Impact of enzymatic reduction on bivalent bispecific antibody fragmentation and loss of product purity upon reoxidation

Abstract Antibody disulfide bond (DSB) reduction during manufacturing processes is a widely observed phenomenon attributed to host cell reductases present in harvest cell culture fluid. Enzyme‐induced antibody reduction leads to product fragments and aggregates that increase the impurity burden on the purification process. The impact of reduction on bivalent bispecific antibodies (BisAbs), which are increasingly entering the clinic, has yet to be investigated. We focused on the reduction and reoxidation properties of a homologous library of bivalent BisAb formats that possess additional single‐chain Fv (scFv) fragments with engineered DSBs. Despite all BisAbs having similar susceptibilities to enzymatic reduction, fragmentation pathways were dependent on the scFv‐fusion site. Reduced molecules were allowed to reoxidize with and without low pH viral inactivation treatment. Both reoxidation studies demonstrated that multiple, complex BisAb species formed as a result of DSB mispairing. Furthermore, aggregate levels increased for all molecules when no low pH treatment was applied. Combined, our results show that complex DSB mispairing occurs during downstream processes while aggregate formation is dependent on sample treatment. These results are applicable to other novel monoclonal antibody‐like formats containing engineered DSBs, thus highlighting the need to prevent reduction of novel protein therapeutics to avoid diminished product quality during manufacturing.