694. Breast Cancer Specificity, Anti-Tumor Activity, and Reduced Toxicity of an Adenoviral Vector Encoding Interleukin-2 under Control of the Human Mammaglobin Promoter

Abstract

Previous studies in our labs have shown that an adenoviral (Ad) vector encoding interleukin-2 (IL-2) under control of the CMV promoter induced tumor regression in an immunocompetent mouse tumor model, but this regression was accompanied by toxicity and mortality (Addison et al., Gene Therapy 5, 1400-09, 1998) . We hypothesized that this toxicity might partially result from inadvertant expression of IL-2 in normal tissues, particularly the liver, of treated animals. To test this hypothesis, we have investigated transcriptionally targeted Ad vectors as a means to localize IL-2 expression to mammary tumors. We have recently isolated a DNA fragment from upstream of the human mammaglobin-1 (MGB) gene that can direct mammary- or breast cancer-specific expression of reporter genes when delivered by helper-dependent adenoviral (Ad) vectors (Shi et al, Mol. Ther. 10, 758-767, 204). We demonstrate here that activity of the MGB enhancer/promoter is also highly specific when inserted into first generation Ad vectors. In human breast cancer cell lines, the Ad-delivered MGB enhancer/promoter was nearly as active as the murine CMV promoter in driving luciferase expression. Activity of the MGB enhancer/promoter was reduced by over 1000-fold in comparison to the mCMV promoter in normal human cell lines. A high degree of specificity of the MGB enhancer/promoter, in contrast to the mCMV promoter, was also demonstrated following intratumoral injection in a mouse tumor model. This high level of activity in combination with specificity has prompted us to examine first generation Ad vector delivery of MGB enhancer/promoter-controlled IL-2 as a potential cancer therapeutic. An Ad vector encoding IL2 under control of the MGB enhancer/promoter (AdMGB-IL2) induced high levels of IL2 secretion in human and murine breast cancer, but not normal, cell lines. Here we demonstrate that AdMGB-IL2 generated little, if any, liver toxicity (assessed by ALT, AST, and GGT levels as well as histology) even after intravenous delivery to immunocompetent mice. Toxic effects of the same dose of AdmCMV-IL2 were severe, and correlated with high levels of serum IL-2. Although significant tumor regression in this mouse tumor model was induced by treatment with AdMGB-IL2, much higher doses of this vector were required than with the AdCMV-IL2 vector. In conclusion, transcriptional targeting of IL-2 gene expression can be achieved using the MGB enhancer/promoter, however high doses of vector are required to achieve tumor regression in a mouse model for breast cancer.