The influence of covalently linked and free polyethylene glycol on the structural and release properties of rhBMP-2 loaded microspheres

Abstract

The current clinical success of therapies with recombinant human Bone Morphogenetic Protein 2 (rhBMP-2) is limited due to inefficient delivery. The high doses applied have frequently been related to severe adverse effects such as tissue swelling, seroma, inflammatory effects and heterotopic ossification. The controlled delivery of lower doses is supposed to reduce adverse effect incidence as well as costs. In this study, novel polyethylene glycol-poly(lactic-co-glycolic acid) (PEG-PLGA) diblock copolymers were used to produce low dose controlled delivery vehicles for rhBMP-2. A method to fabricate a variety of microsphere formulations with a high encapsulation efficiency in high yields was developed. The influence of PEG as an inner phase cosolvent and linked PLGA as copolymer was investigated. Six different microsphere systems with varying PEG amounts in both core and shell were characterised thoroughly with respect to the specific properties of rhBMP-2. The particle size of the microspheres was investigated with both laser diffraction and environmental scanning electron microscopy. Higher PEG/PLGA ratios showed a tendency to increase in size and a wider distribution. Due to the low rhBMP-2 doses, a profound characterisation was very challenging. The growth factor was covalently attached to rhodamine B for the first time. Studies on drug distribution in the microspheres were performed by means of confocal laser scanning microscopy. The addition of PEG to the inner phase was found to impair the formation of spherical microdomains with localized higher growth factor concentrations. Release profiles, determined with ELISA, were linked to the structural changes that were monitored. Distinct, controlled release profiles were achieved in all formulations and showed that PEG is a versatile tool in the effective control of release rates from microspheres. Higher PEG/PLGA ratios in the polymer were shown to increase the release rate from the microspheres. In contrast, PEG administered to the inner phase decreased the release rate. The biological activity of released protein was shown in vitro in an alkaline phosphatase assay. It was demonstrated that PEG-PLGA microspheres are a promising sustained delivery system which allows a reduction of the required rhBMP-2 dose to limit both adverse effects and costs. Furthermore, the data indicated that the use of PEG as an inner phase cosolvent is not suitable for rhBMP-2 in contrast the reported beneficial effects for other growth factors.