WASHINGTON--(Dow Jones) A Food and Drug Administration panel on Thursday unanimously backed a potential Amgen Inc. (AMGN) blockbuster drug for fragile bones, but suggested limits on its use that could slow adoption of the drug.

The expert panel said the drug denosumab should be used to treat osteoporosis but not to prevent the disease in women who are at risk of developing it. The FDA usually follows the advice of its expert panels but isn't required to.

Amgen, the biggest U.S. biotechnology company by sales, is placing a big bet on denosumab, hoping to grab a share of the roughly $7 billion market for osteoporosis drugs. Hopes for FDA approval have helped boost Amgen's stock price by about 30% in recent months.

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.

The panel said treatment with denosumab should be limited to women who have a history of fracture or are at high risk for fracture. Panel members also said women should try other treatments first until more long-term safety data is available for denosumab, in particular about whether the new drug raises risks for serious infections and cancer.

The FDA has a deadline to make a decision on denosumab by mid-October, but that timetable could be delayed, especially if the agency decides it needs a stringent safety monitoring plan for the drug after it goes on the market. The panel called for such a plan.

If approved, the drug would be sold under the brand name Prolia. It is injected twice yearly, which Amgen says represents an advantage over other drugs that have to be taken more often.

"Amgen looks forward to collaborating with the FDA to arrive at the best possible approach to make Prolia available to appropriate patients," Roger M. Perlmutter, Amgen's executive vice president of research and development said in a statement.

The FDA said denosumab was effective at increasing bone-mineral density and reducing the risk of spine fractures by nearly 70% and hip fractures by 40% over three years compared to women not taking the drug. However, the agency raised concerns about its impact on the immune system.

To treat or prevent osteoporosis, denosumab is designed to inhibit a protein known as RANK Ligand that is involved with bone destruction. 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. However, 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."

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

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

The panel was asked whether there are women with low bone mineral density who would benefit from taking denosumab as a preventive step. A yes vote would have expanded the potential market, but the panel voted no by a 13-2 margin.

The panel backed use of the drug for certain prostate-cancer patients but rejected it for women with breast cancer being treated with aromatase inhibitors.

Amgen said it was too early to discuss the pricing of denosumab, but like other sophisticated biotechnology drugs it is expected to cost thousands of dollars a year.

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. Generic versions of Merck & Co.'s (MRK) Fosamax cost less than $20 a month.

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