Nampt inhibition with FK866 induces significant NAD⁺ intracellular reduction and selectively kills MM cells. (A) FK866 cytotoxic effect in primary cells. Purified patient MM cells (CD138⁺) and PBMCs from healthy donors or MM patients were plated in 6-well plates and treated with FK866 at 10 and 30nM concentrations. After 72 hours (CD138⁺ cells) or 96 hours (PBMC cells) were stained with fluorescein isothiocyanate (FITC)–conjugated annexin-V and PI, and analyzed by flow cytometry. (B-C) MM cell lines (B) and healthy donor PBMCs (C) were plated with or without FK866 10nM for the indicated times. Cells were harvested, and intracellular NAD⁺ level was measured using an enzyme cyclic assay. The intracellular NAD⁺ level was normalized relative to total cell number. Data presented are mean ± SD of 2 independent experiments.