Prions Come Under Closer Scrutiny

Drugmakers Are Taking Steps To Ensure Removal of Abnormal Prions, Rogue Proteins Implicated in Mad Cow, Scrapie and variant Creutzfeldt-Jakob Disease

 

In July, the U.S. Food and Drug Administration issued a final interim rule limiting the types of animal-derived materials that can be used to manufacture animal feed, cosmetics, and food supplements. The rule aims to protect humans and livestock from abnormal prions (Graphics), proteins that are found in many animal species, but which, in abnormal form, have been associated with fatal neurological conditions.

 

Materials including gelatin, keratin and tallow, whose derivatives include fatty acids and glycerin, come from cattle and other livestock. The new rule, which will be finalized in October, would forbid manufacturers from using raw materials made from brain, skull, spinal cord, and other designated parts of cattle 30 months of age and older, and would set an impurity ceiling of 0.15% for tallow.

 

The rule does not explicitly cover pharmaceuticals or set any validation requirements for biopharmaceutical manufacturing. Currently, the Atlanta-based biotech firm Serologicals Corp.estimates that over  $20 billion worth of biotech products, representing over 60% of pharmaceuticals now on the market, use bovine derivatives at some point in their development or production. They include:  

 

?         Monoclonal antibodies, which use bovine serum as a nutrient

?         Human plasma derivatives such as gamma globulin

?         Insulin, heparin, and  bovine pituitary hormones. A 1996 study by the U.K. government indicated that all these products could be vulnerable to prion contamination (For more information on this and other studies, see http://www.priondata.org/data/A_drugs.html).

 

Even if the ruling doesn't discuss biotech manufacturing specifically, the Agency has, clearly, been paying more attention to potential risks from prions since the first case of Mad Cow, or bovine spongiform encephalopathy ( BSE) was reported in the U.S. last December, in a cow that had been imported from Canada. In the UK and Europe, where over 150 cases of BSE have been documented, regulatory agencies have mandated removal of prions from blood products, via white blood cell filtration, and manufacturers can make the claim that their products are "prion-free" on their labels. The European Union has codified standards for use of animal derivative materials, as has Japan's Ministry of Health, Labor and Welfare.

 

Biotech manufacturers are taking various approaches to eliminating prions from their operations. Avoiding animal derivatives entirely is a holy grail for many companies, says Jerold Martin, Senior Vice President and Global Technical Director at Pall Corp.'s Life Sciences Division (East Hills, N.Y.). As an interim step, manufacturers are moving to non-bovine materials whenever possible, says Mary Gustafson, Senior Director of the Plasma Protein Therapeutics Assn. (PPTA; Annapolis, Md.; http://www.pptaglobal.org), which represents plasma protein drug manufacturers. 

 

For Serologicals Corp., ensuring product safety has meant using raw materials from younger cattle, using beef not dairy cows, sourcing animals from disease-free regions [such as New Zealand and Australia], and maintaining certificates of suitability, according to CEO David Dodd. The company is also using a new process, developed inhouse and patented earlier this year, to make its EX-CYTE cell culture growth medium. The process uses highly alkaline solutions to deactivate the prions without damaging biological activity of the end product.

 

"Biotech companies are still very confident [about the safety of their animal derivatives supplies], but can't be sure," says Pall's Martin. After all, the U.S. and Canada were, until very recently, on the list of "BSE-free" nations. "Interest  in preventative systems--insurance policies for existing manufacturing operations--is growing," he says. At least one U.S. manufacturer, the heparin producer Celsus Laboratories (Cincinnati) is using inhouse immunoassays to test its products, and is making the claim that its heparin is prion-free

 

Prions pose a number of cleaning and validation questions. Currently, there are no comprehensive cleaning guidelines to ensure complete prion removal, and no single decontamination method has been proven to be 100% effective, as Peter Burke, Vice President and Chief Technology Officer of Steris Corp. (Mentor, Ohio) said when he testified before the FDA last year.

 

Current decontamination methods are based on recommendations from the World Health Organization, he said, but there are no standards. PPTA has launched a long-term study with the contract laboratory Bio-Reliance in Europe, to evaluate the prion-removal effectiveness  of existing cleaning procedures. In the meantime, a number of effective new procedures are being developed. (For information on one case history, see http://www.invitrogen.com/content.cfm?pageid=4287

 

Chemical, Physical Deactivation

The trouble with existing prion deactivation processes is that many of them operate at conditions that would deactivate protein products, too. Safer alternatives are being developed, though. Serologicals Corp. is using its purification process internally and, at this time, has no plans to license the process. However, Genencor International (Rochester, N.Y.) and the U.K.'s Health Protection Agency have developed an alternative using a protease enzyme. Both partners are in the process of commercializing the technology, and have submitted data for approval in Europe for surgical and dental instrument sterilization.

 

Filtration Solutions

In plasma manufacturing, Martin says, charge-depth and viral filtration can be used to remove prions, as can membrane chromatography. However, viral filtration can only remove prions that have aggregated. Ion-exchange column chromatography can be used to remove individual prions, but the columns must be cleaned thoroughly to ensure product safety. As an alternative to column chromatography, Pall has developed disposable Mustang chromatography systems using microporous membranes. Assuming a 50-L column and 100-L batch size, the company estimates that use of Mustang chromatography units would save manufacturers $3,100 per cycle compared with ion-exchange chromatography columns.

 

So far, Pall's Ultipor VF grade DV20 and DV50 filters have been shown to be effective for removing more than four logs, or over 99.99%, of prion proteins.  The European Agency for the Evaluation of Medicinal Products has approved the use of Ultipor DV 50 virus filters to remove prions from Redimune immunoglobulin, which is made by ZLB Behring AG (Bern, Switzerland).

 

For blood transfusion, Pall has also developed a new filter technology that combines white blood cell reduction with prion removal in one device. The limitation with existing leukocyte filtration technologies, Martin says, is that they only remove 70 to 80 percent of prions, so the rest may remain in plasma. Since leukocyte filtration alone would not remove those prions, Pall developed a special adsorption chemistry that would be used in conjunction with the white cell filters. Pall is customizing these filters for manufacturing applications; a more open pore structure would be required, Martin says, for use with viscous materials such as bovine serum

 

Removing prions from blood, thus ensuring the safety of transfusions, has led to other technologies that, in the future, may have application in manufacturing. The membrane manufacturer, Gradipore Ltd. (Sydney; http://www.gradipore.com) offers Gradiflow prion filtration,while Pathogen Removal and Diagnostic Technology, Inc. (Montreal), a venture between ProMetic Life Sciences (http://www.prometic.com) and the American Red Cross,  has developed proprietary ligand technology that can selectively bind and remove prions from blood and blood products.

 

FDA is confident that existing guidance and the new ruling will address issues of prion contamination vulnerability. One question for manufacturers in the future is whether the Agency will require use of materials from controlled herds of cattle, says PPTA's Gustafson.

 

And some argue that the new interim final rule has not gone far enough. Since prion infection travels across animal species,crossing over from sheep with scrapie, to cows, the key to ensuring safety will be banning use of any and all animal tissues in animal feed, says Christine Park, senior director of quality for Serological Corp.'s North American manufacturing operations. "Regulation of pharmaceuticals should focus on the front end,ensuring raw material safety and traceability," she says.

 

Shown here, a "normal" prion molecule. In abnormal proteins, the "tail" portion of the protein molecule folds in on itself

 

Sheep infected with Scrapie, so called because it causes them to itch intensely, and scrape their sides against fences, have been found to be a source of BSE and related diseases in humans. Shown here, a sheep with Scrapie

A USDA scientist uses a new diagnostic test to determine whether prions are present in a sheep tissue sample

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