Amgen's twice-a-year injectable drug to reduce fracture risk from postmenopausal osteoporosis got U.S. Food and Drug Administration approval Wednesday, the Thousand Oaks, Calif.-based company announced.

The news comes only days after the drug, dubbed denosumab, brand name Prolia, received its European okay from the European Commission.

A potential blockbuster with projected multi-billion dollar annual sales, Prolia will be the first biotech product to be prescribed by general practitioners and not specialists, Amgen said.

"The approval of Prolia provides another treatment option for postmenopausal women with osteoporosis who are susceptible to fractures," Dr. Julie Beitz, director of the FDA's Office of Drug Evaluation III, said in a statement.

One out of every two women over age 50 will break a bone because of the disease, according to the National Institute of Arthritis and Musculoskeletal and Skin Diseases.

The drug, a monoclonal antibody, works by targeting RANK ligand, a regulator of cells known as osteoclasts that break down bone, according to Amgen.

A three-year Phase 3 trial involving nearly 7,800 women found that injecting the drug every six months in women with postmenopausal osteoporosis resulted in a 68 percent reduction in spine fractures, a 40 percent reduction in hip fractures, and a 20 reduction in nonvertebral fractures, according to Amgen.

The FDA said common side effects include back pain, high cholesterol levels, and bladder infections. More serious reactions include low calcium levels in the blood. Because Prolia suppresses bone turnover, it carries a rare risk of leading to bone death in the jaw, or osteonecrosis, the FDA said.

"It's a very rare complication of treatment," Dr. Catherine Stehman-Breen, Amgen's vice president of development for Prolia, told DOTmed News by phone. No cases were seen in the pivotal fracture study, she said. The main rival for postmenopausal osteoporosis, oral bisphosphonates, carries a similar osteonecrosis risk, she said.

A tiny, and statistically insignificant, increase in cancer rates found in a 2006 trial might have been caused by unbalanced randomization, as there were more patients in the drug treatment group than the placebo group, Dr. Stehman-Breen said. The current drug label reads, "No causal relationship to drug exposure has been established [for malignancies]," Dr. Stehman-Breen said.

In fact, Amgen hopes the drug could help cancer sufferers. In October, Amgen filed with the FDA for approval to use the drug on patients with prostate and breast cancer ailing from bone loss because of hormonal ablation therapy. The company is working with the FDA for this indication, Dr. Stehman-Breen said. Last month, they also sought approval to use the drug for skeletal-related events, such as fracture and other complications of bone metastases.

Denosumab doesn't come cheap. Amgen says it will run around $1,650 for a year's worth of treatment, around $350-250 more than bisphosphonates, according to Leerink Swann, a health care investment bank.

Still, "AMGN remains our favorite large cap biotech given the potential of dmb in both PMO and cancer indications," Leerink analyst Joshua Schimmer wrote in a letter to investors Wednesday morning, referring the possible role of the drug in preventing bone loss in patients undergoing cancer treatments.

As part of its marketing effort, Amgen will reach out to 60,000 to 80,000 primary care physicians, Dr. Stehman-Breen said.

News of the approval helped boost stocks of Amgen Inc., the world's biggest biotech firm, lifting them 10.50 percent to close at $56.09 a share Wednesday.

Analysts predict the drug denosumab to have annual sales of $2.9 billion by 2013, according to a report by Bloomberg.

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