A Study to Evaluate the Effect of Aficamten in Pediatric Patients With Symptomatic Obstructive Hypertrophic Cardiomyopathy (oHCM).

Inclusion Criteria:

• Period 1: Treatment Period

• Males and females between 12 and < 18 years of age at screening and at Day 1.

• Body weight ≥ 45 kg for the initial cohort and then body weight ≥ 35 kg afterat least 10 participants in the initial cohort have undergone dose titration upto Week 4 without observed events of LVEF < 50% at the starting dose of 5 mgqd.

• Core laboratory confirmation of the following oHCM echocardiographic criteriaat screening:

• Left ventricular (LV) hypertrophy with nondilated LV chamber in the absence of othercardiac disease.

• LV end-diastolic wall thickness that meets a threshold of:

• Z-score > 2.5 in the absence of family history OR

• Z-score > 2 in the presence of positive family history or positive genetictest.

• LVEF ≥ 60% AND Valsalva LVOT-G ≥ 50 mmHg.

• oHCM of sarcomeric origin confirmed by genetic testing or, if unable to confirmby genetic testing, oHCM of sarcomeric origin may be presumed in the absence ofhistory of metabolic disorders, mitochondrial cardiomyopathies, neuromusculardisease, malformation syndromes, infiltrative diseases/inflammation, andendocrine disorders (such as Fabry's disease, Noonan syndrome with leftventricular hypertrophy, and amyloid-cardiomyopathy).

• New York Heart Association (NYHA) Class ≥ II at screening.

• Adequate acoustic windows for echocardiography.

• Participants on beta blockers, verapamil, diltiazem, or disopyramide shouldhave been on stable doses for more than 4 weeks prior to randomization.

• Period 2: Open-Label Extension

• Completed Period 1. If unable to complete Period 1 due to circumstances notrelated to compliance or safety, the Medical Monitor may review and determineeligibility.

• LVEF ≥ 55% after washout.

• Period 3: Long-term Extension • Completed Period 2.

Exclusion Criteria:

• Period 1: Treatment Period

Any of the following criteria will exclude potential participants from the trial:

• Significant valvular heart disease.

• Moderate or severe valvular aortic stenosis or fixed subaortic obstruction.

• Mitral regurgitation that is greater than mild in severity and not due tosystolic anterior motion of the mitral valve (per judgment of PrincipalInvestigator or designee).

• Evidence of fixed left-sided obstruction (eg, subaortic membrane, aortic valvestenosis, or coarctation of the aorta).

• History of LV systolic dysfunction (LVEF < 45%) or stress cardiomyopathy at any timeduring their clinical course.

• History of congenital heart disease other than oHCM (may be enrolled if nothemodynamically significant in the judgement of the Principal Investigator and studyMedical Monitor).

• Has been treated with SRT (surgical myectomy or percutaneous alcohol septalablation) within the preceding 6 months or has plans for either treatment during thetrial period.

• History of paroxysmal or persistent atrial fibrillation or atrial flutter.

• History of syncope, symptomatic ventricular arrhythmia, or sustained ventriculartachyarrhythmia within 3 months prior to screening.

• History or evidence of any other clinically significant disorder, malignancy, activeinfection, other condition, or disease that, in the opinion of the PrincipalInvestigator (or designee) or the Medical Monitor, would pose a risk to participantsafety or interfere with the trial evaluation, procedures, or completion.

• Current or previous use of drugs known to cause cardiomyopathy (eg, anthracyclines,monoclonal antibodies [trastuzumab], alkylating agents [cyclophosphamide], andtyrosine kinase inhibitors [sunitinib and imatinib]).

• Currently participating in another investigational device or drug trial or receivedan investigational device or drug < 1 month (or 5 half-lives for drugs, whichever islonger) prior to screening.

• Implantable cardioverter defibrillator (ICD) implantation within 6 weeks ofscreening or planned ICD implantation during the trial period.

• Has received prior treatment with aficamten or mavacamten.

• Currently listed for heart transplantation or anticipated to be listed for hearttransplantation in the next 12 months.