The purpose of this study is to:

• evaluate the safety, tolerability, pharmacokinetics (PK) and pharmacodynamics (PD) of single ascending doses of ALN-6400 in healthy volunteers
• evaluate the efficacy, safety, tolerability and PD of multiple doses of ALN-6400 in adult patients with HHT

Inclusion Criteria -

Part A:

• Is a healthy adult volunteer

Part B:

• Is an adult patient with a clinical diagnosis of HHT

Exclusion Criteria -

Part A:

• Has alanine aminotransferase (ALT) or aspartate aminotransferase (AST) > upper limit of normal (ULN)
• Has known human immunodeficiency virus (HIV) infection; or known current or chronic hepatitis C virus or hepatitis B virus infection
• Has an estimated glomerular filtration (eGFR) of <90 mL/min/1.73m^2 at screening

Part B:

• Has ALT or AST >2×ULN
• Has total bilirubin >1.5×ULN
• Has eGFR of <30 mL/min/1.73m^2 at screening

Parts A and B:

• Is not willing to comply with the contraceptive requirements during the study period

Note: other protocol defined inclusion / exclusion criteria apply

Phase 2-4:

Access to Anonymized individual participant data that support these results is made available 12 months after study completion and not less than 12 months after the product and indication have been approved in the United States (US) and/or the European Union (EU).

Data will be provided contingent upon the approval of a research proposal and the execution of a data sharing agreement. Requests for access to data can be submitted via the website www.vivli.org.