**CLEOPATTRA Research Study Summary** This study tests a new medicine called NNC6019-0001 for people with heart failure due to a condition called transthyretin amyloid cardiomyopathy (ATTR-CM). ATTR-CM affects the heart by depositing abnormal proteins. Participants will receive either NNC6019-0001 or a placebo, which is a treatment with no active medicine. The assignment is random, and everyone will continue their regular heart treatments. **Key Points:** - **Study Duration:** Participants will be in the study for a period of time, continuing their regular heart treatments. - **Visits and Testing:** Participants might need to visit the study site for tests like heart scans and blood tests to monitor their condition. - **Risks and Benefits:** The study aims to see if the new medicine can reduce heart problems, but there might be risks involved, like reactions to the medicine. Participants should discuss potential risks and benefits with their doctor. **Eligibility:** Participants must be 18 or older with a confirmed diagnosis of ATTR-CM. They should not have other types of amyloidosis or severe health conditions that could interfere with the study.