Introduction
Cation exchange chromatography (CEC) is a well-established unit operation in the downstream processing of monoclonal antibodies. In many processes this type of chromatography is part of a multi-step sequence which starts with an affinity step for capture and then adds further chromatography steps (most commonly cation exchange chromatography, anion exchange chromatography (AEC) or hydrophobic interaction chromatography (HIC)) in different combinations [1] and [2]. The order of the sequence and therefore also the position of the CEC step in the overall process can vary as it depends on several factors, e.g. the composition of the cell culture supernatant (concentration and character of impurities), the stability profile of the antibody, or a manufacturer's preferred approach. When affinity chromatography, most often represented by a resin with immobilised protein A as ligand, is applied for capture, the CEC step will be part of intermediate purification or polishing. In these later stages of a downstream process, step productivity (defined by dynamic binding capacity of a resin and operational flow rate amongst others) remains an issue for the drug manufacturer for the reason of process economy. In a comparative study on different cation exchangers the methacrylate based resins Fractogel® EMD SO3− (M) and Fractogel® EMD SE Hicap (M) were found to have the highest overall binding capacity for the antibody tested [3]. Besides productivity, however, the potential of a step for removal of impurities may have to be rated even higher, as it is an absolute requirement to meet the purity specifications, set by the regulatory authorities, at the end of the process.