After months of delay, Amgen finally won FDA approval for Prolia, an injectable treatment for postmenopausal women with osteoporosis who are at high risk for fractures. The endorsement is a big boost for the biotech, which has been counting on Prolia to reverse a series of setbacks over the past few years, notably a decline in sales of some of its largest-selling products, Aranesp and Enbrel.

Some Wall Street estimates suggest peak annual sales could reach $1 billion or so, although Amgen will face some challenges along the way - convincing patients that a twice-a-year shot is better than taking a pill once a month, such as generic versions of Roche’s Boniva or Merck’s Fosamax. For instance, the twice-yearly injectable will cost about $1,650 compared with the aforementioned pills that can run $1,200 and $1,300.

“The drug will be competing against generic pills and osteoporosis is not a deblitatiing disease like rheuamtoid arthritis where your fingers curl up,” says industry analyst Jon LeCroy of Hapoalim Securities. “Osteo doesn’t have the same level of discomfort and urgency for the patient. It’s a tough market to play in. Typically, biolgoics do better when there’s a sense of urgency about the disease.”

Prolia was also approved in Europe last week although there was an additional indication for treating bone loss associated with hormone ablation in men with prostate cancer at an increased risk of fractures. Amgen, of course, hopes to market Prolia to treat various cancers. To boost its changes of making Prolia a success, Amgen last summer struck a 50:50 profit-sharing deal with GlaxoSmithKline that has them splitting commercialization rights - Glaxo has osteoporosis indications in Europe, Australia and Mexico, and in some emerging markets, while Amgen will commercialize the drug for oncology indications.