Interfacial Stability in Monoclonal Antibody Formulations: The Role of Oleic Acid from PS80 Degradation

The adsorption of monoclonal antibodies (mAbs) at the air–water interface is a major pathway for aggregation and particle formation in therapeutic protein formulations. Polysorbate 80 (PS80) is widely used to mitigate this risk by preferentially occupying the interface; however, in highconcentration formulations PS80 can undergo enzymatic degradation, generating surface-active fatty acids such as oleic acid (OA). This study focuses on the co-adsorption of PS80 and OA at the air-water interface and its potential impact on the composition and structure of the protective excipient layer.

Here, we combine Langmuir trough measurements, pendant-drop tensiometry, and synchrotron Xray reflectivity (XRR) to determine how OA alters interfacial composition in our formulations across a range of PS80 concentrations. To mimic in situ PS80 degradation, PS80 levels were systematically reduced while OA was increased. We use bulk PS80 solutions above the critical micelle concentration, to ensure fast adsorption (necessary in pharmaceutical applications to prevent mAb adsorption), and OA is added to solution in prescribed amounts.

PS80–OA studies show that above the PS80 critical micelle concentration, OA is incorporated into mixed micelles that adsorb at the air–water interface and form mixed monolayers with lower equilibrium surface tension than PS80 alone. X-ray reflectivity (XRR) further reveals the formation of dense mixed OA/PS80 interfacial layers, demonstrating that higher OA content in the micelles leads to greater OA accumulation at the interface and increased interfacial electron density. However, these high-density interfacial layers also exhibit elevated elasticity, which may promote surface-stress–induced particle formation.