Indenoisoquinoline topoisomerase I (Top1) inhibitors are a novel class of anticancer agents. Modifications of the indenoisoquinoline A, B, and D rings have been extensively studied in order to optimize Top1 inhibitory activity and cytotoxicity. To improve understanding of the forces that stabilize drug-Top1-DNA ternary complexes, the five-membered cyclopentadienone C-ring of the indenoisoquinoline system was replaced by six-membered nitrogen heterocyclic rings, resulting in dibenzo[c,h][1,6]naphthyridines that were synthesized by a novel route and tested for Top1 inhibition. This resulted in several compounds that have unique DNA cleavage site selectivities and potent antitumor activities in a number of cancer cell lines.