Methodological aspects of the validation of purification procedures of human/animal-derived products to remove unconventional slow viruses.

This study describes what needs to be considered in order to make a proper risk estimate of a human/animal-derived biological product from the contamination with unconventional slow virus(es). Several factors are important for this estimate. The first points regard the source of raw material (whether of human or animal origin), the kind of tissue (brain and other neural tissues being a higher risk), and in which way the tissue is collected. Then, the possibility, although remote, is taken into consideration of performing a quality control on the raw material through the measurement of PrP27-30, which is considered a specific marker of these diseases. Unfortunately, the detection of PrP27-30 is not yet so sensitive compared to the measurement of infectivity. Finally, the design of the validation experiments on the extraction and purification procedures of human/animal-derived products which gives the best estimate of safety is described. The conclusion of this study is that each biological product needs to be individually evaluated and therefore, it will be difficult to give standard guidelines for the judgement of their safety.