This study will test a Regeneron study drug called ALN-CIDEB to find out whether it may help treat a liver disease called MASH.

In this study, researchers are looking at the effect of ALN-CIDEB on reducing liver fat, liver injury, and liver scarring. The study will compare ALN-CIDEB with placebo to understand how well ALN-CIDEB works to lower the amount of fat in the liver.

The study is looking at:

• What side effects ALN-CIDEB might cause
• How well ALN-CIDEB works to change liver fat, liver injury, and liver scarring
• How the body and the liver change after having ALN-CIDEB, which can help researchers understand why ALN-CIDEB works better for some people than others

Key Inclusion Criteria:

1. A diagnosis of MASH documented in the participant's medical history, or a clinical suspicion of MASH based on non-invasive biomarkers and clinical risk factors, including having a history of 1 or more elements of metabolic syndrome as described in the protocol
2. Screening percutaneous liver biopsy demonstrating a NAFLD Activity Score (NAS) ≥4 and fibrosis stage F2 or F3 as described in the protocol
3. Has a FibroScan Aspartate aminotransferase (FAST) score >0.35 either at Screening Visit 1 or within approximately 3 months of Screening Visit 1 as described in the protocol

Key Exclusion Criteria:

1. Known chronic liver disease other than Metabolic dysfunction-Associated steatotic Liver Disease (MASLD), as determined by the investigator as described in the protocol
2. Prior or current suspected or known drug-induced liver injury within approximately 1 year prior to Screening Visit 1
3. History of liver transplantation, current placement on a liver transplant list, or MELD score >12
4. Known history of alcohol or other substance abuse within the last year or at any time during screening based on investigator's discretion and/or a score on the AUDIT questionnaire ≥8
5. Prior current, or planned future use of a Glucagon-Like Peptide-1 (GLP-1) receptor agonist-based therapy or any medication approved for the treatment of MASH unless used at a generally stable dose and regimen since at least 3 months prior to Screening Visit 1 or the qualifying historical liver biopsy and throughout the screening period with no change to the dose or regimen anticipated during the treatment period as described in the protocol

NOTE: Other Protocol-defined Inclusion/Exclusion Criteria Apply

All Individual Patient Data (IPD) that underlie publicly available results will be considered for sharing.

When Regeneron has:

• received marketing authorization from major health authorities (e.g., FDA, European Medicines Agency (EMA), Pharmaceuticals and Medical Devices Agency (PMDA), etc.) for the product and indication or has globally discontinued development of the product for all indications on or after April 2020 and has no plans for future development
• made the study results publicly available (e.g., scientific publication, scientific conference, clinical trial registry)
• the legal authority to share the data, and
• ensured the ability to protect participant privacy

Qualified researchers can submit a proposal for access to individual patient or aggregate level data from a Regeneron-sponsored clinical trial through Vivli. Regeneron's Independent Research Request Evaluation Criteria can be found at: https://www.regeneron.com/sites/default/files/Regeneron-External-Data-Sharing-Policy-and-Independent-Research-Request-Evaluation-Criteria.pdf