One of the major limitations of CPT and analogues is the severe toxicity, stemming in part from the instability of the α-hydroxylactone E-ring. This ring plays a key role in supporting both efficient Topo I inhibition and in vivo potency. Three-dimensional structure analyses of a ternary complex formed between TopoI, DNA and Topotecan by X-ray crystallography showed that the drug interacts both with the DNA base pairs flanking the cleavage sites and three key aminoacid ...