Wednesday, April 14, 2010

Oops, adventitious viral DNA fragments in a vaccine

by Dr. Ray Nims

On March 22, 2010, a press release from GlaxoSmithKline (GSK) announced that DNA from porcine circovirus had been detected in their rotavirus vaccine. According to GSK, the DNA ”was first detected following work done by a research team in the US using a novel technique for looking for viruses and then confirmed by additional tests conducted by GlaxoSmithKline”. As a result of this finding, the FDA “is recommending that US clinicians and public health professionals temporarily suspend the use of Rotarix as a precautionary measure. The FDA have also stated that they intend to convene an advisory committee, within approximately four to six weeks, to review the available data and make recommendations on rotavirus vaccines licensed in the USA. The FDA will also seek input on the use of new techniques for identifying viruses in vaccines.” The EMEA, on the other hand, does not appear to consider this finding to be a safety concern, citing the fact that porcine circovirus is not infectious for human cells and does not cause disease in humans.

Porcine circovirus
Source: Meat and Livestock Commission, UK

What is porcine circovirus? Porcine circovirus (PCV) is a member of the family Circoviridae, among the smallest of the known animal DNA viruses. It is approximately 17 nm in diameter, non-enveloped, with icosahedral symmetry. The virus was originally identified as a noncytopathic contaminant of the PK-15 porcine kidney cell line (Tischer et al., Zentralblatt Bakt Hyg A 226:153-167, 1974). Like many very small, non-enveloped viruses, PCV represents a challenge for removal and inactivation.

Why is this finding coming to light now, or stated another way, why wasn’t the PCV DNA detected when the vaccine was initially tested and approved for human use? The press release wasn’t specific as to method used. It has subsequently been revealed, however,  that these fragments were detected using sequence-independent amplification (deep sequencing; or as Eric Delwart calls it, metagenomics). The resulting library of amplified sequences is characterized by BLAST searching using identification algorithms. Confirmation in this case was obtained using microarray and PCR analyses. The sequencing techniques have been available for some time, and have proven useful for identification of viruses in an academic setting, though they have not been applied to safety testing of biopharmaceuticals until fairly recently due to the relatively high costs associated with the analyses.

The finding of viral DNA should not be equated with detecting the infectious virus in the product. The sequence-independent amplification, microarray, and virus-specific PCR assays used can detect viral nucleic acid, but as normally performed do not indicate whether infectious virus is present. Generally, with the possible exception of transforming viruses, it is the infectious virus that is of concern, not its DNA. Efforts to assess the presence of intact viral genomic material and of infectious porcine circovirus in this vaccine are most likely underway at this time.

The presence of the PCV viral sequences has provisionally been attributed to the use of porcine trypsin during the culture of the Vero cell substrate in which the vaccine is manufactured. The trypsin used had been gamma-irradiated to inactivate adventitious viruses prior to use. While it would be comforting to believe that the PCV DNA may simply have reflected carryover of non-infectious, lethally-irradiated PCV1 from the trypsin, the fact is that gamma-irradiation is not very effective at inactivating this small, non-enveloped virus (Plavsic and Bolin. Resistence of porcine circovirus to gamma irradiation. Biopharm Int. 14:32-36, 2001). In the case of porcine circovirus, there is little evidence to indicate that the virus is infectious or pathogenic for humans. So regardless of the outcome of the various ongoing studies, it is likely that the use of the GSK rotavirus vaccine will be re-instated after the FDA convenes and discusses the implications of this finding.

I predict that there will be more and more of this kind of revelation in the future as the sequencing techniques display stretches of viral or other contaminant DNA within samples of biopharmaceuticals. I would hate to see revelations like this impede the use of the sequencing technologies going forward, as these technologies are going to be very useful to the industry as rapid detection methods for contaminant identification.

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