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Strategies for Improved dCO2 Removal in Large-Scale Fed-Batch Cultures

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Abstract

Carbon dioxide buildup in large-scale reactors can be detrimental to cell growth and productivity. In case of protein X, a therapeutic glycoprotein, when cultures were scaled up from bench scale to the pilot plant, there was a 40% loss of specific productivity. The dissolved CO2 (dCO2) level was 179 ± 9 mmHg at the pilot plant scale and 68 ± 13 mmHg at bench scale. The authors proposed a comprehensive approach to maintain dCO2 levels between 40 and 120 mmHg throughout the 14-day fed-batch process. A cell-free experiment was used to investigate the impact of the following parameters on dCO2 removal: (1) sparge rate, (2) agitator speed, (3) bubble size, (4) bicarbonate concentration, (5) impeller position, and (6) aeration rate at the headspace of bioreactor. dCO2 was measured using a fiber optic based probe. dCO2 removal rate was a strong function of sparge rate and a weak function of agitator speed. Bubble size was modulated by the presence or absence of a sparge stone (10 μm pore size, 1 cm pipe i.d.). Open pipe provided 3- to 4-fold better dCO2 removal for the same mass transfer coefficient ( kLa) value. A mathematical model and a bench-scale experiment indicated that the benefit of a lower level of sodium bicarbonate in the culture medium was transient for batch and fed-batch cultures. Thus, this strategy was not used at pilot scale. Decreasing top impeller position improved kLa of dCO2 by 2-fold. Changing headspace aeration rate from 0.02 to 0.04 vvm had no impact on dCO2 removal. Two pilot runs were conducted using (A) open pipe and (B) antifoam in the presence of sparge stone, both in conjunction with lower impeller position. The presence of antifoam may interfere in product purification; however, demonstration of antifoam removal can be difficult. Open pipe allowed an alternative to using antifoam, as foam level with open pipe was significantly less. Both strategies successfully reduced dCO2 level by 2.5-fold (179 ± 9 vs 72 ± 9 mmHg). Titer at day 10 of culture improved by 1.5-fold. Specific productivity improved by 41%. Historically, cultures were harvested around day 9–11 because of the high amount of foam; both strategies allowed the cultures to be extended up to day 14, resulting in 2-fold higher titer compared to that of the historical control without compromising protein quality.

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