Project Summary Doxorubicin (DOX) chemotherapy regimens play a prominent role in many cancer treatments. With long term cancer survivorship, a substantial population of cancer patients remain at risk of early cardiovascular morbidity and mortality due to DOX chemotherapy. Moreover, clinical studies have revealed that sequential treatment of DOX with the ErbB2 inhibitor, Trastuzumab has synergistic effects in improving the control of cancer progression and survival in breast cancer patients, and is better than either drug alone. However, the combination therapies with the ErbB2 inhibitor coupled with DOX cause severe and aggressive form of heart failure. To overcome this clinical problem, we propose a novel combination therapy with PDE5 inhibitor, sildenafil (Viagra) and mTOR inhibitor, rapamycin in preventing the severe cardiotoxicity caused by DOX and DOX with sequential use of Trastuzumab in breast cancer-bearing mice. We will evaluate the therapeutic effect of sildenafil and rapamycin on DOX-induced cardiomyocyte death in vitro and cardiac function in vivo. In addition, we will determine the role of inflammation in the development of cardiotoxicity by measuring the expression of TLR4, NLRP3 and pro-inflammatory cytokines including IL-1β and IL-18. The role of cGMP-dependent protein kinase G (PKG) activation and mTOR inhibition with associated downstream signaling pathways in protecting against DOX-induced cardiac dysfunction will be studied. We will also investigate the effect sildenafil and rapamycin treatment in potentiating the anti-tumor efficacy of DOX and DOX with Trastuzumab and improvement of cardiac function in the clinically relevant mouse models of spontaneous and orthotropic breast cancer. Furthermore, we will determine the effect of sildenafil and rapamycin in attenuation of hypertrophy in cardiac specific ErbB2 transgenic mice. Because sildenafil and rapamycin are clinically approved drugs, the proposed studies may help in developing novel combination therapy for treatment of thousands of cancer patients experiencing the lethal and debilitating cardiotoxic effects of DOX and Trastuzumab worldwide.