Pfizer Inc. and Medivation Inc. said Monday that their drug candidate Dimebon failed in a second late-stage clinical trial, as it did not improve symptoms of the neurologic disorder Huntington's Disease.

The companies said Dimebon was no more effective than a placebo at improving patients' cognition. The two companies are also studying Dimebon as a treatment for Alzheimer's disease; the drug also failed in a previous late-stage trial as an Alzheimer's disease treatment in 2010.

The most recent trial involved 403 patients who were given either 20 milligrams of Dimebon three times a day, or a placebo, for six months. Pfizer and Medivation said patients who took Dimebon did not do significantly better than the placebo group in a mental examination, and a survey of caregivers also did not reveal better results for the Dimebon patients.

The most common side effects of treatment with Dimebon were involuntary muscle movements, headache, and fatigue.

Huntington's is a rare inherited disease that usually strikes people in their late 30s or early 40s. It begins with uncontrollable twitches, and later causes deterioration of mental abilities until patients can barely eat, speak or walk. Death occurs a decade or more after symptoms begin. The condition is believed to affect about 30,000 people in the U.S.

Pfizer of New York and Medivation of San Francisco said they will continue studying the drug in mild-to-moderate Alzheimer's disease. They expect to report results from another late-stage trial in the first half of 2012.

Shares of Medivation lost 71 cents, or 3.5 percent, to $19.40 in premarket trading. Pfizer shares gained 3 cents to $20.49.