DNA topoisomerase II (top 2) is the cellular target of several among the most potent anticancer agents (Doxorubicin, etoposides [VP-16; VM-26), mitoxantrone, amsacrine, ellipticines). For this reason, it is one of the key targets in anti-cancer 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 is a very active top 2 inhibitor with a unique DNA sequence selectivity. Interestingly, it is not a DNA intercalator, and it produces protein- linked DNA breaks in cells. Azatoxin derivatives are being investigated and their antitumor activity is being evaluated at the NCI Drug Screening Program. We are also characterizing the effect of anthrapyrazoles derivatives (DU937 & DU941) on purified top 2. DNA topoisomerase I (top 1) has also become an essential target for anti cancer 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 poisons specifically 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 camptothecin-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. 1992;52:1848-54), indicating that the mutated region might be important for both enzymatic activity and camptothecin sensitivity.
