Disclosed herein include microRNA antagonists, therapeutic compositions that include one or more of such microRNA antagonists, and methods of treating and/or ameliorating cardiac diseases and/or muscular dystrophy disorders with the microRNA antagonists. Also included are combination therapies, wherein a therapeutic composition disclosed herein and an additional therapy agent are provided to a subject having or suspected of having cardiac disease and/or muscular dystrophy disorder. In particular, some embodiments disclosed herein relate to compositions and methods for transiently administering a mixture of microRNA antagonists for promoting cardiomyocyte proliferation and cardiac regeneration.