In this work, enaminones 1 a,b were used as precursors for the synthesis of novel fused heterocyclic ring systems (e. g. pyrazolo [3,4-d]pyrimidinones, isoxazolo[5,4-d]pyrimidinones, pyrimido [4,5-d]pyrimidinones and related compounds) via their reactions with some N-nucleophiles. The cytotoxic activity of the designed products was assessed via the MTT colorimetric assay against human liver hepatocellular carcinoma (HepG2) and normal lung fibroblasts (WI-38) cell lines using doxorubicin as a reference standard. Among the screened compounds, pyrazolo[3,4-d]pyrimidinone 3 d and pyrimido [4,5-d] pyrimidinones 6 a,b, 7 b and 8 a,b were selected as lead molecules because they showed significant activity against HepG2 cells and a weak cytotoxic effect against WI-38 cells. In further, all of the compounds were subjected to the bleomycin-dependent DNA damage studies. The results indicated that most of them have a remarkable ability to protect DNA compared to ascorbic acid as a standard compound.