Deregulation of apoptosis, the physiological form of cell death, is closely associated with immunological diseases and cancer. Apoptosis is activated either by death receptor-driven or mitochondrial pathways, both of which may provide potential targets for novel anticancer drugs. Although several ligands stimulating death receptors have been described, the actual molecular events triggering the mitochondrial pathway are largely unknown. Here, we show initiation of apoptosis by gradual depletion of the intracellular coenzyme NAD⁺. We identified the first low molecular weight compound, designated FK866, which induces apoptosis by highly specific, noncompetitive inhibition of nicotinamide phosphoribosyltransferase (NAPRT), a key enzyme in the regulation of NAD⁺ biosynthesis from the natural precursor nicotinamide. Interference with this enzyme does not primarily intoxicate cells because the mitochondrial respiratory activity and the NAD⁺-dependent redox reactions involved remain unaffected as long as NAD⁺ is not effectively depleted by catabolic reactions. Certain tissues, however, have a high turnover of NAD⁺ through its cleavage by enzymes like poly(ADP-ribose) polymerase. Such cells often rely on the more readily available nicotinamide pathway for NAD⁺ synthesis and undergo apoptosis after inhibition of NAPRT, whereas cells effectively using the nicotinic acid pathway for NAD⁺ synthesis remain unaffected. In support of this concept, FK866 effectively induced delayed cell death by apoptosis in HepG2 human liver carcinoma cells with an IC₅₀ of ∼1 nm, did not directly inhibit mitochondrial respiratory activity, but caused gradual NAD⁺ depletion through specific inhibition of NAPRT. This enzyme, when partially purified from K562 human leukemia cells, was noncompetitively inhibited by FK866, and the inhibitor constants were calculated to be 0.4 nm for the enzyme/substrate complex (K_i) and 0.3 nm for the free enzyme (K_i′), respectively. Nicotinic acid and nicotinamide were both found to have antidote potential for the cellular effects of FK866. FK866 may be used for treatment of diseases implicating deregulated apoptosis such as cancer for immunosuppression or as a sensitizer for genotoxic agents. Furthermore, it may provide an important tool for investigation of the molecular triggers of the mitochondrial pathway leading to apoptosis through enabling temporal separation of NAD⁺ decrease from ATP breakdown and apoptosis by several days.