Claims that Eli Lilly’s Evista is safer than the generically-available drug tamoxifen when used to prevent breast cancer in high-risk women have been challenged by experts attending the American Society of Clinical Oncology annual meeting in Atlanta.

The results of the STAR study were first unveiled at a National Cancer Institute press conference in April, and the principle findings - that Evista (raloxifene) was as effective at reducing the risk of invasive breast cancer in older women with osteoporosis as tamoxifen, but was associated with 36% fewer cases of uterine cancer and 29% fewer blood clots - attracted a lot of media attention.

But in a lively session at ASCO, the STAR researchers presented more detail from the study, revealing that the difference in uterine cancer rates was not statistically significant.

There were also suggestions that the number of cases of blood clots was too small to say reliably that raloxifene was better, even though the difference achieved statistical significance, and tamoxifen appeared to be more effective than Lilly’s drug in preventing non-invasive breast cancer.

The STAR trial was undertaken at a time when tamoxifen had been shown to be effective in this form of primary breast cancer prevention, but the drug has generally been used sparingly by primary care physicians in women without established breast cancer because of concerns about its side effects.

“If raloxifene is approved by the Food and Drug Administration for the prevention of breast cancer, primary care physicians may be more willing, given their experience with raloxifene, to prescribe it for breast cancer chemoprevention than they have been to prescribe tamoxifen,” said the study investigators.

The results of the study will be published in the Journal of the American Medical Association on June 21 and have been made available on the journal’s website.

But in an editorial accompanying the study, William Gradishar and David Cella of Northwestern University note that "although media coverage of the early release of data from the STAR trial suggests a clear 'winner' in raloxifene, the data from clinical endpoints and patient-reported symptoms suggest a less clear conclusion.”

Evista brought a little over a billion dollars into Lilly’s coffers last year from use as an osteoporosis treatment, but has started to see its sales momentum slow as a result of increasing competition in the marketplace for bone-building drugs, particularly from more-potent bisphosphonate medicines.

Lilly said after the STAR results were first publicised in April that it would seek approval in the USA to include the prevention of invasive breast cancer in post-menopausal women on Evista's labelling.