The purpose of this study is to evaluate the safety, tolerability and pharmacokinetics of donidalorsen in pediatric participants with hereditary angioedema (HAE) Type I (HAE-1) or Type II (HAE-2).

This is an open-label study to evaluate the safety, efficacy, and pharmacokinetics (PK) and pharmacodynamics (PD) of donidalorsen in pediatric participants age 2 to less than 12 years old with HAE Type I (HAE-1) or Type II (HAE-2). The study consists of 3 parts: 1) a 3-month Screening Period, 2) a one-year Treatment Period, and 3) a 3-month Post-Treatment Period.

Key Inclusion Criteria:

1. Must be between the ages of 2 and less than 12 years, inclusive, at the time of informed consent and, as applicable, assent.

2. Must weigh at least 9 kg at the time of informed consent and, as applicable, assent.

3. Documented diagnosis of HAE-1/HAE-2 based upon both of the following:

   1. Documented clinical history consistent with HAE (SC or mucosal, non-pruritic swelling episodes without accompanying urticaria).
   2. Diagnostic testing results that confirm HAE-1/HAE-2: C1-inhibitor (C1-INH) functional level <50% normal level AND complement factor C4 level below the lower limit of normal (LLN); OR a known pathogenic mutation in the SERPING1 gene.

Key Exclusion Criteria:

1. Must not have any screening laboratory abnormalities or any other clinically significant abnormalities during screening that would render a participant unsuitable for inclusion.
2. Must not have been treated with another investigational drug, biological agent, or device within 1 month of Screening, or 5 half-lives of investigational agent, whichever is longer.
3. Concurrent diagnosis of any other type of recurrent angioedema, including idiopathic angioedema or HAE with normal C1-INH (HAE-nC1-INH or Type III).

Note: Other protocol-specified inclusion/exclusion criteria may apply.

Ionis may share anonymized individual participant data, aggregated clinical data, and other types of data that support the results in this study. Data requests from qualified researchers will be considered once all three of the following criteria are met: (1) 12 months from marketing approval of the study drug in both the United States and European Union; (2) 18 months from conclusion of the study; and (3) 6 months from publication of study article. Access would be via a secure environment and is contingent upon approval of a research proposal and entry into an appropriate data use agreement. Requests to access data can be submitted via the website https://vivli.org/ourmember/ionis/.