As part of our coverage of the 33rd Annual Miami Breast Cancer Conference, held March 10-13 in Miami Beach, Florida, we spoke with Elizabeth A. Mittendorf, MD, PhD, associate professor at the department of breast surgical oncology at the University of Texas MD Anderson Cancer Center in Houston, Texas, who presented at the meeting on cancer vaccines and checkpoint inhibitors.
Cancer Network: How has being both a surgeon and immunologist, shaped your views of the potential clinical roles of cancer vaccines?
Dr. Mittendorf: As a surgeon, I see and treat patients with early-stage breast cancer that is potentially curable. Unfortunately, despite our best treatment—surgery, chemotherapy when indicated, radiation if required—we still see recurrences in up to 20% of these patients. I think it is not unreasonable to hypothesize that this recurrence is in part attributable to a failure of the immune response against the cancer—hence my enthusiasm for vaccines that could potentially augment that antitumor immunity, thereby decreasing the risk of recurrence.
Cancer Network: In what settings do breast cancer vaccines show the most promise?
Dr. Mittendorf: Secondary prevention. There is currently one vaccine that is being investigated in a phase III trial—NeuVax—which is made up of an immunogenic peptide combined with an immunoadjuvant. The trial is vaccinating patients in the adjuvant setting with the goal being to determine if vaccination can decrease the risk of recurrence.
Cancer Network: Is there reason for optimism that cancer vaccines might prove useful against advanced breast cancers?
Dr. Mittendorf: In my opinion, vaccines as monotherapy are not likely to be successful in advanced breast cancer. With that said, it is possible that vaccines could be administered as part of a combination strategy with other drugs that could augment the immune response such as certain chemotherapy regimens, trastuzumab, or other immunomodulatory drugs such as the checkpoint blockade agents.
Cancer Network: What insights do epidemiologic studies, such as those regarding childhood infections and cancer risk, offer for cancer immunotherapy?
Dr. Mittendorf: There is epidemiologic data to suggest that individuals who have had childhood infections (ie, chicken pox, pertussis, and other febrile illnesses) have a decreased risk of developing cancer. It is likely that these individuals develop adaptive immune responses against epithelial antigens. These responses could be augmented in the setting of a premalignant condition (ie, a colonic adenoma, or ductal carcinoma in situ), thereby tipping the scales back in favor of the immune response, leading to elimination of the threat of malignancy.
Cancer Network: Are the KEYNOTE trial reports to date reason for optimism about immune checkpoint blockade’s potential against breast cancer?
Dr. Mittendorf: Absolutely. These trials have confirmed that pembrolizumab (anti-PD-1 antibody) is fairly well tolerated by breast cancer patients and suggest some clinical activity. Through the portfolio of KEYNOTE trials, which have enrolled the different subtypes of breast cancer, we’re likely to learn more about which subtypes of breast cancer are most likely to respond to pembrolizumab as monotherapy, which in turn would suggest which subtypes might need additional immune stimulation (ie, a combination strategy) in order for the checkpoint blockade agent to be effective.
Cancer Network: What is the significance of PD-L1 expression in tumor cells vs the tumor microenvironment?
Dr. Mittendorf: Whether PD-L1 expression on the tumor cells is required for response to anti-PD-1 or anti-PD-L1 therapy remains a subject of much discussion. Data from the JAVELIN trial presented at the San Antonio Breast Cancer Symposium in December suggested that PD-L1 expression on the tumor was less important than PD-L1 expression on immune cells in the microenvironment—what they referred to as “immune hotspots.”
Cancer Network: Do you anticipate clinical roles for checkpoint blockade in secondary prevention? Breast cancer treatment in combination with other agents, like trastuzumab? (Are there other promising combinations? Do you anticipate immunotherapy combinations that exploit different immune system pathways?)
Dr. Mittendorf: I see a potential role for checkpoint blockade in the adjuvant setting (effectively secondary prevention) in high-risk patients in whom the risk/benefit ratio favors using these agents, which do have some toxicity associated with them. As an example, the SWOG cooperative group is developing a trial that will evaluate pembrolizumab in patients with triple-negative breast cancer who have at least 1 cm of tumor or positive lymph nodes after neoadjuvant chemotherapy. With respect to using in combination with other agents—yes; in fact the PANACEA trial currently accruing in Europe is combining pembrolizumab with trastuzumab in patients with HER2-positive metastatic breast cancer.