This study evaluates the safety and efficacy of PHA-022121 administered orally for prophylaxis against angioedema attacks in patients with hereditary angioedema (HAE). The study consists of 2 parts, with patients completing participation in Part 1 prior to initiation of treatment in Part 2. Part 1 of the study has 3 parallel arms and approximately 30 patients will be equally randomized to one of two dose regimens of PHA-022121 or matching placebo. Patients will continue to the single open-label arm in Part 2 of the study after completion of Part 1. The screening period is up to 8 weeks and the treatment periods are 12 weeks (Part 1) and 30 months (Part 2) in duration.

Inclusion Criteria:

• Signed and dated informed consent form
• Diagnosis of HAE type I or II
• Documented history of at least 3 HAE attacks within the last 3 consecutive months prior to screening, or a minimum of 2 HAE attacks during the screening period
• Reliable access and experience to use standard of care acute attack medications

Exclusion Criteria:

• Pregnancy or breast-feeding
• Clinically significant abnormal electrocardiogram
• Any other systemic disease or significant disease or disorder that would interfere with the patient's safety or ability to participate in the study
• Use of C1-esterase inhibitor, oral kallikrein inhibitors, attenuated androgens, anti-fibrinolytics, or monoclonal HAE therapy within a defined period prior to enrolment
• Abnormal hepatic function
• Abnormal renal function
• History of alcohol or drug abuse within defined period, or current evidence of substance dependence or abuse
• Participation in any other investigational drug study within defined period