SAN DIEGO - A SMALL trial of an experimental vaccine designed to trigger the immune system to fight breast cancer suggests it may reduce the risk of death for most patients, US military researchers said on Sunday.

The vaccine, designed to treat women with tumours that generate a protein called HER-2, has been licensed to privately held Scottsdale, Arizona-based Apthera under the brand name NeuVax.

If results from the 163-patient study are validated in a larger trial, the vaccine may offer a new therapy for the largest subset of breast cancer patients, said Linda Benavides, a resident in general surgery at Brooke Army Medical Centre in San Antonio and the study's lead researcher.

About 25 per cent to 30 per cent of women with breast cancer have tumours that express high levels of HER-2. Their tumours tend to grow faster and are more likely to recur than tumours that do not carry the protein.

Most of these women are currently treated with Herceptin, also known as trastuzumab, an expensive antibody-based drug made by Genentech.

Researchers at Brooke Army Medical Centre tested NeuVax in women whose tumours generated low levels of HER-2 as well as women with high levels of the protein.

At 30 months follow-up, the injected vaccine was shown to cut the risk of death for all patients by half, and in the group of patients with low-expressing HER-2 tumours, no deaths were reported. Due to the small size of the study, the survival results were not statistically significant.

The study, presented in San Diego at a meeting of the American Association for Cancer Research, also showed cancer recurred in 10.7 per cent of vaccinated low-expressers, compared with 18.2 per cent of the control group.

Dr Benavides said a planned Phase III trial of NeuVax in more than 700 patients will test the vaccine solely in women with tumours that generate low levels of HER-2 - a group for which immune-targeting therapy is currently unavailable.

But these breast cancer patients already have a better prognosis than women with high levels of the protein, leading to questions about whether the results can be duplicated in a larger trial.

'The surprising result is that it was the low-expressing patients who had the better outcomes ... that makes it difficult to interpret the data,' said Dr William Hait, president of the American Association for Cancer Research and head of oncology research at Johnson & Johnson's Ortho Biotech unit.

Dr Benavides said said the experimental vaccine offers a 'very simplistic approach,' toward targeting HER-2, adding that it would be 'very cheap to mass produce.'

The wholesale price for Herceptin, which had US sales of US$1.3 billion (S$1.8 billion) last year, is about US$40,000 a year. -- REUTERS