Downstream purification (Protein A capture, viral clearance, polishing)

Downstream processing takes the harvested cell culture fluid from a CHO bioreactor and turns it into bulk drug substance. The harvest is clarified by centrifugation and depth filtration, captured on a Protein A affinity column that binds the antibody's Fc region and lets everything else through, held at low pH to inactivate enveloped viruses, polished on ion exchange and mixed-mode columns to strip host-cell proteins, DNA and aggregates, passed through a virus-retaining nanofilter, and concentrated and buffer-exchanged by ultrafiltration and diafiltration before sterile filtration. Regulators require validated viral clearance of several orders of magnitude across the steps, and every batch is tested for host-cell protein, DNA, aggregates, charge variants and glycan pattern.

Protein A resin is the single most expensive consumable in antibody manufacture and is supplied by a small number of makers (Cytiva, Merck KGaA's life science arm, Thermo Fisher, Purolite and Tosoh among them), and column and membrane capacity, not bioreactor volume, often sets how fast a plant can run as titres rise. Continuous and multi-column chromatography, membrane adsorbers and precipitation are the current efficiency frontier. Antibody-drug conjugates repeat parts of this chain after conjugation to remove free payload.