BIODEGRADABLE CATIONIC POLYCARBONATES WITH WELL-DEFINED MOLECULAR ARCHITECTURES FOR GENE DELIVERY

Abstract

Aliphatic polycarbonates are fast emerging as a highly valuable class of biomaterials due to their inherent biocompatibility, biodegradability, facile synthesis, and ease of tailoring functionalities to meet specific requirements of various applications. In the field of gene delivery, polycarbonates are less widely documented and there lies great potential and scope for novel research. The overall goal of this thesis is thus to design and evaluate a versatile class of biodegradable cationic polycarbonates and their copolymers for gene delivery. We hypothesized that the cationic polycarbonates with well-controlled molecular weights and narrow polydispersities imparted by metal-free living organocatalytic ring-opening polymerization of functionalized cyclic carbonate monomers, followed by a subsequent quaternization reaction with bis-tertiary amines, are safe and effective gene carriers.

To test our hypothesis, we explored three specific aims: (1) Rationally design and evaluate the gene transfection ability and cytotoxicity of novel cationic polycarbonates bearing quaternary and tertiary amines in the side chains. (2) Enhance the stability of DNA complexes through the design of block copolymers of poly(ethylene glycol) (PEG) and cationic polycarbonates; and investigate the effects of PEG configuration within block copolymers on key aspects of gene delivery. (3) Examine the efficacy of galactose-functionalized polycarbonate block copolymer for targeted gene delivery to hepatocytes.

In Aim (1), we described the rational design of cationic polycarbonates with key features such as a biodegradable backbone as well as quaternary and tertiary amines in the side chains for DNA binding and endosomal buffering, respectively. It was demonstrated that the cationic polycarbonate efficiently condensed DNA into positively charged nanoparticles that induced high luciferase gene expression that was comparable or superior to the commercially available branched 25 kDa polyethylenimine benchmark in a panel of mammalian cell lines with minimal cytotoxicities. In Aim (2), diblock PEG-b-cationic polycarbonate and triblock cationic polycarbonate-b-PEG-b-cationic polycarbonate block copolymers were designed to capitalize on the highly desirable in vivo properties of the PEG macroinitiator. We demonstrated for the first time that PEG configuration within block copolymers has an influence on the key aspects of gene delivery. DNA complexes formed with the triblock copolymer in particular, exhibited more favourable physicochemical (size, zeta-potential and colloidal stability) and biological properties (cellular uptake and gene transfection in HepG2) than that formed with the diblock copolymer and non-PEGylated control. In Aim (3), galactose-functionalized cationic polycarbonate block copolymer (Gal-APC) was designed to mediate active targeting of DNA to the asialoglycoprotein-receptor (ASGP-R) expressing hepatocytes. It was shown that the Gal-APC/DNA complexes induced significantly higher luciferase expression in the ASGP-R positive HepG2 than the corresponding glucose-functionalized control. Co-incubation with a natural ligand for the ASGP-R, asialofetuin resulted in a decrease in luciferase gene expression, suggesting an inhibition of ligand-receptor mediated endocytosis.

In conclusion, the findings of this thesis supported the hypothesis that amine-functionalized cationic polycarbonates and their copolymers are safe and efficacious non-viral vectors for gene delivery. Gal-APC is presented as a promising candidate for further preclinical evaluations to be used for the gene therapy of hepatocellular carcinoma and hepatitis virus infections.