12 patients with locally advanced or metastatic breast cancer who have received at least one hormonal therapy and at least one chemotherapy in the metastatic setting will be enrolled to receive treatment. Once patients are consented and have completed on study screening, eribulin and AI treatment will be alternated for up to 9 months, until disease progression or unacceptable toxicities, whichever is sooner. Patients will then attend a safety follow-up visit 4 weeks after completing treatment.