The Food and Drug Administration delayed its approval decision on Amgen Inc.'s (AMGN) osteoporosis treatment denosumab, as it seeks more information from the drug maker.

It is unclear how long it will take for Amgen to meet the requests, but Roger Perlmutter, executive vice president of research and development, said in a statement that the company plans to respond to the requests in the "near term." The next step will be whether the FDA requires a two-month or six-month review prior to making its decision.

Amgen shares traded down 2.1% to $60.05. A spokeswoman declined to comment beyond a press release issued early Monday.

Many on Wall Street had expected such a delay for the drug, which is key to Amgen's future growth, because regulators have a history of missing deadlines and may want more time to analyze the safety record of a new drug like denosumab.

Eric Schmidt, an analyst with Cowen & Co., believes that denosumab's approval is likely in mid-2010. He continues to project 2010 denosumab sales of $200 million in osteoporosis but said the Wall Street consensus estimate of about $500 million will likely have to be reduced.

Despite the delay, Schmidt is recommending that clients buy shares of Amgen prior to its third-quarter earnings report on Wednesday, as he expects the company's core business will outshine any delay from denosumab.

Amgen, which had 2008 revenue of $15 billion, already sells five products with sales exceeding $1 billion.

Some analysts, including Lazard Capital Markets' Joel Sendek, project a shorter delay and see denosumab getting approval in early 2010. Sendek project 2010 sales of $439 million, rising to $1.2 billion in 2011.

Amgen has partnered with with GlaxoSmithKline PLC (GSK) to sell denosumab to osteoporosis patients in Europe and other overseas markets.

In the complete response letter, the agency requested several items including information on the company's submitted post-market surveillance plan, Amgen said.

The FDA isn't requiring any additional clinical trials prior to completing the review of the drug, but it did request a new clinical program to support approval for preventing postmenopausal osteoporosis. The FDA has also requested all updated safety data related to denosumab, which has the proposed brand name of Prolia.

Denosumab has the support of six Phase III trials involving more than 11,000 patients, with a relatively clean safety profile. Since the approval submission earlier this year, data from two large studies of denosumab's usefulness in preventing complications from cancer's spread to bone have become available.

Amgen hasn't disclosed its regulatory filing plans for preventing cancer-related skeletal complications, but a third study is coming early next year and many expect approval in late 2010.

The FDA is requiring a Risk Evaluation and Mitigation Strategy, including a medication guide, a communication plan, and a timetable for submission of assessments of the plans. Amgen has already submitted materials for such a program.

JPMorgan analyst Geoffrey Meacham, in a note to clients, said such requirements aren't onerous as compared to a patient registry, which could have restricted usage of the drug and hurt sales.

Denosumab is being reviewed for treating and preventing postmenopausal osteoporosis, and bone loss caused by hormone treatment in breast or prostate cancer.

Amgen expects a separate FDA response for that cancer-related application, something that Schmidt called "surprising" because denosumab was filed using one application with only one FDA advisory committee reviewing it.

"That is probably not a good sign," Schmidt said, who noted that the sales potential in bone loss caused by homone therapies is "fairly modest" in comparison to other potential markets.

-By Thomas Gryta; Dow Jones Newswires; 212-416-2169; thomas.gryta@dowjones.com