The reason why we think a single dose of pembrolizumab plus a tumor targeted radiation may work in patients with early stage breast cancer is that we know that immune checkpoint inhibitor alone or as monotherapy does not work in breast cancer. We also know that we have to combine pembrolizumab with an immune stimulant such as chemotherapy, but because we really want to try to omit chemotherapy to deescalate treatment in this special population, which is early stage breast cancer, we think that radiation boost, a single fraction of radiation boost, may be effective as a combination with a single dose of pembrolizumab prior to surgery. The study is a window of opportunity study. We are evaluating three treatment arms prior to surgery. The arms one and two to examine the combination of radiation boost post pembrolizumab. The difference between arm one and two is the sequence of the timing of the radiation versus pembrolizumab.
Arm one is radiation boost first, followed by pembrolizumab. Arm two is pembrolizumab first, followed by radiation boost. And then, the third arm is to evaluate pembrolizumab alone. That's the treatment, the three arms that we want to evaluate, and we want to look at this in patients with early stage breast cancer without high risk features, so that we can test whether this radiation combination with immunotherapy may be effective in this patient population. So far, 13 patients have completed the study and 10 patients have triple negative breast cancer.
Among the 10 patients with triple negative breast cancer, we observed that three patients have major response defined as tumor shrinkage. Of the three patients with significant tumor shrinkage, the tumor shrunk by about 80% or more, and in the three patients, one patient had complete pathologic response after a single dose of pembrolizumab and one single tumor targeting radiation given prior to surgery. The next step is to continue to enroll patients into this study because this is considered still, a very small sample size. We want to enroll more patients so that we can confirm our preliminary results and we want to do additional molecular analysis to identify biomarkers associated with treatment response so that we can get to the next step to open a second study that look at patients with even earlier stage disease.
