The U. S. Food and Drug Administration (FDA) in 2010 approved the drug denosumab (Prolia), which is the first biologically derived agent to treat bone loss caused by osteoporosis.

Regulators say the drug, which is injected under the skin every six months in a doctor’s office, is appropriate for postmenopausal women who are at high risk for fractures – a group that includes those who have broken a bone in the past because of osteoporosis, those who have multiple risk factors and those who can’t tolerate other kinds of osteoporosis medications.

Like some other medications for osteoporosis, denosumab (Prolia) works by preventing the breakdown of bone by cells called osteoclasts.

But unlike bisphosphonates, including alendronate (Fosamax), ibandronate (Boniva), risedronate (Actonel) and zolendronic acid (Reclast), which hasten cell death after they are ingested, denosumab (Prolia) is a monoclonal antibody that straitjackets a protein called RANKL that is important for the the production, function and survival of osteoclasts.

It is unclear whether denosumab (Prolia) works better than older medications for bone loss, however, because few head-to-head comparisons have been done.

A 2009 study of 7,868 women with osteoporosis who were followed for three years, denosumab (Prolia) reduced the risk of fractures at the spine and hip by 68 and 40 percent, respectively, compared to a placebo injection. The drug also reduced the risk of other kinds of fractures by 20 percent compared to a placebo. The study was published in the New England Journal of Medicine.

Experts say those results seem to be in line with the effects of zolendronic acid (Reclast) or even teriparatide (Forteo) a hormone taken by daily injection that spurs the formation of new bone.

And in two trials, both recently published in the Journal of Bone and Mineral Research, that pitted denosumab (Prolia) against alendronate (Fosamax) in postmenopausal women for a year, found that denosumab (Prolia) reduced markers of bone turnover and slowed bone loss better than alendronate (Fosamax), suggesting that it may work better than bisphosphonate pills.