Although biopharmaceuticals offer high potency and target specificity, the efficient production of therapeutic proteins still faces multiple challenges. These include discovering new approaches to maximize protein expression, developing cost‑effective, flexible, and robust manufacturing processes to boost product yields, and addressing the complex challenge of refolding proteins into their active conformations.
One area of growing industry interest in addressing these challenges is trace elements in cell culture media and upstream processing. Trace elements in cell culture media and supplements can variably promote or inhibit cell growth, as well as protein expression or quality, during upstream bioprocessing.
Effective concentrations of trace elements in cell culture media are typically very low, often falling below the detection limits of standard analytical instruments. Yet their importance is disproportionately high relative to their added amounts. Many trace metals play critical roles in regulating metabolic pathways and the activities of certain enzymes and signaling molecules.
Copper
In CHO cell culture, copper deficiency can lead to the downregulation of lactate dehydrogenase and other mitochondrial oxidases, resulting in histotoxic hypoxia independent of dissolved oxygen concentration. Accordingly, higher copper levels can shift lactate metabolism in CHO cells from net lactate production to net lactate consumption, thereby supporting cell growth and titer. However, elevated copper concentrations can also increase the relative abundance of basic charge isoforms of antibody products. For this reason, copper levels in CHO cell culture media must be carefully optimized based on both culture performance and product quality.
Iron
Iron is widely recognized as an essential component of chemically defined media. It plays a key role in oxygen transfer via heme groups, mitochondrial oxidative pathways, and other vital enzymes. Nevertheless, free iron, especially free iron ions, can induce high oxidative stress even at trace levels. Siderophores or chelating agents may be incorporated into media to reduce toxicity and improve cellular iron uptake.
Transferrin is a highly effective iron carrier that significantly influences iron absorption. In serum‑containing media, transferrin is naturally present as a serum component, whereas in serum‑free media, recombinant transferrin can be supplemented to improve iron stability and uptake. As low‑cost alternatives to transferrin, small‑molecule chelators such as tropolone, citrate, and selenite have also been employed.
Optimal ratios of selenite to iron and citrate have shown that selenite can promote CHO cell growth in iron‑deficient media by enhancing iron uptake, with efficacy comparable to tropolone. In contrast, citrate alone without selenite does not produce the same effect. Subsequent studies, however, demonstrated that the combination of ferrous sulfate and sodium citrate can improve translation efficiency and protein titer.
Zinc
Zinc is one of the most impactful trace elements for monoclonal antibody productivity in chemically defined and protein‑free CHO cell culture media. Supplementation of zinc in commercial chemically defined media has been shown to increase monoclonal antibody yields by up to 1.2‑fold. Furthermore, exposure of CHO cells to zinc can induce the function of stress proteins and subsequently reduce apoptosis.
Key Trace Elements in CHO Cell Culture Media and Their Functions
Copper supports mitochondrial oxidases and regulates lactate consumption, with an optimal concentration range of 0.8–100 μM.
Iron influences the macro‑heterogeneity of product glycosylation and supports cell growth and health, with a concentration range of 10–120 μM.
Zinc enhances monoclonal antibody production and reduces apoptotic progression, with concentrations between 3–60 μM.
Manganese improves galactosylation and reduces product sialylation, at a concentration range of 0.4–40 μM.
Molybdenum increases the production of bioactive substances, with a very low concentration range of 0.001–0.1 μM. Selenium facilitates iron transport and protects cells against oxidative stress and free radicals, at 0.005–0.5 μM.
Vanadium mimics the metabolic functions of insulin or insulin‑like growth factors and promotes cell growth, with a concentration range of 0.1–70 μM.
Cobalt enhances N‑glycan galactosylation and terminal protein glycosylation, as well as improves productivity in chemically defined media, with concentrations ranging from 0–50 μM.
Beyond the major trace elements discussed above, other elements also influence cell growth, productivity, and product quality. Most of these trace elements are present at concentrations below 5 μM in cell culture media.
Manganese, molybdenum, selenium, and vanadium are well‑known requirements for cell culture and are therefore included in most media formulations. Other trace elements, including germanium, rubidium, zirconium, cobalt, nickel, tin, and chromium, may be required or functional for certain mammalian cells and are thus incorporated into some specialized media.
Reported Effects of Metal Ions (Fe, Cu, Zn, Mn) on Critical Quality Attributes of Biopharmaceutical Proteins
Aggregation
For iron, at low concentrations (0–4 ppm) in the presence of chelating agents, high‑molecular‑weight (HMW) species are significantly reduced; increasing concentrations of ammonium ferric citrate in cell culture lead to higher dimer levels, and HMW species increase slightly in a dose‑dependent manner with elevated ferric citrate and ammonium ferric citrate, while no changes are observed with ammonium alone. High copper levels induce increased aggregation of IgG, elevated Cu²⁺ increases protein aggregation of IgG fusion protein FP‑A, excess CuSO₄ in media causes a mild increase in IgG aggregation, and reduced copper sulfate levels increase the monomeric form of IgG fusion protein B0. No changes are observed with varying manganese concentrations.
Fragmentation and Degradation
In the presence of histidine, iron enhances the cleavage of human IgG1, especially under strong oxidative conditions. IgG1 fragmentation increases with rising Cu²⁺ concentration.
Charge Isoforms
Iron increases acidic isoform levels via reactive oxygen species generated through Fenton chemistry. Increased Cu²⁺ elevates proline amidation, leading to higher basic charge isoforms; reduced Cu²⁺ decreases tryptophan oxidation and overall acidic species; and elevated Cu²⁺ increases C‑terminal lysine levels, contributing to higher basic charge isoforms. Increased Mn²⁺ reduces tryptophan oxidation and overall acidic species, likely by competing with iron to inhibit Fenton reactions. Within a specific concentration range (~40–130 μM), Zn²⁺ reduces acidic isoforms and increases main peak levels; as a key cofactor for carboxypeptidases, Zn²⁺ also lowers C‑terminal lysine and overall basic species.
Amidation and Deamidation
Monoclonal antibody deamidation increases with iron concentration. Higher Cu²⁺ levels in production media increase proline amidation, resulting in elevated basic charge isoforms.
Glycosylation
Iron bioavailability determines the glycosylation profile of glycoproteins; increased iron supplementation in CHO cell culture improves galactosylation and site occupancy on glycoproteins. Cu⁺ has been reported as a potent inhibitor of mannosidase I activity. Manganese chloride supplementation affects Man5 availability in CHO cells, and under glucose‑limited or glucose‑free conditions, high mannose content increases with elevated Mn²⁺. Increasing Zn²⁺ relative to Mn²⁺ reduces galactosylation levels due to decreased galactosyltransferase activity, while Mn²⁺ catalyzes the transfer of galactose from UDP‑Gal to N‑acetylglucosamine.
Although providing trace elements in CHO cell culture media is essential, most trace elements can be toxic to CHO cells at relatively high concentrations. If trace element concentrations in media are near critical thresholds, contamination of media powders, other raw materials, or cell culture vessels with certain trace elements — due to impurities or improper manufacturing — may lead to inconsistent performance or cellular toxicity.
Understanding and controlling the potential outcomes of trace element variability enables a more predictable and tightly controlled robust upstream process. This translates to consistent product yields across cell culture batches and higher output purity. Such control can benefit downstream processing by simplifying purification steps and potentially reducing the number of unit operations.
Ultimately, defining and understanding target trace element levels means recognizing their roles in both product manufacturing and processes such as glycosylation. Identifying and adequately characterizing trace element levels will lay the foundation for the tightest possible process control.
