Napabucasin and Related Heterocycle-Fused Naphthoquinones as STAT3 Inhibitors with Antiproliferative Activity against Cancer Cells

Napabucasin ( 6 ) and its angularly anellated isomer ( 7 ), for which the synthesis is described, together with related plant-derived naphthoquinones, were evaluated in vitro against human breast cancer (MDA-MB-231) and chronic myelogenous leukemia (K562) cells. As observed for β-lapachone ( 3 ), the active naphthoquinones all induced apoptosis in a cell-cycle-independent fashion. In contrast to the pyran-fused β-lapachone ( 3 ), however, the most potent furan-fused naphthoquinones were able to redox cycle and generate superoxide in cell-based assays, which was independent of NAD(P)H:quinone oxido-reductase 1. In a homogeneous time-resolved fluorescence (HTRF) assays with MDA-MB-231 cells, both napabucasin ( 6 ) and isonapabucasin ( 7 ) were identified as targeting STAT3 phosphorylation. In addition, drug affinity responsive target stability assays were performed to validate a direct interaction of the naphthoquinones with STAT3. Isonapabucasin ( 7 ) turned out to be twice as potent against STAT3 as napabucasin ( 6 ) in the HTRF assay, with an EC 50 in the submicromolar range, which was in excellent agreement with the potency of both agents to inhibit the growth of MDA-MB-231 cells. Moreover, molecular docking experiments predicted different binding modes to the STAT3 SH2 domain for the linearly anellated napabucasin ( 6 ) and its angularly anellated isomer ( 7 ).