Protein A is so efficient at attracting and binding the antibody product that, says Henrik Ihre, product manager for Protein
A and program manager for mAb products at GE Healthcare BioSciences, "you go from 1% to above 98% purity in one chromatographic
step." And while some companies have tried to engineer more antibody-binding sites onto Protein A, nature it seems, has produced
the best overall variant of the molecule.
"Nature has engineered Protein A to be pretty efficient and robust," marvels Laura Whitehouse Pew, vice-president of market
development at Massachusetts-based Repligen. Repligen produces the recombinant Protein A that other manufacturers immobilize
in their resins.
So if they can't improve upon nature with regard to the affinity of Protein A for antibodies, many resin manufacturers have
tried to improve the durability of the resin to increase its lifetime and, thus, lower its cost per use. Ian Sellick, director
of marketing at Pall Life Sciences in New York, says that a chromatography column can cost as much as $5 million to pack and
is good for up to approximately 200 purification cycles. To increase durability, commercial vendors such as GE have developed
Protein A variants that can hold up for more purification cycles. Resins frequently degrade during the harsh cleaning protocol
in between cycles. Cleaning makes use of very caustic chemicals such as sodium hydroxide—lye—which decouples Protein A from
the matrix. The Protein A in GE's MabSelectSure resins has been genetically engineered to resist such cleansing-induced decoupling.
As a result, the resins may be usable for more than 300 cycles, thus reducing the cost of use per cycle.
Other companies are offering somewhat more rigid matrixes, with the hope of reducing the compressibility of the matrix and
increasing flow-through rates. For example, DSM (Heerlen, The Netherlands) developed EBA resins. These Protein-A-based resins,
developed in partnership with the Danish firm Upfront, use tungsten carbide particles. According to DSM literature, the added
weight allows fluids to flow through the column at increased rates, with the aim of accelerating processing times. Pall, too,
offers a zirconium-based resin.
Pall, in an attempt to reduce resin costs, is offering a product that uses a synthetic substitute to Protein A. According
to Sellick, Pall's "gel-in-a-shell" resin is reconfigured to offer the same level of separation at a rate that's faster and
which holds up better during sodium-hydroxide cleaning than traditional Protein A-based resins. To cap it all off, Pall's
Hypercel product line is a quarter the price of some of the traditional Protein A-based resins. Sellick adds that Pall's new
resin can withstand between 100 and 200 purification cycles.
Pall is not the only manufacturer exploring the use of synthetic capture chemistries, or ligands. BAC (Naarden, The Netherlands),
for example, produces affinity ligands which are derived from Llama antibodies. One of their products is marketed via GE as IgSelect, a complementary product to Protein A-based resins.
Manufacturers stress that different resins offer biopharmaceutical manufacturers a range of options so they can match their
chromatography solution to their particular equipment needs and to the exigencies of their particular monoclonal product.
But as much as manufacturers may differ on their choice of resins, they all seem to have converged on the use of disposables.
Every provider of chromatography resin offers disposable options. In many cases, this means that biopharmaceutical manufacturers
have the choice of purchasing prepacked, disposable columns.
Vendors note that the disposable option eliminates the enormous upfront capital expenditure related to the purchase of stainless-steel
chromatography equipment. Disposables also eliminate the time and expense related to packing the column initially. Most of
these disposable columns can be used through several purification cycles—somewhere in the neighborhood of 10 to 20 cycles.
The disposable option may be especially good for smaller batches of niche products, early-stage process development, contract
manufacturers who have to produce many different products in the same facility, or for any manufacturer who—for whatever reason—does
not need to run hundreds of purification cycles.