Inclusion Criteria:
1. At least 18 years old at screening.
2. Body weight was greater than 45 kg at screening.
3. Diagnosed with obstructive hypertrophic cardiomyopathy (oHCM) consistent with current American College of Cardiology Foundation/American Heart Association, European Society of Cardiology, and Chinese Society of Cardiology guidelines, ie, satisfy criteria below (criteria to be documented by the echocardiography core laboratory): A. Had unexplained left ventricular (LV) hypertrophy with nondilated ventricular chambers in the absence of other cardiac (eg, hypertension, aortic stenosis) or systemic disease and with maximal LV wall thickness ≥15 mm (or ≥13 mm with positive family history of hypertrophic cardiomyopathy), as determined by core laboratory interpretation, and B. Had left ventricular outflow tract (LVOT) peak gradient ≥50 mm Hg during screening as assessed by echocardiography at rest or after Valsalva maneuver (confirmed by echocardiography core laboratory interpretation).
4. Had documented LV ejection fraction (LVEF) ≥55% by echocardiography core laboratory read of screening TTE at rest.
5. Had a valid measurement of Valsalva LVOT peak gradient at screening as determined by echocardiography core laboratory.
6. Had New York Heart Association (NYHA) class II or III symptoms at screening.
7. Had documented oxygen saturation at rest ≥90% at screening.
8. Study participants were able to understand and comply with the study procedures, understand the risks involved in the study, and provided written informed consent according to national, local, and institutional guidelines before the first study-specific procedure.
Exclusion Criteria:
* 1\) Participated in a clinical trial in which the participant received any investigational drug (or was currently using an investigational device) within 30 days prior to screening, or at least 5 times the respective elimination half-life (if known), whichever was longer.
2\) Known infiltrative or storage disorder causing cardiac hypertrophy that mimics oHCM, such as Fabry disease, amyloidosis, or Noonan syndrome with LV hypertrophy. 3) Had a history of syncope within 6 months prior to screening or sustained ventricular tachyarrhythmia with exercise within 6 months prior to screening.
4\) Had a history of resuscitated sudden cardiac arrest (at any time) or known history of appropriate implantable cardioverter-defibrillator (ICD) discharge for life-threatening ventricular arrhythmia within 6 months prior to screening.
5\) Had paroxysmal, intermittent atrial fibrillation with atrial fibrillation present per the investigator's evaluation of the participant's electrocardiogram (ECG) at the time of screening.
6\) Had persistent or permanent atrial fibrillation not on anticoagulation for at least 4 weeks prior to screening and/or not adequately rate-controlled within 6 months prior to screening (note: participants with persistent or permanent atrial fibrillation who were anticoagulated and adequately rate-controlled were allowed).
7\) Previously participated in a clinical study with mavacamten. 8) Hypersensitivity to any of the components of the mavacamten formulation. 9) Current treatment (within 14 days prior to screening) or planned treatment during the study with disopyramide, cibenzoline, or ranolazine. 10) Current treatment (within 14 days prior to screening) or planned treatment during the double blinded treatment with a combination of beta-blockers and verapamil or a combination of beta blockers and diltiazem.
11\) For individuals on beta-blockers, verapamil, or diltiazem, any dose adjustment of that medication within14 days prior to screening or any anticipated change in treatment regimen using these medications during the treatment.
12\) Had been successfully treated with invasive septal reduction (surgical myectomy or percutaneous alcohol septal ablation \[ASA\]) within 6 months prior to screening or planned to have either of these treatments during the study (note: individuals with an unsuccessful myectomy or percutaneous ASA procedure performed \>6 months prior to screening were enrolled if study eligibility criteria for LVOT gradient criteria were met).
13\) ICD placement within 2 months prior to screening or planned ICD placement during the study.
14\) Had QTcF \>500 msec when QRS interval \<120 msec or QTcF \>520 msec when QRS ≥120 msec or any other ECG abnormality considered by the investigator to pose a risk to participant safety (eg, second-degree atrioventricular block type II).
15\) Had documented obstructive coronary artery disease (\>70% stenosis in 1 or more epicardial coronary arteries) or history of myocardial infarction.
16\) Had known moderate or severe (as per investigator's judgment) aortic valve stenosis, constrictive pericarditis, or clinically significant congenital heart disease at screening.
17\) Had any acute or serious comorbid condition (eg, major infection or hematologic, renal, metabolic, gastrointestinal, or endocrine dysfunction) that, in the judgment of the investigator, could lead to premature termination of study participation or interfere with the measurement or interpretation of the efficacy and safety assessments in the study.
18\) Unable to comply with the study requirements, including the number of required visits to the clinical site.
19\) Pregnant or lactating female.
