444. A New Vector Which Targets Breast Cancer and Its Vasculature
Authors: Yanzheng Liu, Tao Ye, Jonathan Maynard, Hakan Akbulut, Albert Deisseroth
Journal: Molecular Therapy (2005)

20% of women diagnosed with breast cancer will succumb to their disease due to resistance of their cancer cells to chemotherapy and hormonal therapy. One of the reasons for resistance to existing chemotherapy is that 70% of breast cancer cells are non-dividing and are therefore relatively resistant to the best chemotherapy for breast cancer. In order to overcome this problem of resistance of non-dividing breast cancer cells to chemotherapy, our laboratory designed the AdCDIRESE1A vector, which carries a chimeric transcription unit composed of the cytosine deaminase gene (CD) linked to the viral E1A proliferation gene. 5-Fluorocytosine (5FC) is converted into 5-Fluorouracil (5FU) by the bacterial gene cytosine deaminase (CD), which is produced in the vector infected cells. The levels of 5FU generated in vector infected cells are so high that even non dividing cells will die due to disruption of protein synthesis. Previous data from our laboratory had shown that the intratumoral injection of this AdCDIRESE1A vector followed by 10 days of ip 5FC induced regressions of existing sc breast cancer nodules. The E1A is an adenoviral gene necessary for replication. We then constructed a conditionally replication-competent adenoviral vector Ad.Lp-CD-IRES-E1A(control) in which the expression of both the prodrug activating cytosine deaminase gene and the viral replication E1A gene were driven by the L-plastin tumor specific promoter. As reported in this abstract this vector has been further modified by simultaneously ablating the native tropism of the vector to the primary CAR receptor and inserting a RGD-4C peptide into the HI loop of the fiber protein, which allows the vector to use the |[alpha]|v|[beta]|3 and |[alpha]|v|[beta]|5 integrins as alternative receptors. The |[alpha]|v|[beta]|3 integrin receptor is increased in breast cancer cells and in dividing endothelial cells. Dividing endothelial cells are increased (15%) in tumor vasculature as opposed to vasculature in normal tissues (0.5%). The resulting vector was named Ad.Lp-CD-IRES-E1A(MRGD). The transduction efficiency of the vector for breast cancer cell lines which have low expression level of CAR was increased both in vitro and in vivo. The Ad.Lp-CD-IRES-E1A(MRGD) vector produces a higher vector particle yield and a greater cytotoxic effect in tumor cells which have a low expression level of CAR, than did the Ad.Lp-CD-IRES-E1A(control) vector. Intratumoral injection of the Ad.Lp-CD-IRES-E1A(MRGD) vector following the intraperitoneal injection of 5FC into xenotransplanted human breast cancer cell lines led to tumor regression in vivo. The inactivation of the CAR receptor and the addition of the RGD will increase the safety of this vector in vivo, and will decrease the wasteful binding of the vector to non-neoplastic cells. In addition, the altered tropism will circumvent neutralizing antibodies, and will target this vector directly to luminal endothelial cells. Thus, since this vector does not need to cross over from the intravascular to the extravascular space, it is possible that it will be useful clinically.