Microspheres based on biodegradable functionalized poly(alpha-hydroxy) acids for the controlled release of bioactive proteins and peptides

Abstract

Pharmaceutical peptides/proteins have proven to be potent molecules for the treatment of a great variety of chronic and life threatening diseases. These molecules however demand a suitable formulation for their successful delivery. For formulation and delivery of such molecules the parenteral route is the preferred one, as other routes of administration (e.g. oral, rectal, lung) have been shown to be inefficient. Another characteristic of many peptides and proteins is that they have a short plasma and tissue half live. This leads to frequent injection schemes to maintain a therapeutic drug level. Consequently, sustained release systems would lead to less frequent injections and an improved pharmacokinetic profile as the blood and tissue levels will show fewer fluctuations over time. The development of a new family of biodegradable polymer-based sustained release systems for injection is the subject of this thesis. The encapsulation and release of macromolecular hydrophilic Dextran Blue as well as peptide/proteins (e.g. lysozyme, BSA and octreotide) from poly(lactic-co-hydroxymethyl glycolic acid) (PLHMGA) microspheres was investigated and results were compared to the data obtained for PLGA microspheres. Microspheres can be successfully made from PLHMGA, by the double emulsion extraction-evaporation method. The in vitro release study showed that the release is governed by the degradation of the microspheres. The structural integrity of the released peptide/protein was investigated with FTIR, HPLC, SEC, CD and Fluorescence. The results demonstrated that the structural and functional integrity of the released peptide/protein was preserved. One of the major challenges with PLGA microspheres is prevention of the drop of pH inside degrading microspheres which is detrimental for the integrity of acid labile molecules such as peptides and proteins. The microclimate pH (μpH) of degrading PLHMGA microspheres was studied and the results were compared with that of PLGA. PLHMGA microspheres prepared from copolymers with different copolymer composition did not show changes in pH during incubation. As the degradation of the microspheres progressed, the acid degradation products (lactic and hydroxymethyl glycolic acid and their oligomers) which are responsible for pH drop, were released into the degradation medium as was confirmed by a decrease in pH of the surrounding medium. On the contrary, confocal images of PLGA microspheres during incubation showed that these microspheres developed a low pH during degradation. This means that the formed acid degradation products were not released in the medium as also evidenced from the observation that the pH of the degradation medium remained constant at 7.4. In conclusion, this thesis describes the application of novel biodegradable microspheres made from a hydrophilic polyester, poly(D,L-lactide-co-hydroxymethyl glycolide) (PLHMGA) for controlled delivery of peptide and protein pharmaceuticals. PLHMGA microspheres showed superior properties over their PLGA counterpart, a FDA approved polymer, regarding the control over degradation and release rate. The release of biopharmaceuticals and degradation of the microspheres could be successfully tailored by synthesis of copolymers differing in copolymer composition. The results reported in this thesis highlight the great potential of PLHMGA microspheres for biopharmaceutical applications.