(Reuters) - Eli Lilly and Co's experimental drug solanezumab, which seeks to treat Alzheimer's disease, may fail its pivotal studies, Jefferies & Co said, downgrading the stock to "underperform" from "hold."

Eli Lilly is expected to unveil by next month data from the highly anticipated studies on solanezumab, which has completed two late-stage trials.

"We expect that solanezumab will outright fail its pivotal studies in Alzheimer's disease ... with only a 25 percent chance of being effective in a niche population," Jefferies analyst Jeffrey Holford said.

Alzheimer's affects some 5 million Americans and as many as 35 million people worldwide.

On Wednesday, Eli Lilly raised its 2012 profit outlook, and said profit margins should improve after 2014, once the worst pain from patent expirations on its top drugs has abated.

Holford said it suspects Eli Lilly's post-2014 forecasts was a "pre-emptive strike ahead of the pivotal read-out for solanezumab in Alzheimer's to support consensus estimates in case it fails."

Holford -- a four-star rated analyst for the accuracy of his earnings estimates on companies in his coverage universe, according to Thomson Reuters' Starmine -- lowered his price target on Eli Lilly to $35 from $37.

According to StarMine, two analysts rate the stock "strong buy," 15 rate it a "hold," and three rate it a "sell," with a mean price target of $39.83.

Shares of the Indianapolis-based company, which touched more than a three and a half year-high earlier this month at $44.67, closed at $44.37 on Friday on the New York Stock Exchange.

(Reporting by Fareha Khan in Bangalore; Editing by Don Sebastian)