Development of indolequinone mechanism-based inhibitors of NAD(P)H:quinone oxidoreductase 1 (NQO1):: NQO1 inhibition and growth inhibitory activity in human pancreatic MIA PaCa-2 cancer cells

NAD(P)H:quinone oxidoreductase 1 (NQO1) is currently an emerging target in pancreatic cancer. In this report, we describe a series of indolequinones, based on 5-methoxy-1,2-dimethyl-3-[(4-nitrophenoxy)methyl]indole-4,7-dione (ES936), and evaluate NQO1 inhibition and growth inhibitory activity in the human pancreatic MIA PaCa-2 tumor cell line. The indolequinones with 4-nitrophenoxy, 4-pyridinyloxy, and acetoxy substituents at the (indol-3-yl)methyl position were NADH-dependent inhibitors of recombinant human NQO1, indicative of mechanism-based inhibition. However, those with hydroxy and phenoxy substituents were poor inhibitors of NQO1 enzyme activity, due to attenuated elimination of the leaving group. The ability of this series of indolequinones to inhibit recombinant human NQO1 correlated with NQO1 inhibition in MIA PaCa-2 cells. The examination of indolequinone interactions in complex with NQO1 from computational-based molecular docking simulations supported the observed biochemical data with respect to NQO1 inhibition. The design of both NQO1-inhibitory and noninhibitory indolequinone analogues allowed us to test the hypothesis that NQO1 inhibition was required for growth inhibitory activity in MIA PaCa-2 cells. ES936 and its 6-methoxy analogue were potent inhibitors of NQO1 activity and cell proliferation; however, the 4-pyridinyloxy and acetoxy compounds were also potent inhibitors of NQO1 activity but relatively poor inhibitors of cell proliferation. In addition, the phenoxy compounds, which were not inhibitors of NQO1 enzymatic activity, demonstrated potent growth inhibition. These data demonstrate that NQO1 inhibitory activity can be dissociated from growth inhibitory activity and suggest additional or alternative targets to NQO1 that are responsible for the growth inhibitory activity of this series of indolequinones in human pancreatic cancer.