Since its first appearance on U.S. shores in 1999, West Nile virus has become another well-known hazard of the mosquito season. Transmitted by mosquitoes, the virus causes no symptoms at all in 80 percent of those infected, but serious illness in about one out of every 150 cases. The serious cases can result in permanent neurological damage or even death.
To date, treatment has been what is euphemistically termed "supportive." But in the May 2005 issue of Nature Medicine, now available online, researchers from Washington University in St. Louis; Rockville, Md.-based MacroGenics Inc.; and the New York State Department of Health have developed a humanized monoclonal antibody against West Nile virus that raised the survival level in mice from 10 percent to 90 percent when administered soon after infection.
"To our knowledge, these experiments are the first successful demonstration of the use of a humanized antibody as a post-exposure therapy against a viral disease," said senior author Michael Diamond, assistant professor at Washington University.
Diamond and his colleagues presented 46 monoclonal antibodies in the paper, but only 12 were able to neutralize West Nile virus both in vitro and in vivo. Since the research was conducted, Diamond and his group have tested additional antibodies, bringing the total to more than 200.
Epitope mapping in yeast showed that all 12 antibodies bound to the same 4-amino acid epitope on the DIII domain of the envelope protein. Plasma from patients infected with West Nile virus but who had only mild illness confirmed that they, too, had antibodies that recognized the same epitope.
When mice were treated with one of the antibodies, E16, which had shown strong neutralizing activity post-infection, the survival rate showed that 10 percent to 90 percent increase if the antibody was given within five days after infection. However if the antibodies were administered on day six or later, the ultimate survival rate plummeted back just as sharply, to 10 percent, though treated mice still survived longer than untreated controls.
The researchers then humanized E16, which, in addition to strong neutralizing activity, had a greater degree of homology between human and mouse versions than an alternate candidate. A second round of tests with the humanized E16 antibodies confirmed that they retained their ability to treat West Nile virus when administered two days post-exposure.
Diamond told BioWorld Today that the sudden decrease in effectiveness was not surprising for several reasons. For one thing, the virus "infects neurons and neurons don't regrow," so treatment after irreversible damage has occurred is unlikely to be effective. For another, most untreated mice die seven to eight days after infection, so five days after infection is late in the game to initiate treatment.
Diamond said that five days is "a long window for a mouse, but I cannot predict what the window will be in humans."
"What I will say is that in humans, the disease is more protracted," he added, which at least makes it possible that there could be a fairly long treatment window for humans as well. "But you don't know until you do the studies."
The University of Washington has licensed the antibodies to MacroGenics Inc., which humanized the E16 used in the Nature Medicine paper. MacroGenics will pursue preclinical development, including optimization, likely testing in a second animal model, and producing sufficient quantities under GMP conditions.
Diamond said that while he might advise the company on specific issues related to development, his lab would not be involved in the process on a regular basis.
"We're a basic science lab," he said.
A vaccine against West Nile virus entered clinical trials at the National Institutes of Health in mid-April. The vaccine was co-developed by National Institute of Allergy and Infectious Diseases' Vaccine Research Center, and San Diego biotechnology company Vical Inc. Vical also manufacturers the vaccine. It is based on an animal vaccine tested at the Centers for Disease Control and Prevention in Atlanta, and was effective in protecting horses against West Nile virus infection.