(Adds details on drug data, share price)
By Julie Steenhuysen and Ankur Banerjee
Oct 25 (Reuters) - Biogen Inc and partner Eisai Co Ltd said on Thursday the highest dose of its experimental Alzheimer's drug showed effectiveness in a new analysis, but investors remained skeptical about the reliability of the data.
Biogen fell 2 percent to $294.71 in early trading.
Results of the highly anticipated mid-stage trial, presented in July, showed that patients in the early stages of Alzheimer's treated with the highest dose of the drug, BAN2401, experienced 30 percent less cognitive decline than those on a placebo.
However, the results were muddied by concerns that the finding may have been skewed by a decision of European regulators to remove patients with an Alzheimer's genetic mutation called APOE4 from the group that got the highest dose, potentially lowering the bar for success in the group.
"I think that the data is promising and intriguing but continues to beg more questions that we don't have all the answers for," Jefferies analyst Michael Yee told Reuters, adding that the drug needs a much larger study to confirm any benefit.
Leerink analyst Geoffrey Porges said the data was unconvincing and confusing, suggesting only limited value for the drug in the population with APOE4.
The small number of patients remaining on the drug after 18 months and a lack of clear dose responses diminish the reliability of Thursday's dataset, Porges added.
The findings, in which the company offered a further analysis of data earlier presented in July, may not be able to fully dampen concerns.
"It's still an early study. They have to do further testing," said Laurie Ryan, chief of the Dementias of Aging Branch at the National Institute on Aging, who is at the meeting in Barcelona, where the data was presented on Thursday.
Ryan said the presentation supported the initial argument of a positive signal in an early phase study, and offered data suggesting the imbalance in patients with APOE4 may not have had an impact.
"Clearly, that still needs to be tested. You can't say this is ready to rock and roll at all," Ryan said on a telephone interview.
Dr. Jeffrey Cummings of the Cleveland Clinic, a consultant and an Alzheimer's expert, said on Thursday the APOE4 genotype had "very little effect on the rate of decline", suggesting that it was not a factor in the results. (Reporting by Manas Mishra and Ankur Banerjee in Bengaluru and Julie Steenhuysen in Chicago; Editing by Sriraj Kalluvila and Shounak Dasgupta)