To evaluate the efficacy of eplontersen compared to placebo in participants with ATTR-CM receiving available standard of care (SoC).

This is a multicenter, double-blind study in 1438 participants, who will be randomized to receive subcutaneous (SC) injections of either eplontersen or placebo once every 4 weeks. Participants will also receive daily supplemental doses of the recommended daily allowance of vitamin A.

Inclusion Criteria:

• Females must be non-pregnant and non-lactating, and either surgically sterile or post-menopausal or abstinent. If engaged in sexual relations of child-bearing potential, agree to use 1 highly effective contraceptive method
• Males must be surgically sterile or, abstinent or, if engaged in sexual relations with a woman of child-bearing potential, the participant or the participant's non-pregnant female partner must be using a highly effective contraceptive method
• Amyloid deposits in cardiac or non-cardiac tissue confirmed by Congo Red (or equivalent) staining OR technetium scintigraphy (99mTc -3,3-diphosphono-1,2- propanodicarboxylic acid [DPD-Tc], 99m Tc-pyrophosphate [PYP-Tc], or 99m Tc-hydroxymethylene-diphosphonate [HMDP-Tc]) with Grade 2 or 3 cardiac uptake in the absence of abnormal light chains ratio, centrally confirmed
• End-diastolic interventricular septum thickness of > 12 millimeters (mm) on Screening echocardiogram
• New York Heart Association (NYHA) class I-III

Exclusion Criteria:

• Acute coronary syndrome, unstable angina, stroke, transient ischemic attack (TIA), coronary revascularization, cardiac device implantation, cardiac valve repair, or major surgery within 3 months of Screening
• Cardiomyopathy not primarily caused by ATTR-CM, for example, cardiomyopathy due to hypertension, valvular heart disease, or ischemic heart disease
• Monoclonal gammopathy of undetermined significance (MGUS) and/or alterations in immunoglobulin free light chain (FLC) ratio unless fat, bone marrow, or heart biopsy confirming the absence of light chain and the presence of TTR protein by mass spectrometry or immunoelectron microscopy. For participants with chronic kidney disease (CKD) and without presence of monoclonal protein in blood and urine, the acceptable FLC ratio is 0.26-2.25. Results different from that may be discussed with local hematologist, Investigator and Medical Monitor if the risks associated with the biopsy outweigh the benefits
• Prior liver or heart transplant, and/or Left Ventricular Assist Device (LVAD) or anticipated liver transplant or LVAD within 1 year after randomization
• Current or previous treatment with Tegsedi™ (inotersen) or Onpattro™ (patisiran) or other oligonucleotide or ribonucleic acid (RNA) therapeutic (including small interfering ribonucleic acid [siRNA]; does not apply to COVID-19 mitochondrial [mRNA] vaccinations)
• Current treatment with diflunisal, doxycycline, with or without ursodeoxycholic acid, and/or non-dihydropyridine calcium-channel blocker (e.g., verapamil, diltiazem). Participants receiving any of these agents must respect a wash-out period of 14 days before randomization.

Ionis may share anonymized individual participant data, aggregated clinical data, and other types of data that support the results in this study. Data requests from qualified researchers will be considered once all three of the following criteria are met: (1) 12 months from marketing approval of the study drug in both the United States and European Union; (2) 18 months from conclusion of the study; and (3) 6 months from publication of study article. Access would be via a secure environment and is contingent upon approval of a research proposal and entry into an appropriate data use agreement. Requests to access data can be submitted via the website https://vivli.org/ourmember/ionis/.