An in-depth examination of fatty acid solubility limits in biotherapeutic protein formulations containing polysorbate 20 and polysorbate 80

Two of the most widely used surfactants to stabilize biologicals against e.g. interfacial stress are polysorbate 20 (PS20) and polysorbate 80 (PS80). In recent years, polysorbate degradation in biopharmaceutical formulations has been observed. Polysorbate (PS) is mainly composed of sorbitan and isosorbide fatty acid (FA) esters, varying in their FA composition. Especially hydrolysis, which can be induced chemically as well as enzymatically, leads to the release of FAs from PS. These FAs are poorly soluble in aqueous buffer systems due to their hydrophobic nature and therefore prone to precipitation and particle formation. Since the emergence of particles in liquid formulations has to be avoided, it is important to prevent their formation. This study evaluates the solubility limits of selected FAs, which are likely to be released during the degradation of PS20 and PS80 in the presence of defined PS concentrations. Our results show that the solubility is highly dependent on the pH, the temperature, the used PS concentration and the aliphatic chain of respective FAs. Solubility of FAs, such as palmitic and oleic acid under the conditions determined in this study, are in the range of 3–130 µg·ml−1 (12–460 µM). Furthermore, the results allow making an estimation to which extent PS may degrade before particle formation in the drug product may be expected.