Two series of indenoisoquinoline topoisomerase I inhibitors have been prepared to investigate optimal substituents on the indenone ring at the 9-position. The more exhaustive series was prepared using a nitrated isoquinoline ring that has been previously demonstrated to enhance biological activity. After preliminary biological evaluation, a more focused series of inhibitors was prepared utilizing a 2,3-dimethoxy-substituted isoquinoline ring. The results of the two series indicate the existence of superior functional groups such as methoxy, fluorine, and cyano for the indenoisoquinoline 9-position. Interestingly, these functional groups coincide with established structure-activity relationships for the 11-position of camptothecin.

The biological activity of indenoisoquinoline topoisomerase I (Top1) inhibitors can be greatly enhanced depending on the choice of substituents on the aromatic rings and lactam side chain. Previously, it was discovered that a 3-nitro group and a 9-methoxy group afforded enhanced biological activity. In the present investigation, indenoisoquinoline analogues were systematically prepared using combinations of nitro groups, methoxy groups, and hydrogen atoms in an effort to understand the contribution of each group toward cytotoxicity and Top1 inhibition. Analysis of the biological results suggests that the nitro group is important for Top1 inhibition and the methoxy group improves cytotoxicity. In addition, previously identified structure-activity relationships were utilized to select favorable lactam side chain functionalities for incorporation on the aromatic skeleton of analogues in this study. As a result, this investigation has provided optimal Top1 inhibitors equipotent to camptothecin that demonstrate low nanomolar cytotoxicities toward cancer cells.