“This is the first time in 20 years that the established benefits of tamoxifen in early breast cancer have been surpassed by another treatment, and marks an important breakthrough in the management of this ever-increasing and distressing disease,” said Chris Brinsmead, Vice President, Oncology Therapeutic Area, AstraZeneca PLC. “Regulatory approval for Arimidex in this new indication will be sought in the United States and Europe in the first quarter of next year; and we look forward to the benefits that Arimidex may bring to this patient population.”

Arimidex sales in 2000 were $156 million, a 19 per cent increase over the previous year. AstraZeneca’s Nolvadex ® (tamoxifen) accounted for $576 million in sales the same year. The early breast cancer market could be worth approximately $2 billion.

The ATAC (Arimidex, Tamoxifen, Alone or in Combination) Trial is an international study of over 9,300 postmenopausal women with early breast cancer  [1], which started in 1996. The trial compares five years of treatment with tamoxifen alone (20 mg daily), Arimidex alone (1mg daily), or a combination of both therapies, following initial surgery.

After an average of 33 month’s follow-up, Arimidex monotherapy was found to be significantly more effective in preventing relapse than tamoxifen, with a 17 per cent reduction in risk of breast cancer recurring with Arimidex treatment compared to tamoxifen.

Arimidex was also found to have many important tolerability advantages over tamoxifen. Arimidex was associated with significantly fewer reports of endometrial cancer when compared with tamoxifen. This finding was supported by a significantly lower incidence of vaginal bleeding and vaginal discharge among Arimidex-treated patients compared to those taking tamoxifen.

Another known risk associated with tamoxifen is thromboembolic events. In the ATAC trial, both the overall incidence of thromboembolic events and that of deep vein thromboses were significantly reduced in the Arimidex group. Important from the patient’s perspective, the incidence of hot flushes and weight gain were also significantly reduced. However, as expected, women taking tamoxifen did have a lower risk of experiencing musculo-skeletal disorders or the types of fractures common in this age group compared with those taking Arimidex.

The Arimidex/tamoxifen combination showed no additional efficacy or tolerability benefits compared with tamoxifen alone. There were no specific safety issues associated with the combination treatment.

Further analysis of the ATAC data will now be carried out, including analysis of sub-protocols designed specifically to investigate the effect of treatment on the endometrium, bone and quality of life. In addition, the trial is continuing, to allow the investigators to collect more data and particularly to confirm if the observed reduction in relapse rates with Arimidex will lead to improved long-term survival, as has been seen in earlier trials of tamoxifen in early breast cancer.