CHICAGO – For the first time, an experimental drug shows promise for halting the progression of Alzheimer’s disease by taking a new approach: breaking up the protein tangles that clog victims’ brains.

The encouraging results from the drug called Rember, reported Tuesday at a medical conference in Chicago, electrified a field battered by recent setbacks. The drug was developed by Singapore-based TauRx Therapeutics.

Even if bigger, more rigorous studies show it works, Rember is still several years away from being available, and experts warned against overexuberance. But they were excited.

“These are the first very positive results I’ve seen” for stopping mental decline, said Marcelle Morrison-Bogorad, director of Alzheimer’s research at the National Institute on Aging. “It’s just fantastic.”

The federal agency funded early research into the tangles, which are made of a protein called tau and develop inside nerve cells.

For decades, scientists have focused on a different protein – beta-amyloid, which forms sticky clumps outside of the cells – but have yet to get a workable treatment.

The drug is in the second of three stages of development, and scientists are paying special attention to potential treatments because of the enormity of the illness, which afflicts more than 26 million people worldwide and is mushrooming as the population ages.

The four Alzheimer’s drugs currently available just ease symptoms of the mind-robbing disease.

TauRx’s chief is Claude Wischik, a biologist at the University of Aberdeen in Scotland who long has done key research on tau tangles and studies suggesting that Rember can dissolve them.

He is an “esteemed biologist,” and the research “comes with his credibility attached to it,” said Dr. Sam Gandy of Mount Sinai School of Medicine in New York. He heads the scientific advisory panel of the Alzheimer’s Association.

In the study, 321 patients were given one of three doses of Rember or dummy capsules three times a day. The capsules containing the highest dose had a flaw in formulation that kept them from working, and the lowest dose was too weak to keep the disease from worsening, Wischik said.

However, the middle dose helped, as measured by a widely used score of mental performance.

“The people on placebo lost an average of 7 percent of their brain function over six months whereas those on treatment didn’t decline at all,” he said.

After about a year, the placebo group had continued to decline but those on the mid-level dose of Rember had not. At 19 months, the treated group still had not declined as Alzheimer’s patients have been known to do.

Two types of brain scans were available on about a third of participants, and they show the drug was active in brain areas most affected by tau tangles, Wischik said.

“This is suggestive data,” not proof, Wischik warned. The company is raising money now for another test of the drug to start next year.

The main chemical in Rember is available now in a different formulation in a prescription drug sometimes used since the 1930s for chronic bladder infections – methylene blue. However, it predates the federal Food and Drug Administration and was never fully studied for safety and effectiveness, and not in the form used in the Alzheimer’s study, Wischik and other doctors cautioned.

On Monday at the International Conference on Alzheimer’s Disease, other researchers reported encouraging results from a test of a different experimental drug that also targets tau tangles. That drug, by British Columbia-based Allon Therapeutics Inc., was tested in people with an Alzheimer’s precursor, mild cognitive impairment.

The tau-drug results are in stark contrast to the flop of Flurizan, which was aimed at blocking enzymes that form the beta-amyloid clumps. Myriad Genetics announced in June that it would abandon development of Flurizan after the failure. Full results were presented at the conference Tuesday.

Also, fuller results were given from a closely watched test of bapineuzumab, an experimental drug that aims to enlist the immune system to clear out the sticky brain clumps.

Its developers – New Jersey-based Wyeth and the Irish company Elan Corp. PLC – previously announced that the 240-patient study missed its main goal of improving patients’ mental performance at 18 months.

But the company found a silver lining – the drug appeared to help the roughly 60 percent of people in the study who did not have a gene that scientists think makes Alzheimer’s disease more severe.

The results back up the company’s claims of potential effectiveness in some patients, but now there are concerns about possible side effects. Twelve cases of a type of brain swelling occurred in those on bapineuzumab and none in the placebo group. The swelling caused few if any symptoms, company scientists said, but outside experts said it may have contributed to other side effects.

Those were two or more times more common in patients on bapineuzumab than people given the dummy drug. For example, cases of anxiety occurred in 11 percent versus 4 percent on placebo; paranoia, 7 versus 1 percent. Other complaints were vomiting, high blood pressure, weight loss, and back pain.

Three deaths occurred among the 124 patients given bapineuzumab, but they were not related to the drug, said Dr. Sid Gilman of the University of Michigan, who headed the study’s data safety monitoring board. One death was due to pneumonia and two others to worsening Alzheimer’s disease.

Investors reacted to the news by driving down Wyeth’s shares $5.01, or 11.1 percent, in after-hours trading.

Wyeth and Elan have already said they will move on to late-stage testing of bapineuzumab in more than 4,000 patients.