Oral Favipiravir Exposure and Pharmacodynamic Effects in Adult Outpatients With Acute Influenza.

backgroundThe pharmacokinetics of oral favipiravir and the relationships of plasma concentrations to antiviral effects are incompletely studied in influenza.

methodsSerial plasma samples were collected from adults with uncomplicated influenza who were randomized to favipiravir (1800 mg twice a day on day 1, 800 mg twice a day on days 2 to 5; n = 827) or placebo (n = 419) in 2 phase 3 trials. Post hoc analyses assessed the frequency of reaching an average minimum concentration (Cmin) ≥20 µg/mL, its association with antiviral efficacy, and factors associated with reduced favipiravir exposure.

resultsWide interindividual variability existed in favipiravir concentrations, and this regimen failed to reach an average Cmin>20 µg/mL in 41%-43% of participants. Those attaining this threshold showed greater reductions in nasopharyngeal infectious virus titers on treatment days 2 and 3 and lower viral titer area under the curve compared to those who did not. Those with average Cmin <20 µg/mL had over 2-fold higher mean ratios of the metabolite T-705M1 to favipiravir, consistent with greater metabolism, and were more likely to weigh >80 kg (61.5%-64%).

conclusionsHigher favipiravir levels with average Cmin>20 µg/mL were associated with larger antiviral effects and more rapid illness alleviation compared to placebo and to favipiravir recipients with lower average Cmin values in uncomplicated influenza. Clinical Trials Registration . NCT1068912 and NCT01728753.