May 1 (Reuters) - Biogen BIIB.O beat first-quarter profit and revenue expectations on Thursday, as strong demand for its rare disease drugs helped offset declining sales of its multiple sclerosis drugs.

The drugmaker has been counting on newer launches such as genetic disorder drug Skyclarys as demand falls for its once top-selling multiple sclerosis medicines in the face of stiff competition in a crowded treatment market.

CEO Christopher Viehbacher, who took the helm in 2022, has focused on deals, cost-cutting measures and newer drugs to address investor pressure for growth.

The company has been doubling down on its Alzheimer's drug Leqembi, but it has failed to live up to lofty expectations due to concerns over cost, efficacy and side effects. It sells for $26,500 annually in the U.S.

The company said it does not expect any material impact from President Trump's sweeping tariffs implemented to date. Biogen said about 75% of its U.S. revenue last year came from products manufactured in the country, which includes Leqembi and spinal muscular atrophy treatment Spinraza.

Biogen expects 2025 profit per share of $14.50 to $15.50, compared with its previous forecast of $15.25 to $16.25.

It reported an adjusted profit of $3.02 per share for the quarter, compared with analysts' expectations of $2.52 per share, according to estimates compiled by LSEG.

U.S. sales of Leqembi, which the company sells with Japan's Eisai 4523.T, were $52 million for the first quarter ended March 31. The Wall Street consensus estimate was at $54 million, according to brokerage Jefferies.

Sales of multiple sclerosis drugs such as Tecfidera fell 11% to $953 million.

Sales of rare disease drugs, including Skyclarys and Spinraza, rose 33% to $563 million in the quarter.

Revenue rose 6% to $2.43 billion, and came above analysts' average estimate of $2.23 billion.

(Reporting by Christy Santhosh and Mariam Sunny in Bengaluru; Editing by Devika Syamnath)

((Christy.Santhosh@thomsonreuters.com;))