A new breast cancer vaccine candidate, GP2, provides further evidence of the potential of immunotherapy to prevent disease recurrence, especially when combined with a powerful immunotherapy drug in high-risk patients. These findings were presented at the American Society of Clinical Oncology (ASCO) 2014 Breast Cancer Symposium, held in San Francisco, California.

One of only a few vaccines of its kind in development, GP2 has been shown to be safe and effective for breast cancer patients, reducing recurrence rates by 57%. Further, women with the highest overexpression of HER2 (known as HER2 +3) had no cancer recurrences when the vaccine was administered after completing treatment with the monoclonal antibody trastuzumab (Herceptin). HER2 is an oncoprotein that promotes tumor growth and is expressed to some extent in 75% to 80% of breast cancers. 

“This is an important and different avenue in immunotherapy research, in that we are investigating ways to prevent cancer recurrence by stimulating the immune system to treat cancer,” said principal investigator Elizabeth Mittendorf, MD, PhD, associate professor of Surgical Oncology at The University of Texas MD Anderson Cancer Center in Houston, Texas. “The ultimate goal is to develop a preventative tool that will minimize the risk of recurrence in women who have already had breast cancer and for whom standard therapies have failed.”

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The findings are the result of a phase II randomized trial that paired the GP2 vaccine, designed to stimulate the CD8+ cells, commonly known as killer or toxic T cells, with an immune stimulant known as granulocyte/macrophage colony stimulating factor (GM-CSF). The trial included 190 patients with varying levels of HER2; 89 women received the GP2 vaccine with a GM-CSF adjuvant and a control group of 91 patients received GM-CSF alone. Eight patients experienced early recurrence or developed a second malignancy and did not complete the vaccine trial. The vaccine is injected subcutaneously and the initial series consisted of monthly inoculations for 6 months, followed by four cycles of booster shots administered every 6 months thereafter. The patients were monitored for nearly 3 years.

For all 190 patients, including those who did not complete the trial, the disease-free survival (DFS) rate was 88% among those who received the vaccine and 81% in the control group, which represented a 37% reduction in recurrence. Excluding the patients who did not complete the vaccine series, the results are higher: 94% in those who received the vaccine vs 85% DFS rate in those who did not receive GP2, which is a risk reduction of 57%.

Women with HER2 +3 who were received trastuzumab as part of the standard of care prior to receiving the vaccine experienced no cases of cancer recurrence. According to Mittendorf, trastuzumab may act like a primer for the vaccine. Trastuzumab stimulates CD4+ T cells to release substances that fight cancer cells and initiates an antibody response. Thus, it may prepare the immune system, making the vaccine even more effective. MD Anderson is now testing this combination of immunotherapies in other clinical trials.