Fig. (4). Molecular structures of camptothecin and derivatives. The activity of camptothecin against topo I requires an intact lactone ring. The carboxylate analog is inactive. Irintotecan is a pro-drug that requires hydrolysis to SN-38 for activity. Irinotecan is active against colorectal cancer and topotecan shows activity against ovarian cancer. Nine-amino camptothecin has not yet been shown to have significant activity against human malignancies