Study to Evaluate the Efficacy and Safety of Eribulin Mesylate in Combination With Pembrolizumab in Participants With Metastatic Triple-Negative Breast Cancer (mTNBC)

The Phase 1b part will evaluate the safety and tolerability of eribulin mesylate in combination with pembrolizumab. Approximately 6 participants may be enrolled in the Phase 1b part of the study. The Phase 2 part will evaluate the tumor objective response rate (ORR) when treated with eribulin mesylate in combination with pembrolizumab in all participants and will also evaluate ORR in stratum 2 participants and compare with the historical response rate of pembrolizumab monotherapy 10 percent (%) in participants with mTNBC previously treated with \>=1 line of prior chemotherapy in the metastatic setting. Approximately 170 mTNBC participants (including participants in Phase 1b who are on RP2D level) will be enrolled in Phase 2.