Distribution of a bovine spongiform encephalopathy-derived agent over ion-exchange chromatography used in the preparation of concentrates of fibrinogen and factor VIII

Biological Safety of a Highly Purified 10% Liquid Intravenous Immunoglobulin Preparation from Human Plasma

Background

A highly purified 10% liquid intravenous immunoglobulin, IQYMUNE^®, has been developed using an innovative manufacturing process including an affinity chromatography step for the removal of anti-A and anti-B hemagglutinins.

Objectives

The pathogen (viruses and prions) clearance efficacy of the manufacturing process and its robustness for critical steps were investigated.

Methods

The manufacturing process of IQYMUNE^® includes two dedicated complementary virus reduction steps: solvent/detergent (S/D) treatment and 20 nm nanofiltration as well as two contributing steps, namely caprylic acid fractionation and anion-exchange chromatography. The clearance capacity and robustness of these steps were evaluated with a wide range of viruses (enveloped and non-enveloped) and with a model of human transmissible spongiform encephalopathies (TSEs).

Results

The IQYMUNE^® manufacturing process demonstrated a high and robust virus removal capacity with global reduction factors (RFs) of relevant and model viruses: ≥14.8 log₁₀ for human immunodeficiency virus type 1 (HIV-1), ≥16.9 log₁₀ for bovine viral diarrhoea virus (BVDV)/Sindbis virus, ≥15.7 log₁₀ for pseudorabies virus (PRV), ≥12.8 log₁₀ for encephalomyocarditis virus (EMCV) and 11.0 log₁₀ for porcine parvovirus (PPV). The process also exhibited a high removal capacity for the TSE agent with an overall RF of ≥12.9 log₁₀ due to the complementary actions of the caprylic acid fractionation, anion-exchange chromatography and nanofiltration steps.

Conclusion

Data from virus and prion clearance studies fully support the high safety profile of IQYMUNE^®, with a minimal reduction of 11 log₁₀ for the smallest and most resistant non-enveloped virus, PPV, and more than 12 log₁₀ for the TSE agent.

The manufacture of blood plasma products in Scotland: a brief history

A number of essential clinical products are derived from human blood plasma, including immunoglobulin products for the treatment of infections and disorders of immunity; albumin for protein and fluid replacement and coagulation factors for the treatment of haemophilia and other disorders of haemostasis. For many years, these protein pharmaceuticals were manufactured by the Scottish National Blood Transfusion Service (SNBTS) at its Scottish Protein Fractionation Centre (PFC) in Edinburgh, a contribution which ended with the closure of the PFC in 2008. The origins and development of plasma fractionation in Scotland are summarised in this article, as well as issues which contributed to the closure of the PFC.

Removal of transmissible spongiform encephalopathy prion from large volumes of cell culture media supplemented with fetal bovine serum by using hollow fiber anion-exchange membrane chromatography

Cases of variant Creutzfeldt-Jakob disease in people who had consumed contaminated meat products from cattle with bovine spongiform encephalopathy emphasize the need for measures aimed at preventing the transmission of the pathogenic prion protein (PrPSc) from materials derived from cattle. Highly stringent scrutiny is required for fetal bovine serum (FBS), a growth-medium supplement used in the production of parenteral vaccines and therapeutic recombinant proteins and in the ex vivo expansion of stem cells for transplantation. One such approach is the implementation of manufacturing steps dedicated to removing PrPSc from materials containing FBS. We evaluated the use of the QyuSpeed D (QSD) adsorbent hollow-fiber anion-exchange chromatographic column (Asahi Kasei Medical, Tokyo, Japan) for the removal of PrPSc from cell culture media supplemented with FBS. We first established that QSD filtration had no adverse effect on the chemical composition of various types of culture media supplemented with 10% FBS or the growth and viability characteristics of human embryonic kidney (HEK293) cells, baby hamster kidney (BHK-21) cells, African green monkey kidney (Vero) cells, and Chinese hamster ovary (CHO-k1) cells propagated in the various culture-medium filtrates. We used a 0.6-mL QSD column for removing PrPSc from up to 1000 mL of Dulbecco's modified Eagle's medium containing 10% FBS previously spiked with the 263K strain of hamster-adapted scrapie. The Western blot analysis, validated alongside an infectivity assay, revealed that the level of PrPSc in the initial 200mL flow-through was reduced by 2.5 to > 3 log10, compared with that of the starting material. These results indicate that QSD filtration removes PrPSc from cell culture media containing 10% FBS, and demonstrate the ease with which QSD filtration can be implemented in at industrial-scale to improve the safety of vaccines, therapeutic recombinant proteins, and ex vivo expanded stem cells produced using growth media supplemented with FBS.

Decontamination of prions in a plasma product manufacturing environment

BACKGROUND

The high resistance of prions to inactivating treatments requires the proper management of decontaminating procedures of equipment in contact with materials of human or animal origin destined for medical purposes. Sodium hydroxide (NaOH) is widely used today for this purpose as it inactivates a wide variety of pathogens including prions.

STUDY DESIGN AND METHODS

Several NaOH treatments were tested on prions bound to either stainless steel or chromatographic resins in industrial conditions with multiple prion strains.

RESULTS

Data show a strong correlation between inactivation results obtained by immunochemical detection of the prion protein and those obtained with infectivity assays and establish effective inactivation treatments for prions bound to stainless steel or chromatographic resins (ion exchange and affinity), including treatments with lower NaOH concentrations. Furthermore, no obvious strain-specific behavior difference was observed between experimental models.

CONCLUSION

The results generated by these investigations show that industrial NaOH decontamination regimens (in combination with the NaCl elution in the case of the chromatography process) attain substantial prion inactivation and/or removal between batches, thus providing added assurance to the biologic safety of the final plasma-derived medicinal products.

Effective virus inactivation and removal by steps of Biotest Pharmaceuticals IGIV production process

The virus validation of three steps of Biotest Pharmaceuticals IGIV production process is described here. The steps validated are precipitation and removal of fraction III of the cold ethanol fractionation process, solvent/detergent treatment and 35 nm virus filtration. Virus validation was performed considering combined worst case conditions. By these validated steps sufficient virus inactivation/removal is achieved, resulting in a virus safe product.

Estimating the pathogen safety of manufactured human plasma products: application to fibrin sealants and to thrombin

BACKGROUND

Plasma fractionators have implemented many improvements over the past decade directed toward reducing the likelihood of pathogen transmission by purified blood products, yet little has been published attempting to assess the overall impact of these improvements on the probability of safety of the final product.

STUDY DESIGN AND METHODS

Safety margins for human immunodeficiency virus (HIV), hepatitis C virus (HCV), hepatitis B virus (HBV), hepatitis A virus (HAV), parvovirus B19, and variant form of Creutzfeldt-Jakob disease (vCJD) were calculated for the two fibrin sealants licensed in the United States and for thrombin. These products were selected because their use in a clinical setting is, in most cases, optional, and both were relatively recently approved for marketing by the US Food and Drug Administration (FDA). Moreover, thrombin and fibrinogen both undergo two dedicated virus inactivation steps and/or removal steps in accord with the recommendations of regulatory agencies worldwide. Safety margins were determined by comparing the potential maximum viral loads in contaminated units to viral clearance factors, ultimately leading to the calculation of the residual risk per vial.

RESULTS

The residual risk of pathogen transmission per vial was calculated to be less than 1 in 10(-15) for HIV, HCV, HBV, and HAV for both fibrinogen and thrombin. Owing to the greater quantities that can be present and its greater thermal stability, the calculated risk for parvovirus transmission was 1 in 500,000 vials for fibrinogen and less than 1 in 10(7) per vial for thrombin. Assuming that vCJD is found to be present in plasma donations, its risk of transmission by these purified and processed plasma derivatives would appear to be very low.

CONCLUSIONS

The pathogen safety initiatives implemented by plasma fractionators over the past 10 to 20 years have resulted in products with excellent pathogen safety profiles. Of the agents examined, parvovirus continues to have the lowest calculated margin of safety. Despite this, parvovirus transmissions should be rare. Manufacturers are encouraged to continue exploring processes to further enlarge parvovirus safety margins and to continue exploring ways of eliminating prions.

Susceptibility of cell substrates to PrPSc infection and safety control measures related to biological and biotherapeutical products

Concerns over the potential for infectious prion proteins to contaminate human biologics and biotherapeutics have been raised from time to time. Transmission of the pathogenic form of prion protein (PrP(Sc)) through veterinary vaccines has been observed, yet no human case through the use of vaccine products has been reported. However, iatrogenic transmissions of PrP(Sc) in humans through blood components, tissues and growth hormone have been reported. These findings underscore the importance of reliable detection or diagnostic methods to prevent the transmission of prion diseases, given that the number of asymptomatic infected individuals remains unknown, the perceived incubation time for human prion diseases could be decades, and no cure of the diseases has been found yet. A variety of biochemical and molecular methods can selectively concentrate PrP(Sc) to facilitate its detection in tissues and cells. Furthermore, some methods routinely used in the manufacturing process of biological products have been found to be effective in reducing PrP(Sc) from the products. Questions remain unanswered as to the validation criteria of these methods, the minimal infectious dose of the PrP(Sc) required to cause infection and the susceptibility of cells used in gene therapy or the manufacturing process of biological products to PrP(Sc) infections. Here, we discuss some of these challenging issues.

Modern plasma fractionation

Protein products fractionated from human plasma are an essential class of therapeutics used, often as the only available option, in the prevention, management, and treatment of life-threatening conditions resulting from trauma, congenital deficiencies, immunologic disorders, or infections. Modern plasma product production technology remains largely based on the ethanol fractionation process, but much has evolved in the last few years to improve product purity, to enhance the recovery of immunoglobulin G, and to isolate new plasma proteins, such as α1-protease inhibitor, von Willebrand factor, and protein C. Because of the human origin of the starting material and the pooling of 10 000 to 50 000 donations required for industrial processing, the major risk associated to plasma products is the transmission of blood-borne infectious agents. A complete set of measures—and, most particularly, the use of dedicated viral inactivation and removal treatments—has been implemented throughout the production chain of fractionated plasma products over the last 20 years to ensure optimal safety, in particular, and not exclusively, against HIV, hepatitis B virus, and hepatitis C virus. In this review, we summarize the practices of the modern plasma fractionation industry from the collection of the raw plasma material to the industrial manufacture of fractionated products. We describe the quality requirements of plasma for fractionation and the various treatments applied for the inactivation and removal of blood-borne infectious agents and provide examples of methods used for the purification of the various classes of plasma protein therapies. We also highlight aspects of the good manufacturing practices and the regulatory environment that govern the whole chain of production. In a regulated and professional environment, fractionated plasma products manufactured by modern processes are certainly among the lowest-risk therapeutic biological products in use today.

[Plasma fractionation in the world: current status]

From 22 to 25 million liters of plasma are fractionated yearly in about 70 fractionation plants, either private or government-owned, mainly located in industrialized countries, and with a capacity ranging from 50000 to three million liters. In an increasingly global environment, the plasma industry has recently gone through a major consolidation phase that has seen mergers and acquisitions, and has led to the closure of a number of small plants in Europe. Currently, some fifteen countries are involved into contract plasma fractionation programs to ensure a supply of plasma-derived medicinal products. The majority of the plasma for fractionation is obtained by automated plasmapheresis, the remaining (recovered plasma) being prepared from whole blood as a by-product of red cell production. Plasma for fractionation should be produced, and controlled following well established procedures to meet the strict quality requirements set by regulatory authorities and fractionators. The plasma fractionation technology still relies heavily on the cold ethanol fractionation process, but has been improved by the introduction of modern chromatographic purification methods, and efficient viral inactivation and removal treatments, ensuring quality and safety to a large portfolio of fractionated plasma products. The safety of these products with regards to the risk of transmission of variant Creutzfeldt-Jakob disease seems to be provided, based on current scientific data, by extensive removal of the infectious agent during certain fractionation steps. The leading plasma product is now the intravenous immunoglobulin G, which has replaced factor VIII and albumin in this role. The supply of plasma products (most specifically coagulation products and immunoglobulin) at an affordable price and in sufficient quantity remains an issue; the problem is particularly acute in developing countries, as the switch to recombinant factor VIII in rich countries has not solved the supply issue and has even led to an increase of the mean price of plasma-derived factor VIII to the developing world. In the last few years, the plasma fractionation industry has improved greatly, and should remain essential in the years to come for the procurement of many essential medicines.

Manufacture of plasma-derived products in France and measures to prevent the risk of vCJD transmission: Precautionary measures and efficacy of manufacturing processes in prion removal

The emergence of the variant Creutzfeldt-Jakob disease in the mid 1990s soon raised concerns about its possible transmission through the use of blood and plasma-derived medicinal products. A risk analysis approach was initiated by health authorities, based on updated scientific knowledge and precautionary measures were implemented in France and other countries for the management of this new possible risk. Assessment of the vCJD risk is based on epidemiology and estimates of the number of potential cases in the future, on blood infectivity data from models of transmissible spongiform encephalopathies and on data from studies of the capacity of manufacturing processes to remove the agent, should it be present in the plasma of infected donors. The transmission of vCJD by non leukocyte-depleted labile blood components has recently been confirmed. There have been no reports of cases associated with the use of plasma-derived products and the scientific data, and risk analyses for those plasma products, which are of the greatest therapeutic interest, support their safety with respect to this transmission risk. The precautionary measures applied in France and the data contributing to the risk assessment of plasma products are reviewed and updated in the present paper. The uncertainties, which remain, are also addressed and discussed, as well as the ongoing research and developments in this area.

Impact of vCJD on blood supply

Variant Creutzfeldt-Jakob disease (vCJD) is an at present inevitably lethal neurodegenerative disease which can only be diagnosed definitely post mortem. The majority of the approximately 200 victims to date have resided in the UK where most contaminated beef materials entered the food chain. Three cases in the UK demonstrated that vCJD can be transmitted by blood transfusion. Since BSE and vCJD have spread to several countries outside the UK, it appears advisable that specific risk assessments be carried out in different countries and geographic areas. This review explains the approach adopted by Germany in assessing the risk and considering precautionary measures. A fundamental premise is that the feeding chain of cattle and the food chain have been successfully and permanently cleared from contaminated material. This raises the question of whether transmissions via blood transfusions could have the potential to perpetuate vCJD in mankind. A model calculation based on actual population data showed, however, that this would not be the case. Moreover, an exclusion of transfusion recipients from blood donation would add very little to the safety of blood transfusions, but would have a considerable impact on blood supply. Therefore, an exclusion of transfusion recipients was not recommended in Germany.

Current strategies to prevent transmission of prions by human plasma derivatives

Protein products prepared from pooled human plasma are an essential class of therapeutics used mostly to control bleeding and/or immunological disorders. Because of the human origin of the starting material, there is a risk that these products may possibly transmit prions causing variant Creutzfeldt-Jakob disease (vCJD). No case of transmission of prions by plasma products has been observed. Case-by-case measures implemented in various countries, and several technical factors may contribute, to various degrees, to the prevention of the risk of transmission of prions by plasma products. Those measures include (a) the epidemiological surveillance of population in countries with cases of vCJD and/or bovine spongiform encephalopathies (BSE), (b) the deferral of blood donors who traveled or resided, for specific periods of time, to countries with BSE, or who received transfusion or tissue transplant, (c) the removal of leucocytes in plasma used for fractionation, and, last but not least, (d) the removal of the prion agents during the complex industrial fractionation process used to prepare plasma products. Numerous experimental infectivity studies, involving the spiking of brain-derived infectious materials, have demonstrated that several fractionation steps, in particular ethanol fractionation, depth filtration, and chromatography, can remove several logs of prions. Removal is explained by the distinct hydrophobic and aggregative properties of the prion proteins. In addition, nanofiltration using multi-layer membranes of 75 nm or smaller, which is commonly used for removing viruses from coagulation factors and immunoglobulins products, can remove more than 3-5 logs of spiked prions, presumably by size-exclusion and trapping mechanisms. Therefore, the risk of transmission of vCJD by human plasma products appears remote, but caution should prevail since the biochemical nature of the infectious agent in human blood is still unknown.

Investigation of prion removal/inactivation from chromatographic gel

BACKGROUND AND OBJECTIVES

Concerns about the potential for prions to be retained on chromatography gels during the manufacture of plasma products prompted development of an investigational strategy for detecting infectious prions bound to gels. The objective was to firstly examine methods of implanting gels intracerebrally (IC) in mice, then to examine prion cleaning from a scaled-down version of the DEAE Sepharose column used in a production process to fractionate immunoglobulins and albumin from human plasma.

MATERIALS AND METHODS

The study consisted of two parts: (i) the pathophysiological impact by IC inoculation of ground gel beads was compared to whole gel beads; (ii) the feedstreams to two DEAE Sepharose columns were spiked with scrapie ME7. One column was subjected to the protein loading and elution portions of the chromatography cycle. The other column was subjected to the full cycle of protein loading and elution, followed by regeneration with 0.5 m NaCl, 1 m NaOH and solvent/detergent washes. The gels were unpacked and bioassayed by IC implantation in mice to quantify infectivity.

RESULTS

IC inoculation of ground gel beads resulted in unacceptably high pathological impact in the mice whereas whole gel bead inoculation resulted in a reduced affect. Accordingly, the whole bead model system was used to assess prion removal/inactivation from chromatography gels at the pre- and postcleaning stage of the chromatography cycle. Infectious prions were detected on the DEAE Sepharose prior to the cleaning step; however, the gel cleaning cycle reduced infectivity by a log reduction factor (LRF) of > or = 2.75, thus reducing infectivity by bioassay to below detectable limits.

CONCLUSIONS

A model system for assessment of prion inactivation/removal from chromatography gels has been established. Spiked prion infectivity does bind to DEAE Sepharose gel; however, the cleaning cycle removed infectivity to levels below that detectable by bioassay.

Prion-removal capacity of chromatographic and ethanol precipitation steps used in the production of albumin and immunoglobulins

BACKGROUND AND OBJECTIVES

Although there is no epidemiological evidence to suggest that classical Creutzfeldt-Jakob disease (CJD) is transmitted through blood or blood products, the variant form (vCJD) has been implicated in transmission via packed red blood cells. The potential threat of the infectious agent contaminating plasma pools has led to manufacturing processes being examined for capacity to remove prions. The objective of these studies was to examine the prion-removal potential of the chromatographic purification and ethanol precipitation steps used to fractionate immunoglobulins and albumin from human plasma.

MATERIALS AND METHODS

Western blot assay was used to examine the partitioning of proteinase K-resistant scrapie prion protein (PrPsc) over DEAE Sepharose, CM Sepharose and Macro-Prep High Q chromatographic columns, utilizing microsomal scrapie 263K spiked into each scaled down feedstream and assayed after each chromatographic step. In further studies, bioassay in C57 black mice was used and spikes of 10 000 g clarified brain homogenate of scrapie ME7 were added to feedstreams before sequences of scaled down chromatographic or Cohn fractionation process steps.

RESULTS

The microsomal spiking study with Western blot detection demonstrated substantial partitioning of PrPsc away from the target proteins in all ion exchange chromatographic steps examined. The log10 reduction factors (LRF) across DEAE Sepharose and CM Sepharose columns for albumin were > or = 4.0 and > or = 3.0 respectively. The reductions across DEAE Sepharose and Macro-Prep High Q for intravenous immunoglobulin were 3.3 and > or = 4.1 respectively. Bioassay demonstrated LRFs of >or = 5.6 across the combination of DEAE Sepharose and CM Sepharose columns in the albumin process and > or = 5.4 across the combination of DEAE Sepharose and Macro-Prep High Q columns in the intravenous immunoglobulin process. Bioassay studies also demonstrated a LRF of > or = 5.6 for immunoglobulin produced by Cohn fractionation.

CONCLUSIONS

Using rodent-adapted scrapie as a model, the studies indicated that ion exchange chromatography, as well as Cohn immunoglobulin fractionation have the potential to effectively reduce the load of TSE agents should they be present in plasma pools.

Possible removal of prion agents from blood products during the manufacturing process

Blood products prepared from human blood theoretically risk contamination with infectious pathogens. Since recent reports now confirm the likely transmission of pathogenic prions through blood transfusion, effective measures to prevent transmission are required globally, although the prevalence of variant Creutzfeldt–Jakob disease outside of the UK is extremely low. Many studies evaluating the manufacturing process have been conducted for the potential removal of the prion protein from plasma derivatives. In this review, we discuss the possibility of removing prions via several processing steps, especially depth and virus-removal filtration. Through a discussion of the limitations and issues associated with such studies, we hope our review will be of help for better study design in the future.

Pall leukotrap affinity prion-reduction filter removes exogenous infectious prions and endogenous infectivity from red cell concentrates

BACKGROUND AND OBJECTIVES

Three recent probable cases of transmission of a variant of human Creutzfeldt-Jakob disease (vCJD) through blood transfusion suggest that the disease can be transmitted through transfusion of blood components from presymptomatic blood donors. In this study, we investigated the performance of a new filter for reducing the levels of infectious prions (PrP(Sc)) from red cell concentrates (RCC).

MATERIALS AND METHODS

Endogenous Infectivity: A pool of 500 ml of whole blood was collected from 263K-strain scrapie-infected hamsters into an anticoagulant, processed into non-leucoreduced RCC (NL-RCC), and then passed through a prion-reduction filter. Pre- and postfiltration samples were tested for PrP(Sc) by Western blot and infectivity by inoculation of healthy hamsters. Results of the endogenous infectivity study after 200 days post-inoculation are discussed. Exogenous (Spiking) Study: Scrapie-infected hamster brain homogenates containing PrP(Sc) were added to human RCC and then filtered. Levels of PrP(Sc) were determined by Western blot assay. The effect of prior leucodepletion of 'spiked' RCC on PrP(Sc) removal by the prion-removal filter was also assessed.

RESULTS

In the endogenous infectivity study, at 200-day observation time, the prefiltered RCC transmitted disease to six of the 187 hamsters, whereas the filtered RCC did not transmit disease to any of 413 animals, P = 0.001. The prion filter also significantly reduced the concentration of leucocytes in the RCC by about 4 logs, P < 0.05. In the exogenous (spiking) study, the level of PrP(res) was significantly reduced in RCC P < 0.05. Prior leucodepletion of the RCC with a leucoreduction filter did not significantly reduce the concentration of exogenously spiked PrP(Sc), P > 0.05.

CONCLUSION

The use of this new prion-reduction filter should reduce the risk of vCJD transmission through transfusion of RCC, the most widely transfused blood component.

Preparation of soluble infectious samples from scrapie-infected brain: a new tool to study the clearance of transmissible spongiform encephalopathy agents during plasma fractionation

BACKGROUND

Concern about the safety of blood, blood components, and plasma-derived products with respect to prions has increased since the report of two blood-related infections of variant Creutzfeldt-Jakob disease in the United Kingdom. Efforts were directed toward the development of procedures able to remove or inactivate prions from blood components or plasma-derived products with brain fractions of transmissible spongiform encephalopathy (TSE)-infected rodents as spiking materials. These spiking materials, however, are loaded with pathological prion protein (PrP(TSE)) aggregates that are likely not associated to blood infectivity. The presence of these aggregates may invalidate these studies.

STUDY DESIGN AND METHODS

Brains from 263K scrapie-infected hamsters were suspended in 10 percent phosphate-buffered saline. After low-speed centrifugation, the supernatant was collected and ultracentrifuged at 220,000 x g at 25 degrees C for 30 minutes. The high-speed supernatants (S(HS)) and pellets were collected; the proteinase-resistant PrP(TSE) was measured by Western blot and infectivity by intracerebral inoculation into weanling hamsters.

RESULTS

A substantial amount of prion infectivity (more than 10(5) LD(50) per mL of a 10% suspension of brain tissues) is present in the S(HS) fraction of 263K scrapie-infected hamster brains. Concomitantly, this fraction contains none or only traces of PrP(TSE) in its aggregate form.

CONCLUSION

This study describes a simple and fast protocol to prepare infectious material from 263K scrapie-infected brains that is not contaminated with PrP(TSE) aggregates. This S(HS) fraction is likely to be the most relevant material for endogenous spiking of human blood in validation experiments aimed at demonstrating procedures to remove or inactivate TSE infectious agents.

Managing the risk of transmission of variant Creutzfeldt Jakob disease by blood products

Whereas plasma-derived clotting factor concentrates now have a very good safety record for not being infectious for lipid enveloped viruses, concern has arisen about the possibility that prion diseases might be transmitted by blood products. There is epidemiological evidence that classical sporadic Creutzfeld Jakob disease (CJD) is not transmitted by blood transfusion. There is now good evidence that the abnormal prion associated with variant CJD can be transmitted by transfusion of fresh blood components and infect recipients. To reduce the risk of the pathological prion in the UK infecting recipients of clotting factor concentrates, these are now only manufactured from imported plasma collected from countries where there has not been bovine spongiform encephalopathy (BSE) in cattle and the risk of variant CJD in the population is, therefore, considered negligible. The safety of these concentrates is also enhanced because prion protein is, to an appreciable extent, excluded by the manufacturing process from the final product. To help reduce the chance of prion transmission by fresh blood products, donations are leucodepleted, there is increasing use of imported fresh frozen plasma (especially for treating children) and potential donors, who have been recipients of blood since 1980 (the beginning of the BSE epidemic in cattle) are deferred.