Amgen shares jumped 6% in late trading on word that a study of its bone drug Xgeva showed that the drug, which is already FDA approved to help patients who already have fractures and complications related to prostate cancer, actually might also delay the spread to the bones by more than 4 months, if taken sooner.

However, the drug did not prolong the lives of men. Dr. Matthew R. Smith, the lead investigator in the trial and a consultant to Amgen, said the results were meaningful because no drug before had been proved to prevent the spread of cancer to the bone, a major cause of pain and disability in men with prostate cancer.

“This study is the first to demonstrate prevention of bone metastasis, the most devastating complication of prostate cancer,” Dr. Smith, an associate professor at Harvard Medical School and the Massachusetts General Hospital, said in an e-mail. “It addresses a critical unmet need.”

The trial involved 1,432 men with so-called castrate-resistant prostate cancer who were randomly assigned to receive injections every four weeks of denosumab or a placebo. The cancer had not spread to the bone at the start of the trial, but that was considered to be a high risk because the men’s P.S.A. levels, markers of disease severity, were rising.

The goal was to see if denosumab could delay the time until either the cancer spread to the bone or the patient died from any cause. The drug did delay the median time for this to occur by 4.2 months, a statistically significant difference. Using a different statistic, the risk of bone metastasis or death was reduced 15 percent.

Major side effects were low calcium in the blood and a destruction of jaw bones that are associated with many bone drugs.

The benefit came entirely from delaying bone metastasis, not death. Dr. Roger M. Perlmutter, executive vice president for research and development at Amgen, said the trial had not been expected to show a survival benefit since most of the patients lived through the course of the trial. Also, men who experienced bone metastasis were removed from the study so they could be treated with a drug that helps prevent fractures.

Denosumab blocks a protein involved in the destruction of bone. That presumably makes the bone a less hospitable environment for cancer cells to take root.

The drug, sold under the name Xgeva, was approved in November to help prevent fractures and other skeletal problems after prostate and other cancers had already spread to the bone. The drug is also sold to treat osteoporosis under the name Prolia.

Amgen, the world’s biggest biotechnology company, has suffered from declining sales of its anemia drug Aranesp because of safety concerns. Sales growth of some of its other drugs is slowing, so denosumab is considered a key to its future. Use in preventing bone metastasis could add several hundred million dollars a year to annual sales of denosumab.