The FDA reviewers' concerns, raised in documents released on Monday, appeared to dim any serious hopes that an advisory panel of outside experts would recommend approval of an expanded use of Xgeva, viewed by analysts as a drug whose sales could reach $1 billion a year.

Amgen shares were down 0.7 percent on Monday afternoon.

Xgeva, an injectable drug known chemically as denosumab, is approved to prevent fractures caused by cancer that has spread to the bone, but not to prevent or delay the spread. It is seen as one of the most important growth drivers for Amgen, the world's largest biotechnology company.

"Based on the many concerns and questions raised by the FDA, we do not expect the advisory committee to endorse approval of denosumab for this indication," Geoffrey Porges, an analyst for Sanford Bernstein, said in a research note.

However, Porges said any downside of a negative vote would be limited. He said his long-term revenue estimates for Amgen would only decline by 2 percent in that case.

The advisory panel will meet on Wednesday to decide whether to recommend approval of the drug for the wider use. The FDA will make a final decision later.

Xgeva delayed the spread of cancer to the bone by a little longer than four months in a clinical trial of 1,432 men with prostate cancer who had stopped responding to hormone therapy.

Amgen said the drug would be targeted at about 50,000 men in the United States at that stage of the disease.

The FDA reviewers said it was unclear whether that length of time without cancer spreading was "an adequate measure of clinical benefit," especially as the drug did not help men live longer or delay the growth of prostate cancer.

SERIOUS SIDE EFFECT

The reviewers were also concerned that about one in 20 men treated with the drug developed osteonecrosis of the jaw, or death of jawbone tissue, and said it was uncertain whether the rate could be even higher if patients take the drug for a longer period.

That rate is far higher than what has been seen with widely used osteoporosis drugs. The FDA will carefully weigh whether such a serious side effect undermines the benefits of Xgeva, even though the agency has a lower bar for toxicity where advanced cancer treatments are concerned.

ISI Group analyst Mark Schoenebaum said the critical review of Xgeva was expected, since the FDA had previously questioned whether a delay in cancer spreading to the bone was a meaningful benefit to patients.

"Even if the panel votes in favor of approval, commercial demand could be modest," he added in a note, saying a favorable outcome would likely contribute about an additional $300 million a year to sales.

In the fourth quarter, sales of Xgeva rose to $134 million, up from $100 million in the prior quarter.

Amgen is hoping a broader use will significantly accelerate sales of the drug. Xgeva, along with related osteoporosis drug Prolia, are expected to offset declining sales of the anemia drugs that had been the company's backbone.

The company said it would tell the advisory committee that Xgeva provided a meaningful benefit to prostate cancer patients by preventing the spread of their disease to the bones, which is not uncommon.

"Bone metastases ... are irreversible and progressive, and contribute major morbidity to these patients," the company said in a statement. Amgen said that if Xgeva gains the wider approval, it would be the first drug to delay the spread of cancer to the bone for men with prostate cancer.

Amgen said it was also testing Xgeva in other cancers, including breast and lung cancer and the blood cancer multiple myeloma.

(Additional reporting by Bill Berkrot in New York; Editing by John Wallace, Lisa Von Ahn and Steve Orlofsky)

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