Self-assembly of polyamine-poly(ethylene glycol) copolymers with phosphorothioate oligonucleotides

The cationic copolymers for DNA delivery were synthesized by conjugating poly(ethylene glycol) (PEG) and polyamines: polyspermine (PSP) and polyethyleneimine (PEI). These molecules spontaneously form electrostatic complexes with a model 24-mer phoshorothioate oligonucleotide, T24 (PS-ODN). The copolymer complexes are water soluble. This is a marked contrast with the complexes formed by nonmodified PSP and PEI, which immediately precipitate out of solution. The potentiometric titration study suggests that the amino groups of the copolymers form a cooperative system of salt bonds with the thiophosphate groups of the PS-ODN. The PEG-PEI complexes are stable at physiological pH and ionic strengths. The PEG-PSP complexes are less stable in the presence of the low molecular mass electrolytes compared to the PEG-PEI complexes. The dynamic light scattering and transmission electron microscopy demonstrate that the complex particles are small-ca. 12 nm for PEG-PSP and ca. 32 nm for PEG-PEI. They can be lyophilized and redissolved or stored in solution for up to several months without changing size. The study suggests that as a result of formulation with the PEG-PEI the interactions of PS-ODNs with serum proteins (using the example of bovine serum albumin) are decreased and PS-ODN is protected against nuclease degradation. The simplicity of preparation and long shelf life make these systems attractive as potential pharmaceutical formulations for oligonucleotides.