Libraries of mixed-mode ligands
The use of specialized sorbents may reduce the number of process steps. (IMAGE COURTESY OF PALL)
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GE Healthcare Bio-Sciences (Uppsala, Sweden,
http://www.gelifesciences.com/) is designing chromatographic ligand libraries following combinatorial chemistry strategies, which take into account a wide
range of parameters and interactions to design ligands for mixed-mode applications.
"This approach provides the possibility to tune the performance of your media," says Jean-Luc Maloisel, PhD, staff scientist.
"We use it to discover new selectivities and to improve discriminate chromatographic media. GE Healthcare has the traditional
media, and we try to introduce new interactions to modulate the performance of these media." The company designs a library
of mixed-mode ligands comprising about 50 prototypes to start with. These first, diverse libraries are used in the initial
screening step to find a set of interactions that will perform according to a desired specification (e.g., for protein recovery, protein binding, elution).
After the identification of promising candidates, a second-generation library is designed by introducing additional interactions,
thereby fine-tuning the modification to the media to reach the final target specification.
"We can screen our libraries of media very rapidly and in different ways using microtiterplate formats. The libraries are
not set in stone, and we are always continuously working to diversify them. The libraries we have now may be slightly different
tomorrow, so we can design them in different ways," says Maloisel.
Traditional methods are robust, but may not be the optimal solutions in terms of process economy and time to market. "In some
cases you really would like to have a wider range of selectivities and have more customized solutions. And that is where the
library of multimodal ligands can be useful. However often, people will not take the time and trouble to screen a lot of media.
This is where the microtiterplate format allow for a high-throughput process development and that is where you have a chance
to win" says Maloisel.
GE has launched multimodal cation exchangers as part of its "Capto" platform. One challenge in designing these media was to
develop a medium allowing for binding proteins under high-salt conditions without the need to dilute the feedstock. Using
computerized programs, the company made a diverse range of cation exchangers and then screened this range with the condition
that the prestandard be salt tolerant. The company also developed anion exchange media for monoclonal antibody capture. "It
doesn't replace the other chromatographic media, but it can replace traditional chromatography media lacking the prestandard
performance."
Process chromatographers continue to work toward systems that are more economical, offer better selectivity, and lead to greater
yields. (IMAGE COURTESY OF PALL)
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UpFront Chromatography (Copenhagen, Denmark,
http://www.upfront-dk.com/) also has developed a library of 200 ligands. The general criteria are that these ligands must be nontoxic, simple to manufacture
to high purities, stable to harsh cleaning, and stable when immobilized. The performance of the ligands can be modified using
various approaches.
"It depends on the constituent groups," explains Rob Noel, business development manager at UpFront. "If you take a group of
ligands that have a common ionic component such as a carboxyl group, which is negatively charged, then you can change the
pI of that carboxyl group depending on its substituents in the ligand."
Mixed-mode ligands can use a combination of aromatic, hydrophobic, ionic, and hydrogen bonding groups. Each combination of
chemical groups may have a particular target on the protein surface. Most mixed-mode ligands bind to the surface of protein.
"We try to mimic in some way how a protein–protein interaction would work even though the chemistry is different," says Noel.
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