Study to Evaluate ALN-CIDEB in Adults With Fibrotic Metabolic Dysfunction-Associated Steatohepatitis (MASH)

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