Disposables have been widely adopted for commercial-scale bioprocessing, but use of these technologies for downstream processing has lagged behind that for other applications. PharmTech spoke with industry experts about the challenges of implementing disposable chromatography systems.

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Barriers to implemetation

Grund (GE Healthcare): The biggest constraint to single-use is probably a mental barrier based on a narrow view of the pros and cons. Chromatography media are often very tolerant to cleaning and withstand re-use, so it's tough to throw them away after single-use, especially if tests show they still perform well after many cycles. The benefits of speed, facility flexibility, facility output, and avoidance of cleaning are not yet fully appreciated.

Bisschops (Tarpon Biosystems): This statement is absolutely true for applications that involve capture of the product and/or some high resolution polishing steps. For flow-through applications (or negative chromatography), membrane adsorbers have already paved the way for disposable chromatography.

One of the most important reasons why chromatography has not been available in a disposable format is caused by the nature of the chromatography process itself: it is essentially a mass driven process, where the size of the column is governed by the amount of product that needs to be bound. For membrane processes and other flow-through applications, the most important system dimensions are determined by the volume that needs to be processed.

As a result, the successful introduction of disposable bioprocessing has largely been enabled by the process intensification that resulted from the increases in expression levels over the past decade. In essence, this has allowed us to produce the same amount of product with much less water and hence with a significantly reduced volume. All volume-driven unit operations have benefited from this, whereas the mass driven processes were not affected.

Everybody acknowledges that the costs of chromatography media currently are too high to justify a single-use application. These costs need to be depreciated over many cycles in order to make the economy work. This hampers the translation of batch-wise chromatographic processes into a single-use or disposable application, unless one uses a technology that would allow one to use the media over so many cycles in a single batch or in a campaign.

Thompson (Polybatics): Columns are very expensive systems, and the cost of buying these large chromatography systems is a cost that companies are reluctant to walk away from. Also, the cost of buying the resins themselves are fairly expensive, particularly Protein A. Protein A has been on patent until around March 2010, so there's been a monopoly on that particular ligand, which has maintained a very high price of the resin. I think those two factors have been a real impediment to going to a disposable chromatography system. And there hasn't been anyone out there who has come up with a format that is truly comparable to traditional packed-bed chromatography in terms of its ability to purify and capture the target.

In terms of implementation, packing of the columns is very fussy. You pump a slurry into the column, and have to let it settle. If it doesn't settle quite right, you can get voids in the column, and you have to pack again. There's a lot of art in packing the column to get it to perform right. One of the problems of implementing a disposable system is finding a medium that can either be pumped into fixed columns or finding a complete cartridge that is kind of plug-and-play. Until recently, there haven't been those kinds of plug-and-play systems.

Mann (Merck Millipore): Chromatography processes, while not fully single-use, have been operating in a hybrid way for some time with the implementation of single-use bags for buffers and product collection. Elimination of stainless-steel tanks and replacement with single-use bags is, together with the use of single-use bioreactors, the biggest contributor to cost savings when comparing single-use to traditional stainless-steel facilities. This is due to the elimination of clean-in-place (CIP) and steam-in-place (SIP) for tank/vessel cleaning. In contrast, the chromatography system is cleaned by process buffers including sodium hydroxide and does not need a separate CIP system.

Constraints of the chromatography process that make it difficult to implement as a disposable system include first, the greater complexity of the flow path in chromatography systems compared with other unit operations, for example the number of valves required to enable multiple buffer inlets, column flow reversal and bypass and fraction outlet. Coupled to this has been the greater number of different sensors deployed and the operating range and accuracy required of those sensors. Second, the cost of chromatography resins, especially the affinity resins such as protein A, has meant they tend to be used for multiple batches requiring cleaning and storage between times and so are not seen as single-use per se.

Tingley (Repligen): If we take a look at the process as a whole, and we look at the adoption curve of single-use technologies, you can essentially split the process into functional and nonfunctional technologies. It's the nonfunctional technologies that have taken the lead because they've been easier to implement and easier to get to an economical cost point than the functional technologies. Examples of nonfunctional technologies would be replacing stainless-steel pipework with plastic tubing, or replacing stainless-steel tanks with plastic bags. When you start looking at the process, for instance, a bioreactor or filtration technology such as ultrafiltration or microfiltration, these are examples of functional technologies, which have to be disposable. Making functional technology costs money, and functional technologies are often reused to defray some of the costs.

It just so happens that one of the most complex of the functional technologies is purification. That includes capture, using Protein A which we know is an extremely expensive chromatography resin, and hydrophobic interaction or ion exchange or multimode resins which are also reasonably expensive. And processes use a lot of them—that's multiple tens of liters multiplied by multiple thousands of dollars. With chromatography, it's a very expensive, very critical functional technology that is hard to get into a single-use format. So, there are two parts of the problem: can you make a disposable or single-use container for the chromatography, that is, a column, and then, can you make a single-use media or functional element to go into that. That's the problem that's made it so intractable.

When people want to move to single-use technologies, they may be reducing column sizes and cycling them harder. What users are doing is making the media work harder, so it's less painful to throw it away. What you're seeing today is companies offering the easy part, the containment part, of the disposable chromatography, the column shells, and packing them. The difficult part of the technology is finding new ways to stretch the economics of running longer, running smaller batches, cycling the columns more often, and things like that.