BOSTON –– In a finding that represents an entirely new approach to treating viral diseases such as smallpox, scientists at Dana-Farber Cancer Institute and collaborating institutions have shown that infections can be stymied by interfering with signals used by viruses to reproduce in human cells.
The results, reported in the February issue of the Journal of Clinical Investigation, point to a possible strategy for broadly treating acute viral infections that affect millions of people worldwide. If the technique leads to a drug capable of treating people infected with the smallpox virus, it could eliminate the virus' potential as a bioterror agent.
"Certain current anti-viral medications have a number of shortcomings that make them less than ideal for treating and/or preventing illnesses," says the study's senior author, Ellis Reinherz, MD, of Dana-Farber. "The existing vaccine against smallpox, for example, poses potential health risks that make it a questionable candidate for protecting the public against an outbreak of the disease. The approach we've taken is based on a new understanding of the basic mechanisms of viral reproduction and movement –– the actual steps that take place once a virus has invaded the body."
In the study, Reinherz and his colleagues at Dana-Farber (Hailin Yang, PhD, Mikyung Kim, PhD, and Pedro Reche, PhD), at the University of Massachusetts Medical School (Sung-Kwon Kim, PhD, and Raymond Welsh, PhD) and at the Centers for Disease Control and Prevention (Tiara Morehead and Inger Damon, MD, PhD) used a "small molecule" drug –– already in development for some kinds of cancer –– to block a key signal in the cell machinery used by smallpox and similar viruses to propel themselves from infected cells and to reproduce in new cells. In laboratory samples of monkey kidney cells, the drug prevented the smallpox virus from spreading to other cells. In lab mice infected by a virus similar to the smallpox virus, a combination of the drug and a single antibody injection resulted in complete clearance of the lungs in just eight days.
"The results demonstrate the principle that viral diseases can be effectively fought by blocking cellular signaling pathways that viruses depend on for reproduction," Reinherz says. "We now have a model of an approach that can potentially be used to treat a wide array of acute viral conditions."
The study was funded by a grant from the National Institute of Allergy and Infectious Diseases (NIAID), as part of an effort to strengthen the country's defenses against bioterrorism, and from the Dana Foundation.
Smallpox has been identified as a potential terror weapon because, with the suspension of universal vaccination against the virus in the 1970s, most Americans are thought to have little immunity to the often lethal disease. The NIAID grant supports research into the basic workings of the immune system, with the expectation that it will lead to new, more effective treatments and vaccines for possible bioterror agents.
Viruses reproduce by entering cells, insinuating themselves into the cells' division and transport machinery to make copies of themselves. Most existing anti-viral drugs consist of small molecules that bind to viruses, hindering the bugs' ability to enter cells or to multiply once inside. The problem with this approach is that viruses mutate rapidly, changing their genetic makeup in a way that often causes drugs to lose their effectiveness against them.
As for the smallpox vaccine, which raises a natural shield of immunity against infection, it poses potential risks to people with weakened immune systems –– either as a result of disease or treatment for cancer –– or with autoimmune disorders such as the skin condition eczema.
"The advantage of targeting signaling pathways is that cells, and the structures that send and receive signals, are far less likely to mutate than viruses themselves, making it improbable that drugs will lose their potency," Reinherz explains. For short time intervals, blockade of selective cell function is feasible.
In a study published in 2004, Reinherz and his colleagues found that the smallpox virus –– formally known as variola –– initiates this process with a signaling protein called smallpox growth factor (SPGF). SPGF flows from the virus to a "receptor" called erb-B1 on the cell surface; when it binds, the cell is primed to become a factory for the virus.
"This suggested that the process of viral replication could be hindered by short-circuiting the SPGF signaling pathway," Reinherz states. To test the idea, they injected SPGF-blocking antibodies into mice infected with a vaccinia virus (it carries a protein much like SPGF), which causes a lethal pneumonia in mice. As hoped, the animals were protected from developing the disease.
In the new study, investigators tested this approach in laboratory-grown cells and in lab animals infected with a pneumonia virus similar to variola. Instead of using antibodies as the SPGF blockers, they used an experimental cancer drug called CI-1033, which is being developed by Pfizer Corp. as a potential cancer treatment. A search of scientific literature revealed that CI-1033 binds to erb-B1, preventing SPGF from signaling through the cellular receptor, even after landing on it.
When they mixed CI-1033 into monkey kidney cells infected with variola, the spread of virus to other cells was blocked. By administering the drug and another compound in mice exposed to the vaccinia virus, the mice not only lived longer than untreated animals but their lungs were cleared of infection. There were also signs that the treated animals produced a stronger immune attack on the virus than the untreated animals did.
Reinherz notes that although CI-1033 itself is not yet an acceptable smallpox treatment -- due to certain side effects -- it represents the type of drug likely to be successful against the disease. Efforts are now under way to identify erb-B1-blocking drugs that can be given safely to patients and healthy individuals.
In addition, the approach identified by Reinherz and his colleagues would not be useful for chronic viral infections, such as those caused by HIV, the virus that gives rise to AIDS, but rather for acute virus-caused conditions. Since drugs like CI-1033 do not kill viruses outright but deter them from reproducing, the aim of therapy is to keep infection at a low enough level that it can be handled by the immune system.
Because variola belongs to the large family of orthopox viruses (which also include the agents for monkeypox and cowpox) and other viruses exploit erb-B, human cytomegalovirus, for example, it's likely that the signal-blocking technique will prove effective in a wide array of conditions, remarks Reinherz, who is also a professor of medicine at Harvard Medical School. These findings open the way for developing inhibitors of other pathways in cells used by different viruses as well.
In a commentary appearing in the same issue of the Journal of Clinical Investigation, Anthony S. Fauci, MD, director of NIAID, and Mark D. Challberg, PhD, emerging viral diseases program officer at NIAID, write that the technique "would be especially useful in developing countermeasures against newly emerging infectious diseases and those that are introduced deliberately, as in a bioterror attack."
Dana-Farber Cancer Institute is a principal teaching affiliate of the Harvard Medical School and is among the leading cancer research and care centers in the United States. It is a founding member of the Dana-Farber/Harvard Cancer Center (DF/HCC), designated a comprehensive cancer center by the National Cancer Institute.
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