In Vitro Release Testing of PLGA Microspheres with Franz Diffusion Cells

Abstract

In the present study, two methods were used to evaluate the in vitro release of leuprolide acetate (LA) from poly(lac tide-co-glycolide) (PLGA) microspheres: Franz diffusion cells, typically referred to as “vertical diffusion cells” (VDC), and rotating bottle apparatus (RBA), both modified with a dialysis membrane. This hydrosoluble peptide was chosen as a model drug to study different possibilities of in vitro testing and analyze the variables that affect drug release, respecting sink and physiological conditions. Microspheres were prepared with a conventional double emulsion–solvent evaporation method using PLGA (50:50) with a relatively low molecular weight. Comprehensive stability tests for LA were performed in the conditions used for in vitro release assays. In phosphate-buffered saline (PBS), LA showed no significant degradation, but in an acidic medium, it degraded dramatically. The release profile of the delivery system was governed mainly by diffusion as explained by the low molecular weight of the polymer and the high water solubility of the peptide. The in vitro release profiles were triphasic in vertical diffusion cells and biphasic in the rotating bottle apparatus. The release kinetics was enhanced in RBA with respect to VDC, probably because the constant movement of a suspension of loose microspheres in a large volume and the large membrane area facilitated drug migration. The smoother, triphasic profiles obtained with VDC can be explained by the partial confinement of microspheres, which is similar to the described in vivo behavior of an injectable delivery system.