For the Phase 1b - to determine safety tolerability and recommended Phase 2 dose (RP2D) of eribulin mesylate in combination with PEGylated recombinant human hyaluronidase (PEGPH20) in participants with Human Epidermal Growth Factor Receptor (HER2)-negative metastatic breast cancer (MBC) previously treated with up to two lines of systemic anticancer therapy in the metastatic setting.