SAN ANTONIO – Adding pembrolizumab to chemotherapy in the neoadjuvant setting increased the likelihood that women with stage III or early node-positive triple-negative breast cancer (TNBC) would have a pathologic complete response and sustained clinical benefit, results of the phase 3 KEYNOTE-522 study showed.
Among 602 patients evaluable in a definitive pathological complete response (pCR) analysis, the pCR rate was 64.8% for those treated with chemotherapy plus pembrolizumab (Keytruda), compared with 51.2% for patients treated with chemotherapy plus placebo, reported Peter Schmid, MD, PhD, from Barts Cancer Institute in London.
“The addition of neoadjuvant pembrolizumab to chemotherapy provided a significant increase in the path CR rate in all patients, but also a larger magnitude of path CR benefit versus chemotherapy alone in patients with higher-risk disease, such as stage III disease or node-positive early triple-negative breast cancer,” he said at the annual San Antonio Breast Cancer Symposium.
The overall pCR results were originally reported at the 2019 annual meeting of the European Society for Medical Oncology. At SABCS 2019, he reported pCR results for specific subgroups in KEYNOTE-522.
Investigators enrolled patients aged 18 years or older with newly diagnosed TNBC of either stage T1cN1-2, or T2-4N0-2 and an Eastern Cooperative Oncology Group performance status of 0 or 1. Patients also had to have at least two separate tumor cores from the primary tumor for assessment of programmed death–ligand 1 (PD-L1).
After stratification for nodal status, tumor size, and carboplatin schedule (once weekly or every 3 weeks), patients were randomized to receive either pembrolizumab 200 mg every 3 weeks or placebo plus neoadjuvant carboplatin and paclitaxel for four 3-week cycles, followed by four cycles of chemotherapy with either doxorubicin or epirubicin plus cyclophosphamide (AC or EC). Patients went on to surgery, then received adjuvant therapy for nine cycles with either pembrolizumab at the neoadjuvant dose and schedule or placebo.
At the first preplanned interim analysis for event-free survival (EFS) based on 1,174 patients followed for a median of 15.5 months, events had occurred in 7.4% of patients on pembro/chemo, compared with 11.8% on placebo/chemo, but this difference did not meet the prespecified P value boundary of .000051 for significance, Dr. Schmid acknowledged.
When the investigators looked at pCR by disease stage, however, they saw the following benefits across all stages in the study:
- Stage IIA: 73.1% with pembrolizumab versus 62.1% with placebo, difference 11%.
- Stage IIB: 56.2% versus 48.4%, difference 7.8%.
- Stage IIIA: 66.7% versus 42.1%, difference 24.6%.
- Stage IIIB: 48.6% versus 23.1%, difference 25.6%.
The greatest benefit for the addition of pembrolizumab to chemotherapy appeared to be in the higher disease stages, Dr. Schmid said.
There was also a benefit from pembrolizumab for patients with both node-negative disease (pCR, 64.9% vs. 58.6% in the placebo arm) and node-positive disease (64.8% vs. 44.1%, respectively).
pCR rates were also superior with pembrolizumab versus placebo in patients who were PD-L1 negative, defined as a combined positive score (CPS) less than 1 (45.3% vs. 30.3%), as well as PD-L1–positive patients at each of three cutoff values: CPS 1 or greater (68.9% vs. 54.9%), CPS 10 or greater (77.9% vs. 59.8%), and CPS 20 or greater (81.7% vs. 62.5%).
Interestingly, adding pembrolizumab boosted pCR rates both in patients with exposure to a full planned course of chemotherapy (69.7% vs. 55.3% with placebo) and in those who received less than the full course (51.1% vs. 35.7%).
The most common immune-mediated adverse events with the largest between-group differences involved the thyroid, including hypothyroidism (in 14.9% of patients on pembrolizumab and 5.7% of those on placebo), hyperthyroidism (5.1% vs. 1.8%), and thyroiditis (1.7% vs. 1.0%).
“Immune-mediated adverse events are consistent with the known profiles of each regimen, and there’s no new safety signal, no new safety concern at this point in time,” Dr. Schmid said.
Further follow-up will be needed to determine EFS benefit and long-term safety. Investigators plan to perform additional biomarker analyses, including tumor-infiltrating lymphocytes and BRCA, he added.
“Will the KEYNOTE-522 regimen be the new standard of care if approved?” asked invited discussant Kevin Kalinsky, MD, MS, from Columbia University Irving Medical Center in New York. “These are exciting data, both in pCR and early event-free survival. But there’s a risk: a risk of overtreatment, as well as potentially [serious] toxicity in patients with curable disease.”
“The take-home is that this regimen will likely be practice changing in some patients, and with the absence of having a predictor, the benefit may outweigh the risk most in patients with high clinical risk,” he added.
The study was funded by Merck Sharp & Dohme. Dr. Schmid reported advising/consulting for and receiving honoraria from Merck. Dr. Kalinsky disclosed has disclosed that he receives salary from Array Biopharma, has received fees from various companies (not including Merck), and has contracted research with multiple companies, not including Merck.
SOURCE: Schmid P et al. SABCS 2019, .