A Phase 3 Study to Evaluate the Efficacy and Safety of ARO-APOC3 in Adults with Familial Chylomicronemia Syndrome

INTERVENTION: ARO‐APOC3 (25 mg or 50 mg) or placebo on Day 1, then Q3M for a total of 4 doses. CONDITION: Familial Chylomicronemia Syndrome (FCS) PRIMARY OUTCOME: Percent change from baseline at Month 10 in fasting TG SECONDARY OUTCOME: 1. Percent Change from Baseline in Apolipoprotein C‐III (APOC3) at Month 10; 2. Percent Change from Baseline in Non‐High Density Lipoprotein Cholesterol (Non‐HDL‐C) at Month 10 ; 3. Percent Change from Baseline in High Density Lipoprotein Cholesterol (HDL‐C) at Month 10 ; 4. Percent Change from Baseline in Fasting TG at Month 12 ; 5. Percent Change from Baseline in Fasting APOC3 at Month 12 ; 6. Percent Change from Baseline in Fasting Non‐HDL‐C at Month 12; 7. Percent Change from Baseline in Fasting HDL‐C at Month 12; 8. Proportion of Patients Achieving TG of < 500 mg/dL at Month 10; 9. Proportion of Patients Achieving TG of < 500 mg/dL at Month 12; 10. Change from Baseline in Fasting TG Over Time; 11. Percent Change from Baseline in Fasting TG Over Time; 12. Number of Participants with Treatment‐Emergent Adverse Events (AEs) and/or Serious Adverse Events (SAEs) ; 13. Number of Participants with Positively Adjudicated Events of Acute Pancreatitis INCLUSION CRITERIA: 1. Fasting TG >=10 mmol/L ( 880 mg/dL) at screening, that are refractory to standard lipid lowering therapy 2. A diagnosis of FCS 3. Willing to follow dietary counseling as per PI judgment based on local standard of care 4. Participants of childbearing potential (male and female) must use highly effective contraception during the study and for at least 24 weeks following the last dose of study medication. Men must not donate sperm during the study and for at least 24 weeks after the last dose of IP. 5. Women of childbearing potential must have a negative pregnancy test at Screening and cannot be breastfeeding 6. Women of childbearing potential on hormonal contraceptives must be stable on the medication for >=2 menstrual cycles prior to Day 1.