Doxorubicin and other anthracyclines are commonly used to treat breast cancer and other types of cancer. Unfortunately, they can cause heart muscle damage, resulting in scarring, abnormal contraction and relaxation, and heart failure symptoms. This side effect occurs more frequently at higher doses, and limits the total dose that can be given to cancer patients. Eplerenone is an oral medication that prevents or reverses heart damage in other disease states, and is commonly used to treat heart failure. This study will investigate the use of eplerenone to protect the heart from these harmful side effects of doxorubicin.