Evaluate Efficacy, Safety and Tolerability of JTT-861 in Subjects With Heart Failure With Reduced Ejection Fraction

Inclusion Criteria: * Has a clinical diagnosis of symptomatic heart failure (HF) ≥90 days prior to the Screening Visit; * Is in New York Heart Association (NYHA) functional class II or III at the Screening Visit; * Is on stable, guideline-directed therapy for HF, consistent with American Heart Association (AHA), American College of Cardiology (ACC), Heart Failure Society of America (HFSA) or European Society of Cardiology (ESC) guidelines for ≥4 weeks prior to the Screening Visit (with at least half of maximal labeled dose of renin-angiotensin-aldosterone system (RAAS) inhibitors and β-blockers, if tolerated); * Has left ventricular ejection fraction (LVEF) ≤35% at the Screening Visit; * Has a serum N-terminal pro b-type natriuretic peptide (NT-pro-BNP) level ≥600 pg/mL (or ≥900 pg/mL if the subject has atrial fibrillation or atrial flutter) at the Screening Visit. Exclusion Criteria: * Has a confirmed acute myocardial infarction (MI) (i.e., Type 1) or unstable angina within 90 days prior to the Screening Visit; * Has a history of coronary revascularization (percutaneous coronary intervention \[PCI\] and/or coronary artery bypass graft \[CABG\]) or other cardiovascular surgery within 90 days prior to the Screening Visit or planned cardiovascular surgery during the study through the Follow-up Visit); * Has started cardiac resynchronization therapy (CRT) within 90 days prior to the Screening Visit or has planned CRT during the study through the Follow-up Visit; * Has clinically significant congenital heart disease, active myocarditis or constrictive pericarditis; * Has current acute decompensated HF requiring additional treatment with diuretics, vasodilators and/or inotropic medications at the Screening Visit; * Has clinically significant chronic renal insufficiency (i.e., estimated glomerular filtration rate \[eGFR\] \<30 mL/min/1.73 m2 calculated by the Chronic Kidney Disease Epidemiology Collaboration \[CKD-EPI\] creatinine equation) at the Screening Visit.