Myofilament-level effects of aficamten increase diastolic chamber volumes and maintain cardiac output through preserved length-dependent force generation in healthy rat and canine myocardium

Abstract Clinical trial experience supports the use of myosin inhibitors in hypertrophic cardiomyopathy (HCM), while early proof-of-concept studies suggest potential benefit in selected patients with heart failure with preserved ejection fraction (HFpEF). However, concerns about systolic dysfunction arising from their negative inotropic effects may limit broader application. In a series of in vivo and in vitro experiments in healthy animal preparations, we investigated how aficamten, a next-generation cardiac myosin inhibitor, interacts with key determinants of myocardial contractile function. Echocardiographic examinations in sedated Wistar rats revealed that bolus administration of aficamten (2 mg·kg −1 i.v.) did not reduce cardiac output (CO) at pacing frequencies of 300 and 400 bpm, despite marked increases in end-diastolic (ED) and end-systolic left ventricular (LV) volumes and the associated decrease in LV ejection fraction (EF). In field-stimulated intact cardiomyocytes from Mongrel dogs, increasing aficamten concentrations (0.1–1 µM) decreased contraction amplitude and diastolic duration. These effects occurred across pacing frequencies (0.25–1.25 Hz) and in the absence of aficamten-specific changes in intracellular Ca 2+ transients. In permeabilized LV cardiomyocytes from dog and rat hearts, aficamten decreased active force production (F max ) and its Ca 2+ sensitivity under isometric conditions in a similar manner. Importantly, the sarcomere length dependence of F max was preserved, while the length-dependent increase in Ca 2+ sensitivity was attenuated at higher (0.1–0.2 µM) aficamten concentrations in both species. Our findings in healthy myocardium characterize aficamten as a myosin inhibitor that increases diastolic chamber volumes through reduced contractility, with diastolic function indices (i.e. E/e', IVRT) reflecting the expected hemodynamic consequences of increased preload rather than direct impairment of myofibrillar relaxation. This enhanced preload recruits preserved sarcomere length-dependent force generation to compensate for negative inotropy, thereby maintaining stroke volume and cardiac output.