AstraZeneca Plc’s Arimidex, one of a new class of breast cancer drugs, has decisively beaten standard treatment tamoxifen in a 5-year clinical study, researchers said on Wednesday.

Women given Arimidex had an extra 10 percent of cancer-free life compared with those on tamoxifen and the newer drug increased the time to disease recurrence by around 20 percent.

In addition, the spread of cancer, called metastasis, was reduced by 14 percent and the risk of cancer occurring in the other breast was cut by more than 40 percent.

Arimidex, which is also known by the generic name anastrozole, had already been shown to have significant benefits over tamoxifen in 3-year tests and the longer-term advantages are likely to increase demand for the newer drug type.

Tamoxifen therapy for 5 years after surgery is currently the established treatment for postmenopausal women with hormone-sensitive breast cancer.

Arimidex belongs to a class of drugs known as aromatase inhibitors. The two other aromatase inhibitors are Swiss drugmaker Novartis AG’s Femara and Pfizer Inc’s Aromasin.

The newer drugs are associated with fewer side effects than tamoxifen - which has been in use for more than 20 years - although researchers said bone fractures and joint pain were more common in women on Arimidex.

The results from the study, which will be presented later on Wednesday at a breast cancer conference in San Antonio, Texas, were released early by The Lancet medical journal.

Aromatase inhibitors work through a different mechanism from tamoxifen, although both reduce the effects of oestrogen, a hormone that can fuel tumour growth.

The newer drugs were first launched in the mid-1990s but until recently their use has been restricted to patients with metastatic breast cancer.

Arimidex sales totalled $578 million in the first 9 months of 2004.