NEW YORK - Biogen Idec said on Friday it had offered to acquire Facet Biotech Corp, a partner in developing multiple sclerosis and cancer drugs, in a deal valued at about $355 million.

Biogen Idec, which sells two leading multiple sclerosis drugs, in a letter to Facet's board offered $14.50 per share, about a 64 percent premium over the closing price of Facet shares on Sept. 3, Biogen Idec said in a release.

Biogen Idec has asked to meet with the Facet board and its advisors and stated that it is prepared to commit all necessary resources to complete a transaction expeditiously, Biogen Idec said. The Cambridge, Massachusetts-based company said its offer is not subject to any financing contingency.

Officials at Facet, which is based in Redwood City, California, could not immediately be reached for comment.

Biogen Idec said the offer was lower than an initial offer of $15 per share it had made on Aug. 17 to Facet Chief Executive Officer Faheem Hasnain, followed by a letter from Biogen Idec to Hasnain and Facet's board on Aug. 21 offering that price.

But Biogen Idec said it reduced its offer to $14.50 per share in a letter sent to Facet on Sept. 4, about a week after Facet announced a collaboration with Trubion Pharmaceuticals Inc .

Biogen said the Trubion deal reduced the value of Facet in its eyes and those of investors, saying that Facet shares had fallen 22 percent since the Trubion collaboration was announced.

Biogen Idec and Facet -- formed in 2008 as a spin-off from PDL BioPharma Inc -- are jointly developing a drug for multiple sclerosis called daclizumab and a treatment for solid tumors, volociximab.

Tysabri, made by Biogen, is considered the most effective drug on the market for multiple sclerosis, a progressive neurological disease which can lead to paralysis. But the drug carries the risk of a potentially deadly brain disease.

Hasnain, in a Reuters interview last week, said Tysabri was plagued by safety concerns, giving Facet's experimental daclizumab treatment potential to enjoy a very sizable market opportunity.

During the interview, Hasnain said Facet has enough cash to last it through the end of 2010.

Biogen shares were up $1.05, or 2.1 percent, to $50.96 in afternoon trading on the Nasdaq. Activity in Facet shares was halted ahead of the news, having last traded down slightly to $8.79, also on the Nasdaq. (Reporting by Ransdell Pierson; Editing by Phil Berlowitz)