In a setback for the drugmaker, the FDA sent Wyeth an approvable letter for Pristiq, an agency spokeswoman wrote Bloomberg News in an e-mail late today. However, the reasons weren’t known, because the FDA doesn’t release details of approvable letters.

Pristiq, a version of Wyeth’s best-selling Effexor antidepressant, may be safer for some women than the drugmaker’s hormones, Premarin and Prempro, which were linked to breast cancer and heart attackS in US government-funded studies. The HRTs generated $1.1 billion last year, and Wyeth hopes Pristiq will hit $2 billion in annual sales, Bloomberg notes. Wyeth needs Pristiq to help offset losses when Effexor, which had $3.7 billion in sales last year, faces cheaper generics in 2010.

“I expect Pristiq to get approved,” Barbara Ryan, an analyst with Deutsche Bank Securities, tells Bloomberg. “After that, how many women use it for menopause symptoms may depend on how much it costs them. Some women may not want to pay more for a drug that doesn’t work as well as Premarin.”

Wyeth plans to charge up to $4 a day for Pristiq, about four times the daily price for Premarin or Prempro, says Roopesh Patel, an analyst with UBS Investment Research, in a note to clients. U.S. health insurers may in turn charge patients higher out-of-pocket fees than for the older menopause drugs, he wrote.

Wyeth didn’t immediately comment, but scheduled a conference call at 8 a.m. EST.