The purpose of this study is to evaluate the safety, tolerability, efficacy, and pharmacodynamics (PD) of multiple doses of ALN-6400 in adult and adolescent patients with VWD and HMB

Inclusion Criteria -

• Is an adolescent or adult patient with a diagnosis of VWD (including Type 1, Type 2, Type 3, and platelet-type VWD) and HMB for 2 cycles during screening

Exclusion Criteria -

• Has alanine aminotransferase (ALT) or aspartate aminotransferase (AST) greater than 2 times the upper limit of normal (ULN)
• Has total bilirubin greater than 1.5×ULN (except for patients with Gilbert's syndrome)
• Has an estimated glomerular filtration (eGFR) less than 30 mL/min/1.73m^2 at screening
• Is not willing to comply with the contraceptive requirements during the study period
• Used routine factor prophylaxis (greater than or equal to 1 infusion of factor per week over 12 weeks) within 4 weeks of screening or plans use for routine factor prophylaxis during the study
• New placement of any hormonal or nonhormonal intrauterine device (IUD) or hormonal implants within 24 weeks of screening
• Known current gynecological conditions causing abnormal uterine bleeding (including infection, fibroids, endometriosis, polycystic ovary syndrome, or dysplasia)

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.