Elucidation of the mechanism of incorporation of insulin in controlled release systems based on complexation polymers

The objective of this study was to investigate the insulin incorporation and release properties of poly(methacrylic acid-g-ethylene glycol) P(MAA-g-EG) microparticles as a function of copolymer composition. These microparticles exhibited unique pH-responsive characteristics in which interpolymer complexes were formed in acidic media and dissociated in neutral/basic environments. The microparticles containing equimolar amounts of MAA and PEG were capable of efficient insulin loading using equilibrium partitioning (>90%). Additionally, insulin release from the gel was significantly retarded in acidic media while rapid release occurred under neutral/basic conditions. In contrast, as the amount of MAA of the polymer was increased, the entrapment efficiency of insulin within the gel greatly reduced and the insulin was readily released from the polymer network in the acidic and neutral/basic media. In addition, in order to evaluate the potential application of the microparticles to other drugs, theophylline, vancomycin, fluorescein-isothiocyanate-labeled dextrans (FITC-Ds) with average molecular weights of 4400 (FITC-D-4), 12,000 (FITC-D-10) and 19,500 (FITC-D-20) were utilized as model hydrophilic drugs. The incorporation profiles showed that the uptake of theophylline and vancomycin to the microparticles was lower than that of insulin. Additionally, polymer microparticles loaded with theophylline and vancomycin exhibited pH-sensitive release behavior, however, the oscillatory behavior is less pronounced than those of insulin. The values of drug incorporation ratio showed that the microparticles were capable of incorporating almost 90% of insulin and 15% of vancomycin from solution. On the other hand, the other hydrophilic drugs showed very low incorporation efficiency to the microparticles. These data suggest that gels containing equimolar amounts of MAA:EG have the potential to be used as an oral carrier of peptide drugs, especially for insulin.