In a key test involving nearly 1,000 women with very advanced disease, the experimental treatment extended by several months the time women lived without their cancer getting worse, doctors planned to report Sunday at a cancer conference in Chicago.

More importantly, the treatment seems likely to improve survival; it will take more time to know for sure. After two years, 65 per cent of women who received it were still alive versus 47 per cent of those in a comparison group given two standard cancer drugs.

A warning to hopeful patients: the drug is still experimental, so not available yet. Its backers hope it can reach the market within a year.

The treatment builds on Herceptin, the first gene-targeted therapy for breast cancer. It is used for about 20 per cent of patients whose tumors overproduce a certain protein.

Researchers combined Herceptin with a chemotherapy so toxic that it can't be given by itself, plus a chemical to keep the two linked until they reach a cancer cell where the poison can be released to kill it.

This double weapon, called T-DM1, is the "smart bomb," although it's actually not all that smart - Herceptin isn't a homing device, just a substance that binds to breast cancer cells once it encounters them.

Doctors tested T-DM1 in 991 women with widely spread breast cancer that was getting worse despite treatment with chemotherapy and ordinary Herceptin. They were given either T-DM1 infusions every three weeks or infusions of Xeloda plus daily Tykerb pills - the only other treatments approved for such cases.

The median time until cancer got worse was nearly 10 months in the women given T-DM1 versus just over 6 months for the others. That is about the same magnitude of benefit initially seen with Herceptin, which later proved to improve overall survival, too, Blackwell said.

T-DM1 caused fewer side effects than the other drugs did. Some women on T-DM1 had signs of liver damage and low levels of factors that help blood clot, but most did not have the usual problems of chemotherapy.

"People don't lose their hair, they don't throw up. They don't need nausea medicines, they don't need transfusions," said Blackwell, who has consulted in the past for Genentech, the study's sponsor.