Prions, the causative agents of Creutzfeldt-Jakob Disease (CJD) and related neurodegenerative disorders in humans including variant CJD (vCJD) have up to now never been demonstrated in human blood although if in UK two probable transmissions of vCJD by blood transfusion were reported (
Llewelyn et al., Lancet 2004;363:417–421
and Peden et al., Lancet 2004;364:527–529
). In order to minimize a theoretical risk of human prion contamination of plasma pools, potential donors at risk of incubating CJD are excluded from donation permanently. Due to the implementation of these precautionary measures, the risk to collect plasma from a donor developing vCJD subsequent to a donation is very low. In addition, the amount of prions present in plasma of a healthy donor (i.e., a donor in the incubation period of (v)CJD) can be considered very low (Comer Risk Assessment of Exposure to vCJD Infectivity in Blood and Blood Products for Department of Health [UK], Revision DE, Det Nordske Veritas, February 2003). Even with this very low risk related to plasma for fractionation, the capacity of selected steps of the manufacturing process of the VWF/FVIII product Haemate® P/Humate-P® to remove prions has been evaluated in prion removal studies. As it has never been possible to isolate infectious prion protein, termed PrPSc, from human blood or plasma, the model prion strain 263K (isolated from brain of deliberately infected hamsters) was spiked to intermediates of scaled-down plasma protein purification processes. As there is no knowledge of the biophysicochemical nature of potential prion contaminants in plasma, we employed two different prion spike preparations in these studies: membrane associated prions in so-called microsomes and purified prion protein PrPSc, an almost native, molecular, non-membrane associated form of prion infectivity. Laboratory studies were performed by spiking either experimental plasma pools or an intermediate at a production stage further downstream with prion preparations and investigating the prion reduction by consecutive scaled-down process steps. These investigational studies resulted in a mean overall prion reduction factor of 5.3 log10 and 6.4 log10, respectively, for the two hamster derived spike preparations.Reduction of prions by selected manufacturing steps for Haemate® P/Humate-P®
| Manufacturing stages
. | Prion Reduction Factor [log10]
. |
|---|
|
. | Microsomes
. | PrPSc
. |
|---|
| Combined steps cryoprecipitation, Al(OH)3, glycine and NaCl precipitation | 3.5 | 3.9 |
| Combined steps ultracentrifugation/sterile filtration | 1.8 | 2.5 |
| Mean Overall Prion Reduction | 5.3 | 6.4 |
| Manufacturing stages
. | Prion Reduction Factor [log10]
. |
|---|
|
. | Microsomes
. | PrPSc
. |
|---|
| Combined steps cryoprecipitation, Al(OH)3, glycine and NaCl precipitation | 3.5 | 3.9 |
| Combined steps ultracentrifugation/sterile filtration | 1.8 | 2.5 |
| Mean Overall Prion Reduction | 5.3 | 6.4 |
Additionally, confirmative data were obtained in studies employing human brain-derived prion preparations (sCJD and vCJD) as spiking material in selected studies of a single manufacturing step. Therefore, it can be concluded that the risk of prion transmission by the VWF/FVIII product Haemate® P/Humate-P® is extremely remote based on complementary safety procedures, i.e., collection of plasma by stringent donor selection and the overall prion reduction factor which clearly exceeds a potential prion load in the manufacturing pool. This high safety margin is confirmed in a probabilistic risk calculation, demonstrating at a probability of 95% a safety factor for each vial of the product of 6.4 log10.
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