WASHINGTON - Wyeth Pharmaceuticals is expected to receive initial approval this week for an antidepressant, and analysts say the drug's greatest market potential could be among menopausal women seeking an alternative to hormone-replacement therapy.

A spokesperson for Wyeth said the company expected the Food and Drug Administration to tentatively approve Pristiq as a treatment for major depression, although the company would likely need to submit additional information to get the final OK.

Pristiq's active ingredient is essentially a derivative of the ingredient in the company's blockbuster drug Effexor, which will lose patent protection in 2010. Drug companies routinely patent slightly adjusted versions of old ingredients as a way to extend the shelf life of their drugs.

Effexor was the No. 1 depression medication worldwide in 2005, with sales of $3.46 billion, nearly 20 percent of Wyeth's total revenue.

Sales of Pfizer Inc.'s Zoloft, the top-selling antidepressant in the United States in 2005, plunged more than 40 percent in the third quarter of 2006 after the company lost its patent. Other leading antidepressants include Eli Lilly & Co.'s Cymbalta and GlaxoSmithKline P.L.C.'s Paxil.

Pristiq is unlikely to expand Wyeth's depression franchise, but it "will be used to capture Effexor patients before the latter's patent expires," Sanford C. Bernstein & Co. analyst Richard Evans said in a research note.

But even if Wyeth can successfully transfer patients from Effexor to Pristiq, the drug will enter an antidepressant market that seems to have passed its peak. Spending on antidepressants fell 4 percent last year amid concerns that the drugs increase risk of suicidal behavior in children.

With the market for antidepressants saturated, Wyeth is looking to steer Pristiq toward an alternative use that shows more financial promise: treating menopausal symptoms.

The FDA is scheduled to issue a decision on Pristiq for menopause in April. While regulatory approval is never a certainty, Leerink Swann analyst Seamus Fernandez wrote last month that the agency would likely see Pristiq for menopause as "a compelling indication given that there is a significant unmet need."

Roughly 40 million U.S. women experience hot flashes and night sweats as a result of lower estrogen levels that come with menopause.

Harvard University professor JoAnn Manson said studies have shown that antidepressants can be somewhat effective for treating menopausal symptoms, and some doctors have already begun prescribing them off-label. A drug is used off-label when it is prescribed for a use other than the one authorized by government regulators.

"I would say there is a strong need for non-hormonal options because we know one in five women going through menopause have severe hot flashes that impair their quality of life," Manson said. "They are looking for a therapy that doesn't pose the risks of hormone replacement."

Shares of Wyeth fell 38 cents Friday to close at $51.50 on the New York Stock Exchange.