Amgen shares are likely to soar once the markets open in the USA later today after the company presented late-stage data showing the superiority of its experimental osteoporosis drug denosumab over Novartis’ Zometa in breast cancer patients with bone metastases.

The Phase III trial involving 2,049 patients with advanced breast cancer met its primary and secondary endpoints and demonstrated denosumab’s superior efficacy compared to Zometa (zoledronic acid), Amgen says. Full details of the study will be reported later this year but the company noted that denosumab significantly delayed the time to the first “skeletal-related event”, such as fractures, bone surgery or spinal-cord compression.

Amgen added that the incidence of serious adverse events was consistent “with what has previously been reported for these two agents”, although osteonecrosis of the jaw which had not been observed in previous Phase III trials of denosumab, was seen infrequently, and with no statistically significant difference, in both treatment groups. Infectious adverse events were balanced between denosumab and Zometa, as was overall survival and the time to cancer progression.

Roger Perlmutter, head of R&D at Amgen, said the firm is “extremely pleased with the outcome of this important study”. It shows that denosumab “can reduce or delay the serious complications of bone metastases in breast cancer patients better than the current standard of care, and with a favourable benefit/risk profile", he added.

Dr Perlmutter claimed that the results “underscore the importance of the RANK ligand pathway in bone disease”, adding that “we look forward to reviewing the results from a second Phase III study of denosumab effects in advanced cancer patients later this year”. The drug has been filed with US regulators as a treatment for post-menopausal osteoporosis.

However, if denosumab is approved to delay bone damage among cancer patients, that could add $1-$2 billion in annual sales of the treatment, said Eric Schmidt, an analyst at Cowen & Co. He projects $750 million in osteoporosis sales in 2013.

Denosumab is vital to Amgen’s future prospects as it faces up to declining sales of its anti-anaemia drugs Aranesp (darbepoetin alfa) and Epogen (epoetin alfa) and a US Food and Drug Administration panel will review the drug on August 13. Investors seem convinced that denosumab will prove successful and Amgen shares leapt 13% in after-hours trading on the news.