This study is enrolling subjects with non-obstructive hypertrophic cardiomyopathy (nHCM). nHCM is typically a genetic condition in which the main pumping chamber of the heart (called the left ventricle) becomes abnormally thickened and stiff, which makes it harder for the ventricle to fill and pump out enough blood. This study involves the investigational medication Aficamten, which means it is not approved for commercial use by the Food and Drug Administration. (FDA) Aficamten is designed to reduced excessive heart pumping function. This is a randomized study which means all subjects are assigned to receive either Aficamten or placebo. Subjects have a 50:50 chance of being assigned to either group, but will not know which group they are assigned. Placebo looks like the medication but does not have any active ingredients in it. Study procedures include exercise testing, echocardiograms (ultrasound test of the heart), blood work, questionnaires and genetic testing. Study risks include risks associated with the study medication including decreased heart pumping, nausea, headache and dizziness. There are also study procedure related risks, and the risk of loss of confidentiality. There may be no benefit but the information learned may benefit others in the future. Study participation will last between 10.5 and 19 months and include up to 13 visits to the study site. Visits will generally last 2-3 hours.

This study is for patients with obstructive hypertrophic cardiomyopathy (oHCM). oHCM is a condition where the heart muscle becomes abnormally thickened, which can sometimes block the blood flow out of the heart and results in the heart muscle working harder to pump blood to the body.

The study is done to compare the side effects and effectiveness of an investigational (not yet approved by the Food and Drug Administration (FDA)) medication with the beta-blocker metoprolol succinate in participants with oHCM. The study medication is known as Aficamten and is a tablet taken by mouth. This is a randomized study (participants will be assigned by chance to the study medication Aficamten and placebo or metoprolol succinate and placebo). A placebo looks just like the study medication but has no active ingredient in it. The medications will be administered in the form of a pill. This study will take about 9 months and include about 11 visits to the study site. Study related procedures include blood work, echocardiograms (ultrasound test of the heart), electrocardiogram (recording of heart's electrical activity), exercise testing, physical exams, questionnaires and optional genetic testing. Risks associated with this study include shortness of breath, nausea, diarrhea, headaches and dizziness.