Population Pharmacokinetics of Ensitrelvir in Healthy Participants and Participants with SARS-CoV-2 Infection in the SCORPIO-SR Study.

Introduction: Ensitrelvir, a novel oral inhibitor of the 3C-like protease of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has shown efficacy and safety in participants with mild to moderate coronavirus disease 2019 (COVID-19) with once-daily multiple doses of 375 mg on day 1 followed by 125 mg on days 2-5. The aims of this study were to characterize the pharmacokinetics of ensitrelvir and to explore its exposure-response relationships on the dose regimen.

Methods: Pharmacokinetic data, including 8034 plasma concentration datasets from 2060 participants, from two phase I and one phase II/III study in healthy participants and participants infected with SARS-CoV-2 were used to develop a population pharmacokinetic model. The correlation between exposure and drug response was evaluated using observed plasma concentrations and estimated pharmacokinetic parameters as pharmacokinetic indexes and viral RNA as drug response.

Results: A two-compartment model with a first-order absorption model effectively described plasma ensitrelvir concentrations. The effects of body weight on clearance and volume of distribution and of food conditions and formulation on the absorption rate constant were selected as significant covariates. The efficacy indexes changed in the active group, but the responses were similar across the exposure range in the phase II/III study (SCORPIO-SR) regardless of the effects of the pharmacokinetic covariates.

Conclusion: Population pharmacokinetics revealed that body weight is the most important covariate in the pharmacokinetics of ensitrelvir. The antiviral effect, independent of ensitrelvir exposure, was demonstrated on the current dose regimen for treatment of SARS-CoV-2 infection.