Effect of Particle Size on Drug Loading and Release Kinetics of Gefitinib-Loaded PLGA Microspheres

Polymeric microspheres have gained widespread application as drug eluting depots. Typically, drug-loaded polymeric microspheres are prepared by oil-in-water emulsification which yields a product with a broad size distribution. The aim of the present study was to investigate the properties of different size-fractions of drug-loaded microspheres, in order to delineate whether particle size governs drug loading efficiency and release profile. Gefitinib-loaded PLGA-based microspheres were prepared using an oil-in-water solvent evaporation method and wet-sieved to obtain well-defined size fractions of 5 ± 1, 32 ± 4, 70 ± 3, and 130 ± 7 μm, respectively. The average drug loading of unfractionated microspheres was 6.3 ± 0.4% w/w, while drug loading of sieved fractions ranged from 2.4 ± 0.3 to 7.6 ± 0.9% w/w for smallest to largest microparticles. X-ray diffraction (XRD) and differential scanning calorimetry (DSC) analysis demonstrated that gefitinib was amorphously dispersed in the PLGA matrix, with no apparent shift in the Tg of PLGA indicating the absence of direct molecular interactions of the drug and polymer due to the formation of small drug particles embedded in PLGA. In vitro drug release was studied with microspheres embedded in dextran hydrogels to avoid their aggregation during the incubation conditions. Microspheres smaller than 50 μm showed rapid diffusion-based release reaching completion within 2 days when particles have not degraded yet. Larger microspheres, however, showed a sigmoidal release pattern that continued for three months in which diffusion (early stage) as well as particle erosion (later stage) governed drug release. Scanning electron microscopy (SEM) and polymer degradation data showed that larger microspheres degraded faster than smaller ones, which is in line with autocatalytic PLGA degradation upon acidification within the core of microparticles. In conclusion, we showed that different size-fractions of drug-loaded microspheres showed quite distinct drug loading and release kinetics. Control of microparticle size by fractionation is therefore an important determinant for obtaining well-defined and reproducible sustained release depots.