Oct 25 (Reuters) - Biogen Inc on Tuesday said the Alzheimer's disease drug it is developing with Japan's Eisai Co Ltd would be used by doctors in a competitive market as investors wait for data from rival therapies made by Eli Lilly and Co and Roche.

The drug, lecanemab was shown to slow the progress of the disease by 27% in a trial last month, and additional data is expected next month.

Lecanemab and experimental drugs for the mind-wasting disease from Lilly and Roche belong to a class of treatments that reduces amyloid beta proteins in the brain believed to be an underlying cause of Alzheimer's.

"The data needs to be seen from the other anti-amyloid therapies before we decide what is going to be meaningful," said Biogen interim research chief Priya Singhal.

Singhal added that safety data from lecanemab - showing rates of a type of brain swelling called ARIA also seen with other Alzheimer's drugs were at about 21% - was in-line with Biogen's expectations. That should help doctors make prescribing decisions along with efficacy data, she said.

Biogen raised its annual profit forecast for the second successive quarter based in part on much lower expenses. Its costs were about halved as the company significantly cut back on commercial infrastructure around the launch of Aduhelm, its previous Alzheimer's drug that has sputtered amid controversy over efficacy and lack of coverage by health insurers.

Biogen is now forecasting a full-year profit of $16.50 to $17.15 per share, up from its prior view of $15.25 to $16.75.

The drugmaker's spending on setting up a sales network and other commercial activities for lecanemab once approved would ramp up over time and be tied to its sales, Chief Financial Officer Michael McDonnell said.

Eisai, which is the leading the partnership, is expected to file for a U.S. approval soon with a decision expected in January.

Investors have been pinning hopes on the Alzheimer's drug as cheaper generics of Biogen's multiple sclerosis treatment Tecfidera have entered the market, hurting sales. Growth driver Spinraza for the rare genetic disorder spinal muscular atrophy is also facing competition from Novartis' newer Zolgensma.

Biogen shares were off more than 1% despite posting a higher-than-expected quarterly profit and raising its 2023 forecast.

The earnings highlighted that Biogen's base business is in decline, said BMO analyst Evan Seigerman. That, along with lack of answers around lecanemab's commercialization, could be hurting Biogen's shares, Seigerman said.

Excluding items, Biogen earned $4.77 per share for the third quarter, topping Wall Street estimates by 63 cents.

Biogen did not provide any update on its search since May for a new chief executive. Michel Vounatsos will remain at the helm until a replacement is found. (Reporting by Manas Mishra and Bhanvi Satija in Bengaluru; Editing by Bill Berkrot)