Cell-permeable. FK-866 is a highly specific non-competitive inhibitor of Nicotinamide phosphoribosyltransferase (NAMPT) (Ki = 0.4 nM), causing gradual NAD+ depletion. NAMPT functions as an intra-cellular and extra-cellular NAD biosynthetic enzyme that is important for regulating metabolism and stress resistance through sirtuins and other NAD-consuming regulators. NAD+ depletion by FK-866 directs delayed cell death by apoptosis in Hep-G2 human liver carcinoma cells (IC₅₀ = ~1 nM).