In this approach, inhibition of gene expression is accomplished by administration of a targeted oligonucleotide that reduces transcription of complementary messenger ribonucleic acid. More recently, plasmid- or viral vector-mediated transfer of small interfering RNA or short hairpin RNA (shRNA) precursors has been employed in animal models. Inhibition of the Akt signaling pathway by aerosol delivery of a lung cancer cell-target shRNA has been reported to suppress lung tumorigenesis. However, it is difficult to achieve adequate concentrations of these molecules within tumors, and their efficacy is constrained by limited bystander effects on nontrans- duced cells within the tumor (Vachani et al. 2011).
The innate ability of bacteria to invade cells has been employed to efficiently deliver DNA. Delivery of genetic material by bacteria is achieved by either of the two approaches: (1) specific replication of bacteria within tumors or (2) intracellular transfer of plasmids within cancer cells (bactofection) by various bacterial species, such as Salmonella, Escherichia coli, and Listeria species. Anaerobic and facultatively anaerobic bacteria are employed to specifically target the relative hypoxic environment within tumors. Bacteria also show chemotaxis to necrotic regions within tumors, and aberrant “leaky” vasculature and local immune suppression may also facilitate bacterial entry and colonization within tumors. Several bacterial species (Bifidobacterium, Salmonella, E. coli, Vibrio cholerae and Listeria monocytogenes) are capable of tumor-specific growth after IV administration, and they transport and amplify genes within tumors (Baban et al. 2010). However, the inhalation of bacteria has not been employed for DNA delivery and the potential for environmental spread of the vector will need to be carefully addressed if such a delivery mechanism is employed in clinical practice.