This study focuses on Hereditary Angioedema, a rare condition that causes unexpected swelling in different parts of the body. This swelling can occur under the skin or in deeper tissues, such as the throat or digestive tract. The study aims to evaluate the safety and how the body processes donidalorsen, an experimental medication provided as a subcutaneous injection, which is an injection given just under the skin.

During the study, participants will receive the medication ISIS 721744. Researchers will monitor how the body handles the drug and look for any side effects. The study will also observe changes in plasma, which is the liquid part of the blood, and levels of plasma kallikrein, a specific protein involved in the swelling process. This investigation will take place over a period of 12 months to see how the treatment affects the frequency and severity of swelling episodes.