Assessment of Safety, Tolerability, Immunogenicity, and Pharmacokinetics of AZD3427

Inclusion Criteria: * Part A will include healthy men and non-pregnant, non-lactating females of non-childbearing potential with a body mass index (BMI) of 18-30 kg/m\^2 and a weight of 55-100 kg. One cohort will require participants be of Japanese descent * Part B will include men and non-pregnant, non-lactating females of non-childbearing potential * Participants have a BMI of 18-40 kg/m\^2 and a weight of 55-136 kg * Participants with a diagnosis of stage C HF New York Heart Association (NYHA) Class I-III on stable medical therapy for at least 12 weeks * Participants with diagnosis of HFrEF will be defined as those with EF ≤ 40% and HF with EF ≥ 41% * Participants either with N-terminal prohormone of brain natriuretic peptide (NT-proBNP) \> 125 pg/mL or BNP \> 35 pg/mL (46) Exclusion Criteria: Both Part A and Part B will exclude participants with any of the following: * Any clinically important illness, medical/surgical procedure or trauma within 4 weeks of the first administration of study drug or planned surgical procedure before study completion * History of vascular and left ventricular aneurysms or prior dissections * Any history of joint hypermobility, Marfan's syndrome, or any connective tissue disorder * Clinical signs and symptoms consistent with Coronavirus disease-19 or confirmed infection within the last 4 weeks * History of severe allergy/hypersensitivity or ongoing clinically important allergy/hypersensitivity, as judged by the Investigator, or history of hypersensitivity injection devices or to drugs with a similar chemical structure or class to AZD3427 or any component of AZD3427 In addition, Part A will exclude participants with any of the following: * Alanine Aminotransferase (ALT) \> Upper limit of normal (ULN) * Aspartate Aminotransferase (AST) \> ULN * Total bilirubin \> ULN (unless due to Gilbert's syndrome) * Creatinine \> ULN * White blood cell (WBC) count \< Lower limit of normal (LLN) * Hemoglobin \< LLN * Prolonged QTcF \> 450 m * Shortened QTcF \< 340 ms * Family history of long QT syndrome * PR (PQ) interval shortening \< 120 ms (PR \> 110 ms but \< 120 ms is acceptable if there is no evidence of ventricular pre-excitation) * PR (PQ) interval prolongation (\> 240 ms) intermittent second (Wenckebach block while asleep is not exclusive) or third-degree atrioventricular (AV) block, or AV dissociation * Persistent or intermittent complete bundle branch block, incomplete bundle branch block, or intraventricular conduction delay with QRS \> 110 ms. Participants with QRS \> 110 ms but \< 115 ms are acceptable if there is no evidence of eg, ventricular hypertrophy or pre-excitation In addition, Part B will exclude participants with any of the following: * Atrial fibrillation or flutter occurring in the past year * Clinically significant ventricular arrhythmias under treatment * High-degree AV block II-III or sinus node dysfunction * Implanted permanent pacemaker or implantable cardioverter defibrillator for which the participant is pacing-dependent * Severe right-sided valvular heart disease; severe mitral regurgitation; moderate or severe mitral stenosis, severe aortic regurgitation and mild, moderate or severe aortic stenosis * Other conditions where vasodilatory therapy may be contraindicated (hypertrophic obstructive cardiomyopathy, and restrictive cardiomyopathy) * Congenital heart disease * NYHA HF Class IV * Occurrence in the last 6 months of acute coronary syndrome, percutaneous coronary intervention, cerebrovascular accident or transient ischemic attack, HF hospitalization; history or suspicion of cardiac amyloidosis * ALT \> 1.5 × ULN * AST \> 1.5 × ULN * Total bilirubin \> ULN (unless due to Gilbert's syndrome) * Impaired renal function, defined as eGFR \< 30 mL/min/1.73m\^2 assessed by the Chronic Kidney Disease Epidemiology Collaboration equation * WBC \< LLN * Hemoglobin \< 10g/L * PR (PQ) interval prolongation (\> 220 ms) * Participants with persistent BBB and QRS (ECG interval measured from the onset of the QRS complex to the J point) duration \> 130 ms. Participants with intraventricular conduction delay and QRS duration \< 130 ms