Introduction:Hypertrophic cardiomyopathy (HCM) is a heritable disease characterized by cardiac remodeling, impaired relaxation, and exertional intolerance. Direct myosin-attenuation with Mavacamten can normalize contractility and improve exercise capacity in patients with obstructed HCM, providing sustained symptomatic relief. However, mava (and its surrogates) can also improve relaxation by limiting residual cross-bridges during diastole, and therefore, may offer cardiac benefits beyond obstruction reprieve. This in vivo study evaluated the chronic effects of MYK-581, a mava surrogate, in a genetic large-animal model of non-obstructed HCM.Methods:Young cloned Yucatan mini-pigs with a heterozygous MYH7 R403Q mutation were randomly assigned to one of two arms: time-controls (n=10) or daily MYK-581 (n=10; PO). After ~14 weeks of treatment, pigs underwent in vivo cMR imaging, including T1 mapping and extracellular volume (ECV) assessments. A subset of animals (MYK: n=6, CTRL: n=5) also underwent invasive hemodynamics assessments, including cardiac output (CO), load-independent systolic/diastolic function (via LV pressure-volume relationships), and ?-adrenergic (?-AR) cardiac reserve (dobutamine, 5 ug/kg/min IV).Results:In R403Q mutant pigs, MYK-581 treatment decreased (P<0.05) both EF (59 ? 2 vs. 65 ? 2%) and LV mass (51 ? 4 vs. 66 ?5 g), while preserving CO. Treated pigs had smaller left-atrial volumes (16 ? 1 vs. 29 ? 4mL, P<0.05) with lower T1-times and ECV (27 ? 1 vs. 32 ? 2%, P<0.05), suggesting improved LV structure/compliance. Indeed, the MYK-group had lower (P<0.05) LV end-diastolic pressures (9 ? 1 vs. 23 ? 4 mmHg) and stiffness (1.3 ? 0.2 vs. 3.5 ? 0.3 mmHg/mL) with faster time-constants of relaxation (45 ? 3 vs. 71 ? 5 ms, P<0.05). Treatment also rescued ?-AR stroke-volume recruitment (+15?4 vs. -14?6%, P<0.05).Conclusions:Chronic direct myosin attenuation with a mavacamten surrogate prevented cardiac remodeling characteristic of disease in a genetic HCM model. Chronic treatment improved diastolic function and cardiac reserve while reducing left atrial size, a known prognostic indicator in HCM. Taken together, these pre-clinical observations show potential salutary effects beyond obstruction relief in patients with HCM.