Understanding the rheology of liquid protein formulations

The work described in this thesis had the main aim of understanding protein solution rheology. This was from a biopharmaceutical perspective, with account of the biophysical properties of proteins and in particular their level of aggregation. Molecular interactions influencing the rheology of a range of protein solutions were studied. Proteins were selected to relate directly to the diversity of protein types used in biopharmaceuticals. In addition, the roles of a surfactant formulation additive and synthetic amphiphilic polymers in the flow behaviour of protein solutions were studied. The effect of protein concentration on solution viscosity in a commercially available biopharmaceutical formulation of a recombinant albumin (rAlbumin) was studied. The effect of the level of protein aggregation, variation in protein concentration and its impact on solution viscosity was revealed. Theoretical models predicting the increase of viscosity with concentration were applied to these data. A recent model that accounts for multiple protein species in solution, predicted the experimental data best. The rAlbumin study, although a relatively simple system, represented a 'real-life' formulation with results highlighting the need to account for heterogeneity in the level of aggregation when addressing the increase of viscosity observed at high concentration of protein solutions. Beta-lactoglobulin (b-LG) excipient-free solutions were characterised by bulk and interfacial shear rheology. Solutions at various concentrations, characterised using conventional rheology instrumentation, evidenced an apparent yield stress behaviour at a low shear rate range (0.01 - 10 1/s), whilst showing constant viscosities throughout higher shear rates. Comparing interfacial shear rheology, air-water interface-free bulk rheology measurements, and tensiometry results, it was demonstrated that the complexity of this protein's solution rheology was due to the formation of a protein viscoelastic film at the air-water interface, as present in conventional rheometry. This is in agreement with literature. Further studies considered the effect of insoluble b-LG aggregates on the solutions' rheology, linking with their characterisation in size and quantication. The presence of insoluble proteinaceous particles was suggested to have an impact on the solution's flow behaviour, particularly at the lower shear rates. Excipient-free monoclonal antibody (mAb) solutions were studied with the aim of generating protein aggregates (soluble and insoluble) to explore their impact on solution rheology. mAb samples were subjected to thermal stress and were characterised for their purity, aggregate content and size. The change in species content did not alter the original protein's yield-stress behaviour at low shear rates. An increase in aggregate content was related to the increase of viscosities observed at high shear rates. Establishing a relationship between species content (in volume fraction) and viscosities, as for the rAlbumin study, was not possible due to this mAbs specific aggregation behaviour. However, from the b-LG and mAb case studies, our results highlight the importance of detailed characterisation of protein solutions with orthogonal biophysical techniques so as to better understand protein solution rheology. An additional study looking at the effect of polysorbate-80 upon protein rheology was made. In agreement with literature, this commonly used excipient in biopharmaceuticals was demonstrated to affect the rheological measurements of globular protein solutions. Amphiphilic brush-like poly(ethylene glycol) methacrylate polymers were also synthesised and tested as novel additives with b-LG and mAb solutions, for their potential effects on protein solution rheology, similar to those observed with polysorbate-80. Preliminary results showed that the effects of these polymers are likely related to competition for the air-water interface, between these and the proteins involved. This competition leads to changes in the yield-like behaviour at low shear rates.