Immunological Considerations of Polysorbate as an Excipient in Botulinum Neurotoxin Type A Formulations: A Narrative...

Recent botulinum neurotoxin type A (BoNT/A) formulations have shifted towards the use of polysorbate 20 (PS20) and polysorbate 80 (PS80) as a non-human-derived excipient to enhance product stability. Polysorbates are a distinct class of synthetic non-ionic surfactants with high heterogeneity in chemical structure and properties. Accumulating mechanistic and clinical evidence suggests that they may trigger immunological reactions, including hypersensitivity and immunogenicity. Such risks are largely associated with their susceptibility to degradation via hydrolysis and oxidation, forming reactive byproducts that can interact with proteins and immune pathways. Despite these mechanistic insights, data on the association between polysorbate excipients and observed immune outcomes in practice is relatively sparse and excipient-related immunogenicity and hypersensitivity is often underrecognized in practice. This review provides a summary of polysorbate excipients in BoNT/A formulations, focusing on their chemical properties and degradation pathways, characterizing downstream immune effects and appraising available clinical data of polysorbate-containing BoNT/A formulations. Finally, we discuss potential risk mitigation strategies including process modifications that could prevent degradation, and consideration of alternative excipients, such as human serum albumin, that has been shown to be immunologically inert and has an established safety profile. By integrating chemical, mechanistic, and clinical perspectives, this review seeks to clarify the implications of polysorbate use in BoNT/A formulations and inform both clinical practice and future formulation strategies.