Amgen Inc. shares gained 5 percent Tuesday, despite a choppy overall market, after reporting widely anticipated clinical studies showing its experimental osteoporosis drug denosumab dramatically cut the risk of spinal and hip fractures in postmenopausal women when compared with a placebo.

The three-year trial of 7,800 women found that denosumab helped prevent more fractures of the vertebrae and hips compared with a placebo, Amgen said in a Securities and Exchange Commission filing in conjunction with a scientific meeting presentation. Data also showed denosumab was safe, with no statistically significant difference in adverse side effects, said Amgen.

Thousand Oaks-based Amgen on Monday released the first head-to-head comparision data for the drug, which it expects to submit for U.S. regulatory approval by the end of the year. The company said patients treated with denosumab achieved significantly greater gains in bone mineral density than those using the current market-leading Merck drug Fosamax.

Several analysts said the latest data suggest that denosumab could become a $1 billion blockbuster drug and help offset the sales declines Amgen has seen in its flagship anemia drug Aranesp, which has been hurt by safety issues.

While Eric Schmidt at Cowen & Co. agreed the drug could become a blockbuster, he cautioned that denosumab's overall effectiveness was comparable or only slightly better than cheaper drugs already on the market. Its greatest attraction for postmenopausal women may be convenience and potentially fewer side effects, he said. Denosumab would only need to be injected twice yearly at a physician's office. Fosamax, which is now available in generic versions, has to be taken orally once a week.

Amgen shares closed up $3.70 to $65.89 in Nasdaq trading.

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