Target Attainment and Population Pharmacokinetics of Nirmatrelvir/Ritonavir in Critically Ill Adult Patients

The population pharmacokinetics of nirmatrelvir/ritonavir (NIR/RIT) has not yet been described for critically ill adult patient.

This was a prospective observational population pharmacokinetic study of nirmatrelvir/ritonavir (NIR/RIT) in critically ill adult patients and identify optimal dosing regimens.

The prescription of NIR/RIT is determined by the attending physician and ranges from 150mg/100mg to 300mg/100mg twice a day. Two to three serial blood samples were collected for each patient after the second doses. We developed and validated PK model for plasma NIR and plasma RIT. Monte Carlo dosing simulations were performed to assess target attainment.

We analyzed 89 plasma samples from 31 adult patients. The data were best described by a one-compartment model. Among the covariates tested on pharmacokinetic parameters, creatinine clearance (CrCL) and area under curve (AUC) of RIT had a significant effect on apparent clearance (CL/F) of NIR. Mean (SD) parameters estimates for the absorption rate constant (Ka), apparent distribution (V/F) and CL/F were 0.42 (0.10) h^-1. 36.5 (8.5) L, 3.6 (0.26) L/h, respectively. Dosing simulations showed that the target in vitro 90% effective concentration (EC₉₀) was more likely to be achieved twice a day than once a day at the same daily dose of NIR. High CrCL, low AUC of RIT were associated with a reduced likelihood of NIR reaching the target EC₉₀.

Based on our dosing simulations, the initial dosage of NIR/RIT was 300mg/100mg twice a day in critically ill patients with CrCL>45 mL/min; When CrCL in critically ill patients is between 15 and 45 mL/min, NIR/RIT is 150mg/100mg twice a day. The maintenance dose is adjusted according to CrCL and AUC of RIT, with the dosages varying between 75mg/100mg and 300mg/100mg.