Optimization of the Lactam Side Chain of 7‑Azaindenoisoquinoline Topoisomerase I Inhibitors and Mechanism of Action Studies in Cancer Cells

Optimization of the lactam ω-aminoalkyl substituents in a series of 7-azaindenoisoquinolines resulted in new anticancer agents with improved Top1 inhibitory potencies and cancer cell cytotoxicities. The new compounds 14–17 and 19 exhibited mean graph midpoint cytotoxicity (GI₅₀) values of 21–71 nM in the NCI panel of 60 human cancer cell cultures. Ternary 7-azaindenoisoquinoline–DNA–Top1 cleavage complexes that persist for up to 6 h were detected in HCT116 colon cancer cells. Ternary complexes containing 7-azaindenoisoquinolines were significantly more stable than those in which camptothecin was incorporated. DNA content distribution histograms showed S-phase block 3 h after drug removal. Drug-induced DNA damage in HCT116 cells was revealed by induction of the histone γ-H2AX marker. The 7-azaindenoisoquinolines were able to partially overcome resistance in several drug-resistant cell lines, and they were not substrates for the ABCB1 drug efflux transporter. Molecular modeling studies indicate that the 7-azaindenoisoquinolines intercalate at the DNA cleavage site in DNA–Top1 covalent complexes with the lactam side chain projecting into the major groove. Overall, the results indicate that the 7-azaindenoisoquinolines are promising anticancer agents that merit further development.