The effect of poly(ethylene glycol)–poly(d,l-lactic acid) diblock copolymers on peptide acylation

Abstract

The combination of poly(ethylene glycol) (PEG) with a biodegradable poly(ester), such as poly(d,l-lactic acid) (PLA), is an approach that has been successfully used for the stabilization of proteins and peptides in several biodegradable delivery devices. The acylation of peptides inside degrading PLA microspheres has been described only recently as another instability mechanism related to the accumulation of polymer degradation products inside eroding PLA. We investigated whether the block copolymerization of PLA with PEG reduces peptide acylation inside degrading microspheres. Diblock copolymers consisting of poly(d,l-lactic acid) covalently bound to poly(ethylene glycol)-monomethyl ether (Me.PEG–PLA) were used for these investigations. Human atrial natriuretic peptide (ANP) was incorporated into microspheres manufactured from Me.PEG5–PLA45, a diblock copolymer with an overall PEG content of 10%. Peptide integrity inside the microspheres was monitored by HPLC–MS analysis during 4 weeks of microsphere degradation in isotonic phosphate buffer (pH 7.4) at 37°C. Inside the degrading Me.PEG5–PLA45 microspheres, acylation products as well as an oxidation product of ANP were formed. The results demonstrate that the combination of PEG with PLA does not necessarily display a favorable effect concerning peptide acylation inside degrading polymer microspheres. However, they also suggested that the acylation reaction is mainly driven by the formation and accumulation of polymer degradation products inside the degrading microspheres.