PLGA-based drug delivery systems: importance of the type of drug and device geometry.

Different types of ibuprofen- and lidocaine-loaded, poly(lactic-co-glycolic acid) (PLGA)-based microparticles and thin, free films of various dimensions were prepared and physico-chemically characterized in vitro. The obtained experimental results were analyzed using mathematical theories based on Fick's second law of diffusion. Importantly, the initial drug loadings were low in all cases (4%, w/w), simplifying the mathematical treatment and minimizing potential effects of the acidic/basic nature of the two model drugs on polymer degradation. Interestingly, the type of drug and device geometry strongly affected the resulting release kinetics and relative importance of the involved mass transport mechanisms. For instance, the relative release rate was almost unaffected by the system size in the case of spherical microparticles, but strongly depended on the thickness of thin, free films, irrespective of the type of drug. Ibuprofen and lidocaine release was found to be primarily diffusion controlled from the investigated PLGA-based microparticles for all system sizes, whereas diffusion was only dominant in the case of the thinnest free films. Interestingly, the type of drug did not significantly affect the resulting polymer degradation kinetics. However, ibuprofen release was always much faster than lidocaine release for all system geometries and sizes. This can probably be attributed to attractive ionic interactions between protonated, positively charged lidocaine ions and negatively charged, deprotonated carboxylic end groups of PLGA, hindering drug diffusion. The determined apparent diffusion coefficients of the drugs clearly point out that the mobility of an active agent in PLGA-based delivery systems does not only depend on its own physico-chemical properties and the type of PLGA used, but also to a large extent on the size and shape of the device. This has to be carefully taken into account when developing/optimizing this type of advanced drug delivery systems.