Caged siRNAs with Single cRGD Modification for Photoregulation of Exogenous and Endogenous Gene Expression in Cells and Mice. 
RNA interference (RNAi) mediated gene silencing holds significant promise in gene therapy.  It is very important to manually regulate the activity of small interference RNAs (siRNAs) in the controllable mode.  Here, we designed and synthesized a series of caged siRNAs through bioconjugation of cyclo(Arg-Gly-Asp-d-Phe-Lys) (cRGD) peptide to the 5' end of siRNA through a photolabile linker.  These cRGD modified caged siRNAs allowed for precise light-regulation of gene expression of two exogenous reporter genes (firefly luciferase and green fluorescent protein, GFP) and an endogenous gene (the mitosis motor protein, Eg5) in the integrin αvβ3 positive cells.  This kind of bioconjugate further enabled photochemical activation of siRNA activity, and the target gene silencing was successfully achieved in tumor-bearing mice by intratumoral injection.  This study also suggested that photomodulation of target gene expression using single cRGD caged siRNA at the 5' end of antisense strand RNA inhibited siRNA activity probably due to three factors: (1) trapping of cRGD modified siRNA in endosome and lysosome, (2) the steric hindrance of cRGD, (3) the binding of cRGD to its corresponding receptor.