Background: Familial chylomicronemia syndrome (FCS) is a rare, autosomal recessive genetic disorder characterized by a marked increase in plasma triglyceride (TG) levels and recurrent episodes of pancreatitis. The response to conventional TG-lowering therapies is suboptimal. Volanesorsen, an antisense oligonucleotide that targets hepatic apoC-III mRNA, has been shown to significantly reduce TGs in patients with FCS., Objective: To further evaluate the safety and efficacy of extended treatment with volanesorsen in patients with FCS., Methods: This phase 3 open-label extension study evaluated the efficacy and safety of extended treatment with volanesorsen in three groups of patients with FCS: Those who had previously received volanesorsen or placebo in the APPROACH and COMPASS studies, and treatment-naive patients not participating in either study. Key endpoints included change in fasting TG and other lipid measurements, and safety over 52 weeks., Results: Volanesorsen treatment resulted in sustained reductions in plasma TG levels in previously treated patients from the APPROACH and COMPASS studies. Volanesorsen-treated patients from the three populations studied had mean decreases in fasting plasma TGs from index study baseline to months 3, 6, 12 and 24 as follows: decreases of 48%, 55%, 50%, and 50%, respectively (APPROACH); decreases of 65%, 43%, 42%, and 66%, respectively (COMPASS); and decreases of 60%, 51%, 47%, and 46%, respectively (treatment-naive). Common adverse events were injection site reactions and platelet count decrease, consistent with previous studies., Conclusion: Extended open-label treatment with volanesorsen in patients with FCS resulted in sustained reductions of plasma TG levels and safety consistent with the index studies., Competing Interests: Declaration of Competing Interest JLW is a consultant to Ionis Pharmaceuticals and ST is an employee of Ionis Pharmaceuticals. JLW and ST are co-inventors and receive royalties from patents owned by the University of California, San Diego on oxidation-specific antibodies and of biomarkers related to oxidized lipoproteins. They are co-founders and have an equity interest in Oxitope, Inc and its affiliates (“Oxitope”) as well as in Kleanthi Diagnostics, LLC (“Kleanthi”). Although these relationships have been identified for conflict of interest management based on the overall scope of the project and its potential benefit to Oxitope and Kleanthi, the research findings included in this particular publication may not necessarily relate to the interests of Oxitope and Kleanthi. The terms of this arrangement have been reviewed and approved by the University of California, San Diego in accordance with its conflict of interest policies. DG reports grants and personal fees from Ionis Pharmaceuticals during the conduct of the study; grants and personal fees from Allergan, Amgen, Amryt, Arrowhead, Eli Lilly, Novartis, NovoNordisk, Regeneron, and Sanofi outside the submitted work; grants from Acasti, Aegerion, Applied Therapeutics, AstraZeneca, Boehringer-Ingelheim, Ceapro, Dalcor, Esperion, Kowa, The Medicine Company, and Uniqure outside the submitted work; and personal fees from CRISPR Therapeutics, Saliogen, and Verve Therapeutics outside the submitted work. MA has received research grant support and lecturing fees from Alfasigma, Amgen, Amryt, Daiichi Sankyo, Ionis/Akcea, Novartis, Pfizer, Regeneron, and Sanofi. AJ has provided services to the following companies: Akcea, Amgen, Sanofi, and SOBI. HS received research and education grants and honoraria from Akcea Therapeutics, Alexion, Amryt, Synageva, Kowa, MSD, NAPP, Novartis, Pfizer, Sanofi, Synageva, and Takeda. IG-B has received personal honoraria from Aegerion, Akcea, Amarin, Amgen, Daiichi Sankyo, Novartis, Regeneron, and Sanofi; and nonfinancial support from Akcea, Amgen, and Sanofi. ESGS has received advisory board/lecturing fees, paid to institution, from: Amgen, Sanofi, NovoNordisk, AstraZeneca, Esperion, Daiichi-Sankyo, and Ionis/Akcea. No patents/stocks. VJA, LW, and SX are employees of Ionis Pharmaceuticals. RJ was an employee of Akcea Therapeutics at the time of the study and manuscript development and may own stock in Ionis Pharmaceuticals, parent company of Akcea Therapeutics., (Copyright © 2023 National Lipid Association. All rights reserved.)