Porcine Insulin Biodegradable Polyester Microspheres: Stability and In Vitro Release Characteristics

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Abstract
The stability of porcine insulin in biodegradable polymers, i.e., poly(dl-lactide-co-glycolide) 50:50 (50:50 DL-PLGA) and poly(l-lactide) (L-PLA) was investigated. Insulin encapsulated microspheres were fabricated from both polymers using double-emulsion–solvent evaporation and emulsion–solvent evaporation techniques and subjected to accelerated stability studies at 40°C and 75% relative humidity. Porcine insulin was found to degrade in all microsphere formulations with an average of <50% of the initial loading amount remaining intact at the end of 4 weeks. The two major degradation products observed in these formulations were determined to be A-21 desamido insulin and covalent insulin dimer with trace amounts of high molecular weight transformation products. In vitro release studies in phosphate buffered saline at 37°C resulted in very slow and incomplete (<30% in 30 days) release kinetics for all microsphere formulations. Extraction and analyses of the unreleased insulin within the microspheres revealed that an average of ∼11% of the encapsulated insulin remained intact. The degradation products observed consisted of ∼15% of three distinct deamidated hydrolysis products including A-21 desamido insulin, ∼22% covalent insulin dimer, and trace amounts of high molecular weight transformation products. The degradation of porcine insulin within biodegradable polyester microspheres during stability and release studies can be attributed to the gradual decrease in the pH within the microspheres due to progressive polymer hydrolysis resulting in the production of dl-lactic and glycolic acids. The encapsulation of an acid–base indicator, bromophenol blue, in 50:50 PLGA microspheres (as a probe to estimate pH within the microspheres during accelerated stability studies) indicated that the pH decreased to ∼3.8 after 3 weeks.