Peptide degradation during preparation and in vitro release testing of poly( l-lactic acid) and poly( dl-lactic- co-glycolic acid) microparticles

Abstract

Biodegradable, tetracosactide-loaded microparticles were prepared by means of (i) spray drying, (ii) w/o/w solvent evaporation method (WOW) and (iii) by the aerosol solvent extraction system (ASES) using poly( l-lactic acid) ( l-PLA) and poly( dl-lactic- co-glycolic acid) ( dl-PLGA) of varying monomer composition or molecular weight. In the absence of the polymer the peptide did not degrade or aggregate irreversibly when in contact with methanol and methylene chloride or under the conditions used in the first step of WOW, as proven by HPLC, electrospray-mass spectrometry (MS) and circular dichroism (CD). During the extraction process, used to isolate the peptide from the microparticles, tetracosactide was partially oxidised. The highest stability of the peptide during microencapsulation was guaranteed with high molecular weight l-PLA, when using WOW or ASES, and with very low molecular weight PLGA, in the case of spray drying and WOW. The burst release of the microparticles, during in vitro release testing, depended on the preparation method as well as on the nature of the polymer and increased in the order ASES<spray drying<WOW and with increasing hydrophilicity of the polymer. Exceptionally, in the case of very low molecular weight PLGA, to which tetracosactide showed a very strong affinity during the in vitro adsorption study, no burst effect was observed. In addition, these microparticles released the peptide continuously, whereas for the others, composed of high molecular weight PLA and PLGA, the burst release was followed by a lag phase. During in vitro release peptide degradation increased with increasing polymer hydrophilicity but could be reduced by increasing drug loading. In polymer-free control solutions tetracosactide degradation was always slower than in the presence of microparticles. Oxidation and hydrolysis were found to be the major degradation pathways.