Stock in two pharmaceutical companies are flying high after their experimental drug to treat Alzheimer's Disease showed just enough promise to raise hopes that a viable drug could be on the horizon.

In a study of 856 participants, the drug BAN2401 developed by Japan's Eisai Co. and US-based Biogen Inc. did not meet its main goal.  Technically, it's a failure.  But company officials said that 161 people who got the highest dose every two weeks for 18 months did significantly better than 245 people who were given a placebo.  The drug removed much of the sticky plaque associated with Alzheimer's in the former group's brains.

"We're cautiously optimistic," said Dr. Maria Carrillo of the Alzheimer's Association, which spoke of the trial results at its recent international conference in Chicago.  "A 30 percent slowing of decline is something I would want my family member to have," she continued, noting that the drug's ability to clear the brain plaques "looks pretty amazing".

It's only a glimmer of hope and caveats abound:  The test was very small, and was conducted by the companies rather than unbiased university researchers; it has not been pier reviewed. 

The companies used a novel new way to measure mental decline, creating a scale that combines parts of three other widely used tests.  But this is the first study to use that measure, and it's unclear how much of a difference a 30 percent slowing of decline makes: for instance, would such a slowing allow the patient to continue to bathe or feed himself.

"It's intriguing, but these are designs we're not used to seeing," said Dr. Julie Schneider of Rush University Medical Center in Chicago, a widely recognized expert on dementia. 

And there were side effects:  19 percent of patients on the high dose had to discontinue treatment, some of them because of brain swelling.  Eisai says the other 81 percent in the high dose group saw all signs of sticky plaques that are the hallmark of Alzheimer's disappear after 18 months.

About 50 million people around the world have dementia.  Alzheimer's is the most common type, and there is no cure.  Current medicines merely ease some symptoms.