Participants will be screened and 480 eligible participants will be randomized: 60 participants per each of six ALN-PCSSC dose groups plus 120 participants total across the placebo groups (20 participants each to match each of the six drug dose groups). Treatment allocation will be stratified by country and by current use of statins or other lipid-modifying therapies. Each participant will receive either one or two injections on Day 1 or a single injection on Day 1 and on Day 90 of blinded ALN-PCSSC or placebo.