The U.S. Food and Drug Administration staff reviewed the pill, tafamidis, ahead of an advisory panel of outside experts, which on Thursday will vote on whether to recommend it for approval. The FDA will make a final decision later, taking into account the panel's recommendation.

Tafamidis, which is already approved in Europe under the name Vyndaqel, is meant to treat familial amyloid polyneuropathy, a fatal condition that affects as many as 10,000 people worldwide, including about 2,500 Americans.

Most people with the genetic mutation that is thought to be the most common cause of the disease live in Portugal, Sweden or Japan. Once symptoms begin, a majority of people die in 11 years, the FDA said. The only current treatment for the disease is a liver transplant, which keeps 80 percent of people alive after five years but requires taking immunosuppressant drugs.

Two FDA reviewers said during a clinical trial that tafamidis was most successful in treating people in Portugal, but it was unclear why, and the data did not prove it helped those with the disease in other locations.

People taking tafamidis during clinical trials also appeared to have a milder disease than those taking a placebo, which could skew the results, they said.

The director of the FDA's division of neurology products, Dr. Russell Katz, emphasized the FDA had not yet made a final decision on tafamidis and wanted to get advice from the committee on Thursday.

Tafamidis is a relatively minor product for Pfizer, the world's largest drugmaker. The company's shares were up about 10 cents at $22.64 in morning trading on the New York Stock Exchange, in line with the wider market.

(Reporting by Anna Yukhananov; Editing by Gerald E. McCormick and Maureen Bavdek)

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