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Daniel J. DeNoon
WebMD Health News
Reviewed By Louise Chang, MD
July 30, 2008 — Dimebon, a 25-year-old Russian antihistamine, seems to stabilize Alzheimer's disease in an 18-month study, surprising experts at this week's International Conference on Alzheimer's disease.
It's not the only experimental Alzheimer's treatment to show promise. That's especially good news, coming on the heels of a conference report that drove the last nail into the coffin of Flurizan, a once-promising Alzheimer's treatment.
Dimebon is now only one clinical trial away from approval. That trial is now recruiting patients. Success — not guaranteed, as the Flurizan flop shows — means Dimebon could be in pharmacies in two or three years.
In Russia, where the drug was tested against a placebo in 183 patients with mild or moderate Alzheimer's disease, Dimebon stabilized disease for at least 18 months. Overall, patients treated with Dimebon tended to have better or not-much-worse mental function, while those given placebo pills got significantly worse.
That difference is the biggest ever seen in Alzheimer's studies, says David T. Hung, MD, president and CEO of Medivation, the U.S. firm developing Dimebon.
"The effect size for Dimebon is the largest ever seen over placebo in a trial of an Alzheimer's treatment," Hung tells WebMD. "And over one year, when placebo patients had declined, our patients were kept at or even above baseline on every parameter tested, including memory, cognitive function, and behavior."
Less impressed with the findings is Gary J. Kennedy, MD, director of geriatric psychiatry at New York's Montefiore Medical Center. Kennedy warns that placebo-treated Alzheimer's patients in Russia get far different care than U.S. patients, who must be allowed access to existing treatments.
That would make any effect of Dimebon seem greater compared with placebo. And Kennedy says that patients' actual improvement on Dimebon is not very different from improvement seen with the existing Alzheimer's drugs Aricept, Razadyne, and Exelon.
"They say people improve over time on Dimebon, and that it has a bigger impact on Alzheimer's disease than we've seen before — but the data don't bear out much enthusiasm for that," Kennedy tells WebMD. "However, what is exciting is that this drug has a different mechanism of action than other dementia drugs. So this is a medication that could be combined with existing drugs to possibly slow down the course of the disease."
Samuel Gandy, MD, PhD, chairman of the Alzheimer's Association's Medical and Scientific Advisory Council, found the Dimebon data "very encouraging," but warns patients not to get their hopes up too high — yet.
Although the Russian study included well-respected Alzheimer's researchers from the U.S., the study must be replicated by independent investigators in American and Western European settings. And it's not at all clear whether Dimebon alters the course of Alzheimer's disease or just makes symptoms better for a while.
"Dimebon does appear to preserve function at an improved state for quite a long time," Gandy tells WebMD. "But it is not clear this is anything more than a symptomatic drug — and if it doesn't modify the disease, it will eventually wear off."
How does Dimebon work? Nobody knows for sure. There's evidence the drug affects mitochondria — the power plants that energize cells — fixing a defect that kills brain cells. That may be why the drug also seems to help patients with Huntington's disease.
"Improvements in mitochondrial health affect all cells," Hung says. "In our trial, we saw fewer adverse events in Dimebon patients than in control patients. So we are not only seeing beneficial effects, but the drug is better tolerated than placebo. That is a very attractive profile for patients."
Anti-Tangle Drug Rember
Plaque isn't the only thing that clogs the brains of people with Alzheimer's disease. There's also a buildup of fibrous tangles, made up of a protein called tau. These tangles appear in the brain before symptoms appear — and the more tangles there are, the worse the disease gets.
TauRX Therapeutics of Singapore is developing a drug called Rember that dissolves tau tangles and prevents tau from forming new tangles.
In a 321-patient trial in the U.K. and in Singapore, patients with mild-to-moderate Alzheimer's disease were treated with Rember or placebo.
After 19 months of treatment, Rember-treated patients had no decline in cognitive function. Brain scans suggested the drug reduced tangle density in parts of the brain critical for memory, according to a conference report by Claude Wischik, chairman of TauRx Therapeutics and professor of psychiatric gerontology at the University of Aberdeen, Scotland.
"This is an unprecedented result in the treatment of Alzheimer's disease," Wischik says in a news release. "We have demonstrated for the first time that it may be possible to arrest the progression of this disease by targeting the tangles which are highly correlated with the disease."
Gandy says the development of an anti-tangle drug is "great," especially since there are dementias other than Alzheimer's in which tangles, not amyloid plaque, are the problem.
"Tau is a legitimate drug target," Gandy says. "I think it may very well be that we will one day have a cocktail of anti-amyloid and anti-tau drugs to offer patients."
A phase III clinical trial of Rember will start soon, as will a planned trial in patients with Parkinson's disease. Should the Alzheimer's trial succeed — always a big "if" — the manufacturer says the drug could be available by 2012.
Anti-Plaque Drug PBT2
The Australian firm Prana Biotechnology is developing the plaque-busting drug PBT2, which attacks the particularly sticky protein called beta-amyloid 42.
In a 12-week trial, the drug significantly reduced beta-amyloid-42 levels in the brains of 78 patients with mild Alzheimer's disease, although it had no effect on mental function, according to a conference report by Jeffrey L. Cummings, MD, of the David Geffen UCLA School of Medicine.
"These results indicate PBT2 is having an impact on the underlying biology of Alzheimer's, which is very exciting," Cummings says in a news release. "This is a critical proof of concept, and the safety and efficacy demonstrated by PBT2 in this study warrant evaluation in a larger-scale clinical trial."
Gandy says that although PBT2 looks promising in this phase II clinical trial, the Flurizan failure shows that success can't be guaranteed until phase III trials are completed.
But Gandy hopes that an anti-amyloid drug such as PBT2 will eventually be available, possibly to combine with an anti-tau drug such as Rember and with a drug like Dimebon that has a totally different effect.
SOURCES: WebMD Health News: "Alzheimer's Drug Nothing to Sneeze At." News release, Alzheimer's Association. News release, TauRx Therapeutics. News release, Medivation. 2008 International Conference on Alzheimer's Disease, Chicago, July 26-31, 2008. Doody, R.S. The Lancet, July 19, 2008; vol 372: pp 207-215.
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