The aim of this study is to alternate eribulin and aromatase inhibitors, examining whether there may be breakthrough relapse during the AI therapy or on the other hand we can extend the duration that eribulin may be used for. Importantly, blood based biomarkers, the tumour derived fraction of cfDNA (ctDNA), and circulating tumour cells will be measured. A major aim of this study is to test whether biomarkers fluctuate between chemotherapy and AI treatment in the setting of advanced breast cancer.