Inactivation of Prions

INACTIVATION OF PRIONS

Prions are the cause of transmissible spongiform encephalopathies, a group of fatal neurological diseases such as scrapie, Creutzfeld–Jakob disease (CJD), new variant Creutzfeld–Jakob disease (vCJD), bovine spongiform encephalopathy (BSE), kuru and Gerstman–Straussler– Sckeinker syndrome (GSS). It is now widely accepted that prions are an abnormal, protease-resistant form of a normal harmless host protein (PrP). The prion protein undergoes a conformational change from four α-helices to an infectious β-sheet form. These conformational changes cause degeneration of nervous tissue which, under the microscope, exhibits a sponge-like appearance. Prions are found in association with a wide range of tissues and are of particular significance in a pharmaceutical context because of the need to decontaminate surgical or other hospital equipment that has been in contact with such diseased tissue. Prions are very stable in the environment, may form aggregates, and are highly resistant to conventional disinfection and sterilization methodologies.

Prions are considered highly resistant to various types of biocidal products (formulations) (Table 20.6); this includes strong acids (e.g. 8 M hydrochloric acid for 1 hour), alkylating agents (e.g. glutaraldehyde, βpropiolactone), iodine and iodophors, phenolics, alcohols, oxidizing agents in their liquid form (e.g. hydrogen peroxide, peracetic acid) and proteolytic enzymes. Mild detergents were also reported inactive although sodium dodecyl sulphate (SDS) has shown some activity.

Alkali (e.g. 1 M NaOH for 1 hour) is usually effective against prions and as such has been widely used in the laboratory, industrial and clinical environments. However, alkali efficacy might depend on the prion’s nature (host), and residual prion infectivity following treatment has been documented. More aggressive treatments combining alkali and gravity-displacement autoclaving at 121 °C for 30 minutes have been used. The use of sodium hypochlorite containing 20 000 ppm of available chlorine for 1 hour has been recommended for use in practice. Sodium dichloroisocyanurate (NaDCC) might not be equally effective against some prion proteins, as some infectivity following treatment has been reported. Hydrogen peroxide in a gaseous form has been shown to be active against prions, although reported efficacy to date depends much on the type of vapourized hydrogen peroxide generator used.

Formulations play an important role in prion decontamination. Complex formulations containing liquid hydrogen peroxide and copper have been shown to be active against prions. The combination of alkali, chelating agents, surfactants and various buffers have been particularly effective as they combined prion removal from surfaces and prion inactivation. A combination of proteinase K, pronase and SDS has been shown to degrade PrP^res material from highly concentrated vCJD-infected brain preparations. Slight changes in formulations might bring a loss of efficacy against prions, however, and new formulations need to be carefully assessed for antiprion activity.