CARDIO-TTRansform: A Study to Evaluate the Efficacy and Safety of Eplontersen (Formerly Known as ION-682884, IONIS-TTR-LRx and AKCEA-TTR-LRx) in Participants With Transthyretin-Mediated Amyloid Cardiomyopathy (ATTR CM)

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.