Some breast cancers do not respond to treatment
A new vaccine has completely eliminated a type of breast cancer tumour in tests on mice, say researchers.
The vaccine targets breast cancer caused by an excess of a protein called HER2 - and even destroyed tumours resistant to current drugs.
The US team said it might also be used to prevent initial development of the tumours in cancer-free women.
But UK experts warned the vaccine was at a very early stage, and it was not known if it would work in humans.
The study, by Wayne State University, appears in the journal Cancer Research.
HER2 receptors promote normal cell growth, and are found in low amounts on normal breast cells.
But HER2-positive breast cells can contain many more receptors than is typical, promoting a particularly aggressive type of tumour that affects up to 30% of all breast cancer patients.
There are drugs to treat this form of the disease, including Herceptin, but they do not work for a significant proportion of patients.
The new vaccine contains genes that produce the HER2 receptor, and a compound which stimulates the immune system.
The researchers used electrical pulses to deliver the injected vaccine into leg muscles in mice.
Once there, the vaccine produced a huge quantity of HER2 receptors which triggered a reaction by the animals' immune systems, and primed them to fight cancer.
The researchers also used an agent that, for a while, suppressed the activity of regulatory T cells, which normally keeps the immune system from over-reacting.
In the absence of regulatory T cells, the immune system responded much more strongly to the vaccine.
Then, when the researchers implanted HER2-positive breast tumours in the animals, the cancer was eradicated.
There were no sign of any side-effects.
Lead researcher Professor Wei-Zen Wei said: "The immune response against HER2-positive receptors we saw in this study is powerful.
"Both tumour cells that respond to current targeted therapies and those that are resistant to these treatments were eradicated.
"This may be an answer for women with these tumours who become resistant to the current therapies."
The researchers have previously developed a similar vaccine which is currently in early clinical trials.
They say this time they have fine tuned the process, and hope the new vaccine will be more effective.
However, Dr Sarah Rawlings, of the charity Breakthrough Breast Cancer, said: "This is very early research that has only been carried out in mice so we don't actually know if it could be used in women.
"Much more research is needed to find out if it works, to either treat HER2 positive breast cancer or prevent the disease, and if there are any side effects."
Several other groups are also working on breast cancer vaccines that target HER2.