Herein we report an efficient synthesis of a series of regioisomeric N,O-syn and N,O-anti 3-diethoxyphosphorylfuroquinoline-4,9-diones combining furoquinoline-5,8-dione skeleton, present in several highly cytotoxic compounds, with diethoxyphosphoryl moiety. The cytotoxic activity of the obtained analogs was tested against two human cancer cell lines: promyelocytic leukemia HL-60 and breast cancer adenocarcinoma MCF-7 and for comparison on human umbilical vein endothelial cells HUVEC and mammary gland/breast MCF-10 A cells. Several diethoxyphosphorylfuroquinoline-4,9-diones proved to be highly cytotoxic for cancer cells with IC<sub>50</sub> values even below 0.1 μM. Interestingly, N,O-syn 3-diethoxyphosphorylfuroquinoline-4,9-diones were 3- to 7-fold more active against HL-60 cells than the respective N,O-anti regioisomers. The most promising analogs 9c and 9i, with the highest cancer/healthy cells cytotoxicity ratio, were further evaluated to establish their mode of action. In HL-60 cells these analogs enhanced intracellular ROS generation and NAD(P)H:quinone oxidoreductase 1 (NQO1) depletion which led to the cell cycle arrest in the S-phase, reduced cell proliferation, DNA damage and apoptosis.