Understanding Particle Formation: Solubility of Free Fatty Acids as Polysorbate 20 Degradation Byproducts in Therapeutic Monoclonal Antibody Formulations

The purpose of this work was to determine the aqueous solubilities at 2-8 °C of the major free fatty acids (FFAs) formed by polysorbate 20 (PS20) degradation and identify possible ways to predict, delay, or mitigate subsequent particle formation in monoclonal antibody (mAb) formulations. The FFA solubility limits at 2-8 °C were determined by titrating known amounts of FFA in monoclonal antibody formulations and identifying the FFA concentration leading to visible and subvisible particle formation. The solubility limits of lauric, myristic, and palmitic acids at 2-8 °C were 17 ± 1 μg/mL, 3 ± 1 μg/mL, and 1.5 ± 0.5 μg/mL in a formulation containing 0.04% (w/v) PS20 at pH 5.4 and >22 μg/mL, 3 ± 1 μg/mL, and 0.75 ± 0.25 μg/mL in a formulation containing 0.02% (w/v) PS20 at pH 6.0. For the first time, a 3D correlation between FFA solubility, PS20 concentration, and pH has been reported providing a rational approach for the formulator to balance these with regard to potential particle formation. The results suggest that the lower solubilities of the longer chain FFAs, generated from degradation of the stearate, palmitate, and myristate fraction of PS20, is the primary cause of seeding and subsequent FFA precipitation rather than the most abundant lauric acid.