Bioprocess development for the production of a recombinant MUC1 fusion protein expressed by CHO-K1 cells in protein-free medium (Dedicated to Prof. Dr. C. Wandrey on the occasion of his 60th birthday)

The mucin MUC1 is a candidate for use in specific immunotherapy against breast cancer, but this requires the large-scale production of a MUC1 antigen. In this study, a bioprocess for the expression of a recombinant MUC1 fusion protein with a cancer associated glycosylation in CHO-K1 cells has been developed. Cells permanently expressing parts of the extracellular portion of MUC1 fused to IgG Fc were directly transferred from adherent growth in serum-containing medium to suspension culture in the protein-free ProCHO4-CDM culture medium. Using the Cellferm-pro system, optimal culture parameter as pH and pO(2) were determined in parallel spinner flask batch cultures. A pH of 6.8-7.0 and a pO(2) of 40% of air saturation was found to give best cell growth and productivity of secreted recombinant protein. Specific productivity strongly depended the pO(2) and correlated with the online monitored oxygen uptake rate (OUR) of the cells, which indicates a positive influence of the rate of oxidative phosphorylation on productivity. The optimised conditions were applied to continuous perfusion culture which gave very high cell densities and space time yields of the recombinant MUC1 fusion protein, allowing production at gram scale. The product degradation was much lower in supernatants from continuous perfusion culture compared to batch mode. Antibodies reacting with cancer associated MUC1 glycoforms strongly bound to the fusion protein, indicating that the desired glycoforms were obtained and suggesting that the recombinant MUC1 protein could be tested for use in immunotherapy