By David N. Leff

Cows aren't the only animals that develop mad cow disease - bovine spongiform encephalopathy (BSE). In sheep and goats, the same fatal brain infection goes by the name of scrapie, because in a frenzy of itching brought on by the disease, these mammals keep scraping their necks against the nearest fence post.

In humans, the identical prion infection takes the form of Creutzsfeldt-Jakob disease (CJD), which already has brought death to half a hundred victims in Britain - and still counting. "Prion" is short for "proteinaceous infectious particle." (See BioWorld Today, May 21, 1999, p. 1; and March 19, 1999, p. 1.)

Normal prions harmlessly inhabit the mammalian bodies of man and beast. What makes them infectious is their flip side - a twist in their protein structure. When a contagion transmits the lethal prion protein, which is rich in pathogenic beta-sheets, to a healthy victim, it latches on to normal prions in the brain of its target, and changes its shape. That change involves breaking a single molecular bond and turning the normal protein into a killer conformation, by endowing it with beta-sheet-structure.

There is no cure for BSE or CJD. Such therapy would require breaking that beta-sheet shape in abnormal prions, leaving only normal proteins.

In this week's issue of The Lancet, dated Jan. 15, 2000, an international task force reports an initial advance toward just such treatment. Its paper is titled: "Reversion of prion protein conformational changes by synthetic b-sheet breaker peptides." Its senior author is protein structuralist Claudio Soto, who heads the research unit of neurodegenerative disorders at the Serono Pharmaceutical Research Institute in Geneva, Switzerland.

"We don't know exactly how the beta-sheet creates disease," Soto told BioWorld Today. "We do know that all the animals and human patients have this abnormal form of the protein, which has some properties that make it more dangerous. For example, resistance to proteolysis - enzyme cleavage - so it is very difficult to remove it from the body. Second, it is very prompt to aggregate, making a similar type of amyloid deposits to those also present in Alzheimer's disease (AD)."

Backing Into Drug Discovery

Here is how Soto and his 12 American, Swiss, Spanish and Italian co-authors discovered their candidate curative compound:

"We took advantage of what happens during the disease process," Soto recounted, "to try to reverse it. We used this process of transmission from one protein to another, to see if we cannot do the same, but the other way around. That is, create something that can interact with the abnormal form of the protein, and convert it into something that looks like the normal form.

"To do that," he went on, "we focused on the region of the prion protein that seems most important for the conformational changes, and synthesized a peptide that is partially homologous to that region. Then we took 13 amino acids from that prion protein, so that this inhibitory peptide, which we call iPrP13, breaks formation of the beta-sheet. At the same time, it cannot fit in this abnormal beta-sheet form. So we hoped that the peptide would bind to the protein and destabilize its abnormal form."

To test this concept in vivo, Soto and his co-authors turned laboratory mice into experimental models of scrapie. To do so, they inoculated the animals' brains with the infective prion form, along with stepwise doses of their breaker peptide. "The mice developed scrapie symptoms significantly later than those that received the abnormal prion alone," Soto recalled. "That delay," he added, "ranged from 12 to 35 to 63 days, depending on escalating dosage of infectious prions. We estimated that infectivity was diminished by 90 percent to 95 percent, after treatment with the breaker peptide, compared with the mice that were not treated."

"What we communicated to The Lancet," he observed, "is a proof of the concept, showing we can convert the abnormal form of the protein - by incubating it with the peptide - into something that looks a lot more like the normal form, biochemically and structurally.

"And also in a biological way," Soto added, "because when we treat this abnormal, infectious form of the protein with the peptide, we find that the protein is much less infectious. The inoculation still produces scrapie, but the time that it takes to get the disease to the animals is much longer. So what we are reporting here is that the abnormal form of the protein is reversible. Then, if iPrP13 really can do the same thing inside the human body, we will have something that can prevent and reverse what seems to be the critical step in the pathogenesis of these prion diseases."

New Peptide: From Prototype To Candidate

"So what we are working on now," Soto said, "is basically transforming this prototype peptide into a drug that can be used in human beings or in animals." In this effort, the team faces two major problems: "One is that peptides usually degrade quickly in the body," Soto noted. "As soon as you put them in the blood, for example, they're gone - degraded by normal enzymes.

"The second problem," he added, "is that to act against these diseases of the neural system, our iPrP13 peptide has to go to the brain to be able to work. And we know that there is a boundary separating the blood from the brain. Peptides have low permeability for this barrier, so they go to the brain very badly.

"As soon as we can resolve these problems," Soto foresees, "we are definitely thinking of conducting human trials, probably in Creutzfeldt-Jakob disease and Alzheimer's as well.

"We started this work with AD," he observed, "and then expanded it to prions. We are actually more advanced in the AD experiments, and we have a paper coming out this month showing we can both prevent and reverse amyloid plaque deposition in animal models of AD. And by doing this we can partially reverse some of the damage characteristic of the Alzheimer's brain."