Assessment of Insulin Stability Inside Diblock Copolymer PEG-PLA Microspheres

Insulin-loaded PEG2-PLA40 and PEG5-PLA20 microspheres containing 5% bovine insulin were manufactured using single emulsion and w/o/w multiple emulsion-solvent evaporation techniques. Microspheres were characterized for their insulin encapsulation efficiency and release characteristics in phosphate-buffered saline (PBS) at pH 7.4 and 37 °C. Moreover, the stability of the peptide during 18 days of release was evaluated using HPLC and HPLC-MS techniques. The results showed that the loading efficiencies were higher in case of insulin loaded PEG2-PLA40 and PEG5-PLA20 microspheres prepared by single emulsion emulsion-solvent evaporation technique. Insulin release was characterized by an initial burst, which was attributed to the amount of protein located on or close to the microsphere surface. The total ion chromatogram (TIC) of insulin samples extracted after 6, 12 and 18 days of PEG2-PLA40 microspheres erosion showed that insulin was intact inside the eroding microspheres. In addition, only small amounts of protein undergo degradation under these conditions (only 11.69% ± 1.13 of the initially loaded insulin loading were detected as degradation products after 18 days. Mass spectra recorded at these retention times confirmed the presence of insulin with a molar mass of 5734 Da and other two products of molar masses of 5587 Da and 5487 Da.