The results of the large-scale STAR clinical trial reveal that Eli Lilly’s osteoporosis drug Evista is as effective as the widely-used anti-oestrogen tamoxifen in reducing the risk of breast cancer in older women, and has fewer side effects.

STAR, which compared Evista with raloxifene in 19,000 women, revealed that both drugs cut the risk of post-menopausal women at high risk of developing the disease by around 50%. But Lilly’s drug was significantly less likely to cause side effects such as blood clots and uterine cancer.

Steven Paul, executive vice president of science and technology at Lilly, said the STAR results “add to the growing body of scientific knowledge about the potential for therapies to reduce breast cancer risk.” Last week, Lilly reported data from the RUTH study which also showed that Evista could reduce the breast cancer rates in post-menopausal women, although it failed to show a hoped-for benefit in reducing cardiovascular risk.

Lilly said it will now press on with plans to file for approval in the USA to include the prevention of invasive breast cancer in post-menopausal women on Evista’s labelling.

If approved, Evista would provide a valuable and more tolerable alternative to tamoxifen for women hoping to fend off breast cancer, said Leslie Ford, associate director for clinical research in the US National Cancer Institute’s division of cancer prevention.

Tamoxifen was originally developed by a forerunner of AstraZeneca, which sells it in the USA under the Nolvadex brand name. But the availability of generic products recently caused demand for the product to dwindle, and from June AstraZeneca will no longer sell it stateside. It will continue to manufacture tamoxifen in the UK for supply to the rest of the world.

And with generic tamoxifen cheaply available, analysts have questioned how much of an impact a breast cancer reduction label will have on sales of Evista, which brought $1.03 billion to Lilly last year from use in osteoporosis but is starting to feel the effects of competition from new formulations of the more-potent bisphosphonate class of osteoporosis medicines.

Lilly’s product could also face competition from newer hormonal therapies, notably the aromatase inhibitors, which are also starting to encroach on territory formerly dominated by tamoxifen thanks to data showing superior efficacy and safety in breast cancer patients.

At present these agents - which include AstraZeneca’s own Arimidex (anastrozole), Novartis’ Femara (letrozole) and Pfizer’s Aromasin (exemestane) - are not yet approved for primary breast cancer prevention, but are already becoming established in the secondary prevention setting and are being tested in risk reduction.