Spongiform Encephalopathies
The emergence of HIV in the 1980’s provided a major driving force for the development of virus screening tests as well as virus-inactivation methods for use with plasma products. The inactivation methods that were developed also proved effective against other major plasma-transmitted viruses e.g. HBV and HCV. A number of factors contribute to the safety of both blood and plasma products. The high quality of blood donated by non-remunerated UK donors, donor selection, and the testing of each donor for various viral markers are initial contributors to virus safety. The main products derived from plasma include coagulation factors, albumin, and immunoglobulin. The use of plasma pools of 10,000 or more donations increases the risk of virus transmission. In addition to HIV, HBV and HCV screening on each donation, additional testing should be done on the plasma pool, process intermediates, and final products. The use of PCR assays requires further evaluation, especially given that PCR positivity does not necessarily equate with infectivity.
Coagulation factors
Coagulation factors are prepared from plasma by various precipitation and adsorption tests. Following the outbreak of the HIV-epidemic, virus-inactivation steps were introduced into the manufacturing process which were effective in inactivating HIV with little loss in product activity. For products of immediate purity, heat treatment was either carried out at 80oC, or pasteurization at 60oC for 10 hours. Treatment with B-propiolactone/UV has also been used in the past but this was discontinued when a single batch of factor IX was found to transmit HIV. High purity coagulation factor concentrates incorporates an in-process virus inactivation step. The most widely used method is the solvent/detergent procedure using Tri-n-butyl phosphate and a non-ionic detergent such as Triton X 100 or Tween 80. This method is currently used for about 70% of all plasma products and has an unblemished safety record for enveloped viruses. The method has no adverse effect on the product, although the chemicals must be removed after treatment. Other virus inactivation methods such as pasteurization or moist-heat treatment are also used. Dry heat-treatment and virus-removing filters are being evaluated as final virus-reducing steps in an effort to control more resistant non-enveloped viruses such as hepatitis A and parvovirus B19. Subsequent purification steps involving chromatographic processes can also contribute to virus safety of the product.
Albumin
Albumin is prepared from plasma by cold-ethanol fractionation, a process which is likely to reduce the virus load in the product. Pasteurization (60oC for 10 hours) can also be carried out on both the bulk product and on the final product. Under these conditions, HIV is rapidly inactivated within 30 minutes. Albumin has been prepared in this manner since the 1940’s and has an excellent safety record.
Intramuscular immunoglobulins
Immunoglobulins are prepared from plasma by cold-ethanol fractionation. Although there are no reports of HIV having been transmitted by immunoglobulins, there have been several reports concerning the transmission of HCV. These reports suggest that the conditions of cold-ethanol fractionation, by themselves, are not sufficient to render immunoglobulins completely safe from virus transmission.
Intravenous immunoglobulins
Further processing is required to make an immunoglobulin product suitable for intravenous use. This is simply accomplished in some products by treating the Intramuscular product with pepsin at pH4 for extended periods of time. Alternative approaches for the production of IVIG involve chromatographic processes. Although no cases of HIV had been reported following the administration of IVIG, high rates of HCV transmission had been reported. As a result, there is now increasing pressure for the incorporate of a specific inactivation step in such products eg. a solvent/detergent step with Tween 80. In addition the CM sepharose ion-exchange chromatographic step can reduce the level of enveloped viruses significantly. It is expected that these steps will contribute further to the safety of the product.
In conclusion, the safety of blood products has increased significantly over the last 10 years with the introduction of virus screening and virus reduction methods. Residual risk estimates are in the order of 1 in 3.5 X 105 per unit of blood and <1 in 106 per unit of solvent/detergent treated factor VIII. However, there has been pressure to introduce additional inactivation steps for plasma products as a result of reported rare instances of HBV and HCV transmission, and also because of the inability of current manufacturing processes to deal with small non-enveloped viruses such as HAV and parvovirus B19. Currently, many new techniques are being evaluated, such as photochemical inactivation, iodine-based chemicals, UV-C, virus filtration, and phase conversion by high pressure.