Drugs and the pharmaceutical sciences

572 Aranha
also virus removal filters have been demonstrated to provide prion clearance; chromatographic
purification in the manufacture of coagulation factor concentrates is extensively
used and these methods have the potential to effect prion removal (Foster et al., 2000).
PROCESS CLEARANCE EVALUATION: CONSIDERATIONS AND
DESIGN ISSUES
As in the case of ensuring viral safety of biologicals, a multifaceted approach involving
adequate sourcing, incorporation of multiple orthogonal clearance strategies and process
evaluation for prion clearance are vital. While there is no specified requirement for
demonstration of prion clearance in the case of biotech products, if the product labeling
includes any reference to the safety of the product from a prion standpoint, the claims
must be substantiated with experimental data.
Currently, there are no in vitro tests directly applicable for detection of low levels
of infectivity either in the raw materials or the finished product; consequently, adequate
sourcing and demonstration of the prion clearance ability of the manufacturing process
constitutes a key paradigm to ensure safety.
The first steps in the design of the clearance evaluation study involve a critical
analysis of the entire manufacturing process to determine the potential sources of prion
contamination followed by process characterization to evaluate which steps in the
manufacturing process possess clearance capability. The principles applied in evaluation
of clearance of conventional viruses are applicable to TSE agents (Hellman and Asher,
2000). CPMP (Committee for Proprietary Medicinal Products) guidance documents
acknowledge that several routine processing steps such as precipitation, chromatography
and nanofiltration can contribute to TSE agent removal and require that whenever TSE
clearance claims are made for a particular step, the process should be validated
(CPMP, 1996; EMEA, 2000).
A number of studies have been undertaken to evaluate purification process steps for
their potential for prion clearance using either a crude brain homogenate (Bader et al.,
1998; Lee et al., 2000; Lee et al., 2001), detergent-solubilized (Pocchiari et al., 1991) or a
microsomal fraction (Forster et al., 2000) of the hamster-adapted sheep scrapie agent. The
kind of spike used will depend on the process step being evaluated.
Issues to address when designing a validation study to document prion removal are
choice of spiking agent, nature or form of the spiking agent (and its relevance), the design
of the study (including appropriate scale down) and the detection method (in vitro or
in vivo assay). Validations are often performed using strains of the sheep scrapie agent
that have been adapted to either mice or hamsters, by direct intracerebral inoculation of
infected sheep brain into mice or hamsters followed by serial passage of the agent in the
same species.
Spiking Agent(s)
Experimental TSE studies have been conducted with a variety of TSE agent spikes—
mouse/hamster passaged scrapie agent, mouse-passaged BSE, mouse/hamster/guinea pigpassaged
CJD. Process clearance evaluation studies are often performed using strains of
rodent-adapted sheep scrapie agent; these models have been accepted by regulatory
authorities (Bader et al., 1998).
In general, the hamster-adapted 263 K strain of scrapie is regarded as the optimal
system because high titers in the brain combined with a short incubation period are a
hallmark of this system. Spike-related considerations include the “relevance” of the spike