Using biochemical, imaging and histological methods, we employed transcriptional targeting to increase the specificity of tumor gene expression in vivo for intravenously administered recombinant adenovirus vectors. Surprisingly, the relative specificity of tumor expression in comparison with other tissues was increased for a constitutively expressing recombinant adenovirus, AdCMVLuc, by simply reducing the viral dose. Even at lower doses, however, the high frequency of viral infection and transgene expression in the liver using constitutive promoters still represents a substantial problem. To further augment tumor specificity, we constructed a series of adenoviruses expressing luciferase from several other promoters and tested their ability to selectively transcribe genes in tumor cells, both in vitro and in vivo. Constitutively active viral promoters (RSV, SRα) varied widely in their tumor selectivity, but hypoxia-responsive promoters (carbonic anhydrase 9, PAI-1, SOD2 and several chimeric constructs) showed the most tumor-selective expression. Our results show that tumor targeting to HT1080 fibrosarcomas was readily achieved using transcriptional targeting mechanisms. We attribute the relatively high level of gene transfer and expression in HT1080 tumors in vivo to increased viral access to the tumor, presumably due to discontinuities in tumor vasculature and augmented expression from stress-responsive promoters in the hypoxic and inflammatory tumor microenvironment.
