Cell culture-related activities take far longer than all the remaining unit operations combined. Yet, bioprocessors recognize the unique issues related to downstream bottlenecks, which are akin to what chemists call a “rate-limiting step” in a reaction mechanism.
Bottlenecks are operations that are time-consuming relative to steps that precede or follow, often resulting in equipment or facility under-utilization, product instability, reduced efficiency, and higher process-related costs.
“When we speak with downstream processing groups, they still mention the classic bottlenecks of chromatography, buffer capacity, and cleaning,” says Kimo Sanderson, vp for client services at Asahi Kasei Bioprocess. “So yes, downstream bottlenecks are real, but we’re seeing some relief in terms of their severity.”
Single-use equipment, in-line buffer dilution, high-capacity chromatography resins, and a deeper appreciation for modeling and scheduling software, have helped to mitigate some of the more serious bottlenecks, Sanderson adds. “And we’re beginning to see the idea of debottlenecking taken to the next level, in the form of continuous downstream processing,” notes Sanderson.
A downstream bottleneck is any step that requires more time or resources than it “should.” Perception plays into this view, however. Rising upstream productivity can make certain downstream operations appear inefficient when in fact they are just as efficient, or in some cases more efficient, than they were a few years back.
“It’s not always fair to call these bottlenecks, but when in the heat of the moment you can see, smell, taste, and hear the inefficiency of a particular step, you want to improve it,” Sanderson remarks.
Despite advances in downstream equipment, the overall perception is that purification will never quite catch up with protein expression due to cost, facility, and time constraints. Separation products are expensive and difficult to scale, and processing floor space cannot be created from thin air.
“And the longer a target protein remains in the presence of contaminating proteins, the greater the risk to its stability, product yield, and purity,” says Yamuna Dasarathy, Ph.D., director of chromatography marketing at Pall Life Sciences. “Any technology that accelerates the target-isolation process will mitigate the risk and de-bottleneck the entire process flow.”
Dr. Dasarathy identifies chromatography as one of the chief downstream bottlenecks. Column packing, testing, qualifying, and validation of the packing followed by processing of the feed stream on the column all add to the cycle time and slow down the purification process flow.
To avoid or mitigate bottlenecks, Dr. Dasarathy suggests using flexible facilities, single-use technologies, pre-packed chromatography columns, and membrane chromatography technology. Reducing the number of purification steps and/or unit operations (process intensification) is another strategy, but difficult to achieve.