A Universal Protein Tag for Delivery of SiRNA-Aptamer Chimeras

Liu, Hong Yan; Gao, Xiaohu

To add endosomal escape functionality, a short histidine (His) oligomer is added to the C-terminus of the dsRBD. His has been incorporated into a number gene carriers because its endosomal buffering capacity promoting drug cytoplasmic release 28, 29 . His molecules have a pKa value of approximately 6. At neutral pH (such as in circulation), they are mainly deprotonated (uncharged), which is desirable over positively charged counterparts due to reduced accumulation within the RES (reticuloendothelial system). In acidic compartments such as endosome, His becomes protonated and facilitates osmotic swelling that leads to cargo release, a mechanism proposed as the proton sponge effect 30 . Overall, this protein tag is equally small, simple, and biodegradable as siRNA-aptamer chimera, while perfectly complementing chimera's functionalities. When complexed together, they remain small in size, discrete and stable in solution, low positive charge for circulation, and simultaneously achieve therapeutic, targeting, and endosomal escaping capabilities.