PROJECT SUMMARY / ABSTRACT Senescence is a state of permanent cell cycle arrest, and can occur during development (programmed senescence), or be triggered by a variety of stressors, like telomere shortening (defined as replicative senescence), oxidative stress DNA damage, and oncogene activation (stress-induced premature senescence). Senescent cells remain metabolically active and secrete high concentrations of signaling molecules into the tissue microenvironment (known as Senescence-Associated Secretory Phenotype, or SASP). These secreted factors can activate immune systems to initiate clearance of senescent cells, which contribute to tumor suppression, would healing and tissue homeostasis. When the immunosurveillance fails to eliminate senescent cells, their accumulation causes inflammation and contributes to a variety of diseases, including cancer, metabolic disorders, fibrosis, diabetes, brain disorders, osteoarthritis, and kidney disease. These complex and important roles in ageing and diseases prompt extensive interest in developing imaging methods to detect senescence cells. A number of optical imaging probes have been developed by targeting senescence-associated beta-galactosidase (SA-b-gal) activity. However, their use in vivo is limited due to poor penetration and scattering photons in living tissues. Several SPECT, MRI, and PET probes have been reported to address this limitation, but they displayed either poor cell permeability and/or sensitivity. In the case of PET tracers, they generally suffer from a lack of efficient signal retention mechanism. This research proposes to develop a novel PET tracer for in vivo imaging of senescence in a mouse model of cancer by targeting a novel lysosomal enzyme and using an in celluo probe assembly retention strategy. There are two Specific Aims: 1) synthesize the PET tracer and assess its capacity to detecting senescence in cell culture; 2) validate the PET tracer in animal models of therapy induced senescence. This project would provide a more specific and sensitive PET tracer for imaging cell senescence in vivo and for future translation into human studies.