Self-Assembled Hydrogels Based on B-Cyclodextrin/Cholesterol Inclusion Complexes : Properties and Pharmaceutical Applications

Abstract

Hydrogels, i.e. hydrophilic polymer networks that are capable of absorbing considerable quantities of water, are applied in a wide variety of biomedical and pharmaceutical applications, including soft contact lenses, drug delivery depots and tissue engineering scaffolds. Their high water content gives hydrogels a rubbery appearance, which minimizes irritation of surrounding tissue, and creates a natural environment for many encapsulated drugs (e.g. protein pharmaceuticals). In aqueous environment, hydrogels are held together by crosslinks, which are based on either covalent bonds or physical interactions between the hydrophilic polymers. The introduction of covalent bonds between the hydrophilic polymer chains requires chemical crosslinking reactions that might potentially affect the structure and biological activity of encapsulated pharmaceuticals. Furthermore, these chemical reactions often require crosslinking reagents or catalysts that are toxic towards cells. Because of these drawbacks, the use of physical crosslinks for the design of hydrogel systems is preferred. In such hydrogels the network is held together by non-permanent, reversible interactions between the polymer chains, such as ionic interactions, hydrophobic interactions, hydrogen bonds and specific biomimetic interactions. In the thesis of Frank van de Manakker, the synthesis and characterization of novel self-assembled hydrogels is described in which physical crosslinking is established by host-guest inclusion complexes between the cyclic oligosaccharide beta-cyclodextrin (betaCD) and the complementary guest molecule cholesterol. Hydrogel building blocks were prepared by end-modification of 8-arm star shaped poly(ethylene glycol) (PEG8) with either betaCD or cholesterol moieties via a hydrolytically cleavable succinyl linker. Mixing the resulting cholesterol- and betaCD-derivatized PEG8-molecules in aqueous solution led to hydrogel formation. Important gel properties, i.e. mechanical properties, gel degradation and protein release kinetics, could be tailored by a broad range of parameters, including the polymer concentration, B-?CD/cholesterol stoichiometry, PEG’s molecular weights and/or architecture, or by adding molecules that form competing inclusion complexes, e.g. adamantanecarboxylic acid and monoamino-derivatized betaCD. The physical nature of the gels did not only offer extra tools to manipulate gel properties, but also rendered the gels responsive towards external stimuli, such as temperature and mechanical stresses, which offered the opportunity to use the hydrogels as injectable, in situ gelling devices. When aqueous media (e.g. buffer and serum) were added on top of the self-assembled gels, hydrogel degradation was primarily mediated by surface erosion of dissociated PEG8 derivatives. This degradation mechanism, which is hardly observed for other hydrogel systems, also controlled protein release from the gels, which occurred at a constant rate and was nearly independent on protein size. Although at this stage, the in vivo stability of the hydrogels requires further improvement, the combination of tunable properties, high gel strengths (compared to other physically crosslinked hydrogels), the unique protein release mechanism, and easy preparation from biocompatible and well-available building blocks make these self-assembled, PEG8-based hydrogels attractive candidates for many pharmaceutical and biomedical applications, such as injectable devices for the delivery of protein pharmaceuticals.