The UK's Medicines Control Agency, which acted as Reference Member State in the European Mutual Recognition procedure, reviewed new evidence from clinical trials involving 1,000 patients worldwide and granted the extension into first line use from Arimidex's® current second line advanced breast cancer claim.

Arimidex® can now be prescribed in the UK, Italy, Germany, Spain, Austria, and Portugal - following previous similar line extensions in Holland and France - for the new first line use, which confirms the treatment as an effective alternative to the gold standard treatment, tamoxifen.

This is the first time that the position of tamoxifen as the routine first-choice endocrine treatment option has been challenged by a drug of the aromatase inhibitor class and means an important increase in treatment options for post-menopausal women with advanced disease.

The data were drawn from two large studies designed to explore the comparative efficacy of the two compounds when used as first-line treatments in advanced disease. The results of both randomised, double blind studies showed that Arimidex® is at least as effective as tamoxifen in first-line treatment. However the results from the North American study also suggested that Arimidex® is significantly more effective in terms of 'time to progression' (p=0.005) and clinical benefit (p=0.0098; retrospective analysis) compared to tamoxifen. Women treated with tamoxifen were found to have a 44 per cent higher risk of their disease progressing in a given period of time than those treated with Arimidex®.

Tamoxifen and Arimidex® have different mechanisms of action. Tamoxifen blocks oestrogen receptors in tumour cells, while Arimidex® reduces oestrogen levels by suppressing its synthesis. The potential impact of these differences in terms of side effect profile was also explored in the studies. Both compounds were generally well tolerated and appeared similar in terms of number of side effects reported.

A major trial comparing tamoxifen and Arimidex® for the treatment of early breast cancer has recently completed recruitment. The ATAC (Arimidex®, Tamoxifen Alone or in Combination) trial is the largest study of adjuvant endocrine treatment in breast cancer ever undertaken, involving over 9,000 post-menopausal women worldwide. The study will answer some vital questions about the comparative efficacy and safety of these two compounds in early breast cancer, and in addition, as Arimidex® and tamoxifen will be given together in one arm of the study, ATAC will investigate the intriguing possibility that giving the two treatments together could produce even better results for these patients. ATAC is due to report in 2001.