Defining Purity in Botulinum Neurotoxin Type A Formulations: A Narrative Review on Past and Future Developments

Botulinum neurotoxin type A (BoNT/A) has become one of the most widely utilized biologics today in both therapeutic and aesthetic applications. Alongside innovative explorations in BoNT/A use, advancements have also been made in manufacturing and formulation practices. The earliest concept of a “pure” BoNT/A referred to the successful isolation of the native neurotoxin complex from Clostridium botulinum, which enabled its clinical development. Subsequently, various definitions of the term “purity” have been applied to BoNT/A, encompassing distinct scientific, regulatory, clinical, and commercial interpretations. Formulation advances first emphasized the removal of non-toxic bacterial components in efforts to reduce protein load and immunogenicity. Increasingly, analytical methods such as SDS-PAGE and ELISA are also commonly cited to support claims for BoNT/A identity and quantity and define purity; however, these provide limited information regarding biological activity. Given that BoNT/A is not a single uniform molecular entity, differences in bacterial strains, manufacturing processes, and the presence or absence of complexing proteins have resulted in distinct product formulations that further complicate direct comparisons relating to purity. This review examines various working definitions of BoNT/A purity, discusses implications for clinicians and patients, and highlights emerging scientific and regulatory efforts to improve product comparability through standardized and biologically relevant measures of potency.