The synthesis and antitumor activity of substituted benzo[e][1,2,4]triazino[2,3-c][1,2,3]triazin-2-ones -- novel electro-deficient tricyclic compounds -- are described. These compounds were prepared by treatment of 3-(2-amino-3-R²-5-R³-phenyl)-6-R¹-1,2,4-triazin-5(2H)-ones with sodium nitrite in acetic acid. Spectral properties of synthesized compounds were studied and compared with spectral data of known 3-R-8-R¹-10-R²-2H-[1,2,4] triazino[2,3-c]quinazolin-2-ones. These compounds are promising antitumor agents. The most active anticancer compound 3d was studied in dose-depended anticancer activity assay and its selective growth inhibition against breast cancer MDA-MB-468 cell line was established (GI50 = 0.41 µm).