This study compares **ALXN2220** to a **placebo** (a substance with no active medicine) for adults with **ATTR-CM**, a type of heart disease caused by a protein build-up. Participants will be checked for heart-related deaths and events. To join, you need to have confirmed ATTR-CM, certain heart and blood test results, and be on a specific heart medicine. Your heart should be functioning at a certain level, and you must be at least 6 months away from predicted life expectancy. You cannot join if you have other serious health problems like certain types of amyloidosis (a condition where abnormal proteins build up in organs), recent heart surgery, or severe kidney issues.

• The study will monitor your health for any heart events or deaths.
• Participants must be using birth control if able to have children.
• Some medical conditions will exclude you from participating.