BACKGROUND: Significant variability in the metabolism of midazolam (MDZ) exists among mechanically ventilated (MV) patients in the intensive care unit (ICU) due to complex clinical conditions and genetic factors. The NR1I2 gene (PXR), which encodes a nuclear receptor that regulates drug-metabolizing enzymes like CYP3A4, plays a critical role in MDZ metabolism. Polymorphisms in NR1I2, along with variations in genes such as CYP3A4, CYP3A5, and ABCB1, may influence enzyme activity and MDZ pharmacokinetics (PK). Understanding these factors is essential for optimizing MDZ dosing in high-risk patient populations. METHODS: We studied 61 MV ICU patients receiving continuous MDZ infusion. A population pharmacokinetic (PopPK) model was used to assess MDZ PK, with genetic factors (NR1I2 rs2461817, CYP3A4, CYP3A5, ABCB1, and other PXR polymorphisms) and clinical covariates (body weight (BW), aspartate aminotransferase (AST) levels) evaluated for their impact on MDZ clearance (CL). RESULTS: The PK of MDZ and its metabolite, 1-hydroxymidazolam (1-OH-MDZ), were accurately described using a one-compartment model. The estimated population means for MDZ and 1-OH-MDZ CL were 22.6 L/h (inter-individual variability [IIV] 59.4%) and 67.1 L/h (IIV 57.7%), respectively. MDZ CL was significantly associated with the NR1I2 rs2461817 polymorphism and AST levels, accounting for 11.3% of the variability. MDZ CL decreased by 32.7% as AST increased from 22 IU/L to 60 IU/L, and by 40.7% in patients homozygous for the NR1I2 rs2461817 variant. BW also influenced the CL of 1-OH-MDZ, demonstrating a 34.2% increase as weight increased from 54 kg to 65 kg. Simulations confirmed the significant impact of NR1I2 rs2461817 on MDZ CL. CONCLUSION: The PopPK model highlights the significant impact of NR1I2 rs2461817 polymorphism on MDZ CL in Chinese MV patients, emphasizing the need to consider genetic and clinical factors for optimizing MDZ dosing in ICU settings.