As the GEN article "Prospects for Downstream Production" notes, perfusion cell culture is most often associated with upstream continuous processing operations.
The two main perfusion techniques are microcarrier-based processes (e.g., iCELLis™ from Pall/ATMI) and those in which cells flow across the membrane through which product is harvested (e.g., GE Healthcare’s hollow fiber microfiltration). A third method, based on centrifugation, was under investigation during the 1990s and early 2000s, but has fallen out of favor.
Benefits of perfusion over fed-batch cultures include higher productivity and, at least theoretically, lower cost of goods.
According to Pall, iCELLis combines the “advantages of single-use technologies with the benefits of a fixed-bed system.” This design, which relies on microcarriers, provides a 500m2 growth area (approximately equal to 3,000 roller bottles) within a 25L volume.
Because cell multiplication occurs in a fixed bed, iCELLis bioreactors are inoculated at low density, with reduced manual operation. iCELLis is available as a fully-disposable product, which scales from bench to production.
“Perfusion cell culture provides higher productivity from a smaller set of assets,” says Nick Hutchinson, market development manager at Parker Hannifin Manufacturing. “Quality may also improve because cells are in a steady state throughout the culture.”
Recent improvements in cell density and product titers should have lessened the appeal for continuous cell culture, but this has not been the case. Hutchinson says “there’s more noise than ever” over perfusion methods.
Perfusion’s major challenge compared with fed-batch is maintaining a controlled, sterile, cell-friendly environment for weeks or months.
“Fed batch is much simpler, and doesn’t require moving volumes of cell culture and product-containing media around,” adds Hutchinson. Movement of process fluids increases the risk of contamination, leakage, and equipment failure.
Perfusion cultures raise additional concerns of handling contamination, specifically the cutoff for retaining or discarding product once contamination is detected. It also requires more intense process development than fed-batch culture.
Development time becomes a serious issue when processors face the need to test numerous culture conditions, which could result in delayed time-to-clinic or time-to-market.
There is also some disagreement on productivity calculations. “OK, you’ve reduced your fixed asset footprint, and all the utility and other requirements that entails, but perfusion cultures use a great deal more cell culture media, which is not cheap,” says Hutchinson. “If you view product titer for overall media usage, perfusion doesn’t always look that great.”