Synthesis of Cytotoxic Indenoisoquinoline Topoisomerase I Poisons

A number of indenoisoquinolines were prepared and evaluated for cytotoxicity in human cancer cell cultures and for activity vs topoisomerase 1 (top1). The two most cytotoxic indenoisoquinolines proved to be cis-6-ethyl-5,6,12,13-tetrahydro-2,3-dimethoxy-8,9-(methylenedioxy)-5,11-dioxo-11H-indeno[1,2-c]isoquinoline (21) and cis-6-allyl-5,6,12,13-tetrahydro-2,3-dimethoxy-8,9-(methylenedioxy)-5,11-dioxo-11H-indeno[1,2-c]isoquinoline (22), both of which displayed submicromolar mean graph midpoints when tested in 55 human cancer cell cultures. Two of the most potent top1 inhibitors were 6-(3-carboxy-1-propyl)-5,6-dihydro-5,11-dioxo-11H-indeno[1,2-c]isoquinoline (26) and 6-ethyl-2,3-dimethoxy-8,9-(methylenedioxy)-11H-indeno[1,2-c]isoquinolinium chloride (27), both of which also inhibited top2, unwound DNA, and are assumed to be DNA intercalators. However, two additional potent top1 inhibitors, 6-allyl-5,6-dihydro-2,3-dimethoxy-8,9-(methylenedioxy)-5,11-dioxo-11H-indeno[1,2-c]isoquinoline (13c) and 5,6-dihydro-6-(4-hydroxybut-1-yl)-2,3-dimethoxy-8,9-methylenedioxy-5,11-dioxo-11H-indeno[1,2-c]isoquinoline (19a), did not unwind DNA and did not affect top2. Some of the DNA cleavage sites detected in the presence of the indenoisoquinolines were different from those seen with the camptothecins. The cleavage sites induced by the indenoisoquinolines were reversed by salt treatment, which is consistent with the reversible trapping of top1 cleavable complexes by the indenoisoquinolines. In general, the potencies of the indenoisoquinolines as top1 inhibitors did not correlate with their potencies as cytotoxic agents, as some of the most cytotoxic agents had little if any effect on top1. On the other hand, the most potent of the indenoisoquinolines vs top1 were not the most cytotoxic. In several cases, moderate activity was observed for both cytotoxicity and activity vs top1.