(Reuters) – Biogen and partner Eisai Co Ltd are ending two late-stage trials for the experimental Alzheimer’s drug aducanumab, in a setback to efforts to find treatments for the disease and for Biogen’s stock, which lost more than $17 billion of its value on Thursday.

Without the potential revenue from an Alzheimer’s drug, the company has poor growth prospects as it faces patent issues over its flagship Tecfidera multiple sclerosis drug and possible competition to spinal muscular atrophy drug Spinraza, Wall Street analysts said.

“We view this as a transformative failure for Biogen’s pipeline,” RBC Capital Markets analyst Brian Abrahams wrote in a research note.

Abrahams said he was reducing the stock’s valuation to $240 per share and further declines were likely given that “investors owned BIIB to not miss out on what could have been one of the biggest blockbuster products in the pipeline of large biopharma.”

Shares of Biogen sank 27 percent to $233.98 in U.S. morning trading, its largest drop since Aug. 1, 2008 when it fell 28 percent. Japanese markets were closed for a national holiday.

Investors believe the successful development of a treatment for Alzheimer’s, which affects about 5.7 million Americans, will result in multi-billion dollar annual sales. Still-experimental treatments have had a dismal track record, with more than 100 failures.

The decision to discontinue the trials was made after an independent data monitoring committee reported the drug was unlikely to be successful, the companies said. The recommendation was not based on safety concerns, they added.

The company had two trials, Engage and Emerge, that were testing aducanumab in patients with mild cognitive impairment due to Alzheimer’s and mild Alzheimer’s disease dementia.

The trials ended just two weeks after Eisai’s chief executive had expressed confidence in the drug. “I believe we have already gone through the riskiest stage,” Haruo Naito had said at a news briefing earlier this month.

Both Eisai and Biogen, which have collaborated on the development and commercialization of the drug, said they would continue to work on Alzheimer’s treatments.

“This disappointing news confirms the complexity of treating Alzheimer’s disease and the need to further advance knowledge in neuroscience,” Biogen Chief Executive Officer Michel Vounatsos said. “We will continue advancing our pipeline of potential therapies in Alzheimer’s disease.”

Guggenheim analyst Yatin Suneja said that would be a mistake for Biogen, which should instead start looking at merger and acquisition opportunities in treatments that target the central nervous system.

“They need to stop wasting or stop investing money in Alzheimer’s now,” Suneja said.

Big drug companies, including Eli Lilly, AstraZeneca Plc, Roche AG, Pfizer Inc, Merck and Co Inc, and Johnson and Johnson, have all previously abandoned Alzheimer drug trials over lack of effectiveness or due to safety concerns.

The companies will also discontinue a mid-stage study and a long-term extension study of aducanumab, which was designed to target the brain-destroying protein beta amyloid. It is still assessing whether to conduct a separate aducanumab late-stage prevention trial.