Preliminary evidence also suggests that the vaccine primed the patients’ immune systems to attack tumour cells and helped slow the cancer’s progression.
The new vaccine causes the body’s immune system to home in on a protein called mammaglobin-A, found almost exclusively in breast tissue.
The protein’s role in healthy tissue is unclear, but breast tumours express it at abnormally high levels, past research has shown.
“Being able to target mammaglobin is exciting because it is expressed broadly in up to 80 per cent of breast cancers, but not at meaningful levels in other tissues,” said senior author William E Gillanders, professor of surgery at Washington University School of Medicine in St Louis.
“In theory, this means we could treat a large number of breast cancer patients with potentially fewer side effects,” Gillanders said.
The vaccine primes a type of white blood cell, part of the body’s adaptive immune system, to seek out and destroy cells with the mammaglobin-A protein.
In the smaller proportion of breast cancer patients whose tumours do not produce mammaglobin-A, this vaccine would not be effective, researchers said.
In the new study, 14 patients with metastatic breast cancer that expressed mammaglobin-A were vaccinated.
According to the authors, patients experienced few side effects, reporting eight events classified as mild or moderate, including rash, tenderness at the vaccination site and mild flu-like symptoms.
Although the trial was designed to test vaccine safety, preliminary evidence indicated the vaccine slowed the cancer’s progression, even in patients who tend to have less potent immune systems because of their advanced disease and exposure to chemotherapy.
“Despite the weakened immune systems in these patients, we did observe a biologic response to the vaccine while analysing immune cells in their blood samples,” said Gillanders.
“That’s very encouraging. We also saw preliminary evidence of improved outcome, with modestly longer progression-free survival,” Gillanders added.