This is a Phase 3, multicenter, randomized, double-blind, placebo-controlled study to evaluate the efficacy and safety of once-daily orally administered deucrictibant extended-release tablet compared to placebo for prophylaxis to prevent angioedema attacks in participants aged ≥ 12 years with hereditary angioedema.

The study consists of a Screening Period during which eligibility is confirmed, a Treatment Period of 24 weeks, and a Follow-up Period of maximum 4 weeks or subjects may roll over into the open-label study PHA022121-C307 (CHAPTER-4). During the Treatment period participants will receive blinded study drug (deucrictibant or placebo randomized in a 2:1 ratio). Participants will undergo regular efficacy and safety assessments, complete an electronic diary daily, and also complete questionnaires at predefined timepoints during the study.

Inclusion Criteria:

1. Provision of written informed consent/assent.
2. Male or female, aged ≥12 years at the time of providing written informed consent/assent.
3. Diagnosis of hereditary angioedema (HAE)
4. History of at least 3 HAE attacks within the 3 consecutive months prior to Screening Visit
5. Predefined number of attacks during the Screening Period
6. Reliable access and ability to use standard of care on-demand treatments to effectively manage acute HAE attacks.
7. Willing and able to adhere to all protocol requirements, including eDiary and ePRO data recording.
8. Female participants of childbearing potential must agree to the protocol specified pregnancy testing and contraception methods.

Exclusion Criteria:

1. Any diagnosis of angioedema other than HAE
2. Participation in a clinical study with any other investigational drug within the last 30 days or within 5 half-lives of the investigational drug at Screening (whichever is longer)
3. Has received prior prophylactic treatment with deucrictibant
4. Exposure to ACE inhibitors or any estrogen-containing medications with systemic absorption within 4 weeks of Screening
5. Prior gene therapy for any indication at any time
6. Use of prophylactic treatment for HAE within 2 weeks of Screening for C1INH, oral kallikrein inhibitors, or anti-fibrinolytics; within 4 weeks of Screening for attenuated androgens; within 5 half-lives of Screening for monoclonal antibodies, or within 7 days of Screening for short-term prophylaxis
7. Any females who are pregnant, plan to become pregnant, or are currently breast-feeding
8. Abnormal hepatic function
9. Moderate or severe renal impairment
10. Any clinically significant comorbidity or systemic dysfunction that would interfere with the participant's safety or ability to participate in the study.
11. History of alcohol or drug abuse within the previous year, or current evidence of substance dependence or abuse
12. Use of medications that are moderate and strong inhibitors or strong inducers of CYP3A4 within the last 30 days or within 5 half-lives (whichever is longer) of the time of randomization
13. Known hypersensitivity to deucrictibant or any of the excipients of the study drug