Vitamin B₁₂ association with mAbs: Mechanism and potential mitigation strategies

Process control for manufacturing biologics is critical for ensuring product quality, safety, and lot to lot consistency of therapeutic proteins. In this study, we investigated the root cause of the pink coloration observed for various in-process pools and drug substances in the antibody manufacturing process. Vitamin B₁₂ is covalently bound to mAbs via a cobalt-sulfur coordinate bond via the cysteine residues. The vitamin B₁₂ was identified to attach to an IgG4 molecule at cysteine residues on light chain (Cys-214), and heavy chain (Cys-134, Cys-321, Cys-367, and Cys-425). Prior to attachment to mAbs, the vitamin B₁₂ needs to be in its active form of hydroxocobalamin. During culture media preparation, storage and cell culture processing, cyanocobalamin, the chemical form of vitamin B₁₂ added to media, is converted to hydroxocobalamin by white fluorescence light (about 50% degradation in 11-14 days at room temperature and with room light intensity about 500-1,000 lux) and by short-wavelength visible light (400-550 nm). However, cyanocobalamin is stable under red light (wavelength >600 nm) exposure and does not convert to hydroxocobalamin. Our findings suggests that the intensity of pink color depends on concentrations of both free sulfhydryl groups on reduced mAb and hydroxocobalamin, the active form of vitamin B₁₂ . Both reactants are necessary and neither one of them is sufficient to generate pink color, therefore process control strategy can consider limiting either one or both factors. A process control strategy to install red light (wavelength >600 nm) in culture media preparation, storage and culture processing areas is proposed to provide safe light for biologics and to prevent light-induced color variations in final products.