AstraZeneca has reported additional evidence that switching patients with breast cancer from the standard treatment tamoxifen to its aromatase inhibitor Arimidex (anastrozole) improves survival.

The new data, presented at the San Antonio Breast Cancer Symposium, showed that women who switched from tamoxifen to Arimidex after two years of a five-year treatment course were 29% less likely to die than those who remained on tamoxifen for the entire period.

The results, achieved in post-menopausal women with oestrogen-sensitive tumours, also indicated that switching to Arimidex reduced the chances of the disease recurring by 45%.

Data from the ATAC trial published at the end of last year showed that five years of treatment with AstraZeneca's drug was more effective than the standard five-year tamoxifen therapy in terms of survival, but the new data suggest doctors should consider swapping their tamoxifen-treated patients over to Arimidex.

“The risk of the cancer returning peaks in the first two years after surgery, and ... women should be given an aromatase inhibitor at the earliest opportunity to minimise the risk in this crucial period,” commented Professor Mike Baum of University College London.

At present, Arimidex is approved for use after surgery in postmenopausal women with hormone receptor-positive early invasive breast cancer, and does not have clearance to treat the full gamut of patients that can be prescribed tamoxifen, although the new data is expected to increase the pressure to expand access to AstraZeneca’s drug. There may be cost barriers to this, however: tamoxifen is generically-available and costs a fraction of Arimidex.

Arimidex is one of the fast-growing drugs in AstraZeneca’s portfolio, with third quarter sales up 36% to $303 million dollars. It competes in the aromatase inhibitor sector with Novartis’ Femara (letrozole) and Pfizer’s Aromasin (exemestane).

In the UK, the National Institute of Clinical Excellence is due to deliver a verdict on the use of all three of these drugs in the post-surgery setting next November, although the Scottish Medicines Consortium has already given its backing to use of Arimidex on the strength of the ATAC data.