Design of hydrogels for delayed antibody release utilizing hydrophobic association and Diels-Alder chemistry in tandem

Biodegradable hydrogels were prepared from furan-and maleimide-functionalized eight-armed poly-(ethylene glycol) with an average molecular mass of 40000 Da (8armPEG40k-furan and 8armPEG40k-maleimide) using the Diels-Alder (DA) reaction as a cross-linking mechanism. Hydrophobic 6-aminohexanoic acid (C-6) and 12-aminododecanoic acid (C-12) spacers were introduced between the polymer backbone and the functional end-groups; the influence on the gel properties was studied. Modification with C-6 and C-12 spacers induced hydrophobic interactions between the macromonomers leading to association and increased viscosity of the polymer solutions; both effects were influenced by the spacer length. In combination with DA cross-linking, hydrophobic derivatives of 8armPEG40k-furan and 8armPEG40k-maleimide led to hydrogels with improved properties. Upon introduction of C-12 spacers, gelation of 8armPEG40k hydrogels occurred twice as fast. Interestingly, no effect was observed when only one of the two components had been modified. Our experiments suggest that the association of macromonomers by hydrophobic interactions facilitates chemical cross-linking via DA chemistry. This hypothesis is supported by calculations of the network mesh size and the Young's modulus of compression, which showed an increased cross-linking density of hydrophobically modified hydrogels. As a consequence of the increased cross-linking density, the degradation stability of C-12-modified hydrogels increased by a factor of 4. Moreover, hydrophobic modification improved the hydrolytic resistance of maleimides; this also contributes to gel stability. The in vitro release of bevacizumab, which served as a model antibody, could be delayed for almost 60 days using modification with C-12. Similar trends were observed for C-6-modified 8armPEG40k hydrogels; however, the effects were considerably weaker. In summary, utilizing hydrophobic association and chemical cross-linking in tandem is a promising approach to create biodegradable hydrogels for delayed antibody release.