Population pharmacokinetics of favipiravir in patients with COVID-19

The antiretroviral drug favipiravir (FPV) inhibits RNA-dependent RNA polymerase. It has been developed for the treatment of the novel coronavirus (severe acute respiratory syndrome coronavirus 2) infection disease, coronavirus disease 2019 (COVID-19). However, its pharmacokinetics in patients with COVID-19 is poorly understood. In this study, we measured FPV serum concentration by liquid chromatography-tandem mass spectrometry and conducted population pharmacokinetic analysis. A total of 39 patients were enrolled in the study: 33 were administered FPV 1600 mg twice daily (b.i.d.) on the first day followed by 600 mg b.i.d., and 6 were administered FPV 1800 mg b.i.d. on the first day followed by 800 mg or 600 mg b.i.d. The median age was 68 years (range, 27-89 years), 31 (79.5%) patients were men, median body surface area (BSA) was 1.72 m² (range, 1.11-2.2 m² ), and 10 (25.6%) patients required invasive mechanical ventilation (IMV) at the start of FPV. A total of 204 serum concentrations were available for pharmacokinetic analysis. A one-compartment model with first-order elimination was used to describe the pharmacokinetics. The estimated mean clearance/bioavailability (CL/F) and distribution volume/bioavailability (V/F) were 5.11 L/h and 41.6 L, respectively. Covariate analysis revealed that CL/F was significantly related to dosage, IMV use, and BSA. A simulation study showed that the 1600 mg/600 mg b.i.d. regimen was insufficient for the treatment of COVID-19 targeting the 50% effective concentration (9.7 µg/mL), especially in patients with larger BSA and/or IMV. A higher FPV dosage is required for COVID-19, but dose-dependent nonlinear pharmacokinetics may cause an unexpected significant pharmacokinetic change and drug toxicity. Further studies are warranted to explore the optimal FPV regimen.