Stability of biologic drug formulations : adsorption to interfaces and aggregation
Keywords: Chemical Engineering.

Monoclonal antibodies (mAbs) are therapeutic proteins that can uniquely recognize and neutralize pathogens in the body, making them well-suited for the treatment of various diseases like cancer or autoimmune disorders. These amphiphilic molecules readily adsorb to air-solution interfaces to form a viscoelastic network, which can lead to aggregation. One of the major challenges faced in the large-scale manufacture of these antibody molecules is their instability and propensity to form aggregates. This aggregation has an adverse effect on the quality and the immunogenicity of the drug and is triggered primarily due to interfacial stresses. Therefore, understanding the interfacial behavior of these antibody molecules is crucial in determining their aggregation propensity. In the biopharmaceutical industry, surfactants are typically added to mAb formulations as they competitively adsorb to the air-solution interfaces and prevent the mAbs from adsorbing and aggregating. First, we studied two mAbs, provided by our industrial collaborator Genentech, with different propensities to aggregate at air-solution interfaces. In this work, we performed interfacial stress relaxation studies under compressive step strain using a custom-built dilatational rheometer. The dilatational relaxation moduli correlated with the aggregation tendencies of the two mAbs. We also measured the time taken for bubbles laden with mAbs to coalesce with another such interface. To study the influence of surfactants in mAb formulations, polyethylene glycol (PEG) was chosen as a model surfactant. The effect of this surfactant in lowering the interfacial elasticity and the coalescence times was studied for both the mAbs. Then, we focused on the mAb that was prone to aggregation and studied the mixtures of these antibodies with three pharmaceutically relevant surfactants: polyethylene glycol, poloxamer-188 and polysorbate-20. We performed agitation studies and characterized the aggregation of both smaller (oligomeric) aggregates (< 100 nm) and larger (sub-visible) particulate aggregates using size exclusion chromatography, flow cytometry and light obscuration techniques. The air-solution interface was visualized through fluorescent confocal microscopic imaging of fluorescently-tagged mAbs and co-adsorbed surfactants. Surface tension and mAb relative surface coverage were quantified for these mixtures. Moreover, we used the dynamic fluid-film interferometer, an automated custom-built instrument, to study the thin film drainage as a bubble approached a flat interface in the presence of mAbs and surfactants in the solution. MAbs immobilized the sandwiched thin film, whereas surfactants triggered Marangoni instabilities, caused by differences in surface tension. The volume of fluid entrapped and the nature of Marangoni surface flows depended on different governing mechanisms - interfacial rheology, surface tension and surface tension gradients for different surfactants. It was shown that the level of aggregation at different length scales correlated with the surface tension, surface relative coverage, interfacial rheology and interfacial fluid mechanics. Many mAb formulations are administered intravenously. We mimicked IV bag conditions by diluting the solution in a physiological, normal saline and studied the conformational and colloidal stability of mAbs in the solution and at the interface through Raman spectroscopy, small angle X-ray scattering and dynamic light scattering, and investigated the differences in aggregation due to the presence of salt. We also studied the adsorption and aggregation of the mAbs in saline in the presence of polysorbate-20 in this high ionic strength buffer and compared it with the regular low ionic strength formulation buffer. We observed that at the same surfactant concentration, the presence of salt in the buffer led to differences in the interfacial aggregation behavior in mAb formulations. Moreover, many mAb products are sold in pre-filled syringes that have a coating of silicone oil to provide lubrication. The silicone oil-solution interface is also an important source of aggregation. In addition to our work on air-water interfaces, we studied the nature of mAb adsorption and aggregation at this oil-water interface. It was shown that the aggregation directly correlated with differences in surface activity of the mAbs at oil-water interfaces, studied with interfacial tension, surface mass adsorption and interfacial rheology. This correlation was further reinforced in the coalescence behavior of oil droplets laden with mAbs. We also looked at the efficacy of surfactants in lowering adsorption and aggregation of mAbs at oil-water interfaces. Finally, we embarked on a more fundamental study of the dynamics of underwater bubbles approaching an ultra-thin, porous ultra-high-molecular-weight polyethylene (UHMWPE) film. When an air bubble is released underneath such a film, the bubble rises up, bounces against the film, makes contact after the liquid film dewets, spreads against the polymer film and eventually shrinks in size as the gas within the bubble permeates through the pores of the film. In our work, these events were recorded using a high-speed camera and were characterized through various shape descriptors of the bubbles. The effect of different surface-active species like surfactants, which exhibit interfacial mobility and proteins, which form a viscoelastic interfacial network, was also studied. The adsorption of these surface-active molecules led to profound differences in the interaction of the bubbles and their ultimate removal through the film