Protected Zwitterionic Polymers for Delivery of Therapeutic Macromolecules

Abstract

Polymeric carriers are ubiquitous in biomedical engineering, especially for the delivery of drugs, proteins and ribonucleic acids (RNA). Both the encapsulation of biomacromolecules within polymeric drug carriers and their controlled release therefrom are equally important. Zwitterionic polymers are a promising class of polymers for controlled adsorption and desorption of macromolecules because their overall charge can be manipulated with transient protecting groups, allowing them to complex oppositely charged therapeutics. These zwitterionic polymers can then be deprotected to release the therapeutics. In this thesis, I used a variety of protecting groups to control the release of both proteins and RNA for biomedical applications. First, I used alkyl ester protecting groups to protect the pendant carboxylate groups on block copolymer poly(lactic acid)-block-poly(carboxybetaine) (PLA-b-PCB) to produce self-assembled cationic nanoparticles (NPs). The NPs lost positive charge at rates determined by the hydrolytic stability of alkyl ester protecting groups. Several anionic proteins were then adsorbed onto the charged NPs, loaded into a hydrogel depot, and their release rate was controlled based primarily on the hydrolysis of the ester protecting groups. This demonstrated that protected zwitterionic polymers could be used for temporal control of protein release. Next, I modified the PLA-b-PCB polymer to bear acid-responsive hemiacetal ester protecting groups such that the cationic to zwitterionic charge transition occurred faster in acidic conditions. These polymers were designed to induce RNA dissociation from the polymeric carrier in the acidic endosome after endocytosis. The polymers were mixed with lipids to produce hybrid polymer lipid nanoparticles (PLNPs) that encapsulated both siRNA and mRNA with high efficiency. The PLNPs increased RNA transfection efficiency over conventional lipid nanoparticles in vitro. The increased efficiency was lost when the acid-responsive polymers were replaced with acid-inert polymers, demonstrating that the acid-responsiveness was necessary for the effect. Thus, the synthesis of novel protected zwitterionic polymers and their applications in both temporal and stimulus-responsive controlled release of therapeutic macromolecules was demonstrated.