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bone drug Xgeva delayed the spread of cancer to the bones of prostate cancer patients but did not prolong their lives, according to results from a closely watched clinical trial reported Monday night.

Roger Perlmutter, Amgen's research chief, said data from the study were a "landmark" because Xgeva is the first drug proven to delay or prevent cancer from spreading to bone. Prostate cancer entering the bone is a signal that the cancer is worsening and patients feel a lot of pain.

Amgen shares rose about 7% to $57.80 in Monday's after-hours session after the company announced the results from the study. Generally, investors were pessimistic about the Xgeva study's outcome, so the results came as a positive surprise.

Amgen shares were trading at $58.25 in Tuesday's pre-market session.

If regulators approve Xgeva for the prevention of bone metastases in prostate cancer, sales of the drug could expand by another $1 billion or more, analysts predict.

Xgeva is already approved to reduce bone fractures in cancer patients and is sold under the name Prolia as a treatment for osteoporosis. Before Monday's results, the drug, whose scientific name is denosumab, was expected to bring in about $2 billion to $2.5 billion in peak sales for Amgen -- revenue the company desperately needs as sales of its anemia drugs have leveled off.

While positive, the Xgeva prostate cancer study data are also likely to be controversial and provoke debate because the drug did not help men live longer -- the most important measure of clinical benefit in cancer treatment.

An Amgen spokesperson says the Xgeva study was not designed to measure overall survival, as patients discontinued treatment with Xgeva when the cancer spread to their bones.

The U.S. Food and Drug Administration and many prostate cancer doctors may want to know if Xgeva prolongs survival before endorsing the drug's use, said Deutsche Bank biotech analyst Robyn Karnauskas, in a note to clients Monday night.

While avoiding the pain that can come from delaying the spread of cancer to the bone might be seen as a clinical benefit, treatment with Xgeva also exposes patients to the risk of other painful side effects, including death of bone in the jaw.

Xgeva's ability to stop the spread of cancer to the bone of prostate cancer patients was also somewhat modest. The study enrolled 1,432 men with prostate cancer that had not yet spread to their bones but who had rising levels of PSA, a marker of disease progression. The men were randomized to receive monthly injections of Xgeva or a placebo.

At the end of the study, men treated with Xgeva had a 15% reduction in the risk of their cancer spreading to bones or death compared to placebo patients. At the median, Xgeva delayed bone metastases by 4.2 months.

ISI Group biotech analyst Mark Schoenebaum called the results "not particularly overwhelming" but said Amgen shares could still trade into the mid- to high $60 range because Wall Street expectations were so low.

--Written by Adam Feuerstein in Boston.

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