Harmony-HHT: ATV-1601 in Participants With Hereditary Hemorrhagic Telangiectasia (HHT)

Inclusion Criteria:

* Ability to provide informed consent prior to any study-specific procedures
* Confirmed diagnosis of hereditary hemorrhagic telangiectasia (HHT) based on Curaçao criteria
* Moderate to severe HHT with an ESS ≥ 4
* Anemia at Screening and/or requirement for at least 1 red-cell unit (RUE) in the previous 6 months
* Adequate hematologic, renal, and hepatic function per protocol-defined laboratory criteria
* Use highly effective contraception during the study and for a protocol-defined period after last dose

Exclusion Criteria:

* Clinically significant abnormalities of glucose metabolism including diagnosed Type 1 or uncontrolled Type 2 diabetes
* Chronic cardiac disease, or cardiac rhythm abnormalities
* History of significant cardiovascular, hepatic, renal, or hematologic disease not related to HHT that may confound study results
* Use of prohibited concomitant medications within a protocol-defined washout period prior to first dose (including strong CYP modulators and certain herbal supplements)
* Recent (within 6 weeks) major surgery or local ablative procedures, or procedures on nasal telangiectasias
* Prior AKT inhibitor
* Pregnant or breastfeeding women

Additional Criteria for Open-Label Extension:

* Participants must complete the double-blind treatment period (Part 1)