Studying spatial drug distribution in golf ball-shaped microspheres to understand drug release

Poly (lactic-co-glycolic acid) (PLGA) microspheres have been one of the most successful products for slow drug release. While distribution of drugs in microspheres might be a fundamental factor affecting drug release, it has been often overlooked. Indeed, very few studies are available on the distribution of drugs in microspheres with complex morphology like golf ball-shaped microspheres. In this paper, the distribution of rotigotine in golf ball-shaped microspheres (GSRM) was investigated by argon ion milling, combined with scanning electron microscopy and energy dispersive X-ray spectroscopy (AIM-SEM-EDS). Rotigotine in GSRM was clearly observed in two forms, respectively in an aggregated state and as a molecular dispersion. The distribution of palmitic acid in the microspheres (used as an additive to reduce burst release) was also demonstrated: 10% was found on the microspheres' surface while 90% separated from the polymer to form small particles inside the microspheres onto which rotigotine aggregated through hydrogen bonding interactions. In in-vitro release studies we observed that first the phase-separated palmitic acid/rotigotine particles dissolved and released the drug, followed by the release of the molecularly dispersed rotigotines by osmosis. We also found that rotigotine accelerated the degradation and reduced the glass transition temperature of PLGA, which played an important role as well in the release of the drug from GSRM. Finally, two linear Level A in vitro-in vivo correlations were established and validated, indicating that the in vitro release testing could be a meaningful predictor for the in vivo performance of GSRM. Our work demonstrates the importance of studying drug distribution in complex microspheres to understand drug release.