No one is completely sure how it works, and the tests on it are far from complete. Yet the drug Herceptin has become a hope for countless women with breast cancer - and a political football too.
So when experts suggest it may even be a cure for some women who fear the return of an aggressive form of breast cancer, there is not only hope and frustration for worried patients and their clinicians, but extra pressure on health authorities that have been accused of effectively rationing it.
Why the excitement? Trial results published yesterday in the New England Journal of Medicine suggested that the drug, which is already used for women with advanced disease where tumours have returned and spread to other parts of the body, could also be a lifesaver for those still in the early stages of breast cancer.
Studies suggested that the risk of a tumour returning after surgery, radiotherapy and chemotherapy, was cut by half when Herceptin, which is administered intravenously, was used for a year either in conjunction with other drugs or afterwards.
The most important of the studies reported yesterday involved more than 5,000 patients in 39 countries, 500 of them from Britain. All involved are to be monitored for possible side-effects for up to 10 years.
There are still limitations. Herceptin is of benefit only to those women who have too much of a protein known as HER-2, which is probably present in 20%-30% of breast cancer patients. But for many of these patients, breast cancer is a death sentence - as many as a quarter to a third may be dead within two to three years.
Not since tamoxifen was introduced - which has been a standard therapy now for 20 years in the treatment of other forms of cancer - has there been such hope raised over a drug. Tamoxifen stopped the hormone oestrogen from helping cancers to grow. It also cut the risk of breast cancer returning by around half, as well as reducing the risk of death by a third over a longer period.
But most of the cancers it has targeted are slower acting. Latest indications from the use of Herceptin on early-stage patients suggest that it is significantly cutting recurrence in the disease in the first two to three years after surgery.
Links between HER-2 and the severe form of breast cancer have only been established over the past 20 years. Scientists at the American company Genentech began investigating whether a molecularly engineered antibody could attack the protein. By the early 1990s trials were under way, the first 15 women being all considered to be terminally ill.
Herceptin soon showed extremely promising results in women with advanced disease, slowing progression and improving survival. At least one woman is known still to be alive nine years after being put on the drug.
In 1998, the drug giant Roche signed agreements with Genentech, becoming the international player in the field, although Genentech retains the US licence. In 2000, Herceptin's use for advanced breast cancer was approved in Europe and two years later the National Institute for Health and Clinical Excellence, Nice, also endorsed its use on these patients, although specialists say many in this group are still not benefiting from its use.
Trials, including those in the UK, soon started on women with early stage breast cancer, some people receiving the drug after surgery, radiotherapy and/or chemotherapy, others getting the drug in conjunction with other drugs.
Quite how the drug works is still not certain. It may block tumour growth by binding to HER-2-positive cancer cells and preventing them dividing and growing. It may make the body's immune system attack cancer cells, or it may, along with chemotherapy, simply destroy the HER-2-positive cancer cells.
There are some side-effects. There has been some evidence that use of Herceptin with chemotherapy can cause heart problems and breathlessness. But some risk is considered acceptable because of the greater importance of halting the cancer.
In fact, the most recent trials where Herceptin has been used after standard therapies, rather than alongside them, has suggested the levels of heart problems can be kept under control. However, the risk of the cancer returning seems slightly less when Herceptin is given with other drugs, suggesting there is still work to do on establishing the best combination of treatments.
There is some concern too about whether the drug can prevent cancer spreading to the brain and central nervous system. Up to a third of women receiving the drug for advanced cancer do develop cancers in the brain, despite controlling tumours elsewhere in the body. It may be that Herceptin does not work efficiently there or it may be that cancer cells there simply reflect very aggressive disease. Again, only longer-term results from trials will help.
Women often do not know what type of breast cancer they have; sometimes they are only tested for HER-2 after the cancer has returned, although the government is promising that this will change with all new breast cancer patients being tested from now on. That and the timescale of treatment should mean they will benefit by the time Herceptin has passed the licensing and effectiveness stages next summer.
That may be too late for women who might have benefited already, some would say.
Footnotes
HER-2 HER-2 is a protein found in increased quantities on the surface of tumour cells, and produced by a specific gene with cancer-causing potential. High levels of HER-2 are present in an aggressive form of the disease. Too much production of the protein, or too many copies of the gene, seem to give cancer cells extra power to grow and spread to other parts of the body.
Tamoxifen Drug which helps prevent the actions of oestrogen. Most women with breast cancer have oestrogen-receptors on tumour cells which stimulates their growth. Research has also suggested the drug can significantly reduce incidence of breast cancer in healthy women at high risk of developing disease.
Nice The National Institute for Health and Clinical Excellence is the NHS body established by the government to end the "postcode lottery" in drug prescribing in England and Wales. It has been criticised for its lengthy decision-making process.
