Potter: Activation of CPT-11



My laboratory has three major interests. The first is to characterize enzymes involved in the activation of prodrugs, the second is to apply this knowledge to develop enzyme/prodrug therapy approaches for the treatment of malignant disease and thirdly, we are exploring the possibility of modulating prodrug activation, and hence therapeutic efficacy, using selective enzyme inhibitors. The model system that we have concentrated on is the activation of the camptothecin analog CPT-11 to SN-38 by esterases. CPT-11 is a relatively non-toxic ester that upon hydrolysis yields SN-38, a potent topoisomerase I poison. We have isolated a cDNA encoding a rabbit liver carboxylesterase that can efficiently activate CPT-11 and expression of this protein in mammalian cells confers sensitivity to CPT-11 when grown either in culture or as xenografts in immune-deprived mice. Interestingly a human homologue of this gene, hCE1, is very inefficient at drug metabolism affording the potential of using the rabbit enzyme to selectively activate CPT-11 in human tumors. Studies performed in collaboration with Dr. Randy Wadkins (Johns Hopkins University School of Medicine) indicate that the lack of activation by hCE1 is due to constraints enforced by the entrance to the active site of the enzyme. This has been confirmed by mutagenesis studies and we have now developed a variant of hCE1, hCE1m6 that is as effective as the rabbit liver carboxylesterase at activating CPT-11.

To determine whether the rabbit liver carboxylesterase, or hCE1m6, in combination with CPT-11 can be used for the selective elimination of tumor cells, we have designed a novel delivery system using neural progenitor cells. These cells exhibit tropism for tumors in vivo and can be used to express high levels of the carboxylesterases at the site of the metastatic lesions. Subsequent treatment with CPT-11 results in enhanced, selective, tumor cell kill due to increased prodrug activation within the milieu of the solid tumor. We envisage that approaches using these neural progenitor cells as well as different combinations of prodrug-activating enzymes may have therapeutic utility against metastatic disease.

The research in my lab is funded in part by NIH Grants CA76202, CA79763,  CA98468, CA108775, CA113446, the Cancer Center Core grant CA21765, and by the American Lebanese Syrian Associated Charities.