Trial to Evaluate ALN-CIDEB in Adults With Metabolic Dysfunction-Associated Steatotic Liver Disease (MASLD)

This study will test a Regeneron study drug called ALN-CIDEB to find out whether it may help treat a liver disease called metabolic dysfunction-associated steatohepatitis (MASH). MASH is more advanced stage of a broader liver disease, MASLD. MASLD is a liver condition caused by too much fat building up in the liver. This can lead to liver scarring and inflammation and can reduce the liver's ability to keep the body healthy. Cell death-inducing DFFA-like effector B (CIDEB) is a protein made by liver cells that helps the liver to store fat. 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 ALN-CIDEB could affect the body and the liver, which can help researchers understand why ALN-CIDEB works better for some people than others

Key Inclusion Criteria: 1. Has a clinical diagnosis of MASLD, as defined in the protocol 2. Has a liver stiffness measurement between 6 and 20 kilopascals (kPa), as measured by a FibroScan® scan during screening or within approximately 3 months of screening, as defined in the protocol Key Exclusion Criteria: 1. Has a known chronic liver disease other than MASLD, as determined by the investigator, as defined in the protocol 2. Has prior or current suspected or known drug-induced liver injury within approximately 1 year prior to screening 3. Has a history of liver transplantation, current placement on a liver transplant list, or Model for End-Stage Liver Disease (MELD) score \>12 4. Has a known history of alcohol or other substance abuse within the last year or at any time during screening based on investigator's discretion 5. Has prior, current, or planned future use (during the treatment period) of a GLP-1 receptor agonist-based therapy or any medication approved for the treatment of MASH (eg, resmetirom) unless used at a generally stable dose and regimen since at least 3 months prior to screening visit and throughout the screening period with no change to the dose or regimen anticipated during the treatment period, as defined in the protocol 6. Has evidence of uncontrolled human immunodeficiency virus (HIV) infection, hepatitis B virus (HBV) infection, or hepatitis C virus (HCV) infection during screening, as defined in the protocol NOTE: Other Protocol-defined Inclusion/Exclusion Criteria Apply