NEW YORK (CNN/Money) - GlaxoSmithKline will lose patent protection on a blockbuster migraine treatment in three years, but it may have figured out how to protect sales by spinning off the drug as a new combination product, analysts say.

GlaxoSmithKline (up $0.54 to $50.67, Research), a British drugmaker with $39 billion in 2004 sales, will lose its patent on Imitrex, a painkiller for migraines, in 2009. The drug has been a profitable part of the company's business: sales from Imitrex and a related drug, Imigran, totaled $1.2 billion in 2004.

GlaxoSmithKline filed a new drug application for Trexima with the Food and Drug Administration on Aug. 8. Trexima is a combination of two drugs: Imitrex, also known as sumatriptan succinate, and naproxen sodium, a generic drug found in the Bayer (down $0.62 to $35.57, Research) painkiller Aleve.

"The strategy is to meet an unmet medical need," said GlaxoSmithKline spokeswoman Mary Anne Rhyne. "We want a full pipeline of products to help migraine sufferers."

The FDA said on Oct. 11 that it had accepted the Trexima application for review.

"This drug has a very high likelihood of success," said Bernstein analyst Gbola Amusa, who estimates a 95 percent chance of FDA approval for Trexima.

Bernstein projects that GlaxoSmithKline sales for Trexima will total $1.4 billion in 2010, the first full year without an Imitrex patent. Amusa said that GlaxoSmithKline has a history of successfully spinning off drugs into profitable new products, having turned Paxil into Paxil CR and Wellbutrin into Wellbutrin XL.

Not every analyst is convinced. Peter Cartwright, analyst for Williams de Broe, described Trexima as "a little tweak to the existing product lineup" and said that GlaxoSmithKline would be able to hold on to some of its sales revenue from migraine treatments if Trexima is approved. However, he said the drug would probably not achieve blockbuster status.

"This combination would not, I don't think, be able to defend $1 billion in sales," said Cartwright. The analyst said the market for migraine painkillers is "mature," with no room to expand.

Irina Stratan, an analyst for WestLB Equity Markets, projects that the FDA will approve Trexima in June 2006, and that sales will reach $390 million in 2009.

Stratan, however, said that Trexima sales could eventually surpass Imitrex sales and reach blockbuster status.

Cartwright owns shares in GlaxoSmithKline stock and Williams de Broe may have conducted business with the company in the last 12 months. The other analysts do not own shares in the company's stock, and their firms have not conducted recent business with GlaxoSmithKline.

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