The long-term objective of this project is to create the next generation of topoisomerase I-targeted chemotherapy, which will be both less toxic and more efficacious in the treatment of human cancer. This objective will be achieved by three integrated approaches. First, more potent and selective inhibitors of topoisomerase I will be synthesized, based upon recent success with camptothecin (CPT) analogs that act as water-soluble prodrugs. These agents enable sustained release of active species to produce slowly reversible and irreversible topo I-DNA crosslinks in tumor cells. Such activity is promoted under acidic conditions, which suggest that the therapeutic index of the tumor microenvironment. This approach exploits the significant but highly variable pH gradient that is known to exist in human tumors and not in normal tissue. For agents that are weak acids, such a pH gradient will also effectively concentrate the agent inside tumor cells, which likely explains how the antitumor activity of certain alkylating agents is potentiated by pH modulation. Preliminary data suggests that the same is true for CPT prodrugs. Since alkylating agents produce synergistic antitumor activity in combination with topoisomerase I inhibitors, a third approach is to identify synergistic combinations of alkylating agents and novel CPT analogs whose selectivity is improved with pH modulation. Specifically, glycinate esters of methylenedioxy or 10-hydroxy derivatives of CPT will be synthesized and evaluated for antitumor activity in vitro and in vivo, either alone or in combination with cyclophosphamide, BCNU or temozolamide. Combination chemotherapy will then be assessed in immunocompromised mice bearing human tumor xenografts that have been selectively acidified by agents that produce transient hyperglycemia and hypertension. These experiments will be interwoven with mechanistic studies to confirm that the therapeutic strategy in fact yields topoisomerase-I mediated DNA damage and subsequent tumor cell apoptosis. Since this strategy is clinically feasible, there is significant potential for rapid translation of this work to clinical oncology.
