By David N. Leff
The poliomyelitis virus used to top the "Most Wanted" list of pathogens - afflicting some 50,000 children a year in the U.S. alone with lifelong paralysis. Then, in the mid-1950s, the Salk and Sabin vaccines - employing killed and attenuated virus respectively - whittled the disease down to near eradication.
Now the runty but vicious poliovirus (PV), which features a single-stranded RNA genome, no envelope and a diameter of only 22 to 30 nanometers, is being called to convert from crime to community service. Virologists have married PV to a viral cousin, the rhinovirus, whose worst offense is causing the common cold. Their union created a chimeric virus with an unexpected knack for killing human brain-tumor cells - fast.
"There are a number of proposals out there to use viruses for cancer therapy," observed virologist Matthias Gromeier, of Duke University in Durham, N.C. "What all these proposals have in common," he pointed out, "is that by understanding how viruses cause disease, we can turn around their ability to harm and eventually destroy their host cell, and use them against cancer cells. So our work is rooted in the study of how neuropathogenic viruses, in this case poliovirus, can cause a brain infection.
"And we are using this knowledge in order to tailor the poliovirus to infect healthy cells," Gromeier added, "and direct them toward attacking malignant tumor cells. This discovery was not a deliberate attempt to create a new treatment modality. We were really studying how the poliovirus can cause neurologic disease. It's in a sense serendipity."
Gromeier is first author of a paper in the current Proceedings of the National Academy of Sciences (PNAS), dated June 6, 2000, which reports his team's novel technology. Its title: "Intergeneric poliovirus recombinants for the treatment of malignant glioma." The article reports research done while he was at the Stony Brook campus of the State University of New York (SUNY), with which he still collaborates.
Paralytic + Sniffles Pathogens = Oncolytic Virus
"Poliovirus," Gromeier told BioWorld Today, "uses a unique mechanism to initiate translation, which means the reading and conversion of genetic information into a protein sequence. And PVs contain specific genetic elements within their non-translated region that are called internal ribosome entry sites - IRES. The virus uses these IRES to initiate translation in a cell's ribosome. This mechanism is now widely used in various commercial applications - even in the biotech community. For example, in gene therapy, to translate two open reading frames.
"What we discovered," Gromeier recounted, "is that these IRES elements can be used to direct gene expression specifically to certain cell types. We took the IRES element of human rhinoviruses - which, unlike PV, are never associated with neurologic disease - and plugged it into polioviruses. The outcome of this transaction," he continued, "is that the resulting chimeric construct is no longer able to cause neurologic disease, because its rhinovirus element will not function in neuronal cells.
"And that is highly desirable in brain tumor therapy," he said, "because you always have to make sure that if you use an infectious agent, it will not lead to collateral damage to healthy neuronal cells. That would be unacceptable."
The acceptability of this SUNY chimeric virus has been tested extensively at the FDA, in monkeys. "This was done by injecting the virus into their spinal cords," Gromeier observed, "and not a single one got any signs of neurologic disease. If done with a wild-type polio virus, the monkeys would die in a very short period."
Gromeier and his co-authors have tested their chimeric virus in large numbers of mice carrying human malignant glioma tumors. Glioma is the most common cancer of the central nervous system.
When a sudden seizure, headaches, vomiting, sensations of pressure inside the skull, flashes of light or diminished vision bring a person to the neurologist, CAT scans or magnetic resonance imaging may confirm malignant glioma and locate its tumor site or tumors sites in the brain. At that point the treatment of choice is not just surgical excision or irradiation or chemotherapy, but all of the above. Once the tumor has been cut out, followed by irradiation, followed by anti-tumor drugs, a patient's life expectancy is typically extended from a very few months to a year or so.
The SUNY team gave nude mice intracranial injections of human malignant glioma cells, followed by their chimeric viral therapeutic. "The results were really very encouraging," Gromeier recounted. "When we injected the virus directly into the xenotransplanted tumors of the mice, they were typically destroyed within two weeks, and the mouse had a completely normal life span thereafter."
PV's Secret Molecule: Tumor-Only Receptor
Gromeier explained the mechanism behind this anti-glioma efficacy:
"For most viruses used in tumor therapy, the exact mechanisms that account for tumor killing are not well known. We know very well which is the receptor for poliovirus. A single binding molecule called CD155 appears to be sufficient.
"We found that CD155 is ectopically overexpressed. That means it's expressed only in tumor cells, but it is not found on the healthy precursor cells of that tumor. Malignant glioma stems from glial cells, and we have evidence that normal glial cells do not express the poliovirus receptor. But once they become cancerous, they start making this CD155 molecule.
"Our oncolytic virus," Gromeier continued, "will infect tumor cells, then replicate. So even if we don't hit all tumor cells at once, we have several rounds of replication that will release very efficient viral particles to infect other tumor cells in the vicinity.
"It is really a coincidence," he remarked, "that the receptor for poliovirus appears to be associated with this type of malignancy, and we can exploit this coincidence to use poliovirus-based therapeutic agents for therapy. The National Cancer Institute's developmental therapeutics program is evaluating our chimeric virus for clinical trial," he concluded. "The main obstacle is production of the agent under GMP [good manufacturing practice] conditions, which is currently our goal."