The purpose of this study is to evaluate the safety and tolerability of extended dosing with eplontersen in participants with hereditary transthyretin-mediated amyloid polyneuropathy (hATTR-PN).

This is a multicenter, open-label, Phase 3 study in up to approximately 165 participants. Participants will also receive daily supplemental doses of the recommended daily allowance (RDA) of vitamin A. This study will consist of the following periods: less than or equal to (≤) 8-week screening period, a 157-week treatment period, and a 24-week post-treatment evaluation period.

Participants may continue to receive treatment in this study for up to 2 years or until ION-682884 becomes commercially available in the patient's country, whichever occurs earlier.

Inclusion Criteria:

1. Satisfactory completion of ION-682884-CS3 (NCT04136184) (Index Study) as judged by the Investigator and Sponsor, or diagnosis of hATTR-PN and satisfactory completion of either study ISIS 420915-CS101 or study 2018-P001436 (NCT03702829) (both are Investigator-Sponsored studies with inotersen - the unconjugated version of eplontersen) as judged by the Investigator and Sponsor.

2. Must have given written informed consent (signed and dated) and any authorizations required by local law and be able to comply with all study requirements.

3. Satisfy the following:

   1. Females: must be non-pregnant and non-lactating and either:

      • Surgically sterile (e.g., tubal occlusion, hysterectomy, bilateral salpingectomy, bilateral oophorectomy);
      • Post-menopausal (defined as 12 months of spontaneous amenorrhea in females > 55 years of age or, in females ≤ 55 years, 12 months of spontaneous amenorrhea without an alternative medical cause and follicle-stimulating hormone (FSH) levels in the postmenopausal range for the laboratory involved;
      • Abstinent*;
      • If engaged in sexual relations of child-bearing potential, agree to use highly effective contraceptive methods from the time of signing the informed consent form until at least 24 weeks after the last dose of eplontersen and agree to receive pregnancy tests per protocol.

   2. Males: Surgically sterile (i.e., bilateral orchidectomy) or abstinent*, if engaged in sexual relations with a woman of child-bearing potential (WOCBP), the participant or the participant's non-pregnant female partner must use a highly effective contraceptive method from the time of signing the informed consent form until at least 24 weeks after the last dose of eplontersen. *Abstinence (i.e., refraining from heterosexual intercourse throughout the duration of study participation) is only acceptable as true abstinence, i.e., when this is in line with the preferred and usual lifestyle of the participant. Periodic abstinence (e.g., calendar, ovulation, symptothermal, post-ovulation methods), declaration of abstinence for the duration of a trial and withdrawal are not acceptable methods of contraception.

4. Willingness to adhere to vitamin A supplementation per protocol.

Exclusion Criteria:

1. Have any new condition or worsening of existing condition that in the opinion of the Investigator or Sponsor would make the participant unsuitable for enrollment or could interfere with the participant taking part in or completing 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/.