Study of Eribulin Mesylate in Combination With PEGylated Recombinant Human Hyaluronidase (PEGPH20) Versus Eribulin Mesylate Alone in Subjects With Human Epidermal Growth Factor Receptor 2 (HER2)-Negative, High-Hyaluronan (HA) Metastatic Breast Cancer (MBC)

The primary objective for the study is as follows: 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. For the Phase 2 - to evaluate objective response rate (ORR) of eribulin mesylate in combination with PEGPH20 in participants with HER2-negative, High-Hyaluronan (HA)-high, MBC previously treated with up to 2 lines of systemic anticancer therapy in the metastatic setting.

Phase 1b will occur in two parts: Part 1: Run-in safety cohort(s) (dose levels 1, 0, and -1, as necessary) of 6 participants each will be conducted until the RP2D is determined. The purpose of the run-in safety cohort(s) is to study the safety of the 2-drug combination and determine a RP2D. Dose limiting toxicity (DLT) will be assessed in the first cycle to determine the RP2D of eribulin mesylate in combination with PEGPH20. Part 2 (Phase 1b "Expansion Part"): A total of 12 additional participants will be enrolled at the RP2D determined in Part 1 (using Phase 1b criteria, i.e., participants previously treated with up to 2 lines of systemic anticancer therapy) in order to assess the safety profile of the combination and identify any potential safety signals and the incidence of thromboembolic events in this population (RP2D determined dosing). In Phase 2, participants will be stratified by triple negative breast cancer (TNBC) status and randomized 1:1 to receive eribulin mesylate and PEGPH20 at the established RP2D level or eribulin mesylate alone at 1.4 milligrams per square meters (mg/m\^2).