This study will test a study drug called ALN-PNP with and without another drug that is used for controlling blood sugar, appetite, and weight (for example, tirzepatide), to see if it can help treat MASLD, also known as fatty liver disease. ALN-PNP reduces the amount of Patatin-like phospholipase domain-containing protein 3 (PNPLA3), a protein that liver cells make, which may help decrease liver fat if there is an abnormal PNPLA3 protein.

The goal of this study is to understand the effect of ALN-PNP with or without tirzepatide on reducing liver fat.

The study is looking at:

• How well ALN-PNP with and without tirzepatide works
• What side effects ALN-PNP might cause
• How much ALN-PNP is in the blood at different times
• How the body and the liver change after having ALN-PNP, which can help researchers understand why ALN-PNP works better in some people than others

Key Inclusion Criteria:

Part A and Part B:

1. Homozygous for the PNPLA3 p.I148M genotype
2. Liver fat by Magnetic Resonance Imaging-Proton Density Fat Fraction (MRI-PDFF) ≥15% at visit 3
3. Has a Body Mass Index (BMI) ≥30 to <45 kg/m^2 at visit 2

Part A: To be eligible for randomization on study day 1:

1. Alanine Aminotransferase (ALT) and Aspartate Aminotransferase (AST) ≤3 × Upper Limit of Normal (ULN) as described in the protocol
2. On a stable dose of tirzepatide at randomization (≥5 mg weekly)

Key Exclusion Criteria:

1. Evidence or diagnosis of portal hypertension or cirrhosis from any cause, including cirrhosis due to MASH, as determined by the investigator, based on medical history, clinical assessment, imaging, and/or liver biopsy
2. Known chronic liver disease other than MASLD, as determined by the investigator, as defined in the protocol
3. Contraindications to MRI examinations, including but not limited to persons with MRI-incompatible cardiac pacemaker and implants made of metal, severe claustrophobia, size restrictions
4. Any contraindication listed in the Zepbound United States Prescribing Information (USPI), as defined 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