Biodegradable branched polycationic polymers as non-viral gene delivery vectors for bone tissue engineering
Chew, Sue Anne
Mikos, Antonios G.
Doctor of Philosophy
In this work, biodegradable branched triacrylate/amine polycationic polymers (TAPPs) were synthesized from different amine and triacrylate monomers by Michael addition polymerization and incorporated into a composite scaffold to evaluate these polymers in a bone tissue engineering system. The effects of the hydrophilic spacer lengths in the polymer on characteristics which are important for gene delivery were evaluated by varying the triacrylate monomer used in the synthesis. The results demonstrated that hydrophilic spacers can be incorporated into polycationic polymers to reduce their cytotoxicity and enhance the degradability. The effects of amine basicities in the polymer on characteristics which are important for gene delivery were also evaluated by varying the amine monomers used in the synthesis. The results indicated that polycationic polymers with amines that dissociate above pH 7.4, which are available as positively charged groups for plasmid DNA (pDNA) complexation at pH 7.4, can be synthesized to produce stable polyplexes with increased zeta potential and decreased hydrodynamic size that efficiently transfect cells. TAPP/pDNA polyplexes were then incorporated into a composite containing gelatin microparticles (GMPs) and a porous poly(propylene fumarate) scaffold. The release of pDNA in vitro was not affected by the crosslinking density of the GMPs but depended, instead, on the degradation rates of the TAPPs. Besides the initial burst release of polyplexes not bounded to the GMPs and the minimal release of pDNA through diffusion and dissociation from the GMPs, the pDNA was likely released as naked pDNA or in an incomplete polyplex as fragments of the polymer had to degrade to release the pDNA. The results indicated that polymeric vectors with a lower degradation rate can prolong the release of pDNA from the composite scaffold. Composite scaffolds loaded with TAPP/pDNA polyplexes may not have delivered enough intact polyplexes, as enhanced bone formation was not observed in a critical-size rat cranial defect at 12 weeks postimplantation compared to those loaded with naked pDNA. A gene delivery system consisting of biodegradable polycationic polymers should be designed to release the pDNA in an intact polyplex form.
Polymer chemistry; Biomedical engineering