This study is testing a new medicine called **AZD0780** for adults with a type of high cholesterol called **HeFH** (Heterozygous Familial Hypercholesterolemia). People with HeFH have a hard time lowering their **LDL-C** (Low-Density Lipoprotein Cholesterol), often known as "bad" cholesterol. **ASCVD** (Atherosclerotic Cardiovascular Disease) includes conditions like heart attack or stroke. The study checks if AZD0780 can lower LDL-C and if it's safe to use. The participants will be given either AZD0780 or a **placebo** (a harmless pill that looks like the medicine) for one year. The study is **double-blind**, meaning neither the participants nor the doctors know who gets AZD0780 or placebo. Key Points: - **Duration**: Up to 56 weeks in total, including a 52-week treatment period. - **Visits**: Participants will have regular visits to the study center. - **Safety**: An independent team will ensure the safety of all participants. To join, you must be over 18, have HeFH, and be on a strong cholesterol-lowering treatment. You cannot join if you have certain other health issues or take specific medicines.