Tracers targeting phosphodiesterase 1 for use in gamma radiation detection-based diagnostic techniques, particularly gamma-emitter labeled tracers for SPECT and positron emitter-labeled compositions for PET are disclosed. Radio-labeled multiple novel scaffolds as PDE1 inhibitors such as substituted pyrazolo-pyrimidin-4-one derivatives, biomarkers for phosphodiesterase 1 [PDE1) in vivo, methods for developing novel therapies for PDE1-implicated conditions such as pulmonary arterial hypertension (PAH), Central Nervous System (CNS) and Cardiovascular (CV) disorders, and methods of detection and treatment are also disclosed.
