The topoisomerase I enzymatic inhibition of hybrid quinolino [4,3-b] (Siegel et al., 2013; Antony et al., 2003) [1,5]naphthyridines and quinolino [4,3-b] (Siegel et al., 2013; Antony et al., 2003) [1,5]naphthyridin-6(5H)-ones was investigated. First, the synthesis of these fused compounds was performed by intramolecular [4 + 2]-cycloaddition reaction of functionalized aldimines obtained by the condensation of 3-aminopyridine and unsaturated aldehydes affording corresponding hybrid 5-tosylhexahydroquinolino [4,3-b] (Siegel et al., 2013; Antony et al., 2003) [1,5]naphthyridine and tetrahydroquinolino [4,3-b] (Siegel et al., 2013; Antony et al., 2003) [1,5]naphthyridin-6(5H)-one compounds with good to high general yields. Subsequent dehydrogenation led to the corresponding more unsaturated dihydro (Siegel et al., 2013; Antony et al., 2003) [1,5]naphthyridine and (Siegel et al., 2013; Antony et al., 2003) [1,5]naphthyridin-6(5H)-one derivatives in quantitative yields. The new polycyclic products show excellent-good activity as topoisomerase I (TopI) inhibitors that lead to TopI induced nicking of plasmids. This is consistent with the compounds acting as TopI poisons resulting in the accumulation of trapped cleavage complexes in the DNA. The cytotoxic effect on cell lines A549, SKOV3 and on non-cancerous MRC5 was also screened. Tetrahydroquinolino [4,3-b] (Siegel et al., 2013; Antony et al., 2003) [1,5]naphthyridin-6(5H)-one 9 resulted the most cytotoxic compound with IC₅₀ values of 3.25 ± 0.91 μM and 2.08 ± 1.89 μM against the A549 cell line and the SKOV3 cell line, respectively. Also, hexahydroquinolino [4,3-b] (Siegel et al., 2013; Antony et al., 2003) [1,5]naphthyridine 8a and dihydroquinolino [4,3-b] (Siegel et al., 2013; Antony et al., 2003) [1,5]naphthyridine 10a showed good cytotoxicity against these cell lines. None of the compounds presented cytotoxic effects against non-malignant pulmonary fibroblasts (MRC-5).