Alzheimer’s is one of the leading causes of death in the U.S. and the only disease among the top five for which there is no treatment. Prospects got even bleaker this week when an Eli Lilly & Co. experimental drug failed.

Still, researchers and investors are urging the industry not to give up. And pharmaceutical companies, even those that had pulled out of the search for a treatment in the past, are responding with a renewed commitment — for now.

“This is such a devastating disease, and it’s so important for scientists to continue to push through this,” said Rita Balice-Gordon, the recently arrived head of neuroscience at pharmaceutical company Sanofi. “I’m committed to beating the drum for doing well-reasoned and well-researched clinical experiments, which will help drive the field collectively forward.”

Alzheimer’s research has already consumed more than $3 billion in spending over 27 years at Lilly alone, and the failure of its solanezumab treatment sent shares tumbling Wednesday. Lilly’s drug, which is targeting the amyloid protein that builds up in patients’ brains, didn’t slow their inexorable mental decline, adding to growing evidence that finding a way to treat the leading cause of dementia in the world may be even more herculean than experts expected.

Balice-Gordon is under no illusion about the effort to find a treatment, predicting additional failures are coming for a disease that is already littered with setbacks. The Paris-based company is moving gradually, and perhaps seeking a partner, for a compound in early-stage development. Yet her commitment is particularly apt because Sanofi pulled back from Alzheimer’s research a few years ago.

Lilly, too, plans to stick with its Alzheimer’s research. The Indianapolis-based company has one of the broadest pipelines in the industry, with a half a dozen Alzheimer’s drugs in development, said incoming CEO David Ricks.

Biogen Inc., Merck & Co. and Roche Holding AG, which all have late-stage trials, offered similar perspectives, saying that they remained confident in their clinical programs and stressing that each therapy is designed to attack the condition in different ways.

“We don’t think that it — by itself, one therapeutic — negates the amyloid hypothesis,” said Samantha Budd Haeberlein, head of Alzheimer’s clinical development at Biogen, which has generated promising early results with its drug, aducanumab. “Disappointing, for sure, but for us it doesn’t shake our confidence going forward.”

The jury is still out for the amyloid theory, which argues that the accumulation of the sticky protein in the brains of patients with Alzheimer’s is the root cause of the disease.

While there was a suggestion in Lilly’s trial that patients getting solanezumab may have done better than those given a placebo, the difference wasn’t meaningful, and Lilly gave up.