Porcine insulin biodegradable polyester microspheres: Stability and in vitro release characteristics

The development of controlled release dosage forms for peptides and proteins has been a major area of research in recent years. With advances in recombinant DNA technology, a large number of therapeutic peptides and proteins are now commercially available. Most of these agents have a short half-life in vivo such that a cumbersome multi-dose therapeutic treatment is required and controlled release technology could alleviate such a problem. A variety of degradable and nondegradable polymers have been utilized as matrices to incorporate protein molecules. However, the most promising approach is to microencapsulate the protein molecules in injectable biodegradable polymer microspheres. As the polymer degrades, the protein diffuses out in a sustained manner through the enlarged pore channels over the desired period of time. Among the various biodegradable polymers, poly(L-lactic acid) and its copolymers with D-lactic acid or glycolic acid provide a wide range of degradabilities from months to years and have a long history of biocompatibility from their use as surgical sutures. However, as polymeric carriers for high molecular weight protein drugs, it is still questionable whether they provide a benign microenvironment for the encapsulated protein drug. This research investigated the stability of porcine insulin, a model protein drug, in biodegradable polyester microspheres. Since these polymers degrade into acidic end-products, preliminary studies were conducted to characterize insulin stability as a function of pH. Based on these studies, insulin was found to be most stable in the pH range of 6.5–7. Insulin microspheres were fabricated from 50:50 DL-PLGA and L-PLA using double-emulsion-solvent-evaporation and emulsion-solvent-evaporation techniques and subjected to accelerated stability studies at 40°C/75% RH. Insulin degraded in all formulations into A-21 desamido insulin and covalent insulin dimer, with an average of <50% of the initial loading amount remaining after 4 weeks. In vitro release studies conducted at 37°C resulted in slow and incomplete release profiles (average cumulative release ∼14.1% in 30 days) for all microsphere formulations which was associated with significant covalent dimerization (∼22%) of the unreleased insulin. Intra-microsphere pH estimation studies confirmed that the progressive drop in the pH within the microspheres during polymer hydrolysis (pH ∼1.8 in 7 weeks) was responsible for insulin degradation. The incorporation of a buffering agent, sodium bicarbonate as an additive in 50:50 DL-PLGA microspheres resulted in an improved in vitro release profile (cumulative release ∼47.3% in 30 days) as well as a significant reduction in covalent dimerization of the unreleased insulin to barely detectable levels.