Cultivations of mammalian cells are routinely conducted in shake flasks. In contrast to instrumented bioreactors, reliable options for non-invasive, time-resolved monitoring of the culture status in shake flasks are lacking. The Respiration Activity Monitoring Respiration Activity Monitoring System system was used to determine the oxygen transfer rate (OTR) in shake flasks. It was proven that the OTR could be regarded as equal to the oxygen uptake rate as the change of the dissolved oxygen concentration in the liquid phase over time was negligibly small. Thus, monitoring the oxygen transfer rate (OTR) was used to increase the information content from shake flask experiments. The OTR of a Chinese hamster ovary cell line was monitored by applying electrochemical sensors. Glass flasks stoppered with cotton plugs and polycarbonate flasks stoppered with vent-caps were compared in terms of mass transfer characteristics and culture behavior. Similar mass transfer resistances were determined for both sterile closures. The OTR was found to be well reproducible within one experiment (standard deviation <10%). It correlated with changes in cell viability and depletion of carbon sources, thus, giving more profound insights into the cultivation process. Culture behavior in glass and polycarbonate flasks was identical. Monitoring of the OTR was applied to a second culture medium. Media differed in the maximum OTR reached during cultivation and in the time when all carbon sources were depleted. By applying non-invasive, parallelized, time-resolved monitoring of the OTR, the information content and amount of data from shake flask experiments was significantly increased compared to manual sampling and offline analysis. The potential of the technology for early-stage process development was demonstrated.

Background: Shake flasks are frequently used for mammalian cell suspension cultures. For process development and routine culture monitoring, information on culture behavior is needed early on. Main methods and major results:Here, cell-specific oxygen uptake rates (qO 2 ) of two CHO cell lines were determined from shake flask experiments by simultaneous measurement of oxygen transfer rates (OTR) and viable cell concentrations (VCC). For cell line one, qO 2 decreased from 2.38⋅10 −10 to 1.02⋅10 -10 mmol cell −1 h −1 during batch growth. For cell line two, qO 2 was constant (1.90⋅10 −10 mmol h −1 ). Determined qO 2 values were used to calculate the VCC from OTR data. Cumulated oxygen consumption and glucose consumption were correlated for both cell lines and enabled calculation of glucose concentrations from OTR data. IgG producing cell line one had an oxygen demand of ∼15 mmol oxygen g glucose −1 , cell line two consumed ∼5 mmol oxygen g glucose −1 .The established correlations for determination of VCC and glucose were successfully transferred to subsequent cultivations for both cell lines. Combined measurement of the OTR and the carbon dioxide transfer rate enabled quantitative determination of the lactate concentration (production and consumption) without sampling. Conclusions and implications:Taken together, non-invasive measurement of the respiration activity enabled time-resolved determination of key culture parameters for increased process understanding in shake flasks. K E Y W O R D Scarbon dioxide transfer rate, Chinese hamster ovary cells, oxygen transfer rate, respiratory quotient, shake flasks INTRODUCTIONChinese hamster ovary (CHO) cells are the most important mammalian cells used for the stable production of therapeutic proteins. [1] Even though traditionally applied for microbial cultivations, shake flasks areThis is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Background Determining the cytotoxicity of test substances is essential for the safety assessment of chemicals. To quantify the cytotoxicity, dose–response curves are determined and the half-maximum inhibitory concentration (IC50) is subsequently calculated. Results often rely on a single endpoint evaluation which typically requires manual sampling and subsequent sample analysis to determine the IC50. Hence, no information on culture behavior are available during treatment. Here, measurement of the oxygen transfer rate (OTR) was applied as a method for time-resolved assessment of the cytotoxicity to (a) provide information on culture behavior during treatment and (b) quantitatively assess the cytotoxic effect of a test substance. Results To investigate the applicability of the method, different types and concentrations of test substances were added and the OTR was measured for two different CHO suspension cell lines in shake flasks: (a) For CHO cell line one, culture behavior was not affected when 0.1 mM ZnCl2 was added. However, for both cell lines, adding up to 0.2 mM CoCl2 led to a decrease in the OTR that was concentration dependent. For CHO cell line two, adding up to 5% DMSO led to a concentration-dependent decrease in the OTR and in the slope of the OTR; (b) From the measured OTR, dose–response curves were established and used to calculate the IC50. For cell line one, the IC50 for CoCl2 was 0.03 mM while the IC50 from a comparative measurement based on trypan blue exclusion was 0.06 mM. Cell line two was less susceptible to CoCl2 as the IC50 was not reached at the concentrations tested. For DMSO, single OTR values and determined slopes were used for evaluation. In both cases, the IC50 was calculated to 2.3%. Conclusions Our approach provides a method for analyzing the cytotoxicity of a test substance based on OTR measurements. This method provides (i) insights on culture behavior, (ii) information on the progression of cytotoxicity, (iii) dose–response curves, and (iv) a first indication of IC50 values. Quantitative assessment of cytotoxicity is possible non-invasively and in real-time during treatment. Compared to traditional endpoint evaluation, our method simplifies experimentations and enhances the comparison of assay results.

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Cultivating Chinese hamster ovary (CHO) cells in microtiter plates (MTPs) with time‐resolved monitoring of the oxygen transfer rate (OTR) is highly desirable to provide process insights at increased throughput. However, monitoring of the OTR in MTPs has not been demonstrated for CHO cells, yet. Hence, a CHO cultivation process was transferred from shake flasks to MTPs to enable monitoring of the OTR in each individual well of a 48‐well MTP. For this, the cultivation of an industrially relevant, antibody‐producing cell line was transferred from shake flask to MTP based on the volumetric oxygen mass transfer coefficient (kLa). Culture behavior was well comparable (deviation of the final IgG titer less than 10%). Monitoring of the OTR in 48‐well MTPs was then used to derive the cytotoxicity of dimethyl sulfoxide (DMSO) based on a dose–response curve in a single experiment using a second CHO cell line. Logistic fitting of the dose–response curve determined after 100 h was used to determine the DMSO concentration that resulted in a cytotoxicity of 50% (IC50). A DMSO concentration of 2.70% ± 0.25% was determined, which agrees with the IC50 previously determined in shake flasks (2.39% ± 0.1%). Non‐invasive, parallelized, and time‐resolved monitoring of the OTR of CHO cells in MTPs was demonstrated and offers excellent potential to speed up process development and assess cytotoxicity.