SCIENCE PHOTO LIBRARY environmental control including a reproducible composition of the culture medium , pH , level of agitation and level of oxygenation . 5 Batch to batch variation can occur due to differences in the source of nutrients , the presence of trace metals or other impurities , inducing subtle variations in the qualitative or quantitative composition of variants ( isoforms ), particularly on the carbohydrate fraction of glycoproteins or at the COOH terminus of the protein chains . 6 For instance , the DNA sequence for IgG1 antibodies codes for lysine at the C-terminus of each heavy chain although during the cell culture , one or both of these lysine moieties are usually removed . This removal occurs due to hydrolysis and is largely dependent on the culture conditions . 6 Overall , these reactions explain a natural batch-to-batch variability in the composition of the final biological molecule that can be observed , from one batch to another , and will occur for both the originator or a biosimilar . 7 Consequently , manufacturers are required to characterise the molecular source of the variations and demonstrate that they have no effect on potency or safety . For glycoproteins , the variants can also originate from the carbohydrate moiety and which can increase the number of co-existing variants in the same product .
A further and important problem is the presence of impurities from the host cells – the host cell proteins ( HCPs ). The importance of these HCPs will depend on the kind of cell used and on the modalities of extraction of the target protein from the cell culture medium . Indeed , if after the synthesis of the target protein inside the host cell , it remains sequestered into the cytoplasm ( i . e ., not secreted ), it will be necessary to destroy the host cell to extract it 8 , 9 and which can pose a challenge to isolate and purify the target protein .
Isolation and purification After culture of the cells under optimal conditions and time , the target protein must be isolated from the culture media . Isolation can be done via filtration , sedimentation and / or centrifugation . Subsequent purification of the target protein usually is done through complex and expensive chromatography techniques ( e . g ., exclusion , ion exchange , and affinity ) and concentration , buffer exchange or desalting related processes . Moreover , HCPs derived from prokaryotic cells are particularly immunogenic and while these may be
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