Cardiotoxicity remains a serious problem in anthracycline-treated oncologic patients. Therapeutic modulation of microRNA expression is emerging as a cardioprotective approach in several cardiovascular pathologies. MiR-34a increased in animals and patients exposed to anthracyclines and is involved in cardiac repair. In our previous study, we demonstrated beneficial effects of miR-34a silencing in rat cardiac cells exposed to doxorubicin (DOXO). The aim of the present work is to evaluate the potential cardioprotective properties of a specific antimiR-34a (Ant34a) in an experimental model of DOXO-induced cardiotoxicity. Results indicate that in our model systemic administration of Ant34a completely silences miR-34a myocardial expression and importantly attenuates DOXOinduced cardiac dysfunction. Ant34a systemic delivery in DOXO-treated rats triggers an upregulation of prosurvival miR-34a targets Bcl-2 and SIRT1 that mediate a reduction of DOXO-induced cardiac damage represented by myocardial apoptosis, senescence, fibrosis and inflammation. These findings suggest that miR-34a therapeutic inhibition may have clinical relevance to attenuate DOXO-induced toxicity in the heart of oncologic patients. The anthracycline doxorubicin (DOXO) is a very powerful antineoplastic drug whose clinical use is limited by cardiotoxicity, its main side effect that may occur both acutely and chronically, affecting the quality of life of otherwise successfully treated oncologic patients 1,2. Anthracycline cardiotoxicity begins with subclinical myocardial damage, progresses to an early asymptomatic deterioration in left ventricle (LV) ejection fraction (EF) and ends, if not properly treated, with a symptomatic and often intractable heart failure (HF). For what concerns myocardial function, diastolic dysfunction may represent a precocious manifestation of DOXO cardiotoxicity, associated with ventricular relaxation and chamber wall stiffness leading to an alteration of ventricular function that first involves diastole and then eventually affects systole 3-6. Given the growing successful of chemotherapy with the significant increase in cancer survival, the clinical significance of DOXO cardiotoxicity is by no means small 7. Therefore, there is a serious need of an efficacious cardioprotective strategy to prevent or reduce ventricular complications. MicroRNAs (miRNAs) are small noncoding RNAs that suppress protein expression through binding and silencing specific mRNAs. A single miRNA inhibits many different mRNAs simultaneously, thus allowing an amplification of biological responses. A fine manipulation of miRNA expression and function through systemic or local delivery of miRNA inhibitors (antimiRs) or mimics, has triggered the interest for miRNAs as innovative therapeutic targets 8,9. MiRNAs are emerging as a novel treatment for cardiovascular diseases 9-11 and recently, several studies have investigated the role of miRNAs in DOXO-induced cardiotoxicity 12,13. MiR-34a is involved in several cellular processes, such as apoptosis, senescence ...