INTRODUCTION: Whether long-term treatment with the twice-yearly, siRNA therapeutic inclisiran, which reduces hepatic production of proprotein convertase subtilisin/kexin type 9 (PCSK9), results in sustained reductions in LDL cholesterol with an acceptable safety profile is not known. The aim of this study was to assess the effect of long-term dosing of inclisiran in patients with high cardiovascular risk and elevated LDL cholesterol. METHODS: ORION-3 was a 4-year open-label extension study of the placebo-controlled, phase 2 ORION-1 trial, conducted at 52 sites across five countries. Patients with prevalent atherosclerotic cardiovascular disease or high-risk primary prevention and elevated LDL cholesterol despite maximally tolerated statins or other LDL-lowering treatments, or with documented statin intolerance, who had completed the ORION-1 trial were eligible. Patients receiving inclisiran in ORION-1 received twice-yearly 300 mg subcutaneous inclisiran sodium throughout ORION-3 (inclisiran-only arm), whereas patients receiving placebo in ORION-1 first received subcutaneous evolocumab 140 mg every 2 weeks until day 360 thereafter transitioning to inclisiran twice-yearly for the remainder of ORION-3 study (switching arm). The primary efficacy endpoint was the percentage change in LDL cholesterol with inclisiran from the start of ORION-1 through to day 210 of the open label extension phase in the inclisiran-only arm (approximately 570 days of total inclisiran exposure in the modified intention-to-treat population). Secondary and exploratory endpoints included changes in LDL-C cholesterol and PCSK9 concentrations levels up to day 1440 (4 years) in each arm, and safety. ORION-3 is registered with ClinicalTrials.gov, NCT03060577. FINDINGS: Of the original ORION-1 cohort of 497 patients, 290 of 370 patients allocated to drug continued into the inclisiran-only arm and 92 of 127 patients allocated to placebo entered the switching-arm in the ORION-3 extension study conducted between March 24, 2017, and Dec 17, 2021. In the inclisiran-only arm, LDL cholesterol was reduced by 47·5% (95% CI 50·7-44·3) at day 210 and sustained over 1440 days. The 4-year averaged mean reduction of LDL-C cholesterol was 44·2% (95% CI: 47·1-41·4), with reductions in PCSK9 ranging from 62·2% to 77·8%. Adverse events at the injection site were reported in 39 (14%) of 284 patients in the inclisiran-only arm and 12 (14%) of 87 patients in the switching arm. The incidence of treatment-emergent serious adverse events possibly related to the study drug was 1% (three of 284) in the inclisiran-only arm and 1% (one of 87) in the switching arm. INTERPRETATION: Twice-yearly inclisiran provided sustained reductions in LDL cholesterol and PCSK9 concentrations and was well tolerated over 4 years in the extension study. This is the first prospective long-term study to assess repeat hepatic exposure to inclisiran. FUNDING: Novartis Pharma.