Control, ENDX-R, 4HT-R, and ICI-R cells were treated with serially-diluted concentrations of drugs commonly used as second- and third-line therapies for endocrine-resistant breast cancer. Drug concentration ranges were chosen individually for each therapy, based on published IC50 values from other breast cancer cell lines. Cells were treated for 7–10 days, after which time, dose response curves were plotted and IC50 values calculated (n=4). N.D.: not defined; an IC50 was unable to be calculated using the chosen drug concentrations. Cmax: maximum concentration reported to be achievable in patient plasma, indicated by a vertical dashed line. Values shown indicate mean +/− SEM.