DNA topoisomerase II (top 2) is the cellular target of several among the most potent anticancer agents (Doxorubicin, etoposides [VP16; VM-26], mitoxantrone, amsacrine, ellipticines). For this reason, it is one of the key targets in anticancer drug development. By comparing the structures of the VP-16 chromophore and ellipticine, we have rationally designed azatoxin, a new drug which has been made by total synthesis. Azatoxin was found to be an active top 2 inhibitor with a unique DNA sequence selectivity in the presence of purified top 2. Interestingly, it is not a DNA intercalator, and it produces protein-linked DNA breaks in cells. A patent has been filed for azatoxin and its antitumor activity is being evaluated. DNA topoisomerase I (top 1) has also become an essential target for anticancer research since the discovery that camptothecin and several of its derivatives are specific top 1 poisons and that water-soluble camptothecin analogs exhibit promising anticancer activity. One of our goals is to identify the molecular mechanism(s) of top 1 inhibition by camptothecins. Our recent studies using top 1 cDNA demonstrate that camptothecin specifically poisons top 1 at the cleavage sites which have a guanine at their 5'-terminus. This observation is consistent with our other results that photoactivated camptothecin induces cleavage specifically at guanines. Together, these data support our previous drug stacking model for drug-induced topoisomerase inhibition (Jaxel et al., Nucleic Acids Res. 1991;266:20418-23; Pommier et al., Nucleic Acids Res. 1991;19:5973-80). Another approach to the molecular pharmacology of camptothecin has been to develop and analyze campothecin-resistant cells. We are finding a single point mutation in the top 1 cDNA from drug-resistant Chinese hamster cells (Tanizawa and Pommier, Cancer Res. 1991; 52: 1848-54), indicating that the mutated region might be important for both enzymatic activity and camptothecin sensitivity.
