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Sustained release microparticles used for parenteral drug delivery must be well characterized in terms of their size range, morphology and function. It is widely understood that the chemistry and morphology of microparticles have a degree of interdependence which strongly affects drug release behaviour from microparticles. This thesis investigates, for the first time, the use of mid-IR imaging along with the development and optimisation of relevant multivariate image analysis methods for studying the real-time degradation of pharmaceutically relevant biodegradable polymer microparticles and the real-time release of protein based drugs from such microparticle systems. The application of attenuated total reflection - Fourier transform infrared spectroscopic(ATR-FTIR) imaging and analysis to monitor the degradation of a single microparticle is optimised and the developed methodology is detailed. A series of time resolved images of a PLGA microparticle undergoing hydrolysis at 70 °C are obtained using ATR-FTIR imaging for the first time. A novel partially supervised non-linear curve fitting (NLCF) tool is developed and the output from the NLCF is evaluated by direct quantitative comparison with a traditional peak height (PH) data analysis approach and multivariate curve resolution alternating least squares (MCR-ALS) analysis for the same images, in order to develop an image analysis strategy. The NLCF method is shown to facilitate the calculation of hydrolysis rate constants for both the glycolic (kG)and lactic (kL) segments of the PLGA copolymer. This results in improved spatial resolution on time-resolved microparticle images, so providing better insight into the dimensions of hydration layers and particle dimension changes during hydrolysis when compared to images derived from both PH measurements and MCR-ALS. The MCRALS routine is shown to be faster than NLCF and its images are found to provide sufficient contrast to be used for qualitative comparison. The optimised mid-IR-ATR procedures are then applied to investigate several factors influencing the hydrolytic degradation of a family of PLGA microparticles. Degradation rate constants for glycolic and lactic units are shown to increase (whilist maintaining a ~1.3 ratio between each other) with increasing initial glycolic content of the copolymer,temperature or γ-radiation exposure. Differential scanning calorimetry (DSC) and gel permeation chromatography (GPC) results indicate a chain scission based degradation in PLGA upon γ exposure. The distribution of lactic acid is probed with IR during the hydrolysis of a PLA microparticle for the first time, showing a diffusional pathway from the degrading microparticle outwards into surrounding water. Utilising the chemical selectivity of the infrared methodology, ATR-FTIR imaging is applied for the first time to monitor the redistribution and release of human growth hormone (hGH) from a range of CriticalMixTM processed PLGA/PLA microparticles during a set of dissolution experiments at 37 °C in D2O. Increasing the γ dose is shown to have a profound influence on the release mechanism, with higher γ doses leading to a dramatic increase in the initial burst release followed by retardation in the sustained release and a lower total level of hGH release over the dissolution experiment. These changes are shown to be the result of: (i) protein aggregation as a function of applied γ-dose as studied by size exclusion chromatography; (ii) decrease in overall porosity as studied by SEM; (iii) decrease in Mw of all of the component polymers post γ irradiation indicating a chain scission mechanism as studied by GPC and DSC; and (iv) the increase in the number of oxygenated components in the Poloxamer 407 excipient, thereby increasing the strength of interaction between the microparticle and the entrapped hGH. These findings suggest that any γ sterilisation dose should be less than 25 kGy and that other sterilisation methods may need to be considered, due to the stability of the studied formulations. |
