Abstract by Anas Samir Mohammad Aljabbari

The adsorption of proteins and lipids at the interface between blood and particles, installs a new “surface identity” on the particles termed the “biomolecular corona”. The analogous phenomenon that occurs in the gastrointestinal (GI) tract, the biomolecular gastrointestinal corona, has received considerably less attention. Adsorption of biomolecules onto drug particles, where the fluid also contains bile salts and phospholipids in addition to proteins, peptides, and carbohydrates, has not been studied in this context. This is despite the fact that oral drug delivery remains the preferred method of administration, and that poorly soluble drugs (i.e. drugs that are prone to form particles in the GI tract) make up an increasing percentage of emerging therapeutics. Furthermore, the GI tract is a complex environment with a variety of surface-active biomolecules such as proteins and bile salts, coupled with a compartment-specific pH, osmolarity and ionic strength. Therefore, given the importance of drug particles in oral drug delivery, there is a need for further knowledge on how the nature of the particle and GI environment affect the composition and structure of the GI corona. The solid-state form of the drug is also important for its performance, and interaction with gastrointestinal biomolecules and formation of a corona during precipitation or crystallisation has the potential to influence performance.

Therefore, the aim was to I) confirm the formation of a GI biomolecular corona on poorly soluble drug particles, II) elucidate the factors that impact its formation and III) investigate a potential connection between adsorption of GI biomolecules onto drug particles, and changes in their recrystallization behaviour. The rationale for the formation of a GI corona on drug particles is proposed in the first part of the thesis, which also serves as the main hypothesis: hydrophobic drug particles adopt a biomolecular gastrointestinal corona when exposed to GI fluids.

In support of this hypothesis, the experimental methods comprised those for quantitative determination of the composition of the corona (including HPLC and proteomics) and those supporting a kinetic study of the structural and solid-state changes that drug particles undergo upon incubation with GI biomolecules (including time-resolved X-ray diffraction and low frequency Raman spectroscopy). To quantify the adsorption of biomolecules onto particles, the model hydrophobic drug particle ritonavir was incubated with bile salts in simulated intestinal medium. After separation of particles from the medium, the depletion of bile salts from solution was quantified. To identify the impact of compositional variables on the extent of depletion (an indication of adsorption from solution), the molecular hydrophobicity, solution ionic strength, and bile salt aggregation state were varied. Additionally, model drug particles of ritonavir, clotrimazole and indomethacin were incubated in ex vivo porcine gastric, intestinal and colonic fluids. After separation of particles from the medium, the proteins adsorbed to the particles were eluted and digested, prior to proteomic analysis. The results confirm that bile salts and proteins adsorb onto the surface of drug particles, forming a complex biomolecular corona that is impacted by the physicochemical properties of the particles and the medium.

For the determination of the kinetics of solid state changes in the presence of GI molecules, phase transformations of amorphous indomethacin drug particles were probed in simulated intestinal fluid, with and without micelles of bile salts and phospholipids. Results show that the crystallization behaviour of amorphous particles was affected by the presence of GI biomolecules. In the presence of bile salts and phospholipids, model amorphous drug particles recrystallized directly into their most stable crystalline form, bypassing intermediate polymorphic states typically observed in the absence of these biomolecules. This highlights that the adsorption of these compounds to the particle surface can influence their properties beyond just the surface identity.

In summary, this work has established that the formation of a biomolecular gastrointestinal corona occurs on hydrophobic drug particles, that both the properties of the particles and the media are important in determining the extent and composition of corona formation, and that its formation can impact the solid-state of recrystallized drug particles. The potential impact of the thesis is in alerting the field to this phenomenon and stimulating future studies to determine the impact of the GI corona on other aspects of processing of drug particles, such as their interactions with surfaces in the gastrointestinal environment (including mucus) and the gut microbiota.