Biogen shares plunge as Alzheimer's drug trial disappoints

Biogen shares plunged Thursday after the biotechnology company said it is halting late-stage trials of an experimental Alzheimer's drug because results show the treatment is unlikely to help those with the brain disease.  

Investors fled the company's stock after the Cambridge, Massachusetts-based company and Japan's Eisai announced they were pulling the plug on further research into the drug, aducanumab, they'd been jointly developing. 

Biogen's stock price was down more than 29 percent in afternoon trading, lopping some $18 billion from the drugmaker's market value. 

It's the latest blow to Biogen's work to find a treatment for Alzheimer's, a common form of dementia. "This disappointing news confirms the complexity of treating Alzheimer's disease and the need to further advance knowledge in neuroscience," Michel Vounatsos, Biogen's CEO, said in a statement. 

Beyond a few medications that can curb memory decline for a couple of months, Alzheimer's has no effective treatment. It afflicts about 5.5 million Americans, with cases expected to triple by 2050.

Pfizer last year ended research on therapies for Alzheimer's and Parkinson's diseases, the latter of which limits a person's motor functions. In 2012, Pfizer and partner Johnson & Johnson ended further work on a drug that failed to help Alzheimer's patients in clinical trials.