By David N. Leff
Besides their cloven hoofs, one thing that cows, sheep and pigs have in common is the pestivirus - a global pathogen of domestic livestock.
"The four-member estivirus family," observed molecular virologist Marc Collett, "is classically associated with infections of bovine, ovine and porcine species. In the case of the bovine virus," he went on, "it's very difficult to find herds of cattle that are completely free of the virus. It's very ubiquitous, not only in the U.S., but in most countries around the world."
Pestivirus belongs to the Flaviviridae family, which means it's an enveloped, single-strand, positive sense RNA virus. Other viral family members include the human-disease purveyors of dengue fever, yellow fever - and hepatitis C and West Nile virus.
Collett is vice president of discovery research at ViroPharma Inc. in Exton, Pa. "In the 1990s," he pointed out, "there have been recent pestivirus disease outbreaks in the U.S., Canada, Germany and the Netherlands. In these animals and their progeny" he noted, "pestivirus causes a range of pathogenic manifestations, including abortion, teratogenesis [fetal malformations], respiratory problems, chronic wasting and immune-system dysfunction. This then exposes animals to secondary viral or bacterial infections. In a smaller proportion that become infected," he pointed out, "this immune deficiency can lead to an acute, fatal disease."
Charles Rice, a molecular virologist at Washington University in St. Louis, explained: "These animals are actually infected in utero, because the infectious virus is transmitted transplacentally. So they are born persistently infected and immuno-tolerant to BVDV, and therefore don't recognize the resident pestivirus as foreign. They have severely pathogenic phenotypes that kill them."
Collett resumed: "The majority of infected animals will recover, and then be immune to reinfection. There are probably hundreds of vaccines commercially available against BVDV," he said. "In some countries, vaccines are used with varying degrees of success. In others, culling, quarantine and slaughter are used to contain outbreaks. Currently," he pointed out, "there are no antiviral pharmaceuticals for controlling pestivirus diseases."
At the U.S. Department of Agriculture's National Animal Disease Center, in Ames, Iowa, specialist Keith Murray told BioWorld Today: "Pestivirus comes in four persuasions, of which the two best known are BVDV and hog cholera. Three years ago in the Netherlands," he recalled, "the porcine version caused an outbreak that resulted in the slaughter of 12 million hogs, at a cost to the country's economy of $2.2 billion."
Scientists at ViroPharma and Washington University are co-authors of a paper in the current Proceedings of the National Academy of Sciences (PNAS), dated July 5, 2000. Its title: "Mechanism of action of a pestivirus antiviral compound." Collett is its senior author.
'Interesting Molecule' Fills Drug Therapy Void
"We report finding some interesting small-molecule compounds," he told BioWorld Today, "that inhibit replication of the prototype pestivirus, BVDV - the bovine viral diarrhea virus." Their drug discovery involved "traditional screening for inhibitors of viral replication in cell culture. We call one promising candidate VP32947. It's a small molecule that we found to be a very specific inhibitor of pestivirus replication. And we subsequently determined that VP32947 acts by affecting RNA replication at the level of the viral RNA polymerase."
As for its potential utility in protecting livestock, Collett observed, "We find that the compound is potentially useful for applications in either treating or preventing pestivirus infections. So far, these findings derive from in vitro experiments. At the present time," Collett said, "we're certainly contemplating in vivo studies."
In vitro, the co-authors challenged bovine kidney cells with wild-type BVDV pestivirus, then treated them with VP32947. "The drug showed antiviral activity," Collett recounted, "against all four pestivirus species, but did not affect distant or unrelated viruses - such as dengue and yellow fever, influenza and herpes simplex. When we added the compound to the cell culture up to eight hours after infection," he continued, "it inhibited up to 95 percent of the viral yield. These results suggest that the drug acts on a process after viral attachment and penetration, but before its assembly and release."
He made the point, "Our pestivirus collaboration with the university is more of an academic exercise at this point, but it could have commercial applications. Pestiviruses in general, and BVDV in particular," Collett reiterated, "cause very significant and economically important epizootic and enzootic diseases throughout the world. So the potential extensions and applications of our work could be in any of those areas."
Animal Virus Wisdom Helps Hepatitis C Research
Then the ViroPharma scientist took off on a new tangent: "The other aspect, which is probably equally as important, is that this virus - BVDV in particular, and pestiviruses in general - is a cousin of the human hepatitis C virus [HCV]. Pestivirus belongs to the same taxonomic virus family - Flaviviridae - as hepatitis C, but unlike HCV, we're able to culture this animal virus. HCV shares many molecular and virological similarities with pestiviruses, but it can't be efficiently cultured. Both have RNA genomes, approximately 9,600 and 12,600 nucleotides in length, respectively. So almost all research groups that are working in HCV have adopted the BVDV of pestiviruses as a model experimental system to facilitate their understanding of hep C. We are pursuing that line, too. ViroPharma has a separate program in HVC.
"We're trying to treat hepatitis C with small-molecule pharmaceuticals, looking for different chemicals, obtained by setting up either biochemical or cellular assays, and screening large numbers of compounds, using high-throughput screening technologies." He noted that the World Health Organization estimates that 170 million people worldwide are chronically infected with HCV.
The PNAS paper concludes: "Given the seriousness of the hepatitis C epidemic and the urgent need for new and effective therapies, it is hoped that compounds such as the one described here may provide insight toward identification of HCV antiviral drugs. The advancement of these compounds into clinical evaluations is eagerly anticipated."