The study was done to compare the efficacy of eribulin plus pembrolizumab vs eribulin alone in patients with HR-positive, ERBB2–negative MBC.

The primary endpoint was PFS. Secondary endpoints were ORR and OS. Exploratory analyses assessed the association between PFS and PD-L1 status, TILs, TMB, and genomic alterations.

Eighty-eight patients started protocol therapy; the median number of prior lines of chemotherapy was 1 (0-2), and the median number of prior lines of hormonal therapy was 2 (0-5). The Median follow-up was 10.5 months. Median PFS and ORR were not different between the 2 groups. Fourteen patients started crossover treatment with pembrolizumab; 1 patient experienced stable disease. All-cause adverse events occurred in all patients including 2 treatment-related deaths in the combination group, both from immune-related colitis in the setting of sepsis, attributed to both drugs. The PD-L1 22C3 assay was performed on archival tumor samples in 65 patients: 24 had PD-L1–positive tumors. Analysis indicated that PD-L1 status, TILs, TMB, and genomic alterations were not associated with PFS.

The study concluded that the addition of pembrolizumab to eribulin did not improve PFS, ORR, or OS compared with eribulin alone in either the intention-to-treat or PD-L1–positive populations.