Synthetic polyspermine imidazole-4, 5-amide as an efficient and cytotoxicity-free gene delivery system

A chemically dynamic spermine-based polymer: polyspermine imidazole-4, 5-amide (PSIA, Mw > 7 kDa) was designed, synthesized, and evaluated in terms of its ability to deliver nucleic acids. This polymer was made from an endogenous monomer professionally condensing genes in sperms, spermine, and a known safety drug metabolite, imidazole-4, 5-dicarboxylic acid, through a bis-amide bond conjugated with the imidazole ring. This polymer can condense pDNA at a W/W ratio above 10 to form polyplexes (100–200 nm in diameter), which is consistent with the observation by transmission electron microscopy (TEM), and the zeta potential was in the range of 10–20 mV. The pDNA packaged polymer was stable in phosphate buffer solution (PBS) at pH 7.4 (simulated body fluid) while the polyplexes were releasing pDNA into the solution at pH 5.8 (simulated endo-lysosomes) due to the degradation of the bis-amide linkages in response to changes in pH values. PSIA-polyplexes were able to achieve efficient cellular uptake and luciferase gene silencing by co-transfection of pDNA and siRNA in COS-7 cells and HepG2 cells with negligible cytotoxicity. Biodistribution of Rhodamine B-labeled PSIA-polyplexes after being systemically injected in BALB/c nude-mice showed that the polyplexes circulated throughout the body, accumulated mainly in the kidney at 4 hours of sample administration, and moved to the liver and spleen after 24 hours. All the results suggested that PSIA offered a promising example to balance the transfection efficiency and toxicity of a synthetic carrier system for the delivery of therapeutic nucleic acids.