UPDATE 3-Biogen's 2021 view disappoints on multiple sclerosis drug sales

(Adds details from conference call, analyst comment, updates share movement)

Feb 3 (Reuters) - Biogen Inc on Wednesday forecast full-year adjusted profit below Wall Street estimates as it expects a sharp drop in sales of its multiple sclerosis treatment to counter any benefit from a potential approval of its Alzheimer’s drug.

Shares dropped over 2% in morning trading after the company also missed fourth-quarter earnings estimates.

Biogen said it expected to launch the Alzheimer’s disease drug, aducanumab, immediately if approved by the U.S Food and Drug Administration by June 7 but estimated only modest revenue in the first few months.

The “painful” guidance was contrasted by positive expectations on aducanumab, said Baird analyst Brian Skorney.

Biogen forecast full-year adjusted profit of $17 to $18.5 per share, while analysts were expecting $24.67, according to Refinitiv IBES data.

Still, company executives on a post-earnings call warned of a hit to 2021 results if the drug was not approved as a large portion of manufacturing capacity had been allocated toward the therapy.

A panel of experts to the FDA voted against the drug in November and the agency is now expected to decide after extending the review period by three months, the latest in a rocky road to approval.

The agency is not bound to follow the advice of its experts, but typically does.

Biogen reiterated its data supported an approval, and pointed to a rival study by Eli Lilly that underpinned the theory that targeting amyloid beta - a protein believed to be a major cause of Alzheimer’s - could help patients.

Investor attention has been focused on aducanumab, which is expected to generate sales of over $4 billion by 2024 and cushion the blow from generic drug competition for Tecfidera.

Quarterly sales of the multiple sclerosis drug tumbled 47.8% to $607.9 million in the fourth quarter, while adjusted profit of $4.58 per share fell short of estimates by 29 cents. (Reporting by Dania Nadeem and Manas Mishra in Bengaluru; Editing by Sriraj Kalluvila)