BACKGROUND: MK-0616 is an oral macrocyclic peptide inhibitor of proprotein convertase subtilisin/kexin type 9 (PCSK9) in development for the treatment of hypercholesterolemia. OBJECTIVES: This Phase 2b, randomized, double-blind, placebo-controlled, multicenter trial aimed to evaluate the efficacy and safety of MK-0616 in participants with hypercholesterolemia. METHODS: This trial was planned to include 375 adult participants with a wide range of atherosclerotic cardiovascular disease risk. Participants were assigned randomly (1:1:1:1:1 ratio) to MK-0616 (6, 12, 18, or 30 mg once daily) or matching placebo. The primary endpoints included percentage change from baseline in low-density lipoprotein cholesterol (LDL-C) at Week 8 and the proportion of participants with adverse events (AEs) and study intervention discontinuations due to AEs; participants were monitored for AEs for an additional 8 weeks after the 8-week treatment period. RESULTS: Of the 381 participants randomized, 49% were female, and the median age was 62 years. Among 380 treated participants, all doses of MK-0616 demonstrated statistically significant (P < 0.001) differences in least squares mean percentage change in LDL-C from baseline to Week 8 vs placebo: -41.2% (6 mg), -55.7% (12 mg), -59.1% (18 mg), and -60.9% (30 mg). AEs occurred in a similar proportion of participants in the MK-0616 arms (39.5% to 43.4%) as placebo (44.0%). Discontinuations due to AEs occurred in 2 or fewer participants in any treatment group. CONCLUSIONS: MK-0616 demonstrated statistically significant and robust, dose-dependent placebo-adjusted reductions in LDL-C at Week 8 of up to 60.9% from baseline and was well tolerated during 8 weeks of treatment and an additional 8 weeks of follow-up. (A Study of the Efficacy and Safety of MK-0616 [Oral PCSK9 Inhibitor] in Adults With Hypercholesterolemia [MK-0616-008]; NCT05261126).