for improving survival in patients with early triple negative breast cancer (TNBC).
The findings were presented in Chicago June 4 and 5 at the annual meeting of the American Society of Clinical Oncology by study author Lajos Pusztai, MD, D.Phil, director of Breast Cancer Translational Research at Yale University, New Haven, Conn.
KEYNOTE-522 is the first prospective, randomized, placebo-controlled phase 3 trial of pembrolizumab for early-stage TNBC in the neoadjuvant and adjuvant setting.
The study included 1,174 patients (median age 49 years) with previously untreated stage II or III triple-negative breast cancer. Patients were randomly assigned to receive neoadjuvant therapy with four cycles of pembrolizumab (200 mg) or placebo every 3 weeks plus paclitaxel and carboplatin, followed by four cycles of pembrolizumab or placebo plus doxorubicin-cyclophosphamide or epirubicin-cyclophosphamide. After surgery, patients received pembrolizumab or placebeo for 9 cycles or until recurrence or unacceptable toxicity. The primary end points were pathological complete response and event-free survival.
A total of 784 patients were treated with pembrolizumab and chemotherapy, and the second group of 390 patients received a placebo and chemotherapy. After surgery, patients received adjuvant pembrolizumab (pembrolizumab-chemotherapy group) or placebo and chemotherapy for every 3 weeks for up to nine cycles.
The estimated event-free survival at 36 months was 84.5% in the pembrolizumab-chemotherapy group, compared with 76.8% in the placebo-chemotherapy group (hazard ratio for event or death, 0.63; 95% confidence interval, 0.48 to 0.82; P <0.001). Adverse events occurred predominantly during the neoadjuvant phase and were consistent with the established safety profiles of pembrolizumab and chemotherapy.
At the first interim analysis, 64.8% achieved pathological complete response in the pembrolizumab group versus 51.2% in the placebo group. At the fourth interim analysis at 36 months, event-free survival was 76.8% in the placebo arm and 84.5% in the pembrolizumab arm. RCB-0 status was achieved by 63.4% and 56.2% of patients in the pembrolizumab and placebo arms, respectively.
Pembrolizumab did contribute immune-related adverse events, mostly grades 1-2, in about 17% of patients with thyroid function abnormalities most common with most occurring 20 weeks prior to surgical treatment.
Treatment with pembrolizumab added to chemotherapy, compared with chemotherapy alone, shifted residual cancer burden to lower categories across the entire spectrum of patients in the trial.
The hazard ratio for event-free survival with RCB-0, which Dr. Pusztai said is equivalent to a pathologic complete response (pCR), was 0.70 (0.38-1.31). For RCB-1 (minimal residual disease) it was 0.92 (0.39-2.20); for RCB-2 (moderate residual disease) it was 0.52 (0.32-0.82); and for RCB-3 (extensive residual disease) it was 1.24 (0.69-2.23).
“The most important finding is that patients in RCB-2, a group with a moderate amount of residual disease, experienced significant improvement with pembrolizumab. This clearly indicates not only that pembrolizumab leads to higher pCR rates but also that the pembrolizumCR/RCB-0 ... extends to patients who do not achieve pCR,” Dr. Pusztai said.
The benefit, he suggested, could be a result of the adjuvant pembrolizumab maintenance phase.
Patients in the RCB-3 category do poorly regardless of treatment (EFS of 34.6 % and 26.2% in the pembrolizumab and placebo arms, respectively).
“The RCB-3 population represents an unmet medical need, and they will need better drugs, and additional postoperative adjuvant therapy,” Dr. Pusztai said. The current standard of care is capecitabine for 6-8 cycles. Emerging new therapies, such as antibody drug conjugates, will be tested, he said.
In terms of limitations, adjuvant capecitabine was not allowed. “It remains uncertain how much better the RCB-2 and -3 patient outcomes would have been if capecitabine were administered,” he said.
The study was funded by Merck Sharp and Dohme, a subsidiary of Merck. Dr. Pusztai has received consulting fees and honoraria from Merck.