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Merck's Keytruda Plus Chemo Improves Progression-Free Survival in Triple-Negative Breast Cancer

NEW YORK – Merck yesterday announced positive progression-free survival data for its anti-PD-1 therapy pembrolizumab (Keytruda) combined with chemotherapy as a first-line treatment for patients with metastatic triple-negative breast cancer.

According to results from the Phase III KEYNOTE-355 trial comparing pembrolizumab in combination with chemotherapy against chemotherapy and placebo, patients treated with the pembrolizumab combination experienced a significant improvement in progression-free survival, translating to a reduced risk of disease progression or death.

KEYNOTE-355 enrolled 847 patients who were randomized to receive either pembrolizumab and chemo or placebo and chemo. The specific chemotherapy was investigator's choice of nab-paclitaxel, paclitaxel, or gemcitabine combined with carboplatin. The PFS improvement in the pembrolizumab arm was statistically significant only in patients whose tumors had PD-L1 expression levels greater than or equal to a combined positive score of 10.

In this subgroup, the median progression-free survival with the pembrolizumab and chemotherapy combination was 9.7 months compared to 5.6 months for those receiving chemotherapy alone. The risk of disease progression or death was reduced by 35 percent for those receiving the immunotherapy.

In patients whose tumors expressed a PD-L1 score beneath the threshold of 10 — but still equal to or greater than a combined positive score of one — the progression-free survival was improved as well, but to a lesser extent that was not considered statistically significant. In these patients, median progression-free survival with pembrolizumab plus chemotherapy was 7.6 months compared to 5.6 months with chemotherapy alone.

Though overall survival data are not yet available, Merck noted in a statement that the trial will continue without changes to evaluate this endpoint, which has been designated a dual primary endpoint for KEYNOTE-355 together with progression-free survival.

Pembrolizumab is not currently specifically approved for metastatic triple-negative breast cancer patients. "There is a significant need for treatment regimens that can help women with metastatic triple-negative breast cancer, an aggressive disease," Javier Cortes of Quironsalud Group, first author of KEYNOTE-355, said in a statement. "The results of this study demonstrate that, if approved, Keytruda in combination with chemotherapy may offer certain women a new option for first-line treatment."

The data will be presented in an oral abstract session during the American Society of Clinical Oncology's 2020 Virtual Annual Meeting.