The surprise positive effect shown by an experimental Alzheimer’s drug “excited” executives at Eli Lilly and Co., but it raised as many questions as it answered.

The drug, solanezumab, appeared to slow the mental decline of patients with mild Alzheimer’s disease, according to the pooled results of two clinical trials, which included more than 2,050 patients from 16 countries, Indianapolis-based Lilly announced Friday morning.

But solanezumab did not meet its primary goal in either trial alone of slowing the decline in thinking and daily functioning in both mild and moderate Alzheimer’s patients.

Those mixed results surprised Wall Street, where stock analysts gave solanezumab less than a 20 percent chance of showing any benefit. Investors bid up Lilly’s shares as much as 6 percent after markets opened Friday. Shares were trading at $43.55, up 2.7 percent, near midday.

“We are encouraged by the data we have seen in patients with mild Alzheimer’s disease,” Dave Ricks, president of Lilly’s bio-medicines’ business unit, said during an online press conference Friday.

But he quickly moved on to talk about Lilly’s other pipeline drugs, which include a different Alzheimer’s drug. “We have a deep pipeline in Alzheimer’s disease, as well as 11 other drugs in Phase 3 developments, that are of interest to us.”

Lilly officials declined to quantify how much of a difference solanezumab made for mild Alzheimer’s patients. Nor would it say what it’s next move will be, other than to discuss the data with U.S. and foreign regulators.

Lilly could ask the U.S. Food and Drug Administration to put the drug on the market based solely on these two trials and some data showing that solanezumab removes the protein believed to cause Alzheimer’s from patients brains.

Or FDA regulators might demand a new clinical trial focused on patients with only mild Alzheimer’s disease or even with pre-Alzheimer’s symptoms. Or Lilly could scrap solanezumab altogether and focus instead on other drugs in its pipeline.

Dr. Tim Anderson, a pharmaceutical analyst at Sanford C. Bernstein & Co., wrote in a research note Friday that the FDA might accept only partially successful results because Alzheimer’s patients have no good options now—and because solanezumab had only modest side effects.

But, he added, "Our best guess is that more studies would have to be done before FDA or other regulatory agencies would consider approving solanezumab."

If Lilly is able to launch the first effective treatment for Alzheimer’s, it could reap as much as $9 billion in annual sales, according to analyst predictions. The memory-sapping disease requires enormous resources to care for the 5 million Americans and 18 million people worldwide who suffer from it.

Lilly could use the money. Sales of its bestseller Zyprexa, an antipsychotic medicine, have fallen off dramatically since it lost patent protection in the United States and Europe nearly a year ago. And Lilly will suffer similar losses at the end of 2013 when patents on its antidepressant Cymbalta expire.

Those two drugs combined had peak sales of $9 billion. And Lilly faces the patent expiration on two more drugs that could steal away even more of its sales.

Ricks reiterated Friday that the company will not let its annual revenue fall below $20 billion and will maintain profit of at least $3 billion per year. In 2011, Lilly brought in revenue of $24.3 billion with a profit of $4.35 billion.

Lilly’s drug solanezumab fights Alzheimer’s by trying to remove a protein called amyloid beta from patients’ brains. The build-up of amyloid into clumps and plaques is believed by many to stop the functioning of neurons, leading to memory loss and eventually death.

A similar-acting medicine developed by New York-based Pfizer Inc., New Jersey-based Johnson & Johnson and Ireland-based Elan Corp. showed no benefit in patients in Phase 3 clinical trial data released earlier this month. Half a dozen other companies also have experimental Alzheimer’s drugs in some stage of human testing.

The soonest to report results will be Germany-based Baxter International Inc. Its drug GammaGard, which is already on the market to treat immune system deficincies, has shown some evidence that it blocks amyloid proteins.

Expectations were low for Lilly’s solanezumab because so many previous drugs to treat Alzheimer’s have failed. Also, recent research indicates the disease actually begins 10 or 20 years before it can be diagnosed. So, many researchers believed Lilly was giving solanezumab to patients who were too far gone to be helped.

“There’s been a lot of discussion in the field, that even for patients with mild to moderate Alzheimer’s disease, that might be too late,” Dr. Eric Siemers, Lilly’s medical director for its Alzheimer’s team, said during the Friday press conference. “For us to see what appears to be a signal in this pooled data, in the mild patients, we’re actually quite pleased with. Because that would suggest you don’t have to move much earlier in the pathology of the disease.”

Siemers said Lilly is having independent academic researchers connected with the Alzheimer's Disease Cooperative Study analyze Lilly’s raw data from its clinical trials of solanezumab. They will release their conclusions Oct. 8 in Boston at a conference of the American Neurological Association.