Acrylic terpolymer microcapsules for colon-specific drug delivery: effect of molecular weight and solubility of microencapsulated drugs on their release behaviors

In our previous study, new microcapsules (MCs) with a membrane of terpolymer of ethyl acrylate, methyl methacrylate, and 2-hydroxyethyl methacrylate were prepared by the Wurster coating process to perorally deliver macromolecular drugs such as peptides and proteins to the colon in a delayed-release manner. The present study focused on the investigation of the release characteristics of acrylic terpolymer MCs containing fluorescein isothiocyanate-labeled dextran (FITC-Dex) of different average molecular weights or small molecular drugs of different solubilities for further understanding the permeation nature of the acrylic terpolymer membrane. FITC-Dex of average molecular weight less than approximately 20,000 atomic mass unit (amu) could pass smoothly through the MC membrane after a lag time of 7 h. FITC-Dex of molecular weight of up to approximately 1,000,000 amu could pass via a membrane of terpolymer at a rate that decreased with an increase in the molecular weight; the release rate then decreased with an increase in the membrane thickness. Thus, the delayed release of FITC-Dex from the MCs was not simply due to crack formation or membrane bursting. In the case of MCs encapsulating small molecular drugs of different solubilities, when the water solubility of the drug was higher than 1 mg/mL, a fast release was observed after the lag time; more than 82% was released after 18 h. The delayed release from these MCs was explained by the water intake during the lag time, drug release through the membrane pores after the end of water intake and final shrinking of the particle.