␤-Lapachone activates a novel apoptotic response in a number of cell lines. We demonstrate that the enzyme NAD(P)H:quinone oxidoreductase (NQO1) substantially enhances the toxicity of ␤-lapachone. NQO1 expression directly correlated with sensitivity to a 4-h pulse of ␤-lapachone in a panel of breast cancer cell lines, and the NQO1 inhibitor, dicoumarol, significantly protected NQO1-expressing cells from all aspects of ␤-lapachone toxicity. Stable transfection of the NQO1-deficient cell line, MDA-MB-468, with an NQO1 expression plasmid increased apoptotic responses and lethality after ␤-lapachone exposure. Dicoumarol blocked both the apoptotic responses and lethality. Biochemical studies suggest that reduction of ␤-lapachone by NQO1 leads to a futile cycling between the quinone and hydroquinone forms, with a concomitant loss of reduced NAD(P)H. In addition, the activation of a cysteine protease, which has characteristics consistent with the neutral calciumdependent protease, calpain, is observed after ␤-lapachone treatment. This is the first definitive elucidation of an intracellular target for ␤-lapachone in tumor cells. NQO1 could be exploited for gene therapy, radiotherapy, and/or chemopreventive interventions, since the enzyme is elevated in a number of tumor types (i.e. breast and lung) and during neoplastic transformation.

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Experiments using purified recombinant human NAD(P)H:quinone oxidoreductase 1 (NQO1) revealed that the auto-oxidation of fully reduced protein resulted in a 1:1 stoichiometry of oxygen consumption to NADH oxidation with the production of hydrogen peroxide. The rate of auto-oxidation of fully reduced NQO1 was markedly accelerated in the presence of superoxide (O 2 . ), whereas the addition of superoxide dismutase greatly inhibited the rate of auto-oxidation.

The quinone pharmacophore is present in many drug classes but is particularly common among antitumor drugs. Many quinones serve essentially as pro-drugs and exert their activities after reduction. Reduction of quinones may generate semiquinones or hydroquinones with subsequent generation of reactive oxygen radicals and oxidative stress, quinones can be designed so they lose a leaving group when reduced to the hydroquinone generating a reactive electrophile or the hydroquinone form of the molecule may have greater pharmacological activity than the parent quinone against a particular target. Enzyme systems that reduce quinones therefore become critically important in the pharmacological activity of this class of drugs. There are a number of enzyme systems that can catalyze reduction of quinones including cytochrome P450 reductase, cytochrome b5 reductase, NAD(P)H:quinone oxidoreductase 1 (NQO1), NAD(P)H:quinone oxidoreductase 2 (NQO2), carbonyl reductases, and thioredoxin reductase. In this context, one of the most extensively studied reductases has been NAD(P)H:quinone oxidoreductase 1 (NQO1). In this review we will focus on the role of NQO1 in the bioactivation of clinically important quinones mitomycin C, β-lapachone and 17AAG as well as the influence of the NQO1*2 polymorphism on the sensitivity and resistance to these agents.