Late Friday, the biotech released partial study results showing its experimental osteoporosis drug, denosumab, significantly reduced the risk of bone fracture in post-menopausal women in a large number of patients, triggering a rise of nearly 18 percent in its shares in after-hours trading.

That’s because denosumab, which targets a protein involved with bone-destroying cells, is seen as a key to Amgen’s future since the growth of its flagship anemia drugs - Epogen and Aranesp - have faltered over safety concerns that have brought Black Box warnings.

Even before the after-hours run-up to $63.49, Amgen shares have climbed about 28 percent over the last three months. In a research note this morning, Rodman & Renshaw analyst Michael King writes that “the worst is behind Amgen,” and that the upbeat news about denosumab more than compensates for disappointments over the anemia drugs, such as restricted Medicare coverage.

Whether this means Sharer no longer faces a crisis is unclear. Before the stock began climbing, he was named one of the worst ceo’s, and morale at Amgen has taken a hit among the setbacks of the past 18 months and subsequent layoffs. Shareholders, however, are obviously another story.

The 7,800 patient, three-year trial found that treatment with denosumab led to a statistically significant reduction in the incidence of new vertebral fractures, compared with treatment with a placebo. The study also showed the drug reduced the incidence of new non-spinal and hip fractures. Side effects included serious infections and malignancies, but were similar to the placebo group.

Some earlier trials of denosumab, which is also being developed as a treatment for bone loss in cancer patients, showed higher risk of infection with denosumab, leading to questions about whether the drug’s safety profile will be sufficient for regulatory approval. Full details of the trial will be presented at a medical meeting in September.

UPDATE: Steve Silver, a biotech analyst at Standard & Poor’s equity research, writes in an investor note that “adverse safety events were comparable to placebo, which we believe removes a key share-price overhang, positions Amgen for (denosumab’s) approval, and provides a platform for re-igniting earnings growth after 2010, following recent safety and competitive concerns with existing products.”

Hat tip to Reuters