PLGA-based microparticles: elucidation of mechanisms and a new, simple mathematical model quantifying drug release.

The two major aims of this study were: (i) to elucidate the underlying release mechanisms from drug-loaded, erodible microparticles based on poly(lactic-co-glycolic acid) (PLGA) showing biphasic drug release behavior: an initial 'burst' effect, followed by a zero order release phase; and (ii) to develop a new, simple mathematical model that allows the quantitative description of the observed in vitro drug release patterns from this type of delivery system. PLGA-based microparticles offer various advantages, such as the possibility to control the resulting drug release rate accurately over prolonged periods of time, easiness of administration (e.g., by stereotaxic injection), good biocompatibility and complete erosion (avoiding the removal of empty remnants). Consequently, the practical importance of these advanced drug delivery systems is remarkably increasing. However, only little knowledge is yet available concerning the processes controlling the release rate of the drug out of these devices. Various chemical and physical phenomena are involved, rendering the identification of the crucial mechanisms and the mathematical description of the resulting drug release kinetics difficult. In the present study, different physicochemical characterization methods (e.g., DSC, SEM, SEC, particle size analysis) were used to monitor the changes occurring within anticancer drug-loaded PLGA microparticles upon exposure to phosphate buffer pH 7.4. Based on these experimental findings, the most important underlying drug release rate controlling mechanisms were identified and a new mathematical model was developed that allows the quantitative description of the resulting release patterns.