The report, from Decision Resources, estimates the market will grow from its current level of $6.3 billion but will plateau at $7.5 billion in 2015.

The reason for this is that Merck's Fosamax is the market leader, so when it goes off patent in 2008, cheaper generic versions of the drug will be readily embraced by formularies, patients and physicians, Ramya Kollipara, an analyst at Decision Resources, told United Press International.

Fosamax has already gone off patent in Europe, and there's already an increasing shift to generics in that region, Kollipara noted.

Sanofi-aventis/Proctor & Gamble's Actonel goes off patent in 2013, but this is unlikely to have much impact as patients and physicians will probably already have shifted to generic versions of Fosamax.

Amgen's denosumab should launch in 2009 and is expected to be accepted by both patients and physicians because data indicates it is at least as effective in increasing bone mineral density as Fosamax and it offers greater convenience because it requires only twice-a-year dosing.

But denosumab's uptake will still be somewhat limited because it is expected to carry a high price tag and generic Fosamax will remain first-line therapy.

Still, denosumab should achieve blockbuster status, yielding more than $1 billion in peak-year sales, Kollipara said.

Jim Reddoch, an analyst with Friedman, Billings and Ramsey, told UPI he expects denosumab peak sales to be significantly higher at more than $3 billion.

"Denosumab is becoming an important driver for Amgen, in terms of why investors are buying the stock," Reddoch said.

Because this is a new class of drug and offers more convenience, he expects it to capture "a big chunk" of patients who are new to osteoporosis preventative treatments.

Amgen also is expected to present data at the American College of Rheumatology meeting in November on the potential of denosumab to treat rheumatoid arthritis. If the drug looks beneficial for this condition, it could add another $1 billion to sales projections, Reddoch said.

Still, there are some unanswered questions that remain about denosumab, including its safety profile and whether there's a change in fractures out to two years.

"We just don't know the safety profile of the drug, but it stimulates a type of cell in the body that rebuilds bone," Reddoch explained. "Anytime you stimulate anything in the body, you really want to guard against the risk of cancer."

Kollipara said another emerging class in the osteoporosis market is parathyroid hormone analogues, which are expected to bring in sales of $1.4 billion by 2015.

Unfortunately, most are still in early stage development, and it's difficult to predict which ones in the pipeline will prove successful.

Eli Lilly's Forteo, which initially had sluggish sales due to its high price and concerns about its potential to induce osteosarcoma, will remain the primary player in this class.

"Sales of Forteo are picking up over the past year and physicians are becoming more accepting of the drug," Kollipara said. Forteo sales amounted to $380 million in 2005.

But another limiting problem for Forteo is that it requires daily injections. "So, many companies are trying to formulate more convenient formulations, including intranasal, oral, transdermal or inhaled formulations," Kollipara said.

Most of them are in early stages of development, but Nastech and Proctor & Gamble have an intranasal formulation that could reach the market as early as 2008 or 2009.

If these other formulations pan out, they "could steal significant market share from Forteo," Kollipara said.

Lilly has its own formulation in the works -- an inhaled formulation it is developing with Alkermes. But that is still in the preclinical stage and it's too soon to know how it will look in clinical trials, she said.

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