Molecular Interactions in PLGA Delivery Systems and Their Effects on Microencapsulated Peptides

Abstract

Poly(lactic-co-glycolic acid) (PLGA) is the current injectable delivery vehicle of choice for controlled release of peptide drugs. However, this class of drug product is usually associated with high costs of development as well as slow approval process owing to difficulties of microencapsulation, drug stability, and a poor understanding of the differences between in vitro and in vivo product performance. Molecular interactions between PLGA and its degradation products, the peptide drug itself, and molecules in the in vitro/in vivo release environment all play a role in the ability to improve development and performance of PLGA peptide depots. The purpose of this thesis was to characterize those interactions. The molecular interactions of peptides with PLGA were characterized by using isothermal titration calorimetry to improve understanding of the role of these interactions on peptide microencapsulation, acylation and release. Measurements were carried out in dimethyl sulfoxide to study effects of the hydrophobicity of the PLGA and the ionic strength on the thermodynamics of PLGA binding to the model cationic peptide octreotide as well as several other therapeutic peptides. The anionic carboxylate acid end-group of the PLGA was found to be the main binding site with the primary amine groups of octreotide, irrespective to the hydrophobicity of the PLGA. Binding was weakened as the ionic strength increased, consistent with octreotide-PLGA-COOH binding by ion-pairing. Binding predicted that net positively charged peptides could be successfully loaded by remote absorption and acidic/neutral peptides were not. Endogenous compounds that could potentially interact with PLGA microspheres during in vivo implantation were investigated. Alkaline phosphatase was used as the model large molecules to study the uptake, and the success of its entry into microspheres appeared to be dependent on the surface porosity of the microspheres. The uptake of small organic amines was found to be correlated to the acid end-group content in microspheres in vitro; however, the correlation was less obvious in vivo. Dimethylamine, ethanolamine and ethylamine were found in microspheres, and the uptake efficiency was irrespective to the surface porosity. The content of each amine detected in microspheres was low compared to the acid end-group content in microspheres. Lipids were found entering the microspheres readily in vivo. The dominant species of different lipid subclasses partitioning into microspheres were identified and included phosphatidylcholines (or phosphatidylethanolamines) and ceramides. Water-soluble degradation products of PLGA partition in, and diffuse out of, the polymer matrix while producing a complex time and position-dependent profile of drug stability-determining microclimate pH (µpH), i.e., the pH in the pores of the polymer. The effect of in vitro and in vivo µpH in microspheres on the pH dependent-stability of exenatide was studied by combining the cage implant system and confocal imaging technique. In vivo µpH in microspheres was highly similar to that of the in vitro condition with higher microsphere concentration, but very different from that of the sample-and-separate method with lower microsphere concentration. The µpH in microspheres was found to be sensitive to the stability of the in vitro incubation media pH. The rise in µpH was shown to slightly correlate with the increase in the level of pH-dependent exenatide acylation. In summary, this thesis advances the understanding of how drugs, endogenous molecules and polymer degradation products interact with PLGA in important ways to help our understanding of key aspects of the development of PLGA controlled release of peptides.