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)