Biogen Idec is halting the development of an experimental treatment for Lou Gehrig's disease after a late-stage study.

Company shares slid more than 6 percent before the market opened Thursday after it announced early results from an international study of 943 patients. Biogen said the experimental drug, dexpramipexole, showed no effectiveness in an analysis of patient function and survival.

Lou Gehrig's disease, or amyotrophic lateral sclerosis, affects nerve cells in the brain and spinal cord that control voluntary muscle movement. There is no known cure.

"As a physician who has treated people with ALS, I hoped with all my heart for a different outcome," said Dr. Douglas Kerr, the company's director of neurodegeneration clinical research.

"While these results were not what we expected, we hope these data will provide a foundation for future ALS research."

Biogen said it will present detailed results from the research at a future medical conference.

Shares of Biogen Idec Inc., based in Weston, Mass., fell $9.12 to $140.88 in premarket trading.