A Study of Olezarsen (Formerly Known as AKCEA-APOCIII-LRX) Administered to Adults With Familial Chylomicronemia Syndrome (FCS) Previously Treated With Volanesorsen

Key Inclusion Criteria 1. Participants with FCS (clinical or genetic diagnosis) currently on or previously treated with volanesorsen (ISIS 304801) o Study participants in countries where Waylivra® is commercially approved and available for participants should not be deprived of the treatment option with Waylivra®. Participation in this study for such participants will only be allowed when Waylivra® was discontinued due to AEs 2. The following concomitant medications will be allowed if dosing regimen is expected to remain constant through the end of the study (occasional or intermittent use of over-the-counter (OTC) medications will be allowed at Investigator's discretion): * Statins, omega-3 fatty acids (prescription and OTC), fibrates, or other lipid-lowering medications. Participants taking OTC omega-3 fatty acids should make every effort to remain on the same brand through the end of the study * Antidiabetic medications * Oral anticoagulants (e.g., dabigatran, rivaroxaban, or apixaban, and warfarin with regular clinical monitoring) * Tamoxifen, estrogens or progestins Key Exclusion Criteria: 1. Treatment with another investigational drug (non-oligonucleotide), biological agent, or device within 4 weeks of Screening, or 5 half-lives of investigational agent, whichever is longer 2. Concomitant medication/procedure restrictions: 1. Systemic corticosteroids or anabolic steroids within 6 weeks prior to Screening and during the study unless approved by the Sponsor Medical Monitor 2. Plasma apheresis within 4 weeks prior to Screening or planned during the study