Monday, September 12, 2011

Ridding serum of viruses with gamma irradiation: part 1

by Dr. Ray Nims

Blood serum, while at times required as a medium component for cell growth in vitro, is an animal-derived material that can introduce contaminating viruses such as Cache Valley virus, REO virus, vesivirus, and epizootic hemorrhagic disease virusinto a biological product. If animal serum must be used in upstream manufacturing processes, the risk of introducing a virus may be mitigated by gamma irradiation of the frozen serum prior to use. How effective is this treatment, and against which viruses?

To answer this question, I have surveyed the literature from the past two decades. A number of investigations have been conducted and the results are in the public domain. The most useful of these studies have examined the dose-response relationships for viral inactivation (rendering of the virus as non-infectious) by gamma irradiation.

In the table below, I have assembled the results obtained for 7 viruses, including four that might be expected to be found in bovine serum: (bovine viral diarrhea virus [BVDV], infectious bovine rhinotracheitis virus [IBR], respiratory-enteric orphan virus [REO virus], and parainfluenza type 3 virus [PI3]). The other three (canine adenovirus, porcine parvovirus [PPV], and mouse minute virus [MMV]), while perhaps not expected to be found in bovine serum, have been studied as model viruses for the adenovirus and parvovirus families (click on table to enlarge).

The efficacy of gamma irradiation for viral inactivation is reported as log10 reduction in titer per kGy, rather than the more commonly employed D10 (Mrad dose required to reduce the titer by 1 log10), as I find the former value to be more useful. To estimate the effectiveness of a given dose of gamma radiation for inactivation of a virus, just multiply the dose in kGy by the log10 reduction in titer per kGy value from the table. The result is the number of logs of inactivation estimated to be achieved for that virus at that radiation dose.

These data tell us that the mid- to large-sized viruses BVDV, IBR, PI3, REO, and CAV should be readily inactivated at the gamma radiation doses normally applied to frozen serum for risk mitigation (25-45 kGy). On the other hand, the two parvoviruses, PPV and MMV, are more difficult to inactivate, presumably due to their small size. Parvoviruses are often used to challenge viral removal and inactivation processes due to their size and lack of an envelope. Higher kGy dose levels may increase the effectiveness of inactivation for these viruses, although at such levels the performance of the animal serum being irradiated may be adversely impacted.

Gamma-irradiation can effectively mitigate the risk of introducing other potential contaminants of bovine serum, including Cache Valley virus, blue tongue virus, and epizootic hemorrhagic disease virus. Like the parvoviruses, however, other relatively small non-enveloped viruses of the calicivirus, picornavirus, polyomavirus, and circovirus families may represent cases where gamma irradiation is less effective at the doses normally applied. Other means of mitigating the risk associated with these viruses may need to be considered.

< This information was excerpted in part from Nims, et al. Biologicals (2011) >

Information sources: Daley et al., FOCUS 20(3):86-88, 1998; Wyatt et al. BioPharm 1993: 6(4):34-40; Purtel et al., 2006; Hanson and Foster, Art to Science 16:1-7, 1997; Hyclone Labs Art to Science 12(2): 1-6, 1993; Gauvin and Nims 2010; Plavsic et al. BioPharm 2001: 14(4):32-36.

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