Mammalian cell cultures. Part II: Genetic engineering, protein glycosylation, fermentation and process control

For expression of human genes in mammalian cell culture regulatory sequences such as promotor or terminator region of viral origin are required. The most widely used expression system uses dihydrofolic acid reductase (DHFR) as a selection marker in conjunction with a DHFR deficient Chinese hamster ovary (CHO) cell using methotrexate as selection pressure. Alternatively the glutamine synthetase amplification system seems to be one of the most efficient expression systems using methionine sulphoximine (MSX) as selection pressure. Folding and glycosylation takes place in mammalian cell cultures at the sites of endoplasmatic reticulum and the Golgi apparatus and is comparable to synthesis in human cells. Most large scale manufacturing processes for products derived from mammalian cell cultures are fed batch suspension culture processes up to 15,000 l. Important factors for productivity are media composition and feeding strategies. Sterility of the entire system during the fermentation period is a decisive factor for success rate. Because mammalian cell cultures reacting very sensitive to small changes in temperature, pH, osmolality and agitation the fermentation system requires a reliable process control system. Validation of the entire manufacturing process is required to ensure consistent product quality which meets predetermined specifications and to provide a basis for an economic process. In a joint effort equipment qualification, process validation, in-process controls and quality controls provide the basis for product consistency from batch to batch.