Biogen Inc beat first-quarter profit estimates on Thursday as higher-than-expected sales for spinal muscular atrophy treatment Spinraza cushioned the blow from eroding sales of its multiple sclerosis drug Tecfidera due to cheaper rivals.

As Tecfidera, once a cash cow for Biogen, battles intense competition, investors are squarely focused on an upcoming regulatory decision on the company's experimental Alzheimer's disease drug aducanumab.

The drug, which could become a blockbuster quickly if approved, has had a bumpy regulatory road, with outside experts to the U.S. Food and Drug Administration voting against the treatment last year.

Biogen said it was ready to launch aducanumab, with a decision from the FDA on the drug's marketing application expected on June 7.

"We see investors looking past the quarterly results and continuing to focus on the upcoming aducanumab review in Alzheimer's disease," said Wedbush analyst Laura Chico.

While sales of Spinraza fell 8% to $521 million in the quarter due to more competition in the United States, it came in above Street expectations of $483 million, according to Refinitiv data.

Roche's ROG.S Evrysdi and Novartis's Zolgensma have been taking away sales from Biogen's Spinraza in the United States.

Sales of Tecfidera, on the other hand, more than halved to $479.3 million, but nudged past estimates of $463.37 million.

Biogen raised its 2021 adjusted profit forecast to $17.50 to $19.00 per share, from the previous expectation of $17 to $18.5 per share.

The company reiterated that the forecast for the year includes anticipated sales from its Alzheimer's drug and if granted approval, the company said there were more than 600 sites in the United States ready to treat patients.

Excluding items, Biogen earned $5.34 per share, above Refinitiv IBES estimates of $5.04.

(Reporting by Mrinalika Roy and Manas Mishra in Bengaluru; Editing by Anil D'Silva and Sriraj Kalluvila) ((mrinalika.roy@thomsonreuters.com; within U.S. +1 646 223 8780, outside U.S. +91 806749 8325;))