Amgen Inc  pledged on Thursday to closely monitor the long-term safety of its novel osteoporosis injection as U.S. advisers weighed the future of the company's most important experimental drug.

Denosumab is seen as a potential blockbuster and the key to growth at the world's largest biotechnology company after safety concerns hit its flagship anemia medicines.

Food and Drug Administration reviewers are asking a panel of outside experts to consider serious infections and a slight increase in some types of cancer seen in Amgen's studies of denosumab, whose proposed brand name is Prolia.

The overall safety profile compares favorably to other osteoporosis drugs, Dr. Paul Eisenberg, Amgen's senior vice president for global regulatory affairs and safety, told the panel.

It will be important that there's long-term follow-up, he said, adding that the company would monitor the health of some patients for up to 10 years.

Amgen will need to compete in an $8 billion osteoporosis market with several rival drugs, including low-cost generics, to treat and prevent the bone-thinning disease, which afflicts an estimated 10 million Americans.

Denosumab works differently from current medicines by targeting a protein that activates bone-destroying cells. It is given as a twice-a-year injection.

The company said denosumab's benefits, including a 68 percent reduction in spine fractures in an osteoporosis study, outweighed any risks. About 7 percent of placebo patients suffered a spine fracture within three years, compared with 2 percent of denosumab patients in the study.

Our data are clear with respect to benefits, Eisenberg said.

The panel is set to vote Thursday afternoon on whether to recommend denosumab's approval for treating and preventing osteoporosis, and for use with breast and prostate cancer therapies that can cause bone loss.

The FDA also will ask the outside experts what steps Amgen should take to minimize side effects if the drug is approved.

Industry analysts predicted before the meeting that the panel would support the drug despite some safety concerns and that the FDA would approve it. Analysts see annual sales for denosumab reaching $2 billion or more.

The FDA usually approves drugs that win the backing of advisory panels. A ruling by the agency is due by October 19, but it has missed several drug deadlines in the past year, leading some analysts to anticipate a delay.

Denosumab would compete with osteoporosis medicines such as Novartis AG's (NOVN.VX) Reclast, Actonel from Sanofi-Aventis (SASY.PA) and Procter & Gamble Co (PG.N), and Boniva from Roche Holding AG (ROG.VX) and GlaxoSmithKline PLC (GSK.L).

Global sales of osteoporosis drugs hit about $8.4 billion in 2008, according to data from IMS Health.

Glaxo made a deal with Amgen in July to sell denosumab in Europe and emerging markets for osteoporosis.