Modeling the Microenvironment-Dependent Degradation of Drug-Loaded Polylactic-co-glycolic Microparticles

Abstract

Aliphatic polyesters gained an important role in the biomedical field, finding applications as drug delivery devices, fixation screws, and suture threads, to name a few. Indeed, they merge several properties of interest, such as biocompatibility, tunable properties, and in situ hydrolytic degradation. Crucial parameters for device performances, like drug release rate and mechanical properties, are strongly influenced by polymer degradation. The main phenomena governing hydrolysis are now wellassessed and rationalized in literature; a relevant role is played by the microenvironmental pH (μpH), since hydrolysis is catalyzed in both acid and alkaline environments. A reliable μpH estimation is challenging because the microenvironment is the result of several factors: generation of acidic polymer fragments (kinetics), their partition in the water-filled pores and their dissociation (thermodynamics) and their accumulation in the device core (transport phenomena). This scenario is further complicated by the presence of an acid/basic active compound. In this work, we propose a detailed model for the microenvironmental-dependent degradation of microparticles made of polylactic-co-glycolic acid as a reference system for their relevance; in particular, μpH is explicitly calculated taking into account the dissociation, the partition and the diffusion of monomer and the drug in the water-filled pores. Comparison with experimental data showed that the model provides a quantitative estimation of molecular weight decay over time; intraparticle μpH is in good agreement in average terms, although results suggested that the dissociation of heavier water-soluble polymer fragments must be also accounted for. Model outcomes also showed that the presence of an active compound can promote acid or basic catalysis and enhance degradation rate, as observed experimentally. A quantitative agreement (in terms of molecular weight decay and cumulative release over time) could be obtained with further refinement of input parameters; overall, results suggested that a reliable description of particle morphology and a more accurate thermodynamic model (to properly account for dissociation and partition as a function of composition and μpH in the water-filled pores) are envisaged to obtain a more robust evaluation of microenvironmental effects.