The following article features coverage from the American Society of Clinical Oncology 2020 meeting. Click here to read more of Cancer Therapy Advisor‘s conference coverage.

Combining pembrolizumab with chemotherapy resulted in significant improvements in progression-free survival compared with chemotherapy alone among women with previously untreated PD-L1–positive metastatic triple-negative breast cancer (TNBC), according to results from the KEYNOTE-355 trial.

“These findings suggest a role for the addition of pembrolizumab to standard chemotherapy for the first-line treatment of metastatic TNBC,” said Javier Cortes, MD, of Ramón y Cajal University Hospital, Madrid, Spain, who presented the results as part of the ASCO20 Virtual Scientific Program.

KEYNOTE-355 randomly assigned 847 previously untreated patients in a 2:1 ratio to receive pembrolizumab plus chemotherapy (investigator’s choice of nab-paclitaxel, paclitaxel, or gemcitabine/carboplatin) or placebo plus chemotherapy. Dual primary endpoints were progression-free and overall survival in patients with PD-L1–positive tumors. Crossover was not allowed. Outcomes were examined among patients with a combined positive score (CPS) or 10 or greater and a CPS of 1 or greater. However, patients were eligible for the study regardless of PD-L1 expression.

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The median follow-up for patients assigned to receive pembrolizumab was 17.5 months; it was 15.5 months for the placebo arm.

The pembrolizumab combination significantly improved progression-free survival in patients with CPS of 10 or greater. The median progression-free survival was 9.7 months for pembrolizumab plus chemotherapy compared with 5.6 months for chemotherapy alone (hazard ratio [HR], 0.65; 95% CI, 0.49-0.86; P =.0012). At 12 months, progression-free survival was 39.1% for the pembrolizumab arm compared with 23% for placebo.

Among patients with CPS of 1 or greater, 67.8% of patients and almost 77% of patients in the placebo group had experienced disease progression or death. The median progression-free survival was 7.6 months for pembrolizumab plus chemotherapy compared with 5.6 months for the placebo plus chemotherapy arm (HR, 0.74; 95% CI, 0.61-0.90; P =.0014).

“However, according to the prespecified statistical criteria, there was no statistically significant difference between treatment groups in patients with CPS 1 or higher,” Dr Cortes said during the video presentation.

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Among the entire intention-to-treat population, the median progression-free survival was 7.5 months pembrolizumab plus chemotherapy compared with 5.6 months for chemotherapy plus placebo (HR, 0.82; 95% CI, 0.69-0.97). Statistical significance was not tested in the ITT population, Dr Cortes said.

The evaluation of overall survival is still ongoing.

More than 60% of patients assigned to pembrolizumab plus chemotherapy (68.1%) or placebo plus chemotherapy (66.9%) experienced grade 3 to 5 adverse events. Two deaths occurred in the pembrolizumab arm because of treatment-related adverse events. Additionally, 5.2% of patients assigned to be administered pembrolizumab had grade 3 or higher immune-related adverse events.

Read more of Cancer Therapy Advisor‘s coverage of the ASCO 2021 meeting by visiting the conference page.


Cortes J, Cescon DW, Rugo HS, et al. KEYNOTE-355: Randomized, double-blind, phase III study of pembrolizumab + chemotherapy versus placebo + chemotherapy for previously untreated locally recurrent inoperable or metastatic triple-negative breast cancer. Presented at: ASCO20 Virtual Scientific Program. J Clin Oncol. 2020;38(suppl):abstr 1000.