Favipiravir - 13 C3 is the 13 C labeled isotope of Favipiravir - 13 C3.Favipiravir (T-705) is a potent viral RNA polymerase inhibitor, it is phosphoribosylated by cellular enzymes to its active form, Favipiravir -ribofuranosyl-5′-triphosphate (RTP). Favipiravir -RTP inhibits the influenza viral RNA-dependent RNA polymerase ( RdRP ) activity with an IC 50 of 341 nM.