The goal of this project is to elucidate the mechanism of action of camptothecin analogs as topoisomerase poisons in upper GI cancer patients. DNA topoisomerase -1 (topo-1) is a nuclear enzyme essential for normal DNA replication, transcription, and a host of other cell functions. This enzyme functions topologically as a swivel by causing single strand DNA breakage. Topo-1 therefore serves as an attractive sub-cellular target for cancer chemotherapy. While many active chemotherapy compounds act through inhibition of topoisomerase II, one unique compound, camptothecin, and its analogs inhibit topoisomerase-1. Ample preclinical data demonstrate that camptothecin and its analogs bind to the topoisomerase-1 : DNA complex to form a """"""""cleavable complex."""""""" The cleavable complex stablizes the unwound and broken DNA, converts topo-1 to a cellular poison, and leads subsequently to disruption of DNA continuity and cell death. Other preclinical studies have shown the importance of structure-function relationships for the camptothecin analogs and their correlation with in vivo activity in cell lines and human xenografts. We have also demonstrated increased topoisomerase-1 levels in surgical colon cancer specimens, which may serve as a basis for an improved therapeutic index. Indeed, analogs effective in curing human-xenograft bearing mice cause only marginal toxicity. Despite this abundant pre-clinicai information, existence of the cleavable complex in humans has yet to be demonstrated. In this project we will develop phase II protocols with clinically available camptothecin analogs for the treatment of patients with upper GI cancers. We will refine our techniques for determination of topo-1 levels and cleavable complex for application to endoscopic biopsy specimens. We will investigate the pharmacokinetics of the camptothecin analog in patients. All patients will have endoscopically accessible tumors and will undergo biopsy in the course of treatment to determine the pharmacodynamics of cleavable complex formation. Finally, we will use appropriate statistical techniques to correlate clinical anti-tumor response with topo-1 and cleavable complex levels to prove the hypothesis that increased levels of topo-1 and cleavable complex underlie activity of the camptothecin analogs.
