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Amanda Engler Chemical Engineering Grad Student |
Functionalized Graft and Block Polymers for Drug and Gene Delivery
Synthetic methods:
Synthetic polypeptides have attracted significant interest in the areas of polymer drug and gene delivery systems and tissue engineering. Similar to natural polypeptides, these polymers possess the ability to assemble into well defined, ordered structures. They also have several additional features that make them very attractive for biological applications including low toxicity, biodegradability, tunable structures, and well-controlled dimensions. Synthetic homo polypeptides are synthesized using a well-studied N-carboxyanhydride (NCA) ring-opening polymerization (ROP) which utilizes a wide variety of monomers containing various functional side groups. In particular, carboxylic acid and amino groups have been used to add chemical moieties such as pharmaceuticals, small molecules that dictate hydrophobicity or pH responsiveness, and loosely grafted polymers. Functionalization of polypeptides synthesized by NCA ROP has several limitations: (1) when creating polypeptides with functional carboxylic acid or amino groups, protection and deprotection steps are necessary before functionalizing the polymer; and (2) the addition of polymeric side chains using a “grafting onto” method at a high grafting density is not attainable. To overcome the limitations of NCA ROP, a new polymer (Poly(Propargyl-L-glutamate (PPLG)) which contain an alkyne group that is available for “click chemistry” has been developed. “Click chemistry” refers to a series of highly efficient reactions including, the 1,3-dipolar cycloaddition reaction between an alkyne and an azide to form a triazole.1 These reactions are advantageous because they have a high reaction efficiency, mild reaction conditions, functional group tolerance, and few byproducts.
Applications:
Tissue Engineering
A cell’s extracellular matrix consists of macromolecules, such as glycolproteins, proteoglycans, and collagen, which control both the cell’s mechanical structure and their microenvironment. Glycoproteins are loosely grafted brush polymers that contain short oligosaccharide side chains that can contain 1-60 wt% carbohydrates. Proteoglycans contain polypeptides that are heavily grafted with unbranched glycosaminoglycan chains, typically 80 sugars long and contain as much as 95 wt% carbohydrates. One of the challenges of tissue engineering is to mimic these complex molecules. We have used the new polymer, PPLG and click chemistry to demonstrate that we can synthesize densely grafted polypeptides. We used a model system, PPLG-g-PEG, to demonstrate that a grafting efficiency of over 96% can be obtained.2
Gene Delivery
One of the key challenges of the systemic delivery of siRNA is the ability to effectively complex the small charged oligonucleotides. Linear and branched cationic polymers are often selected for complexation because of their ability to bind and condense nucleic acids and stimulate endosomal escape. However, some of these polymers, such as poly(ethyleneimine), have limited success due to their high toxicity. We have functionalized polypeptides with various amine groups for siRNA delivery. For the amine-functionalized polymers, we have investigated the effects of pH on the secondary structure of the polymer as well as the polymer hydrophobicity. We have also performed studies to confirm that these polymers are able to complex with siRNA.
1-) Kolb, H. C.; Finn, M. G.; Sharpless, K. B., Click chemistry: Diverse chemical function from a few good reactions. Angewandte Chemie-International Edition 2001, 40, 2004
2-) Engler, A. C.; Lee, H. L.; Hammond, P. T., Highly Efficient "Grafting onto" of Polypeptides. Angewandte Chemie International Edition 2009, in press.
