Plasma protein adsorption on biodegradable microspheres consisting of poly(d,l-lactide-co-glycolide), poly(l-lactide) or ABA triblock copolymers containing poly(oxyethylene): Influence of production method and polymer composition

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Abstract

Biodegradable particulate systems have been considered as parenteral drug delivery systems. The adsorption of plasma proteins on micro- and nanoparticles is determined by the surface properties and may, in turn, strongly influence the biocompatibility and biodistribution of both carriers. In the present study the influence of the polymer composition and the production method of microspheres on the in vitro plasma protein adsorption were investigated using two-dimensional electrophoresis (2-DE). Microparticles were prepared from poly(l-lactide) (l-PLA), poly(d,l-lactide-co-glycolide) (PLGA), and ABA triblock copolymers containing hydrophilic poly(oxyethylene) (B-blocks) domains connected to hydrophobic polyesters (A-blocks). Two different microencapsulation methods were employed, namely the w/o/w emulsion solvent evaporation method and the spray-drying technique. It could be demonstrated that the polymer composition and, especially, the encapsulation technique, influenced the interactions with plasma proteins significantly. For example, the percentages of several apolipoproteins in the plasma protein adsorption patterns of spray-dried PLGA- and l-PLA-particles were distinctly higher when compared to the adsorption patterns of the particles produced by the w/o/w-technique. Some adsorbed proteins were found to be characteristic or even specific for particles produced by the same method or consisting of identical polymers. Polyvinyl alcohol used as stabilizer in the w/o/w-technique may decisively influence the surface properties relevant for protein adsorption. The plasma protein adsorption on particles composed of ABA copolymers was drastically reduced when compared to microspheres made from pure polyesters. The adsorption patterns of ABA-particles were dominated by albumin. The plasma protein adsorption patterns detected on the different microspheres are likely to affect their in vivo performance as parenteral drug delivery systems.

Introduction

Biodegradable polymers have been used in parenteral drug delivery systems for the past two decades [1]. These polymers do not require removal of the spent device due to their biodegradation to water-soluble monomeric or oligomeric breakdown products [2]. Polyesters of lactic and/or glycolic acid are attractive biomaterials due to their well known safety and tissue compatibility [3]. For the prolonged delivery of peptides and proteins the need for faster degrading systems still, however, exists. In these systems the release profile is significantly affected by the degradation rate. Additionally, the interaction of hydrophilic protein drugs with relatively hydrophobic polyesters can lead to pharmacologically inactive protein aggregates [4]. Therefore, new biodegradable materials with faster degradation rates, improved compatibility with proteins (less aggregates) are desirable. ABA triblock copolymers consisting of hydrophilic poly(oxyethylene) (PEO) B-blocks attached to hydrophobic polyester A-blocks, such as l-(+)-lactic-co-glycolic acid (l-PLGA) have shown interesting release and swelling properties under in vitro conditions [5]. The higher rate of water uptake seems to prevent protein aggregation and influences both the release and the degradation behavior of the carrier. Increased release rate and good encapsulation efficiencies were demonstrated in previous studies 6, 7. An improved compatibility between proteins and ABA was achieved for erythropoietic microspheres [8].

The biocompatibility and tissue reaction of ABA microspheres after intramuscular injection in rats demonstrated that these devices were well tolerated 9, 10. The compatibility and biodistribution of ABA or polyester nanospheres will be influenced by preferentially adsorbed plasma proteins 11, 12, 13, 14, 15. The pattern of adsorbed proteins strongly depends on the surface properties of the carriers and may decisively influence the in vivo distribution after i.v. administration 12, 16, 17, 18. Therefore, the analysis of plasma protein adsorption on biodegradable microspheres used as parenteral drug delivery system is of great interest with regard to their in vivo performance. Recently, two-dimensional electrophoresis (2-DE) was established for a detailed determination of adsorbed proteins on particulate surfaces 18, 19.

The present study addresses the effect of polymer composition and production method on the adsorption of plasma proteins on biodegradable microspheres under in vitro conditions. Different polyesters (poly(l-lactide) (l-PLA), poly(d,l-lactide-co-glycolide) (PLGA)), and ABA triblock copolymers, as well as two different microencapsulation techniques, namely the w/o/w double emulsion evaporation method and the spray-drying technique, were investigated with respect to their influence on the in vitro plasma protein adsorption.

Section snippets

Materials

Two different ABA triblock copolymers were synthesized as described previously [5]with a monomer ratio of lactic acid–glycolic acid–poly(oxyethylene) 56:14:30 and an average molecular mass (MW) of 15 kDa (ABA I) and 41:9:50, MW 15 kDa (ABA II), respectively. Poly(d,l-lactide-co-glycolide) (PLGA), with a monomer ratio of 50:50 (Resomer® RG 503, MW 33.1 kDa), was purchased from Boehringer Ingelheim (Germany). Poly(l-lactide) (l-PLA, MW 40 kDa) was a gift of Novartis Pharma LTD (Switzerland).

Characterization of microparticles and polymers

Fig. 1 shows the scanning electron micrographs of the microspheres prepared from various polymers and by different preparation methods. The particles exhibited acceptable sphericity and size homogenicity in all cases. The spray-drying method produced microparticles with a smooth and poreless surface. Scanning electron micrographs of the microparticles prepared by the double emulsion method revealed that the polymer had a considerable effect on the surface structure. PLGA produced microparticles

Conclusion

Using 2-DE, a detailed analysis of protein adsorption on biodegradable microspheres of varying composition and preparation was possible. It could be demonstrated that the polymer composition and, especially, the method of production of PLGA- and l-PLA-particles had a strong influence on the interactions with plasma proteins. Some adsorbed proteins were found to be characteristic or even specific for particles produced either by the same method or consisting of identical polymers. Thermal

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