The manufacture of protein therapeutics is only as efficient as the expression system. Despite the emergence of several platform approaches (e.g., CHO for monoclonal antibodies), drug developers are constantly tweaking cells and microorganisms to optimize productivity and product quality.
One focus of CHI’s upcoming “Peptalk” will be expression, and this article features several scheduled speakers on the topic.
Kevin Sunley, Ph.D., senior scientist at XBiotech, discusses some remarkable observations on the effect of GE Healthcare’s Cytopore™ microcarriers on CHO cell productivity. Cytopores are porous, hollow, positively charged (due to DEAE functionalization) spheres into which anchorage-dependent cells enter and grow. The idea is to provide very high surface areas for attachment, an environment that protects cells against shear forces, and porosity to allow nutrients to enter and waste products to exit. Dr. Sunley refers to the microcarriers as “mesh snowballs.”
He says the microcarriers work as advertised: Cell-specific productivity rose in proportion to the increase in cell density within the carriers, providing a multifold increase in protein yield compared with stirred-tank suspension culture. The surprise is that the dramatic improvement was not due to cell attachment within the carriers. Upon examination the cells “look like suspension cells, so the yield improvement must be due to some other biological function,” Dr. Sunley says.
He theorizes that the 200-micron spheres become a sort of spa and resort for the cells, which enter and form very dense colonies while remaining in suspension mode. Cell densities reach 4x108/cc, slightly above the theoretical density. When carriers become overgrown, cells leave and populate empty carriers—like aliens seeking out new worlds—which would not be possible with attached cells.
“The key idea from this project is not just that cells are inside the carriers and protected, but that they are in an environment that is different from the outside environment. It’s a very favorable local cell environment independent of the bioreactor.”
Large stirred-tank reactors, he adds, are far from homogeneous environments. “If you measure pH, for example, you’re obtaining a reading at the probe, which may not be what the cells are seeing at all. Here we’re giving the cell a unique environment that is protected, high-density, and nutrient rich. And since the carriers are open, the cells almost have their own perfusion system.”
These findings led Dr. Sunley’s group to consider cell “happiness” as a primary consideration in the design of cell culture. He believes that Wave bioreactor bags, another GE product, provide greater opportunity for creating a sustaining, heterogeneous environment within the culture than much larger stirred stainless steel tanks.