Size matters: the effects of varying zinc oxide nanoparticle sizes on human cytochrome P450 enzyme activity and gene expression

Zinc oxide (ZnO) nanoparticles have sparked considerable interest in recent years due to their potential across various biomedical applications. However, their distinct physicochemical properties, including nanoscale size, high surface area-to-volume ratio, quantum confinement effects, increased reactivity, ion release, and photocatalytic activity, may result in toxicological effects or biological impacts that may not be manifest in their larger-scale counterparts. In this study, we aimed to investigate the complex interactions between ZnO nanoparticles and major drug-metabolizing cytochrome P450 (CYP) enzymes, providing insights into the physicochemical characteristics of ZnO nanoparticles and their inhibitory effects on CYP enzyme activity in vitro and gene expression in HepG2 cells. Our findings revealed that smaller ZnO nanoparticles (< 50 nm) exhibit significant size-dependent inhibition on CYP enzymes, with CYP2C9 being the most susceptible (IC₅₀ of 12.76 µg/ml; K_i of 8.20 µg/ml), followed by CYP3A4 (IC₅₀ of 40.31 µg/ml; K_i of 20.14 µg/ml), CYP2D6 (IC₅₀ of 56.03 µg/ml; K_i of 40.31 µg/ml), and CYP2C19 (IC₅₀ of 64.24 µg/ml; K_i of 46.52 µg/ml). The molecular docking analysis corroborated these findings, revealing strong binding interactions between ZnO nanoparticles and key residues in CYP active sites. Furthermore, ZnO nanoparticles, particularly those < 50 nm, significantly (p < 0.05) upregulated the mRNA expression of CYP2C9, CYP3A4, and CYP2D6 in HepG2 cells. These findings suggest that ZnO nanoparticles can potentially impact drug metabolism by inhibiting CYP enzyme activities and altering their gene expressions, highlighting the need for further evaluation in clinical and pharmaceutical settings.