TauRx Pharmaceuticals' experimental Alzheimer's drug LMTX failed to improve cognitive and functional skills in patients with mild to moderate Alzheimer's disease, a large, late-stage study showed.

But the drug did show a benefit in a small group of patients who were not taking other standard Alzheimer's drugs, according to the findings released on Wednesday at the Alzheimer's Association International Conference in Toronto.

Claude Wischik, co-founder of Singapore-based TauRx and a professor of geriatric psychiatry at Aberdeen University, said the drug reduced the rate of decline in cognitive and functional skills by 85 percent in that so-called "monotherapy" group.

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While the overall study involved 891 elderly subjects, only 15 percent, or some 136 people, were not taking standard Alzheimer's drugs.

"It's a small proportion that showed a benefit. That is why they weren't able to achieve significance," said Dr. Laurie Ryan, chief of the Dementia of Aging Branch at the National Institute on Aging.

"It's intriguing, but it doesn't tell us a lot yet at this point," Ryan said in a telephone interview.

Mayo Clinic Alzheimer's expert Dr. Ronald Petersen said the study begs the question of why patients with mild or moderate Alzheimer's were not receiving standard therapies.

The findings are the first from a large-scale trial of a drug that targets the Alzheimer's-related protein tau, which forms toxic tangles of nerve fibers associated with the fatal disease. Current treatments help improve Alzheimer's symptoms, but no drug has yet been shown to slow disease progression.

After several years of failed drug trials targeting beta-amyloid, the Alzheimer's protein that forms sticky plaques in the brain, several companies, including Eli Lilly, Biogen and AbbVie, have begun targeting tau, a protein linked with cell death.

Petersen said the TauRx findings do not mean that pursuing tau therapies is ill-advised.

"The notion that tau is still a very viable target for treatment in Alzheimer's disease remains live and well."

TauRx is currently studying LMTX in an 800-patient trial of patients with mild Alzheimer's. Results are expected to be presented later this year.

More than 5 million Americans have Alzheimer's, according to the Alzheimer's Association.