we evaluated the pulmonary disposition and transformation of iron oxide (Fe2O3) nanoparticles in rats following 28 d of repeated nose-only inhalation exposure, followed by a 28-day recovery phase. A 14-day repeated oral dosing study was conducted in parallel to compare systemic distribution patterns. Inhalation concentrations were 0.56, 1.67, and 5.00 mg/m3, and oral doses were 250, 500, and 1,000 mg/kg body weight. Blood and organ samples were collected at 1-, 7-, and 28-days post-exposure. Proteinase K digestion was used to quantitatively determine iron levels and qualitatively assess nanoparticle transformation. Approximately 21% to 54% of Fe2O3 was cleared from the lungs by day 7, and 11% to 40% remained by day 28. No significant accumulation was observed in extrapulmonary tissues or oral groups. Transmission electron microscopy revealed a progressive conversion of Fe2O3 into smaller, more uniform particles over time. Although no overt toxicity was detected, these findings demonstrate that inhaled Fe2O3 nanoparticles undergo notable biotransformation during clearance, suggesting subtle interactions with the pulmonary microenvironment that warrant further mechanistic investigation.