Biomedical Engineering Reference
In-Depth Information
7.6.5 Viral assays
A range of assay techniques may be used to detect and quantify viral contaminants in both raw
materials and fi nished-biopharmaceutical products. No generic assay exists that is capable of de-
tecting all viral types potentially present in a given sample. Viral assays currently available will
detect only a specifi c virus, or at best a family of closely related viruses. The strategy adopted,
therefore, usually entails screening product for viral particles known to be capable of infecting
the biopharmaceutical source material. Such assays will not normally detect newly evolved viral
strains, or uncharacterized/unknown viral contaminants. This fact underlines the importance of
including at least one step in downstream processing that is likely to inactivate or remove viruses
indiscriminately from the product. This acts as a safety net.
Current viral assays fall into one of three categories:
•
immunoassays;
•
assays based on viral DNA probes;
•
bioassays.
Generation of antibodies that can recognize and bind to specifi c viruses is straightforward. A
sample of live or attenuated virus, or a purifi ed component of the viral caspid, can be injected
into animals to stimulate polyclonal antibody production (or to facilitate monoclonal antibody
production by hybridoma technology). Harvested antibodies are then employed to develop specifi c
immunoassays that can be used to screen test samples routinely for the presence of that specifi c
virus. Immunoassays capable of detecting a wide range of viruses are available commercially.
The sensitivity, ease, speed and relative inexpensiveness of these assays render them particularly
attractive.
An alternative assay format entails the use of virus-specifi c DNA probes. These can be used
to screen the biopharmaceutical product for the presence of viral DNA. The assay strategy is
similar to the dot blot assays used to detect host-cell-derived DNA contaminants, as discussed
earlier.
Viral bioassays of various different formats have also been developed. One format entails incu-
bation of the fi nal product with cell lines sensitive to a range of viruses. The cells are subsequently
monitored for cytopathic effects or other obvious signs of viral infection.
A range of mouse-, rabbit- or hamster-antibody production tests may also be undertaken.
These bioassays entail administration of the product to a test animal. Any viral agents present
will elicit production of antiviral antibodies in that animal. Serum samples (withdrawn from
the animal approximately 4 weeks after product administration) are screened for the presence
of antibodies recognizing a range of viral antigens. This can be achieved by enzyme immu-
noassay, in which immobilized antigen is used to screen for the virus-specifi c antibodies. These
assay systems are extremely sensitive, as minute quantities of viral antigen will elicit strong
antibody production. A single serum sample can also be screened for antibodies specifi c to a
wide range of viral particles. Time and expense factors, however, militate against this particular
assay format.
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