Simultaneous determination of Favipiravir and its active metabolite, Favipiravir Ribofuranosyl-5'-triphosphate, in plasma and buccal cells using HPLC.

Favipiravir, recently approved for the treatment of tickborne severe fever with thrombocytopenia syndrome, is a prodrug that can be metabolized in vivo into its active phosphorylated form. Given that the accurate pharmacokinetic analysis of favipiravir and its active metabolite is crucial for dosage adjustment and, hence, therapeutic efficacy optimization, we herein developed an HPLC-based method for the simultaneous quantification of favipiravir and its active metabolite in plasma and buccal cells. Separation within 17 min was achieved using a mixed-mode C18 column with an anion-exchange functionality as the stationary phase, a phosphate buffer (pH 6.38) as the mobile phase, and gentisic acid as the internal standard. Fluorescence-based detection (excitation/emission = 370/440 nm) enabled the quantification of both compounds within the therapeutic range of 10-500 μM without interference from endogenous biological substances. The developed method, which allows for the rapid and reliable simultaneous determination of the prodrug and its active metabolite, is expected to be useful for improving the therapeutic outcome of favipiravir while minimizing side effects.