A Research Study to Look at How a New Medicine Called NNC6019-0001 Works and How Safe it is for People Who Have Heart Disease Due to Transthyretin (TTR) Amyloidosis

Efficacy and Safety of NNC6019-0001 at Two Dose Levels in Participants With Transthyretin Amyloid Cardiomyopathy (ATTR CM)

This study is testing a potential new medicine, NNC6019-0001, for people who have a heart disease due to TTR amyloidosis.The study will look at if this medicine can reduce the symptoms of a heart disease due to TTR amyloidosis, such as heart failure. Participants will either get NNC6019-0001 (apotential new medicine) or placebo (a medicine which has no effect on the body). Which treatment participants get is decided by chance. The chance of getting NNC6019-0001 is two times higher than getting placebo. NNC6019-0001 is not yet approved in any country or region in the world. It is a new medicine that doctors cannot prescribe yet. Participants will get an infusion of the study medicine 13 times, once every 4 weeks. The study will last for about 64 weeks after the first dose of medicine. Participants cannot participate in this study if they have a heart disease other than a heart disease due to TTR amyloidosis.

Inclusion Criteria:

• Male or female.
• Age greater than or equal to (>=) 18 to less than (<) 85 years at the time of signing informed consent.
• Have an established diagnosis of Transthyretin amyloid cardiomyopathy (ATTR CM) with either wild-type transhyretin (TTR) or hereditary transthyretin (TTR) genotype as per local standards.
• Expected to be on stable doses of cardiovascular medical therapy 6 weeks prior to the randomisation visit.
• Known end-diastolic interventricular septal wall thickness greater than or equal to (>=) 12 millimeters (mm).
• Presently classified as New York Heart Association (NYHA) Class II-III.
• N-terminal-pro brain natriuretic peptide (NT-proBNP) concentration greater than or equal to (>=) 650 picograms per milliliter (pg/mL) in sinus cardiac rhythm and greater than (>) 1000 pg/mL in atrial fibrillation at screening.
• Completed greater than or equal to (>=) 150 meters to less than or equal to (<=) 450 meters on the 6-minute walk test (MWT) at screening.
• Estimated glomerular filtration rate (eGFR) greater than or equal to (>=) 25 milliliter per minute per 1.73 meter square (mL/min/1.73 m^2) at screening.

Exclusion Criteria:

• Cardiomyopathy not primarily caused by transthyretin amyloid cardiomyopathy transthyretin amyloid cardiomyopathy (ATTR CM), for example, cardiomyopathy due to hypertension, valvular heart disease, or ischemic heart disease.
• A prior solid organ transplant.
• Planned solid organ transplant during the study.
• Presence or history of malignant neoplasm (other than basal or squamous cell skin cancer, insitu carcinomas of the cervix, or in-situ/high grade prostatic intraepithelial neoplasia (PIN) or low-grade prostate cancer) within 5 years before screening.
• Current treatment with calcium channel blockers with conduction system effects (example [e.g.], verapamil, diltiazem). The use of dihydropyridine calcium channel blockers is allowed. The use of digoxin will only be allowed if required for management of atrial fibrillation with rapid ventricular response.
• Acute coronary syndrome, unstable angina, stroke, transient ischemic attack (TIA), coronary revascularization, cardiac valve repair, or major surgery within 3 months of screening.
• Body weight >120 kilogram (kg) (264.6 pounds [lb]) at screening.
• History of contrast allergy or adverse reactions to gadolinium-containing agents.