UPDATE 1-Biogen moves long-acting Avonex to final trial-CEO

(Adds CEO comments, other details, byline)

By Deena Beasley

LOS ANGELES, Jan 7 (Reuters) - Biogen Idec Inc (BIIB.O) has moved a long-acting version of its multiple sclerosis drug, Avonex, directly into trials designed to meet requirements for regulatory approval, the company's chief executive said on Wednesday.

CEO Jim Mullen said during an investor conference held in New York that the company has completed Phase 1 trials and will study dosing the drug once every two weeks as well as once monthly.

"I think we've got a good chance of similar efficacy as interferons," he said.

Patients in the trials will be treated for one year with the injectable drug and full results will likely be available in two years, Mullen added.

Avonex, with sales of $573 million in the third quarter of 2008, is the leading drug for multiple sclerosis, an autoimmune disease in which the body mistakenly attacks the fatty myelin coating surrounding nerve cells.

Biogen, along with marketing partner Elan Corp Plc (ELN.I), also sells MS drug Tysabri, which was taken off the market after its 2004 introduction when it was linked to a potentially fatal brain infection, but reintroduced beginning in 2006 because there were so few good options for patients with MS.

Mullen said investors continue to underestimate the newer drug's sales potential: "We have a much more bullish view on Tysabri than the Street does."