JTV-519 free base (K201 free base) is a Ca²⁺-dependent blocker of sarcoplasmic reticulum Ca²⁺-stimulated ATPase (SERCA) and a partial agonist of ryanodine receptors in striated muscle. Antiarrhythmic and cardioprotective properties^[1][2]. In Vitro: JTV-519 (K201) inhibits inward Ca²⁺ movement into large unilamellar vesicles (LUV) caused by annexin V in a dose-dependent manner. In the presence of 50 nM annexin V and 400 μM Ca²⁺, 3 μM JTV-519 shows significant inhibition of Ca²⁺ movement due to annexin V, and 50% inhibition is achieved at 25 μM K201^[2]. In Vivo: JTV-519 (0.5mg/kg/h, i.v., 2 h before the surgery) improves cardiac function in CLP mice, where the fractional shortening (FS) and ejection fraction (EF) are significantly increased as compared with CLP mice without JTV-519 treatment^[3].