Efficient intracellular siRNA delivery by ethyleneimine-modified amphiphilic macromolecules

New materials that can bind and deliver oligonucleotides such as short interfering RNA (siRNA) without toxicity are greatly needed to fulfill the promise of therapeutic gene silencing. Amphiphilic macromolecules (AMs) were functionalized with linear ethyleneimines to create cationic AMs capable of complexing with siRNA. Structurally, the parent AM is formed from a mucic acid backbone whose tetra-hydroxy groups are alkylated with 12-carbon aliphatic chains to form the hydrophobic component of the macromolecule. This alkylated mucic acid is then mono-functionalized with poly(ethylene glycol) (PEG) as a hydrophilic component. The resulting AM contains a free carboxylic acid within the hydrophobic domain. In this work, linear ethyleneimines were conjugated to the free carboxylic acid to produce an AM with one primary amine (1N) or one primary amine and four secondary amines (5N). Further, an AM with amine substitution both to the free carboxylic acid in the hydrophobic domain and also to the adjacent PEG was synthesized to produce a polymer with one primary amine and eight secondary amines (9N), four located on each side of the AM hydrophobic domain. All amine-functionalized AMs formed nanoscale micelles but only the 5N and 9N AMs had cationic zeta potentials, which increased with increasing number of amines. All AMs exhibited less inherent cytotoxicity than linear polyethyleneimine (L-PEI) at concentrations of 10 μM and above. By increasing the length of the cationic ethyleneimine chain and the total number of amines, successful siRNA complexation and cellular siRNA delivery was achieved in a malignant glioma cell line. In addition, siRNA-induced silencing of firefly luciferase was observed using complexes of siRNA with the 9N AM and comparable to L-PEI, yet showed better cell viability at higher concentrations (above 10 μM). This work highlights the promise of cationic AMs as safe and efficient synthetic vectors for siRNA delivery. Specifically, a novel polymer (9N) was identified for efficient siRNA delivery to cancer cells and will be further evaluated.