NIAID Researchers Identify New Drugs to Treat "Mad Cow"-Like Disease in Mice
Researchers from the National Institute of Allergy and Infectious Diseases (NIAID) have identified a new class of compounds that slows the development of a "mad cow"-like prion disease in mice, they report in the February 25 issue of Science. Prion diseases involve aggregation of toxic proteins in the brain, a process blocked by these compounds. If future studies extend these findings to humans, this research may one day benefit not only patients with Creutzfeldt-Jakob disease (CJD), but also patients with non-prion diseases involving protein aggregation such as Alzheimer's disease and type 2 diabetes.
Prions are special proteins believed to be the infectious agents responsible for transmissible spongiform encephalopathies (TSEs), a group of rare, fatal disorders that slowly destroy the brain and nervous system. These diseases include bovine spongiform encephalopathy (BSE, "mad cow disease") in cattle, scrapie in sheep, chronic wasting disease (CWD) in deer and elk, and CJD in humans.
In each case, the brain accumulates abnormal forms of prion protein. Every protein in the body must be folded into a specific three-dimensional shape to be functional. When proteins fold incorrectly, as in prion diseases, they tend to aggregate and cause disease.
Suzette Priola, Ph.D., and colleagues from NIAID's Rocky Mountain Laboratories (RML) in Hamilton, MT, studied the effect of cyclic tetrapyrroles, compounds that include drugs used in cancer therapy, on a TSE disease in mice. A critical step in the development of TSE diseases occurs when normal prion protein converts to an altered, disease-causing form. Previously, a team from RML had reported that cyclic tetrapyrroles block this conversion in the test tube; now Dr. Priola's team has shown that these same agents significantly slow TSE disease progression in mice.
Dr. Priola's team injected mice with a high dose of scrapie, which causes normal prion proteins to convert to the disease-causing form. The researchers then gave one of three candidate treatment compounds at different times during the 80-day incubation period. When given at the time of injection or when mixed with the injection, the compounds extended survival time dramatically, in some cases by 300 percent. When given later during disease, they had minimal effect. These results suggest that the compounds might delay disease if given prior to or early during infection. They also might be used to treat blood products to inactivate infectious prions. According to this study, however, they would likely be ineffective once the disease develops.
"Since diagnosis of CJD in humans can be made only after symptoms appear, we are trying to identify a compound that works later in the disease," explains Dr. Priola. "The extremely wide variety of these compounds available for testing makes our current search for one that can affect disease after the onset of symptoms much more promising."
Treatment for prion diseases has received renewed interest of late. A new variant of CJD appeared in England in 1996, coinciding with a BSE epidemic in that country. Because the new variant closely resembles BSE, health officials believe the disease jumped from cattle to humans. In the western United States, CWD in wild deer and elk has become an increasing concern because of fears it might spread to other species such as cattle and humans. Other diseases, such as Alzheimer's disease and type 2 diabetes, show a similar accumulation of misfolded proteins, although they do not involve prions. Therefore, drugs that block this process might prove effective in treating these disorders as well.
Reference: SA Priola, A Raines, and WS Caughey. Porphyrin and phthalocyanine anti-scrapie compounds. Science 287:1503-06 (2000).
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