The purpose of this study is to evaluate the long-term safety and efficacy of donidalorsen in people with HAE and the effects of donidalorsen on the number of HAE attacks and their impact on quality of life (QoL).

This is a Phase 3, multi-center, open-label, global study with donidalorsen in up to approximately 144 participants with HAE-1 (Type I) and HAE-2 (Type II). There are two groups in this study: 1) participants who roll over from another study of donidalorsen (open-label extension [OLE] participants), and 2) new participants who are not rolling over from another study of donidalorsen and were previously maintained on HAE prophylactic therapy with lanadelumab, berotralstat, or a C1-esterase inhibitor (C1-INH). The length of participation in the study is approximately 70 weeks for OLE participants and 76 weeks for other participants. Participants will receive donidalorsen in an Extended Treatment Period for up to an additional 104 weeks.

Inclusion Criteria

1. Participants and, as applicable, legally authorized representatives (i.e., parent(s)/legal guardian), must provide written and signed informed consent form (ICF).

2. Participants must have access to, and the ability to use, ≥ 1 acute medication(s) (e.g., plasma-derived or recombinant C1-INH concentrate or a bradykinin receptor (BK) 2-receptor antagonist) to treat angioedema attacks.

   Open-Label Extension Participants ONLY:

3. Satisfactory completion of ISIS 721744-CS5 (randomized placebo-controlled index study) through Week 25 or participants who are allowed to exit ISIS 721744-CS5 study per protocol with an acceptable safety and tolerability profile.

   New (not previously on donidalorsen) Participants ONLY:

4. Participants must be aged ≥ 12 years at the time of informed consent and, as applicable, assent.

5. Participants must have a documented diagnosis of HAE-1/HAE-2.

6. Participants must be on a stable dose (≥ 12 weeks) of prophylaxis treatment with lanadelumab or berotralstat or SC C1-esterase inhibitor prior to the Screening Period.

Exclusion Criteria

Open-Label Extension Participants:

1. Have any new condition or worsening of an existing condition or change or anticipated change in medication.

   New (not previously on donidalorsen) Participants ONLY:

2. Concurrent diagnosis of any other type of recurrent angioedema, including acquired, idiopathic angioedema or HAE with normal C1-INH (also known as HAE Type III).

3. Anticipated change in the use of concurrent androgen or tranexamic acid prophylaxis used to prevent angioedema attacks.

4. Any clinically-significant abnormalities in screening laboratory values.

5. Malignancy within 5 years of Screening, except for non-melanoma skin cancers, cervical in situ carcinoma, breast ductal carcinoma in situ, or stage 1 prostate carcinoma that has been successfully treated.

6. Hypersensitivity to the active substance (donidalorsen) or to any of the excipients.

7. Treatment with another investigational drug (non-oligonucleotide) or biological agent within 1 month of Screening or 5 half-lives of investigational agent, whichever is longer.

8. Recent history of, or current drug or alcohol abuse.

9. Participated in a prior donidalorsen study.

10. Exposure to any of the following medications:

    1. Angiotensin-converting enzyme (ACE) inhibitors or any estrogen containing medications with systemic absorption.
    2. Oligonucleotides (including small interfering ribonucleic acid [siRNA]) within 4 months of Screening if single dose received, or within 12 months of Screening if multiple doses received. This exclusion does not apply to vaccines.

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/.