UPDATE 2-Biogen's Spinraza fuels third-quarter profit beat

(Adds background on Spinraza, Tecfidera, Alzheimer's drugs)

Oct 23 (Reuters) - Biogen Inc beat analysts' estimates for third-quarter profit on Tuesday, driven by double-digit sales growth of its muscle disease treatment Spinraza.

Spinraza sales topped expectations in the quarter and crossed $1 billion in annual revenue for the first time. Investors have been expecting the drug to cushion the impact of sluggish demand for the company's money spinner, the multiple sclerosis treatment Tecfidera.

Biogen has also been beefing up its product pipeline, including on treatments for Alzheimer's disease, a space that has been littered with clinical failures amid desperate need of new therapies.

The company will present data on its experimental Alzheimer's drug, aducanumab, as well as safety data from an early stage study of the treatment later this week.

"These results are generally consistent with previous interim analyzes, and there were no changes to the risk-benefit profile of aducanumab," Biogen said on Tuesday.

Sales of Spinraza, the first approved treatment for spinal muscular atrophy, a muscle wasting disorder, came in at $468 million for the third quarter, exceeding Refinitiv estimates of $448.1 million.

The growth was driven by increased demand among adults in the United States, Biogen said.

Competition for Spinraza is also looming, with analysts expecting a U.S. regulatory approval for a rival treatment from Novartis AG by spring next year.

The company's multiple sclerosis treatments had a mixed quarter, with Tecfidera missing and Tysabri topping estimates.

Tecfidera sales rose 1.9 percent to $1.09 billion, but came in below analysts' estimate of $1.11 billion. Tysabri brought in sales of $470 million, above estimates of $453.2 million.

"Our core MS business was relatively resilient during the quarter," Chief Executive Officer Michel Vounatsos said in a statement.

Tecfidera is facing stiff competition from Roche AG's Ocrevus and Sanofi SA's Aubagio.

Excluding items, the company earned $7.40 per share, above analysts' average estimate of $6.78, according to Refinitiv data. (https://bit.ly/2CZuePD)

The U.S. biotech company said net income rose to $1.44 billion, or $7.15 per share, in the third quarter ended Sept. 30, from $1.23 billion, or $5.79 per share, a year earlier.

Total revenue rose 11.7 percent to $3.44 billion, edging past analysts' estimates of $3.33 billion. (Reporting by Manas Mishra and Saumya Sibi Joseph in Bengaluru; Editing by Shounak Dasgupta and Sriraj Kalluvila)