Effects of pH and Ionic Strength on Aggregation of Surfactants and Preservatives in Biologic Drug Formulations

Multi-injection biologic drugs, such as insulin, must be formulated with different excipients to remain stable and sterile. In particular, use of nonionic surfactants and small molecule preservatives increases drug stability and prevents antimicrobial contamination, respectively. Nonionic surfactants or preservatives on their own are well soluble in water and cause no aggregation. However, when combined, aggregation occurs creating adverse turbidity and instability. Previous work shows that preservatives such as phenol in combination with the surfactants such as poloxamer 188 (P188) result in micelle formation and ultimately unstable turbidity. This occurs at concentrations well below the solubility limits of each individual material. In this project we hope to understand how solvent properties, including pH and other ionic effects, may increase or suppress aggregation of surfactants and preservatives. We will alter the solvent conditions by changing buffer identity, concentration, and pH in various combinations of nonionic surfactants and preservatives commonly used in biologic drug formulations. Specifically, we will test combinations of the surfactants P188, polysorbate 20, and polysorbate 80, with the preservatives phenol and benzyl alcohol, under an array of solvent conditions. This will allow us to determine how aggregation is affected by changes in relation to buffer identity. We will analyze samples visually and with ultraviolet-visible light spectrophotometry, with the eventual goal of detailed structural characterization using small angle scattering. Our results will enhance understanding of the relationship between surfactants and preservatives which will allow for further research of biologic drug stability when both classes of excipients are needed.