Elevated cytoplasmic [Ca 2+ ] is characteristic in severe skeletal and cardiac myopathies, diabetes, and neurodegeneration, and partly results from increased Ca 2+ leak from sarcoplasmic reticulum stores via dysregulated ryanodine receptor (RyR) channels. Consequently, RyR is recognized as a high-value target for drug discovery to treat such pathologies. Using a FRET-based high-throughput screening assay that we previously reported, we identified small-molecule compounds that modulate the skeletal muscle channel isoform (RyR1) interaction with calmodulin and FK506 binding protein 12.6. Two such compounds, chloroxine and myricetin, increase FRET and inhibit [ 3 H]ryanodine binding to RyR1 at nanomolar Ca 2+. Both compounds also decrease RyR1 Ca 2+ leak in human skinned skeletal muscle fibers. Furthermore, we identified compound concentrations that reduced leak by > 50% but only slightly affected Ca 2+ release in excitation-contraction coupling, which is essential for normal muscle contraction. This report demonstrates a pipeline that effectively filters small-molecule RyR1 modulators towards clinical relevance. In striated muscle, contraction requires an intracellular Ca 2+-release event mediated by ryanodine receptors (RyR) that are embedded in the sarcoplasmic reticulum (SR) membrane. Dysregulation of skeletal (RyR1) and cardiac (RyR2) isoforms, via mutations or excess posttranslational modification, has been linked to severe muscle pathologies, including malignant hyperthermia (MH), central core disease, muscular dystrophy (MD), sarcopenia, catecholaminergic polymorphic ventricular tachycardia, heart failure, and more recently RyR2 has been recognized as a potentially significant contributor to diabetes and Alzheimer's disease 1-9. In most of these clinical indications, pathogenesis can be fueled by excess SR Ca 2+ "leak" via RyR under resting cellular conditions, which leads to toxic intracellular basal [Ca 2+ ] and insufficient SR Ca 2+ load. As a result, RyR is intensely studied as a therapeutic target. Indeed, the therapeutic potential of pharmaceutically targeting RyR1-mediated SR Ca 2+ leak in skeletal muscle has been shown in animal models of Duchenne MD, limb-girdle MD, and sarcopenia 5,10,11. The therapeutic potential of targeting RyR2-mediated SR Ca 2+ leak for treating heart failure and arrhythmia is also very well documented 12-16. Additionally, targeting RyR2 (which is abundant in the brain 17,18) may have therapeutic potential for treating neurodegenerative diseases. To introduce a systematic and efficient approach for identifying novel small-molecule chemical scaffolds with potential to mitigate RyR1 dysfunction, we developed and implemented a high-throughput screening (HTS) assay that uses fluorescence lifetime (FLT) detection of FRET 19. This assay was designed to identify compounds that bind to the RyR1 channel complex to allosterically correct its pathologically leaky state (without affecting normal channel function) 19. This FRET-based method is based on monitoring RyR binding of fluorescent...