The invention relates to novel inhibitors of phosphodiesterase 1 (PDE1), according to Formula I, below, wherein R1, R2, R3 and R4 are as defined herein, useful for the treatment of diseases or disorders characterized by disruption of or damage to certain cGMP/PKG mediated pathways (e.g., in cardiac tissue). The invention further relates to pharmaceutical composition comprising the same and methods of treatment of cardiovascular disease and related disorders, e.g., congestive heart disease, atherosclerosis, myocardial infarction, and stroke.