Merck has announced positive results from the Phase 3 KEYNOTE-355 trial investigating pembrolizumab in combination with chemotherapy as first-line treatment in patients with metastatic triple-negative breast cancer (mTNBC).

In patients whose tumours expressed PD-L1 with Combined Positive Score (CPS) ≥10, pembrolizumab plus chemotherapy demonstrated a statistically significant and clinically meaningful improvement in progression-free survival (PFS), reducing the risk of disease progression or death by 35% (HR = 0.65 [95% CI, 0.49-0.86], p=0.0012) and improving PFS to a median of 9.7 months compared to 5.6 months for those receiving chemotherapy alone.

In patients whose tumours expressed PD-L1 with CPS ≥1, pembrolizumab plus chemotherapy improved PFS versus chemotherapy alone (median PFS = 7.6 months versus 5.6 months; HR = 0.74 [95% CI, 0.61-0.90], p=0.0014), however these results did not meet statistical significance.

As previously announced, the trial will continue without changes to evaluate the other dual primary endpoint of overall survival (OS).

“The progression-free survival results observed in KEYNOTE-355 have the potential to impart real change for certain patients with metastatic triple-negative breast cancer in the first-line setting,” said Dr. Roy Baynes, senior vice president and head of global clinical development, chief medical officer, Merck Research Laboratories.

“Merck is committed to evaluating innovative treatment approaches, anchored by pembrolizumab, across multiple settings and stages of breast cancer. We are encouraged by the positive results from the Phase 3 KEYNOTE-355 and neoadjuvant/adjuvant KEYNOTE-522 studies, which demonstrate the potential of pembrolizumab in combination with chemotherapy for the treatment of triple-negative breast cancer.”

“There is a significant need for treatment regimens that can help women with metastatic triple-negative breast cancer, an aggressive disease,” said Dr. Javier Cortes, Head Breast Cancer program, IOB Institute of Oncology, Quironsalud Group.

“The results of this study, demonstrate that if approved, pembrolizumab in combination with chemotherapy may offer certain women a new option for first-line treatment.”

These results are being presented as an oral abstract session of the virtual scientific program of the 2020 American Society of Clinical Oncology (ASCO) Annual Meeting (Abstract #1000).

Source: Merck