Filter targets smallest viruses

Sheila Galatowitsch

EAST HILLS, NY—Drug manufacturers looking for a way to remove the smallest known viruses from protein-derived products have a new weapon in their arsenal. The latest filter technology from Pall Corp. (East Hills, NY) is fine enough to efficiently remove 20 nanometer (0.02 micron) viruses, while at the same time allowing free flow of liquid protein particles as large as immunoglobulin G, a common antibody.

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Pall's new filter efficiently removes the B19 parvovirus.

Since the development of the original Salk polio vaccine, viral contamination of vaccines, blood plasma and other biopharmaceuticals has posed a potential public health threat. The spread of HIV and hepatitis C in blood products in the late 1980s, and the growth of the biotech industry in the 1990s, only increased the concern, leading the industry to seek new means of removing viruses from products and raising the level of safety assurance.

Today manufacturers use a variety of techniques, from pasteurization to chemical additives to filtration, to inactivate or remove infectious viruses. But the tiniest viruses, such as the human parvovirus B19, hepatitis A and potentially unknown threats, resist many of the traditional removal approaches.

And in the case of filtration, the valuable proteins are removed just as often as the viruses.

The Ultipor VF DV20-grade virus filter, a sponge-like material made of polyvinylidene fluoride membranes, is formed into a thin sheet and pleated for use in a standard cylindrical filter cartridge. It builds upon Pall's Ultipor VF DV50-grade filter, which allows less than one in a million 50nm viruses, such as HIV or retroviruses, to pass through it, but is ineffective against 20nm viruses. In addition to its smaller pores, the new technology increases the number of pores in the filter membrane so that the viruses are removed more efficiently without sacrificing the flow rate of the proteins. The filter membrane is also treated so that proteins don't stick to the walls of the pores.

A study of the DV20-grade filter performed by the Centers for Disease Control and Prevention (CDC; Atlanta, GA) and funded by Pall tested the filter's removal of the B19 parvovirus from plasma derivatives. B19 is a frequent contaminant of blood and vaccine products, and while not life-threatening to healthy individuals, does pose a threat to people with lowered immunity and can lead to other, more serious infections. CDC researcher Usha Varadarajan found that the Pall filter allows less than one in a thousand of the 20nm-25nm-sized B19 viruses to pass through its pores, yet it is sufficiently porous to allow 95 percent transmission of medium to large size proteins. Varadarajan plans to conduct additional tests to determine what impact, if any, different carrier fluids have on these results.

Another Pall-funded study at New Mexico State University (Las Cruces, NM) tested the filter's ability to remove hepatitis B and C viruses from plasma derivatives. Virologist Kevin Oshima, Ph.D., an assistant professor in NMSU's biology department, found that the DV20-grade filter removed more of these blood-borne viruses than did the DV50-grade filter, which was previously tested at NMSU.

Both the CDC and NMSU studies showed that using two of the DV20-grade filters one after the other in the manufacturing process could double the odds of removing the viruses. After the second filtering, the odds of a 20nm virus getting through both filters becomes one in a million. Other inactivation or removal methods used consecutively don't produce the same odds, says Jerry Martin, Pall's vice president of marketing.

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The Ultipor VF Grade DF20 virus removal filter allows less than one in a million 50 nm viruses to pass through it.

In an independent study, Protein Design Labs (Minneapolis, MN), a manufacturer of antibody products for treatment of cancer and other illnesses, tested the DV20-grade filter to determine how well three different antibodies flowed through the membrane and also found a 95 percent flow rate. The company, which is considering implementing the product in its manufacturing process, plans to hire a contract lab to conduct virus removal studies on the filter, says senior process engineer Ping Huang, Ph.D.

Although Protein Design Labs already uses two methods to remove potential virus contamination from its products, Huang believes it's only a matter of time before the FDA and European agencies require even more process steps to ensure drugs are safe from virus contamination, particularly from the smaller, so-called “non-envelop” viruses. These viruses, size 27nm to 40nm, are harder to destroy than viruses with an envelop or coating around the infectious nucleic acids at the core.

Pall's Martin says the DV20-grade filter was designed for manufacturers who “either have observed the possibility of contamination with smaller viruses or simply want to have something in their process that will remove them should they appear in the future.” But even if the new technology lives up to its promise of removing the smallest viruses, manufacturers will never know if their products are completely free of viral contamination because of the poor sensitivity of the current measurement tools. So, no matter what their size, detecting the presence of these elusive contaminates will remain one of the industry's greatest challenges.


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