The study, published in Clinical Cancer Research, also suggests the vaccine primes patients’ white blood cells to attack tumor cells, slowing down the progression of the cancer.
The new vaccine works by targeting a protein called mammaglobin-A that is predominantly found in breast tissue. Its role in healthy tissue is currently unknown, but previous research has shown that breast tumor cells express the protein at abnormally high levels.
“Being able to target mammaglobin is exciting because it is expressed broadly in up to 80% of breast cancers, but not at meaningful levels in other tissues,” says senior author Dr. William Gillanders. “In theory, this means we could treat a large number of breast cancer patients with potentially fewer side effects.”
Many pre-existing drug treatments for breast cancer target another protein called human growth factor receptor 2 (HER2). These treatments, including trastuzumab and pertuzumab, can have side effects that include diarrhea and heart problems.
Vaccine also found to slow cancer progression
Specifically, the new vaccine primes white blood cells, an integral part of the body’s immune system, to target cells that have the mammaglobin-A protein.
Unfortunately, the vaccine would be ineffective in the small proportion of breast cancer patients whose tumors do not produce mammaglobin-A.
The study took the form of a small-scale phase 1 trial, during which the vaccine was screened for safety. A total of 14 patients with metastatic breast cancer – cancer that has spread to other parts of the body – whose tumors expressed mammaglobin-A received the vaccination.
Only a small number of side effects were observed; the authors reported eight cases that were identified as mild or moderate. These included the development of mild flu-like symptoms, rash and tenderness at the site of the vaccination. No severe or life-threatening side effects occurred.
In addition to testing the vaccine’s safety, evidence from the trial also suggested that the vaccine managed to slow the progression of the breast cancer.
Around half of the 14 patients who received the vaccine showed no progression of their cancer a year on from having the vaccine. This finding compared favorably with one from a similar control group of 12 patients who did not receive the vaccine, within whom only around one-fifth showed no cancer progression after 1 year.
Results deemed ‘very encouraging’
Although the sample sizes in both groups are small, the difference is considered significant, especially considering that patients with metastatic cancer typically have less potent immune systems due to their treatment and the progression of their disease.
Dr. Gillanders describes the findings as very encouraging. “Despite the weakened immune systems in these patients, we did observe a biologic response to the vaccine while analyzing immune cells in their blood samples,” he says. “We also saw preliminary evidence of improved outcome, with modestly longer progression-free survival.”
The team are now looking to follow up their findings with a larger clinical trial in patients with newly diagnosed breast cancer – patients whose immune systems would be expected to be less compromised than those with advanced forms of the disease.
“We also will be able to do more informative immune monitoring than we did in this preliminary trial,” says Dr. Gillanders. “Now that we have good evidence that the vaccine is safe, we think testing it in newly diagnosed patients will give us a better idea of the effectiveness of the therapy.”