This study aims to investigate whether eplerenone protects the heart from the harmful effects of doxorubicin chemotherapy. Specifically, it will measure the effect that eplerenone has on heart muscle relaxation. It will randomly assign women undergoing chemotherapy with doxorubicin to one of two groups: one group will receive eplerenone, and the other group will receive placebo (sugar) pills. The subjects will not know which type of pills they are taking. Heart muscle relaxation will be measured at baseline, after completion of chemotherapy (8-12 weeks), and after 6 months. There will also be various blood tests measured in the study subjects, to determine whether there might be certain blood tests that identify patients at particularly high risk of heart toxicity after doxorubicin therapy.