Novel analytical approaches for solid dispersion characterization

The overall aim of the thesis was to introduce new analytical techniques to characterize solid dispersion formulations. Solid dispersion formulations are employed to enhance the dissolution behavior and apparent solubility of poorly soluble compounds. This formulation strategy uses typically an amorphous physical form of a poorly soluble drug and combines it with a carrier for stabilization. The amorphous form presents higher free energy compared to a crystalline drug form thereby yielding a higher dissolution rate and possibly more complete oral absorption as well as bioavailability. The selection of appropriate excipients is crucial to guarantee the formulation performance and stability during the shelf life of the final product. To investigate drug formulation characteristics and predict their performance, different analytical techniques are needed. Along with the classical characterization techniques, novel approaches such as fluorescence spectroscopy and diffusing wave spectroscopy are introduced in the present thesis.
The chapters 1 and 2 of this thesis cover fundamental aspects of poorly soluble drugs: an overview is given on amorphous solid dispersion (ASD) manufacturing technologies and characteristics of polymers and surfactants used in ASD. Moreover, analytical tools to characterize solid dispersions are presented. Among them, special emphasis is given to novel approaches such as Diffusing Wave Spectroscopy (DWS) and Fluorescence Spectroscopy.
As for the selection of excipients, drug polymer miscibility is a crucial requisite for the performance of an ASD formulation. One of the methods to predict drug-polymer miscibility is to employ solubility parameter approach; its application in solid dispersion formulations is outlined in the Chapter 3.
The first study introduces a novel fluorescence quenching approach together with size exclusion chromatography to study drug-polymer interactions that emerge from ASDs drug release in an aqueous medium. Celecoxib was combined with different pharmaceutical polymers and the resulted solid dispersion was evaluated by the (modified) Stern-Volmer model. Drug accessibility by the quencher and its affinity to the drug were compared in physical mixtures as well as within the ASDs using different polymer types. It was possible to gain knowledge about specific drug-polymer interactions and the amount of drug embedded in the evolving drug-polymer aggregates upon formulation dispersion and drug dissolution. More research in the future will show how such in vitro findings translate into performance of an ASD in vivo.
The second study of this thesis has also a biopharmaceutical focus and investigates formulation differences from a microrheological perspective by considering further an in vitro absorption sink using a biphasic dissolution equipment. Indeed, biphasic dissolution testing can simulate an intestinal absorption from dispersed formulation by using an organic layer. This study employed ketoconazole, a poorly soluble drug, together with different grades of HPMCAS and formulations were produced by hot melt extrusion (HME). Diffusing wave spectroscopy highlighted microrheological differences among the different polymer grades and plasticizers in the aqueous phase. These differences were found to influence drug release and finally the uptake in the organic layer that was intended to mirror the absorption process. There is surely more research needed before final conclusions can be drawn but the obtained findings point already to an important contribution of microrheological differences that evolved upon formulation dispersion.
The third study also emphasized microrheology but with a focus on non-dispersed solid dispersions. It was aimed to investigate microstructuring during phase transitions in drug-polymer solid dispersions. This formulation microstructuring is critical for the physicochemical properties such as stability of the final dosage form. In this study, eutectic mixtures of polyethylene glycol (PEG) were investigated using two drugs: fenofibrate and flurbiprofen. Unlike fenofibrate, the drug flurbiprofen was strongly interacting with the polymer and this was also confirmed by the rheological characterization. Therefore, broadband DWS provided valuable mechanistic information on the drug-polymer interactions and macromolecular structuring during the cooling of the eutectic melts.