Gene Directed Enzyme Prodrug Therapy (GDEPT)

Gene and Oncogene Targeting Team

Section: Cancer Research UK Cancer Therapeutics Unit

GDEPT is a gene-based two-step treatment for cancer. In the first step, the gene for the exogenous enzyme carboxypeptidase G2 (CPG2) is targeted to the tumour by the use of a selective vector, followed by administration of a prodrug that is activated by the enzyme. A large number of prodrugs including novel self-immolative prodrugs have been designed that are converted to a range of different classes of drugs. Thus the prodrug/drug system selected can be tailored for the tumour type.

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CPG2 has been expressed in a variety of forms, both intracellular and tethered to the outer surface of mammalian cells. The former requires intracellular activation of prodrugs, whereas the latter allows this to be extracellular. Excellent cytotoxicity differentials are obtained between those cell lines that express the enzyme compared with the parent cell lines without the CPG2. Good bystander cytotoxicity is obtained in vitro, when only 2% of the cells need to express the enzyme in order to achieve total cell ablation. Tumour xenografts of breast and colorectal carcinoma cell lines stably expressing the enzyme CPG2 have been developed for in vivo analyses of the GDEPT systems. A range of prodrugs causes a dramatic decrease in xenograft tumour volume and many cures are observed in both xenograft models. Tumours that re-grow after initial treatment respond to further administration of the prodrug, indicating that the gene is stable and active in the long term.

An adenoviral gene vector has been developed. In collaboration with Cell Genesys Inc (South San Francisco, CA), adenoviral vectors expressing the gene for CPG2 have been engineered to be under the control of a cancer-selective promoter. This restricts expression of CPG2, and thus prodrug sensitivity to cancer cells. In vivo models of human hepatocellular carcinoma treated with a single intravenous systemic dose of adenoviral vectors showed tumour-selective replication of adenoviruses and expression of CPG2, not found in normal tissues. Levels of CPG2 greatly exceeded those previously shown to be sufficient to cause regression of the tumours following administration of the prodrug.  Thus the adenoviral vectors show great promise for efficacy in GDEPT protocols. A proposal for a GDEPT Phase I clinical trial has been accepted by Cancer Research UK New Agents Committee.