Immunotherapy is one of the most important avenues of clinical research for patients with advanced triple-negative breast cancer.

This subtype of breast cancer has the worst prognosis and accounts for around 15% of all breast cancer cases diagnosed.

Pembrolizumab is a monoclonal antibody which targets the PD-1 receptor, found on the surface of lymphocytes, and works by stimulating the immune system to destroy cancer cells.

The addition of pembrolizumab to first-line chemotherapy treatment increases the antitumour activity in patients with triple-negative advanced breast cancer, compared to chemotherapy treatment alone, as demonstrated by the clinical study KEYNOTE-355, published in The Lancet, whose main author is Dr. Javier Cortés Castán, director of the International Breast Cancer Center (IBCC).

This phase III clinical trial was conducted in 209 centres in 29 countries, and involved 847 patients with triple-negative advanced breast cancer who had not received any prior treatment for metastatic disease. Patients were randomized (2:1) to receive either chemotherapy and pembrolizumab (566 patients) or chemotherapy and placebo (281 patients). The primary endpoints of the study were progression-free survival and overall survival. For the goal of progression-free survival, a hierarchical statistical strategy, widely used in this type of clinical study, was employed.

In this interim analysis, the addition of pembrolizumab to chemotherapy significantly reduced the risk of progression or death by 35% compared to chemotherapy and placebo in patients whose tumours had PD-L1 protein expression, using a score called CPS, with a result of 10 or higher.

Based on the results of this study, the US Food and Drug Administration (FDA) approved, on an accelerated basis, the combination of chemotherapy and pembrolizumab for the treatment of this group of patients in November 2020. The study is ongoing and results for the other primary endpoint, overall survival, will be reported soon.