Removal of empty capsids from adeno-associated virus preparations by multimodal metal affinity chromatography

Separation of empty and full adeno-associated virus capsids by multimodal metal affinity chromatography was investigated using a positively charged metal affinity ligand. A subpopulation of empty capsids eluted first, followed by full capsids, and later by more empty capsids and debris. Empty and full capsid composition of chromatography fractions was evaluated by cesium chloride density gradient centrifugation followed by stratigraphic flow analysis of the centrifuge tube contents, monitored by intrinsic fluorescence. Columns charged with barium, calcium, magnesium, zinc, manganese, and ferric ions gave similar results with respect to capsid separation. Charging with cupric ions maintained resolution between early-eluting empty capsids and full capsids but caused them to elute at lower conductivity. Empty and full capsids were fractionated with Tris-borate gradients, sodium chloride gradients, and magnesium chloride gradients. Recovery of full serotype 9 capsids was 100% with complete elimination of empty capsids. All metal ions bound contaminant subsets that required sodium hydroxide for removal. Columns charged with ferric iron and manganese bound more contaminants than all other metals. Columns charged with calcium, magnesium, barium, and copper bound the least. Contaminant binding on zinc-charged columns was intermediate between the two groups.