New Biodegradable Peptide-based Polymer Constructs

Abstract

Peptide-based polymers are of increasing interest, since they can be applied for a variety of purposes such as drug delivery devices, scaffolds for tissue engineering and -repair, and as novel biomaterials. Peptide-based polymers are common in nature and often exhibit special characteristics. However, when it is very difficult and expensive to harvest peptide-based biopolymers in large quantities from nature, and when it is desired to introduce modifications to modulate their properties, the development of synthetic methods to obtain such peptide-based polymers is necessary. In the ideal situation, the polymerization reaction should be performed with unprotected amino acids or oligopeptides, in high yield and under mild conditions. Until now, no polymerisation method has been found that can meet these requirements. The reaction between acetylenes and organic azides catalyzed by copper (I) yields 1,4-disubstituted 1,2,3-triazoles. The possibility of chemoselective copper(I)-catalyzed alkyne-azide cycloaddition (CuAAC) reaction in the presence of other (unprotected) functional groups and the geometrical similarities between peptide amides and 1,2,3-triazoles was the rationale to explore the CuAAC reaction for the synthesis of peptide-based polymers. In this research, terminal acetylene- and azide-substituted model peptides were used. It was found that these peptides could be converted into high molecular weight amino acid-based polymers (up to 45,000 Da) by a microwave-assisted CuAAC reaction. The optimal reaction conditions were used to synthesize peptide-triazole-based polymers that have sites for enzymatic degradation as well as for chemical hydrolysis. It was demonstrated that the synthesized polymers were sensitive to both enzymatic- and chemical hydrolysis. Depending on the reaction conditions, it was found that the outcome of the CuAAC reaction could be directed either to large linear polymers (up to 300 amino acid residues) or toward medium-sized peptide macrocycles (4 to 20 amino acid residues). Such cyclic oligomers were used as building blocks in a self-assembly process to obtain supramolecular polymers. Furthermore, the CuAAC reaction was used to synthesize enzymatically degradable PEG-based hydrogels. It was shown that the rheological properties of the hydrogels could be tailored by varying the reaction conditions and that the hydrogels could be degraded by enzymatic degradation by the protease trypsin. Finally, LCST (lower critical solution temperature) polymers based on the esters of serine and threonine monomers were synthesized by free radical polymerization. LCST polymers are soluble in water below the LCST and become insoluble when the temperature is raised above the LCST. It was proven that the LCST of the polymers could be tailored by varying the ester moiety and/or the amino acid itself. The esters could be degraded by chemical hydrolysis, which resulted in an increase in the LCST of the polymer. By synthesizing a block-copolymer with PEG, thermoresponsive nanoparticles were created. These particles destabilized upon hydrolysis of the ester bonds. The chemically and enzymatically degradable peptide-based polymers, hydrogels and nanoparticles presented in this thesis are interesting candidates for further explorations of the pharmaceutical and biomedical applications.