Process variable implications for residual solvent removal and polymer morphology in the formation of gentamycin-loaded poly(L-lactide) microparticles

Purpose. The purpose was to determine the influence of process parameters in the precipitation with a compressed antisolvent (PCA) process on the morphology and residual dichloromethane (DCM) levels in gentamycin-loaded PLA microparticles. Methods. The three variables studied were the rate of CO2 co-flowed during the polymer and drug co-precipitation, the post-precipitation pure CO2 flush rate, and the post-precipitation CO2 flush volume. Residual DCM levels were determined from headspace gas chromatography - mass spectroscopy (GC-MS) with single ion monitoring. Xray diffraction (XRD) and differential scanning calorimetry (DSC) were used to estimate the crystallinity within microparticles. DCM was extracted from drug-loaded microparticles by both supercritical CO2 extraction and vacuum drying for up to two days to determine a lower limit for solvent removal. Results. Increasing either the post-precipitation CO2 flow rate or flush volume resulted in lower residual DCM levels in the microparticle. The CO2 co-flow rate showed an opposite trend. Increasing its value resulted in a higher DCM value after precipitation. XRD and DSC analysis on these samples suggest that those produced at lower CO2 co-flow rates have a higher degree of crystallinity, which increases the diffusivity of DCM through the polymer matrix. Finally. samples subjected to extended (48 hr) CO2 extraction resulted in DCM levels on the order of one to three ppm. Conclusions. Specific PCA process conditions during microparticle formation have a strong influence on the residual solvent levels within the microparticles. Polymer morphology affects the diffusivity of solvent through the polymer matrix, which in turn determines the solvent removal rates.