Influence of polymer architecture on the structure of complexes formed by PEG-tertiary amine methacrylate copolymers and phosphorothioate oligonucleotide.

The influence of polymer structure on the characteristics of complexes of a phosphorothioate antisense oligonucleotide (ISIS 5132) was studied, using well-defined cationic copolymers based on 2-(dimethylamino) ethyl methacrylate (DMAEMA) and poly(ethylene glycol) (PEG). The three related copolymer structures were: DMAEMA-PEG (a diblock copolymer) DMAEMA-OEGMA 7 (a brush-type copolymer), DMAEMA-stat-PEGMA (a comb-type copolymer); each of these were examined together with DMAEMA homopolymer, which served as a control. The results revealed that all the polymers exhibited good binding ability with the oligonucleotide (ON). Interestingly, the comb-type polymer DMAEMA-stat-PEGMA demonstrated the highest binding ability and DMAEMA homopolymer the lowest, as judged by a dye displacement assay. DMAEMA homopolymer produced large agglomerates of smaller individual complexes as observed by optical density, photon correlation spectroscopy and transmission electron microscopy studies. In contrast, two PEG-block copolymers, DMAEMA-PEG and DMAEMA-OEGMA 7, formed compact complexes of 80-150 nm which had good long-term colloidal stability. This is attributed to the steric stabilisation effect of the PEG chains on the ON-copolymer complexes. These two copolymers are believed to form complexes with ON that have a micellar structure. Comb-type DMAEMA-stat-PEGMA copolymer formed highly soluble complexes with the ON that did not phase separate from the buffer solution. This study clearly demonstrates that varying the copolymer architecture allows access to a range of ON complexes. In vitro cytotoxicity experiments on HepG2 cells showed that all of the tertiary amine methacrylate copolymers displayed lower cytotoxicity than the control poly(L-lysine).