UPDATE 2-Biogen's multiple sclerosis drug powers quarterly profit beat

(Adds background on Alzheimer's drug, 2020 revenue guidance, shares)

Jan 30 (Reuters) - Biogen Inc's top-selling multiple sclerosis drug helped the company's quarterly profit beat market expectations on Thursday, sending its shares up about 2% before the bell.

Sales of Tecfidera rose 4.5% to $1.16 billion, beating estimates of $1.12 billion and despite the threat of patent challenges and increasing competition from newer treatments like Roche Holding AG's Ocrevus.

JP Morgan analyst Cory Kasimov called the quarterly performance "a good enough result", and said investor focus this year may not be on the company's commercial performance.

Investors expect Biogen to be the first drugmaker to bring a treatment for Alzheimer's to market as it revived plans to apply for approval of experimental treatment aducanumab in October based on a detailed analysis of data, months after ending two trials.

A successful treatment is likely to rake in billions of dollars in sales, given numerous failures to develop a drug for the memory-robbing disease.

Biogen said on Thursday it hoped to apply for a U.S. approval "as soon as possible", but did not specify a time.

The company said it expects 2020 full-year adjusted earnings per share of between $31.50 and $33.50. Analysts on average were expecting $33.03 per share.

Sales of Spinraza, the first approved treatment for spinal muscular atrophy, rose to $543.2 million, but missed the average analysts' estimate of $549 million, according to Refinitiv data.

Biogen forecast revenue of between about $14 billion to $14.3 billion this year, with the midpoint coming in above estimates of $14.06 billion.

Net income attributable to the company rose to $1.44 billion, or $8.08 per share, in the quarter ended Dec. 31, from $946.8 million, or $4.73 per share, a year earlier when it paid more in impairment charges and income tax.

Excluding items, Biogen earned $8.34 per share, above estimates of $8.02 per share.

Total revenue rose 4% to $3.67 billion, beating estimates of $3.54 billion.

Shares of the company were trading higher at $288 before the bell. (Reporting by Trisha Roy and Tamara Mathias in Bengaluru; Editing by Arun Koyyur)