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A U.S. Food and Drug Administration panel on Thursday unanimously backed the use of denosumab, a proposed Amgen Inc. (AMGN) drug, to treat osteoporosis in postmenopausal women, but said use of the product should be initially limited to women who've failed other treatments.

The panel also overwhelmingly suggested the product not be used to prevent osteoporosis in women with low bone mass who are at risk of developing osteoporosis.

The panel is currently voting on a series of questions including whether denosumab should be used in patients undergoing hormone therapy to treat breast or prostate cancer. Hormone therapy increases the risk of bone loss.

The panel voted 15-to-0 on a question that asked if there were a population of postmenopausal women with osteoporosis in which the benefit of treatment with denosumab is likely to outweigh the risks.

However, the panel said treatment should be limited to women with a history of fracture or who are at high risk for fracture. Panel members also said women should try other treatments before denosumab until there are more long-term safety data about the product and whether it affects the immune system and raises risks for serious infections and cancer.

The panel voted 13 to 2 against a question that asked whether there was a population of women with low bone mineral density in which the benefit of prevention of osteoporosis with denosumab is likely to outweigh the risks.

The outcome of the panel votes will amount to a recommendation about whether it thinks the FDA should approve denosumab.

About 10 million Americans have osteoporosis, or bone weakness that places people at high risk of suffering bone fractures while an additional 34 million have low bone mass that puts them at risk for developing osteoporosis.

Investors have high expectations for denosumab to grab a share of the roughly $7 billion market for osteoporosis drugs, which has helped boost Amgen's stock price by about 30% in recent months. Approval of denosumab, which would be injected twice yearly, is key for Amgen's growth.

While the FDA said denosumab was effective at increasing bone-mineral density and reducing the risk of fractures, it raised concerns about the product's impact on the immune system and whether that, in turn, raises risks for serious infections and cancer.

In order to treat or prevent osteoporosis, denosumab is designed to inhibit a protein known as RANK Ligand that is involved with bone destruction. However, the same protein also plays a role in the body's immune system.

The agency said clinical trials involving denosumab showed a slightly higher rate of serious infections and the development of certain types of cancer. The agency also said there is a "potential for tumor progression in patients with cancer." While clinical data showed numerical differences for infections and cancer, the data didn't reach statistical significance, meaning the differences could be a chance finding.

Amgen officials said there was no evidence seen in clinical trials that denosumab suppresses the immune system and increases risks for infections and cancer.

Catherine Stehman-Breen, Amgen's vice president of global development, told the panel that denosumab has a "favorable safety profile," with most adverse events being "mild-to-moderate" as well as balanced between patients who received denosumab and patients who received placebo.

Amgen officials said they already have long-term studies in progress that can be used to track safety issues. They also said they plan additional studies if denosumab were to be approved.

The FDA usually follows its panel's advice but isn't required to do so. The agency has a deadline to make by mid-October but that time table could be delayed if the agency decides to implement a post-marketing safety plan formally known as a Risk Evaluation and Mitigation Strategy, or REMS.

If approved, the drug would be sold under the brand name Prolia.

Denosumab is the first type of drug in its class and acts in a different manner than many other drugs, such as Boniva, Fosamax, and Reclast, that are considered bisphosphonates in order to prevent bone loss. Bisphosphonates, which are delivered orally or through an infusion, have their own side effects, including a risk of rare jaw decay problem.

Ethel Siris, director of the Toni Stabile Osteoporosis Center at Columbia University who presented on behalf of Amgen, said more than half of patients who are prescribed daily oral pills aren't taking them more than a year later partly because of gastrointestinal side effects.

"Adherence is a critical issue" in treating osteoporosis, Siris said, noting that doctors can determine if patients are compliant with medication if they have to come in to the office to receive their medication.

Reclast, made by Novartis AG (NVS), is an infusion that is given once a year and would be the most direct competitor to denosumab, if approved. That drug has a wholesale cost of $1,000; the generic version of Fosamax is less than $20 a month. Amgen said it was too early to discuss pricing details.

-By Jennifer Corbett Dooren, Dow Jones Newswires; 202-862-9294; jennifer.corbett@dowjones.com