Bifunctional siRNAs for tumor therapy.

Double-stranded RNA molecules carrying a triphosphate moiety represent a molecular structure by which the host recognizes viral infections. Such RNA molecules can be generated synthetically by chemical synthesis or by in vitro transcription (see Chapter 2, Hornung et al.). Similar to viruses, they initiate an antiviral immune response, e.g., by stimulation of the immune system. Short, double-stranded RNA in the cytosol can also trigger the RNA interference mechanism, which also has been considered as an antiviral response. Notably, synthetic RNAs that are designed to be specific for a certain host mRNA inhibit expression of the respective gene, leading to specific gene silencing. Both effects-gene silencing and immunostimulation-are interesting from a therapeutic perspective, e.g., for cancer therapy. Notably, both effects can be activated by a single molecule, an siRNA carrying a triphosphate moiety. This chapter provides information how to design such compounds with respect to the associated signaling pathways and the techniques to evaluate bifunctional RNAs in the context of tumor therapy.