Extension Study of Oral PHA-022121 for Acute Treatment of Angioedema Attacks in Patients With Hereditary Angioedema

Key Inclusion Criteria: 1. Provision of written informed consent. If the participant is a minor (i.e., \<18 years of age or as determined by local law), consent will be obtained from the participant's parent/legally designated representative/guardian and written assent will be obtained from the participant, per country regulations. 2. For participants from Study C201, received at least one dose of study drug (including the non-attack visit) in Study C201. For participants from Study C306, participant was randomized (and for adolescent participants 12 to \<18 years received a dose of study drug in a non-attack state at Visit 1) and completed Study C306, with 2 attacks treated, or after closure of that study by the Sponsor. Enrollment of adolescents (≥12 to \<18 years or age of adulthood as defined locally) from these studies is with consideration of local age requirements. 3. Female participants of childbearing potential (or who become of childbearing potential during the study) must agree to the protocol-specified pregnancy testing and to be abstinent from heterosexual intercourse or to use an acceptable contraception method as defined in the protocol and as available locally from enrollment until 30 days after the last study drug administration. 4. In the opinion of the Investigator, the participant (and parent/caregiver for adolescent participants) is willing and able to comply with the protocol. Key Exclusion Criteria: 1. Any female who is pregnant, plans to become pregnant, or is breast-feeding. 2. Any other systemic disease (e.g., cardiovascular, gastrointestinal, renal, respiratory, neurological) or significant disease or disorder that, in the opinion of the Investigator, would interfere with the participant's safety or ability to participate in the study. 3. Use of lanadelumab for long-term HAE prophylactic therapy within 12 weeks prior to enrollment in Part A. 4. For Part A: Use of C1-esterase inhibitor, oral kallikrein inhibitors, attenuated androgens, anti-fibrinolytics, or monoclonal HAE therapy within a defined period prior to enrolment. For Part B: If a participant is receiving long-term prophylactic therapy with one of the following medications indicated for HAE: plasma-derived C1-INH, danazol at less than or equal to 200 mg/day, anti-fibrinolytics, berotralstat, or lanadelumab, they must be on a stable dose and regimen for at least 3 months before screening and intends to remain on the same dose for the duration of the study. 5. History of alcohol or drug abuse within defined period, or current evidence of substance dependence or abuse 6. Participation in any other investigational drug study within defined period 7. Discontinued from parent study after enrollment for any study drug-related safety reason or non-compliance including significant protocol deviation. 8. Use of concomitant medications that are strong CYP3A4 inhibitors (e.g., clarithromycin, erythromycin, itraconazole, ketoconazole, ritonavir) or strong CYP3A4 inducers (e.g., carbamazepine and phenytoin).