Self-assembled materials from thermosensitive and biohybrid block copolymers

Abstract

In this research, several block copolymers were synthesized and characterized with regard to possible pharmaceutical applications. All block copolymers were thermosensitive and self-assembled at 37 °C into structures like micelles and hydrogels, which can be used for innovative drug delivery purposes. Some of the synthesized polymers were biohybrid, in the sense that they contained peptide segments which enabled their cleavage by enzymes that are upregulated in diseased tissues. First, methods were explored for the synthesis of polymer (bio)conjugates. One suitable and popular reaction for synthesizing such conjugates is the copper-catalyzed azide-alkyne cycloaddition ‘click’ reaction. A novel method was developed to introduce a reactive azide functionality on polymers after their synthesis by Atom Transfer Radical Polymerization (ATRP). The newly developed reaction is catalyzed by the ATRP copper catalyst and can be followed by a ‘click’ conjugation reaction using again the same catalyst. Another, relatively new, method to synthesize polymer bioconjugates is to grow a polymer from a peptide or protein. In this research, a peptide containing an enzymatic cleavage site for the tumor-associated matrix metalloproteases (MMPs) 2 and 9 was functionalized at its C-terminus with an ATRP initiator, followed by the polymerization of a hydrophilic polymer block, poly(oligo(ethylene glycol) methyl ether metharcrylate).Then, another ATRP initiator was conjugated to the N-terminus of the peptide followed by the polymerization of a thermosensitive block, poly(N-isopropylacrylamide). At 37 °C in water, all polymers formed micelles with a favorable size for drug delivery purposes. It was shown that the coronas of the micelles could be cleaved off enzymatically. This system may function as a long-circulating drug delivery vehicle with enzyme-triggered destabilization in the target tissue to release e.g. a cytotoxic payload. Subsequently, a study of the physico-chemical differences between ‘star-like’ micelles and ‘flower-like micelles’ was performed. Star-like micelles are formed from amphiphilic diblock copolymers, whereas flower-like micelles are formed from triblock copolymers that consist of two hydrophobic blocks separated by a hydrophilic block. Both Static Light Scattering and 1H NMR T2 relaxation measurements indicated that hydrophilic polymer blocks in flower-like micelles are less flexible than in star-like micelles. The last part of this research focused on a particular property of flower-like micelles, namely the fact that at high concentration they are in equilibrium with a hydrogel. When a thermosensitive triblock copolymer hydrogel was brought into water, flower-like micelles were released. When the hydrogel was pre-loaded with the cytostatic drug paclitaxel, this was co-released with the micelles. The system was tested in a tumor xenograft mouse model: paclitaxel-loaded hydrogels were injected i.p., after which paclitaxel was slowly released and could be measured in the circulation for two days. Furthermore, the released paclitaxel was able to inhibit tumor growth for at least three weeks