The Food and Drug Administration said a proposed new type of osteoporosis drug from Amgen Inc. (AMGN) was effective at increasing bone mineral density and reducing the risk of fractures but could increase the risk of serious infections.

The FDA said it was concerned denosumab "has the potential to affect multiple layers of the immune system," which could result in the development of serious infections and cancer. In order to treat or prevent osteoporosis, denosumab is designed to inhibit a protein known as RANK Ligand that's 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's a "potential for tumor progression in patients with cancer."

The drug, denosumab, faces a review Thursday by an outside panel of medical experts, and the FDA posted a review of the product on its Web site Tuesday.

Amgen is seeking FDA approval for denosumab to treat or prevent osteoporosis in postmenopausal women and bone loss caused by hormone treatment in patients with breast or prostate cancer. The company said denosumab has a "favorable benefit-risk profile in the indications sought."

The FDA said it would ask the panel "whether the risk/benefit balance for the osteoporosis prevention indication, both for patients with and without cancer, supports approval." The outcome of that and other expected votes on the product will amount to a recommendation about whether denosumab should be a approved.

The FDA usually follows its panel's advice but isn't required to. The agency is expected to make a final decision whether to approve denosumab by about Oct. 19. 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 like Boniva, Fosamax, Reclast and Zometa that are considered bisphosphonates in order to prevent bone loss. Denosumab would be injected twice a year.

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