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

Prior treatment

• A prior solid organ transplant.
• Planned solid organ transplant during the study.

Other illness

• 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.
• 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.
• Acute coronary syndrome, unstable angina, stroke, transient ischemic attack (TIA), coronary revascularization, cardiac valve repair, or major surgery within 3 months of screening.
• History of contrast allergy or adverse reactions to gadolinium-containing agents.

Lab values

• 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.

Other

• Body weight \>120 kilogram (kg) (264.6 pounds \[lb\]) at screening.

• 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.