Updated with new information.

Eli Lilly (LLY) - Get Report said Wednesday that its experimental drug solanezumab failed to improve cognition of patients with mild Alzheimer's disease in a large phase III clinical trial.

Shares of Lilly are down 14% to $65.14 in early trading on the solanezumab study failure, which is also a big setback for the entire field of Alzheimer's drug research. Solanezumab is the latest in a long string of drugs that have proven too weak to stop or even slow the progression of the terrible, memory-wasting disease. 

Lilly will not pursue regulatory approvals for solanezumab -- known as "sola" -- because of the phase III study failure, the company said.

"The results of the solanezumab EXPEDITION3 trial were not what we had hoped for and we are disappointed for the millions of people waiting for a potential disease-modifying treatment for Alzheimer's disease," said John Lechleiter, president and chief executive officer of Lilly, in a statement. "We will evaluate the impact of these results on the development plans for solanezumab and our other Alzheimer's pipeline assets."

Here's how Lilly described the study results:

Patients treated with solanezumab did not experience a statistically significant slowing in cognitive decline compared to patients treated with placebo (p=.095), as measured by the ADAS-Cog14 (Alzheimer's Disease Assessment Scale-Cognitive subscale).

While the study results, including many secondary clinical endpoints, directionally favored solanezumab, the magnitudes of treatment differences were small. There were no new safety signals identified in the study. Lilly will not pursue regulatory submissions for solanezumab for the treatment of mild dementia due to Alzheimer's disease.

Sola is a monoclonal antibody that targets amyloid beta, a sticky protein that clumps together to form plaques in the brain. These amyloid plaques damage and eventually kill neurons which leads to the memory loss and dementia in Alzheimer's patients, according to the most prevalent scientific theory known as the "amyloid hypothesis." 

Sola is one of many amyloid-targeted antibodies in clinical development. There are important differences between them, but all these drugs are designed, essentially, to eliminate amyloid beta from the bloodstream, brain or both. Eliminating these plaques is supposed to slow or even reverse the progression of Alzheimer's.

As investors digest the sola failure, they're also continuing to debate the validity of the amyloid hypothesis. Biogen (BIIB) - Get Report is developing its own amyloid-targeted antibody, aducanumab, which the company claims is more potent and therefore does a better job of more directly sopping up amyloid plaques. A phase III study of aducanumab in mild Alzheimer's patients is underway. Analysts Wednesday defended the company, advising investors not to give up on aducanumab just because of Lilly's sola setback.

But Biogen shares were down 8% to $293.51 on increased fears that the amyloid hypothesis might not be the solution to Alzheimer's.

Alzheimer's is a progressive neurological disease that damages and destroy nerve cells in the brain, leading to memory loss, dementia and the inability to function. More than five million Americans (35 million people worldwide) are diagnosed with Alzheimer's -- the sixth-leading cause of death in the U.S.

There are no medicines approved today capable of stopping or even slowing the underlying progression of Alzheimer's. The only drugs available today treat the symptoms of the disease.

Adam Feuerstein writes regularly for TheStreet. In keeping with company editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.