Gene-mediated enzyme prodrug therapy (GDEPT) seeks to increase the therapeutic index of anti-neoplastic agents by promoting selective activation of relatively nontoxic drug derivatives at sites of specific enzyme expression. Glucuronide prodrugs are attractive for GDEPT due to their low toxicity, bystander effect in the interstitial tumor space and the large range of possible glucuronide drug targets. In this study, we expressed human, murine and Esherichia coli beta-glucuronidase on tumor cells and examined their in vitro and in vivo efficacy for the activation of glucuronide prodrugs of 9-aminocamptothecin and p-hydroxy aniline mustard. We show that (1) fusion of beta-glucuronidase to the Ig-like C(2)-type and Ig-hinge-like domains of the B7-1 antigen followed by the B7-1 transmembrane domain anchored high levels of active murine and human beta-glucuronidase on cells, (2) strong bystander killing of tumor cells was achieved in vitro by murine beta-glucuronidase activation of prodrug, (3) potent in vivo anti-tumor activity was achieved by prodrug treatment of tumors that expressed murine beta-glucuronidase and (4) the p-hydroxy aniline prodrug was more effective in vivo than the 9-aminocamptothecin prodrug. Our results demonstrate that surface expression of murine beta-glucuronidase for activation of a glucuronide prodrug of p-hydroxy aniline mustard may be useful for more selective therapy of cancer.

Development of nonimmunogenic and specific reporter genes to monitor gene expression in vivo is important for the optimization of gene therapy protocols. We developed a membrane-anchored form of mouse b-glucuronidase (mbG) as a reporter gene to hydrolyze a nonfluorescent glucuronide probe (fluorescein di-b-D-glucuronide, (FDGlcU) to a highly fluorescent reporter to assess the location and persistence of gene expression. A functional b-glucuronidase (bG) was stably expressed on the surface of murine CT26 colon adenocarcinoma cells where it selectively hydrolyzed the cell-impermeable FDGlcU probe. FDGlcU was also preferentially converted to fluorescent probe by (bG) on CT26 tumors. The fluorescent intensity in bG-expressing CT26 tumors was 240 times greater than the intensity in control tumors. Selective imaging of gene expression was also observed after intratumoral injection of adenoviral bG vector into carcinoma xenografts. Importantly, mbG did not induce an antibody response after hydrodynamic plasmid immunization of Balb/c mice, indicating that the reporter gene product displayed low immunogenicity. A membraneanchored form of human bG also allowed in vivo imaging, demonstrating that human bG can be employed for imaging. This imaging system therefore, displays good selectivity with low immunogenicity and may help assess the location, magnitude and duration of gene expression in living animals and humans.