A transfectant mosaic xenograft model for evaluation of targeted radiotherapy in combination with gene therapy in vivo

For gene therapy to be efficacious in the treatment of cancer, therapeutic transgenes must be limited in their expression to tumor cells and must be expressed at sufficiently high transcriptional levels. Moreover, the inadequacy of gene delivery must be overcome by induction of toxicity to neighboring nontargeted cells. Combining targeted radionuclide therapy with gene therapy using human telomerase promoters has shown promise in these respects, and the efficacy of this scheme has been assessed in vitro using transfectant mosaic tumor spheroids. To enable the evaluation of targeted radiotherapy combined with gene transfer in vivo, we have developed a transfectant mosaic xenograft (TMX) model. Methods: Human telomerase promoters were used to drive expression of the noradrenaline transporter (NAT) transgene in 2 human cell lines (UVW and EJ138). Promoter activity was assessed in xenografts in nude mice by determination of the uptake of the radiopharmaceutical ¹³¹I-metaiodobenzylguanidine (¹³¹I-MIBG) and by measurement of tumor growth. The efficacy of ¹³¹I-MIBG treatment was also assessed in TMXs to determine the delay in growth of tumors composed of various proportions of NAT-expressing cells - a likely clinical scenario after gene delivery in vivo. Results: In terms of induction of the capacity for active uptake of ¹³¹I-MIBG and the resultant inhibition of tumor growth in vivo, both telomerase promoters (hTR and hTERT) were similar in potency to the CMV (cytomegalovirus) promoter as controlling elements for the expression of the NAT transgene. In TMXs derived from UVW and EJ138 cells, ¹³¹I-MIBG uptake was proportional to NAT gene expression (r_s = 0.910, P < 0.001 for UVW; r_s = 0.971, P < 0.001 for EJ138). Inhibition of the growth of these tumors correlated with the fraction of NAT-transfected cells (r_s = 0.910, P < 0.001 for UVW; r_s = 0.971, P < 0.001 for EJ138), and substantial tumor growth delay was observed when 5% of the xenograft was composed of NAT-positive cells. Conclusion: TMXs constitute a suitable model to measure the efficacy of cancer gene therapy strategies when <100% of the tumor mass can be targeted to express the therapeutic transgene.