Dextran-based microspheres as controlled delivery systems for proteins

Abstract

Dextran-based microspheres as controlled delivery systems for proteins Dextran based microspheres are investigated as controlled delivery system for proteins. Microspheres were prepared by polymerization of dex-HEMA in an aqueous two-phase system of dex-HEMA and PEG. Protein loaded microspheres are obtained by addition of proteins prior to polymerization. Due to the presence of hydrolytically sensitive carbonate ester groups linking HEMA to dextran, these gels degrade under physiological conditions gradually releasing the protein. In this thesis, several important aspects in the development of these dex-HEMA based microspheres as controlled delivery system of protein pharmaceuticals were investigated: (1) The in vitro in vivo correlation of a hydroxy ethyl methacrylated dextran (dex-HEMA) microsphere formulation. Therefore a controlled release formulation for human growth hormone (hGH) based dex-HEMA microsphere was developed and tested in preclinical and clinical studies. The results of those studies revealed that a strong correlation between in vitro release and pharmacodynamic or pharmacokinetic effects was obtained. (2) To obtain a a fundamental understanding, to optimize and to predict the release of proteins from dex-HEMA microspheres, a mathematical model was developed describing the mechanisms of protein release. The general effects of diffusion, crosslink density, protein loading, and clustering of proteins on the release were investigated. It was found that quantitative predictions of the release curves of BSA dimer, immunoglobulin G and hGH were possible. Therefore, this model may play an important role in the optimization, understanding and prediction of the release of various proteins from degradable hydrogels in general. (3) The effects of the initiator concentration, reaction temperature and pH on the mechanical properties of the microspheres were investigated. The results showed that higher initiator concentrations and a lower polymerization temperature resulted in a more rapid polymerization and a higher Young's modulus of the gels. This demonstrates that the network properties of dex-HEMA hydrogels and microspheres can be tailored by the polymerization conditions, which opens the possibility to modulate the release rate of entrapped compounds. (4) The possibility was investigated to modulate the encapsulation efficiency and release hGH-loaded dex-HEMA microspheres by using excipients. A biphasic release profile with delay time was observed for microspheres prepared without excipients, release profile without delay time was obtained for microspheres prepared with excipients. It was demonstrated that smaller hGH precipitates were formed in the presence of excipients, which explains the difference in protein release profiles of microspheres prepared with and without these additives. (5) The in vitro degradation of dex-HEMA microspheres was investigated by FT-IR, NMR, mass spectrometry, SEC analysis and XPS (X-ray photoelectron spectroscopy). It was found that in vitro degradation of dex-HEMA microspheres results in the formation of water-soluble degradation products (mainly dextran), leaving a small water-insoluble residue mainly consisting of pHEMA. (6) A series of dextrans with hydrophilic polymerizable groups were synthesized to investigate whether hydrogels based on these derivatized dextrans degrade under physiological conditions into fully water-soluble degradation products. It was demonstrated that the degradation of hydrogels based on the degradation of both macrogels and microspheres based on dex-(lac1,2-)HPMA resulted in the formation of only water-soluble degradation products. In conclusion, this thesis demonstrates that dextran based microspheres are attractive systems for the controlled release of pharmaceutically active proteins such as hGH.