Japanese pharmaceutical firm Eisai's (OTC BB: ESALY.PK) ambition to expand the use of its dementia drug may have suffered a setback, after a trial testing the drug against vascular dementia revealed a higher incidence of death among patients.

Second to Alzheimer's, vascular dementia is the next most common form of dementia striking the elderly, primarily those between the ages of 60 and 75. In Japan, it accounts for 50% of all dementia cases in people 65 or older. It's usually caused by a series of "mini-strokes" which individually cause only temporary impairment, but cumulatively cause brain tissue damage that impairs basic cognitive functions.

There is no way to undo the brain damage caused by a stroke, but there are currently a handful of drugs used to slow the progression of the disease: Aricept, manufactured by Eisai and Pfizer (NYSE:PFE); Reminyl by Johnson & Johnson (NYSE:JNJ) and Shire Pharmaceuticals (NASDAQ:SHPGY); Exelon from Novartis (NYSE:NVS); Namenda by Forest Labs (NYSE:FRX); and Ebixa by Netherlands-based Lundberg.

Eisai's Aricept is the most widely prescribed drug for the treatment of mild-to-moderate forms of Alzheimer's disease, and the company is hoping to pierce the moderate-to-severe Alzheimer's market. That's currently the exclusive domain of Namenda, the only drug with FDA approval for those with more advanced symptoms. Eisai is also trying to expand the use of Aricept to other forms of dementia -- including vascular dementia -- and it reported the results of phase 3 clinical trials with seemingly mixed results.

While Aricept appears to have significantly improved the cognitive functions of patients who received it in comparison with those who received a placebo, there was no noticeable improvement in a patient's global functions; the drug would have an apparent effect on speech, movement, or memory, but not on multiple areas. More importantly, there was a higher incidence of death among those taking Aricept, while there was an absence of deaths for those on the placebo.

Surprised, Eisai reported the results to the FDA. The company had seen a similar incidence of death among Aricept patients in prior trials, but it had been roughly cancelled out by deaths in the placebo group. In these latest trials, the difference in death rates is statistically significant, forcing Eisai to consider its next steps. Aricept is already approved for vascular dementia in several countries, including India, Thailand, Romania, and South Korea, but it would seem that FDA approval for use here in the U.S. won't be coming any time soon.

Johnson & Johnson encountered similar problems when it tested Reminyl for the treatment of mild cognitive impairment in Alzheimer's patients; the company withdrew its plans to apply for that expanded use. Eisai, it seems, may also need to forget about expanding Aricept beyond its currently approved purposes.

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Fool contributor Rich Duprey owns shares of Eisai, but does not own any of the other stocks mentioned in this article. You can see his holdings here. The Motley Fool has a disclosure policy.