PSI-7409 inhibits the enzymatic activities of these NS5BΔ21 polymerases in a dose-dependent manner. The IC ₅₀ s for PSI-7409 against GT 1b, 2a, 3a, and 4a NS5B polymerases are 1.6 μM, 2.8 μM, 0.7 μM, and 2.6 μM, respectively. PSI-7409 is a weak inhibitor of DNA Pol α (IC ₅₀ =550 μM). DNA Pol β and γ are not inhibited by 1 mM PSI-7409. A significant amount of RNA product is made in the presence of 500 μM PSI-7409, about 85%. In clone A cells, the levels of PSI-7409 gradually increases to a maximum concentration of about 25 μm over a period of 48 h. PSI-7409 forms at a much faster rate in primary human hepatocytes, achieving a maximum intracellular concentration of ～100 μM at 4 h and remains at that concentration for 48 h.