Napabucasin (2-acetylfuro-1,4-naphthoquinone) is a small molecule currently being clinically evaluated in several different cancer types. It has mostly been recognized for its ability to inhibit STAT3-mediated gene expression with activity against bulk tumor cells, as well as cancer stem cells. However, based on its chemical structure, we hypothesized that napabucasin is a substrate for reduction by NAD(P)H:quinone oxidoreductase-1 (NQO1) and therefore may exert its anti-cancer effect through redox cycling, resulting in reactive oxygen species (ROS) production and cell death.
Binding of napabucasin to NQO1, as well as other oxidoreductases, was measured. Pancreatic cancer cell lines and xenografts were treated with napabucasin, and cell survival, ROS generation, glutathione to glutathione disulfide (GSH:GSSG) ratios and changes in STAT3 signaling were assayed. Genetic knockout or pharmacological inhibition with dicoumarol was used to evaluate the dependency on NQO1 in vitro and in vivo.
Napabucasin was found to bind with high affinity to NQO1 and to a lesser degree to cytochrome P450 oxidoreductase (POR). Differential cytotoxic effects were observed, where NQO1-expressing cells in particular were highly sensitive. Treatment resulted in significant induction of ROS with reduced GSH:GSSG ratios, increased DNA damage and an NQO1-dependent decrease in STAT3 phosphorylation. Cells with low or no baseline NQO1 expression also produced ROS in response to napabucasin treatment, albeit to a lesser degree, through the one-electron reductase POR.
Napabucasin is bioactivated by oxidoreductases, in particular NQO1 and to a lesser extent POR, resulting in futile redox cycling and generation of cytotoxic levels of ROS. The increase in ROS has multiple intracellular effects, one of which is a reduction in STAT3 signaling, ultimately resulting in cell death.
Clinical trial identification
Legal entity responsible for the study
Lustgarten Foundation, National Institutes of Health, Donaldson Charitable Trust, Boston Biomedical Inc.
J. Li: Full / Part-time employment: Boston Biomedical, Inc. A. Chang: Full / Part-time employment: Boston Biomedical, Inc.. H.A. Rogoff: Full / Part-time employment: Boston Biomedical, Inc.. J.D. Watson: Advisory / Consultancy, The consultancy occured in the past: Boston Biomedical, Inc. All other authors have declared no conflicts of interest.