To borrow an expression from the world of competitive chess, “every Russian schoolboy knows” what factors affect biomanufacturing product yield and quality. Start with a robust cell line, select for viability and expression level, then experiment around media, feed, and bioreactor/fermentor conditions. Throw in some analytics and you’re set.
Yet assembling these building blocks is a balancing act, requiring science, timing, logistics, the manufacturer’s and/or sponsor’s experience, efficiency of technology transfer from premanufacturing (whether in-house or not), and needs for material quantity and quality.
The ultimate goal of cell-culture optimization is to create a robust, reproducible, predictable process. “Optimization has traditionally focused on cell growth and productivity, and higher throughput” says Bretta Erskine, who heads cell culture supplements marketing for EMD Millipore. Strategies for improving cultures include cell-line optimization, media and feed development, and optimization of bioreactor conditions. Where automation has improved how cells are selected and assayed, single-use containers for both cell culture and media/buffer-related activities improve throughput and flexibility, particularly during the early production.
Productivity is still a top priority, but a change in mind-set leading to greater appreciation of quality-by-design (QbD) has caused a shift in how manufacturers approach cell culture. “Once you’re past cell-line development you still want to produce a lot of cell mass and product, but here the tweaking and optimization is more focused on quality,” Erskine explains.
The tradeoff between quality and productivity remains quite real, to the point where one must sometimes sacrifice cell mass and titer. “But that’s a good thing. In the past the problem was that people were not focusing enough on quality. If you produce a ton of protein and you have to throw half of it away, what good is it? It’s a trade-off, but one that’s worth it in the end.”
Millipore, for example, uses a flow cytometer from its Guava Technologies division to monitor cultures for evidence that significant numbers of cells are dying. Cell death is accompanied by release of proteases, which degrade proteins, impurities that may complicate downstream processing. Apoptosis is particularly troublesome in fed-batch cultures.
Kevin Gilbride, Ph.D., of Gilbride Consulting agrees that cell viability is a huge factor in product quality. “You have to monitor cells constantly, not just during manufacturing, but in process development.”
At one of his earlier industrial positions, Dr. Gilbride replaced a trypan blue stain with a much more complex, but indicative assay for externalized phosphatidylserine. “Trypan blue can underestimate apoptosis. It’s a large molecule that dying cells can’t always incorporate. Whereas healthy cells never externalize phosphatidylserine.”