The purpose of the study is to evaluate the safety, tolerability, pharmacokinetic (PK) and pharmacodynamic (PD) effects of olezarsen (formerly known as AKCEA -APOCIII-LRX) in participants with FCS previously treated with volanesorsen.

This is a Phase 3, multi-center, open-label safety study in 24 participants with FCS, previously treated with volanesorsen. The study consists of an 8- week screening period, treatment period up to week 209 and a 13-week follow-up period. Participants enrolled will receive olezarsen every 4 weeks during the 209-week Treatment Period.

Treatment period was extended to allow participants to receive olezarsen for an additional 52 weeks for a total of a 209-week treatment period until the drug may be commercially available in the patient's country, or until the Sponsor discontinues the olezarsen development program, whichever occurs earlier.

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

Ionis may share anonymized individual participant data, aggregated clinical data, and other types of data that support the results in this study. Data requests from qualified researchers will be considered once all three of the following criteria are met: (1) 12 months from marketing approval of the study drug in both the United States and European Union; (2) 18 months from conclusion of the study; and (3) 6 months from publication of study article. Access would be via a secure environment and is contingent upon approval of a research proposal and entry into an appropriate data use agreement. Requests to access data can be submitted via the website https://vivli.org/ourmember/ionis/.