First-line treatment with Keytruda (pembrolizumab) plus chemotherapy reduced the risk of disease progression or death for women with advanced triple-negative breast cancer (TNBC), researchers reported at the American Society of Clinical Oncology Annual Meeting.

The Phase III KEYNOTE-355 trial compared Keytruda versus a placebo, both taken with chemotherapy, in more than 800 women with locally recurrent or metastatic TNBC, a type of breast cancer that can’t be treated with hormone therapy or HER2 inhibitors. Keytruda is a PD-1 checkpoint inhibitor that helps T cells fight cancer.

Keytruda plus chemotherapy reduced disease progression or death by 35% in people with at least 10% PD-L1 expression—a biomarker of response to checkpoint immunotherapy. The median progression-free survival (PFS) time increased from 5.6 months in the placebo group to 9.7 months in the Keytruda group. PFS increased to 7.6 months for Keytruda recipients with at least 1% PD-L1 expression, but this difference wasn’t statistically significant.

Keytruda plus chemotherapy led to “clinically meaningful improvement” in women with this hard-to-treat cancer, the researchers concluded.