CONTROVERSIAL arthritis drugs have the unexpected benefit of greatly cutting the risk of breast cancer, a new trial indicates.

Women who took drugs known as COX-2 inhibitors for at least two years had their risk of breast cancer reduced by 71 per cent. The drugs included Vioxx, a best-seller in the US that has now been withdrawn because of a link with heart disease, and Celebrex, which has a clean bill of health and is still on the market.

Women taking 200mg of Celebrex a day, a normal dose for somebody suffering arthritic pain, showed the best results of all, reducing their risk of breast cancer by 83 per cent.

With many cases of breast cancer occuring in older women who may well be prescribed COX-2 inhibitors, the results suggest a real opportunity to reduce the incidence of the disease. But specialists are likely to react cautiously unless the results are confirmed.

A team from Ohio State University led by Randall Harris looked at the health records of 323 women with invasive breast cancer, and 649 healthy women. By monitoring whether or not they had been prescribed COX-2 inhibitors, or similar older drugs such as ibuprofen or naproxen, they could ascertain whether taking the drugs increased or decreased the risk of getting the disease.

They decreased it sharply, they report in BMC Cancer, an online journal. Ibuprofen and naproxen were also effective, showing a 63 per cent reduction in risk.

The difference between the newer and older agents is that the COX-2 inhibitors have a more selective action and are claimed to be kinder to the stomach. But case-control studies of this type, though useful, are not considered definitive evidence. They are open to errors of various kinds, generally involving whether the controls truly match the cases in risk profile.

Sceptics may also point out that the study was part-funded by Pfizer, which makes Celebrex. The remainder of the funding came from the US National Cancer Institute. But the results would be hard to explain away, and there are plausible reasons why the COX-2 inhibitors and their older cousins should have such effects.

They are designed to inhibit the action of cyclooxygenase, an enzyme involved in the synthesis of prostaglandins. Studies of breast tumours have shown that they make too much cyclooxygenase, and that the excess amounts of the enzyme are linked to various processes favouring the spread of cancers. The enzyme is also linked to the creation of oestrogen, the female hormone that feeds breast cancer.

The team concludes that the creation of too much of the enzyme plays a significant role in the initiation and growth of breast tumours, and that blocking this process has a strong potential for reducing the risk.

However, no physician would prescribe COX-2 inhibitors for cancer prevention solely on this evidence. Vioxx is already withdrawn, though many doctors think that the decision was too hasty. Celebrex and other COX-2 inhibitors remain on the market, though under a Vioxx-induced cloud. For the moment, they are likely to be given only for arthritis — though patients will be encouraged to hear that they may be protected against breast cancer as a side-effect.

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