Takeda TAK-999-3001

The main aim of this study is to learn if fazirsiran reduces liver scarring (fibrosis) compared to placebo. Other aims are to learn if fazirsiran slows down the disease worsening in the liver, to get information on how fazirsiran affects the body (called pharmacodynamics), to learn if fazirsiran reduces other liver injury (inflammation) and the abnormal Z-AAT protein in the liver, to get information on how the body processes fazirsiran (called pharmacokinetics), to test how well fazirsiran works compared with a placebo in improving measures of liver scarring including imaging and liver biomarkers (substances in the blood that the body normally makes and help show if liver function is improving, staying the same, or getting worse) as well as to check for side effects in participants treated with fazirsiran compared with those who received placebo.

Participants will either receive fazirsiran or placebo. Liver biopsies, a way of collecting a small tissue sample from the liver, will be taken twice during this study.

Inclusion criteria:

• The participant must have a diagnosis of the Z allele homozygotes (PiZZ) genotype AATD. PiZZ diagnosis from source verifiable medical records is permitted. Otherwise, participants must undergo PiZZ confirmatory testing (genotyping for PiS and PiZ alleles) at screening. PiMZ or PiSZ genotypes are not permitted.

• The participant, of any sex, is aged 18 to 75 years, inclusive.

• The participant's liver biopsy core sample collected should meet the requirements of the protocol.

• The participant has evidence of METAVIR stage F2, F3, or F4 liver fibrosis, evaluated by a centrally read baseline liver biopsy during the screening period; or confirmed as meeting all the entry criteria by central reading of a previous biopsy conducted within 6 months before the estimated enrollment date using an adequate liver biopsy and slides as defined in the study laboratory manual.

• The participant has a pulmonary status meeting the protocol's requirements.

• It must be confirmed that the participant does not have HCC. Participants will be screened for HCC with alpha-fetoprotein (AFP) and abdominal ultrasound. If the participant has any of the following, they will be required to have contrast-enhanced CT or MRI imaging to exclude HCC before randomization.

• An adult participant must have a body mass index (BMI) between 18.0 and 39.0 kilograms per meter square (kg^m2), inclusive.

• The participant is a nonsmoker for at least 12 months before screening.

Exclusion Criteria

• The participant has a history of liver decompensating events (overt hepatic encephalopathy [West Haven Grade >=2] documented by a physician, clinically significant ascites, spontaneous bacterial peritonitis, GI bleeding from varices, hepatopulmonary syndrome, hepatorenal syndrome, portal pulmonary hypertension, or portal gastropathy).

• The participant has a history of the presence of medium or large varices or varices with red wale signs based on a previous esophagogastroduodenoscopy (EGD) within 6 months before the estimated enrollment date. For certain participants, an EGD will be required at screening if there is no EGD available within 6 months before the estimated enrollment date. Presence of small varices with no red wale signs on EGD and no history of bleeding will be acceptable for study eligibility.

• The participant has evidence of other forms of chronic liver diseases, including viral hepatitis B or C, primary biliary cirrhosis, primary sclerosing cholangitis, Wilson disease, alcoholic hepatitis, hemochromatosis, liver cancer, history of biliary diversion, or autoimmune hepatitis.

• The participant has alanine transaminase (ALT) or aspartate transaminase (AST) levels >250 units per liter (U/L).

• The participant has a platelet count