Introduction of a novel strategy in virotherapy for breast cancer

Background: In view of the limited success of available treatment modalities for a wide array of cancer such as metastatic breast cancer, alternative and complementary therapeutic strategies need to be developed. Virotherapy employing conditionally replicative adenoviruses (CRAds) based on the adenovirus serotyp 5 (Ad5) represents a promising targeted intervention relevant to a wide array of neoplastic diseases. Critical to the realization of an acceptable therapeutic index using virotherapy in clinical trials is the achievement of oncolytic replication in tumor cells, while avoiding non-specific replication in normal tissues. In this report, we exploited cancer-specific control of mRNA translation initiation in order to achieve enhanced replicative specificity of CRAd virotherapy agents. We aimed specifically to reduce background replication activity in non-cancer cells by introducing three different levels of cancer targeting for the Ad5 E1A gene expression which is essential for Ad replication. We constructed a CRAd with three levels of targeting; combining the novel E1A mRNA translation control with Ad5 infectivity enhancement and E1A gene transcription control that selectively targets breast cancer cells. Heretofore, the achievement of replicative specificity of CRAd agents has been accomplished either by viral genome deletions or incorporation of tumor selective promoters. We show herein, the utility of a novel approach that combines three strategies: transcriptional, transductional and translational regulation strategies for the key goal of replicative specificity. Methods: We describe the construction of a CRAd with cancer specific gene transcriptional control using the CXCR4 gene promoter (TSP) and cancer specific mRNA translational control using a 5'-untranslated region (5'-UTR) element from the FGF-2 (Fibroblast Growth Factor-2) mRNA and transductional control using the serotype 3 knob vector modifications. Results: In vitro and initial in vivo studies demonstrated that our CRAd agent retains anti-tumor potency for breast cancer. Importantly, assessment of replicative specificity demonstrated significant improvement in tumor selectivity. Conclusions: Our study addresses a conceptually new paradigm: Triple level targeting of transgene expression to cancer cells. Our novel approach addresses the key issue of replicative specificity and can potentially be generalized to a wide array of tumor types, whereby tumor selective patterns of gene expression and mRNA translational control can be exploited.